perception, action, and experience: unraveling the golden braid
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Neuropsychologia 47 (2009) 1460–1468
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erception, action, and experience: Unraveling the golden braid
ndy Clarkdinburgh University, Edinburgh, United Kingdom
r t i c l e i n f o
rticle history:eceived 25 April 2008eceived in revised form 14 October 2008ccepted 24 October 2008vailable online 5 November 2008
eywords:onsciousnessisionual visual systemsccessttentionemory
a b s t r a c t
Much of our human mental life looks to involve a seamless unfolding of perception, action and experience:a golden braid in which each element twines intimately with the rest. We see the very world we act inand we act in the world we see. But more than this, visual experience presents us with the world ina way apt for the control and fine guidance of action. Or so it seems. Milner and Goodale’s [Milner, D.,Goodale, M. (1995). The visual brain in action. Oxford: Oxford University Press; Milner, A., Goodale, M.(2006). Epilogue: Twelve years on. In Milner, A., Goodale, M. (Ed.), The visual brain in action (2nd ed., pp.207–252). Oxford: Oxford University Press] influential work on the dual visual systems hypothesis castsdoubt on certain versions of this intuitive vision. It does so by prising apart the twining strands of consciousvisual perception and the fine control of visuomotor action. Such a bold proposal is of major interest bothto cognitive science and philosophy. In what follows I first clarify the major claims that the bold proposalinvolves, then examine three sets of worries and objections. The first set concern some important matters
erceptionof detail. The second set concern a certain kind of conceptual or philosophical worry to the effect that theperception/action model unfairly equates visual experience itself with what are in fact certain elementswithin visual experience. The third set concern the very idea of conscious experience as a well-definedconceptual or experimental target. I conclude that the boldest versions of the Dual Visual Systems (2VS)story underestimate the variety and richness of visual experience, but that the general picture of visualuptake as a fragmented, multi-stream, multipurpose adaptation is correct, and still revealing after all these
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. A bold proposal: vision-for-perception, vision-for-action
Milner and Goodale’s bold proposal offers a certain func-ional gloss on the anatomical complexity of the visual system.his gloss takes as its starting point the existence of two majorrocessing streams (the ventral and the dorsal) projecting fromarly visual areas to the rest of the human brain. Ungerleidernd Mishkin famously depicted these as the ‘what’ and ‘where’treams, each specialized to perceive different aspects of the visualorld (Ungerleider & Mishkin, 1982). On this view, both streams
ontributed contents (though different ones) to human visualxperience. Milner and Goodale later described and defendedn alternative functional gloss, according to which one streamupports ‘vision-for-action’ and the other ‘vision-for-perception’Milner & Goodale, 1995). Thus the dorsal stream, projecting to
he posterior parietal cortex, is said to support the kinds of visuo-otor transformation in which visual input leads to fluent actionsuch as reaching and grasping, while the ventral stream, project-ng to the temporal lobe, seems to be especially implicated in the
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ecognition and identification of objects and events. Milner andoodale further suggest that the dorsal stream computes ‘vision-
or-action’ in a way that is fast, transient, and unconscious, and thathe ventral stream computes ‘vision-for-perception’ in a way that islower, more enduring, and at least sometimes conscious (GoodaleMilner, 2004; Milner & Goodale, 1995). The contents of conscious
isual experience, for Milner and Goodale, are thus strongly associ-ted with the coding and processing operations carried out by theentral stream.
To illustrate the way these two streams are then meant to inter-ct, Goodale and Milner develop an analogy with tele-assistancepproaches to the remote control of robots in distant or hostilenvironments (Goodale & Milner, 2004). Here, a conscious humanperator and a non-conscious semi-intelligent distal robot combineorces so as to perform actions in some environment (for example,n the control of a Mars rover, where the human operator reviewsmages on a screen in Texas, flagging items of interest that the robotan locate and retrieve using its own on-board sensory systems
nd sensorimotor routines). Such approaches are contrasted withele-operation solutions, in which the conscious human operatorontrols all the spatial and temporal aspects of the robots move-ents (perhaps via a joystick or a set of sensors that allow theperators own arm and hand movements to be relayed to the robot).
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2 For example, it has been shown that some visual illusions do affect visuomotor
A. Clark / Neuropsycho
he tele-assistance analogy thus identifies the role of the consciousuman operator with the role of the ventral stream (working inoncert with stored memory and various ‘executive control’ sys-ems). The task of this coalition, the analogy suggests, is to identifybjects and to select types of action that are appropriate givenhe agent’s current goals, background knowledge, and currentlyttended perceptual input. The task of the dorsal stream (and asso-iated structures) is then to turn these high-level specificationsnto metrically accurate, egocentrically specified forms of world-ngaging action. The dorsal stream (plus associated structures) thuslays the non-conscious robotic Mars Rover to the ventral coali-ion’s conscious human operator, such that:
“Both systems have to work together in the production of pur-posive behavior-one system to select the goal object from thevisual array, the other to carry out the required metrical com-putations for the goal-directed action.” (Goodale & Milner, 2004,p.100)
This picture of the distinct but interlocking contributions of thewo visual streams explains some of the coarse dissociations andsychophysical effects described in the literature. In particular, thisccount of the division of labour offers a neat explanation of the dif-ering pattern of deficits seen in patients with lesions affecting oner other of the two streams. Thus ventrally compromised patientssuch as DF) display severe impairments in recognizing shapes, ori-ntations, and objects. Not only can DF not recognize most everydaybjects, or faces, she cannot distinguish between squares, rectan-les, triangles and circles. In the famous ‘mailbox’ task DF was prettyell at chance for turning a handheld card to match the perceived
rientation of a posting slot. Yet asked to actually post the cardhrough the slot she was almost indistinguishable from normal con-rols (see Goodale, Milner, Jakobson, & Carey, 1991; Milner et al.,991, and (for a review) Milner & Goodale, 1995, pp. 128–133). Onn intuitive model, DF’s apparent lack of conscious visual awarenessf features such as shape and orientation might suggest that she hasfor whatever reason) simply failed to compute the very informa-ion needed to guide the relevant visuomotor actions. Yet her fluenterformance belies this. The dual streams/tele-assistance modelccounts for this, since the visual information required to supporthe world-engaging action is computed independently (though onhe basis of the same retinal input) of the information required toupport conscious identification.1
Moving to the case of normal, unimpaired subjects, the sametory neatly accounts for work by Aglioti, Goodale, and DeSouza1995). In this work, the experimenters set up a graspable versionf the famous Ebbinghaus or ‘Titchener Circles’ visual illusion inhich two central circles are presented, each surrounded by a ring
f other circles. In one case, the surrounding circles are larger thanhe central one. In the other, they are smaller. This leads to theell-known illusions in which subjects misjudge the relative size
f the two central circles. Such mistaken estimates of relative sizeo not, however, affect subjects’ abilities (in the physical, poker chipersion) to form precision grips that perfectly anticipate the trueize of the centre discs. The explanation (Goodale & Milner, 2004,
p. 88–89) is that the conscious scene is computed by the ventraltream in ways that are at liberty to make a variety of assumptionsn the basis of visual cues, e.g. attempting to preserve size con-tancy by treating the smaller circles as probably further away than1 Conversely, optic ataxics, with dorsal stream lesions, are adept at visually iden-ifying objects that they cannot fluidly reach and grasp. Optic ataxics: ‘have littlerouble seeing [i.e. identifying] objects in a visual scene, but a lot of trouble reach-ng for objects they can see. It is as though they cannot use the spatial informationnherent in any visual scene’ Gazzaniga (1998, p. 109).
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he larger ones. The dorsal stream, by contrast, uses only the kinds ofnformation that are metrically reliable and that thus present spe-ific opportunities for elegant, fast, metrically accurate diagnosis.or example, the dorsal stream may make great use of binocularepth information (Roy, Komatsu, & Wurtz, 1992). These differ-nces in processing, combined with the quasi-independent modesf operation of the two streams, account for the illusion’s ability tompact conscious visual experience while leaving our visuomotorngagements intact.
More recently, a similar effect has been shown using the so-alled ‘hollow face illusion’. In this illusion a concave model ofhuman face appears convex, due to the influence of top-down
nowledge concerning normal human faces. This suggests it is aurely ventral stream based illusion. Kroliczak, Heard, Goodale, andregory (2006) showed that in a task where subjects were asked toick small targets off the actually hollow (though visually convex)
ace, the flicking movements found the real (non-illusory) locationsf the targets. According to Milner and Goodale:
“This demonstrates that the visuomotor system can use bottom-up sensory inputs. . .to guide movements to the veridicallocations of targets in the real world, even when the perceivedpositions of the targets are influenced, or even reversed, by top-down processing” Milner & Goodale, 2006, p. 245
Such demonstrations, and the more general issue of perception-ction dissociations and visual illusions, have spawned a large andomplex literature devoted to the search for counter-examples,lternative explanations, exceptions, refinements, and additionalupport (for some useful reviews, see Carey, 2001; Clark, 2001;oodale & Westwood, 2004).2
Aspects of the dual visual systems picture are supported by aarge and impressively varied body of evidence including a swathef neuroimaging experiments (e.g. James, Culham, Humphrey,ilner, & Goodale, 2003; Le et al., 2002), a wide variety of single
ell recordings and other experimental interventions in monkeyse.g. Sakata, 2003; Taira, Mine, Georgopoulos, & Sakata, 1990),nd psychophysical experiments involving normal human subjectsBridgeman, Lewis, Heit, & Nagle, 1979; Fecteau, Chua, Franks, &nns, 2001; Goodale, Pelisson, & Prablanc, 1986, and (as we justaw) Aglioti et al., 1995). More generally, it may be observed thathe online control of motor action requires the extraction andse of radically different kinds of information (from the incomingisual signal) than do the tasks of recognition, recall and reasoning.he former requires a constantly updated (multiply) egocentricallypecified, exquisitely distance- and orientation-sensitive encodingf the visual array. The latter requires the computation of object-onstancy (objects do not change their identity every time they
ngagement. Importantly, however, this seems to be the case only when the illusions rooted in very early stages of visual processing (in primary visual cortex) and ishus ‘passed on’ to both streams when they subsequently diverge (Dyde & Milner,002; Milner & Dyde, 2003). This is, of course, fully compatible with the strongual systems view. Moreover, several other perceptual illusions have subsequentlyeen shown to affect conscious experience without impacting visumotor acts ofrasp scaling and reaching including the Ponzo (’railway lines’) and Müller-Lyerllusions (see Goodale & Milner, 2004, p. 89). In such cases, motor effects are observed
hen delays are introduced between viewing the illusion and producing the motoresponse. But this is as predicted by the model, which treats time-delayed actionss ‘pantomimed’ in that they cannot rely on the here-and-now computations of theorsal stream and are instead driven by the illusion-prone deliverances of the ventraltream (see Milner & Goodale, 1995, pp. 170–173).
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tsosome of them more plausible, and some of them less elusive (andsome both), than others. What they all have in common is thethought that the bold proposal illicitly identifies conscious visual
462 A. Clark / Neuropsycho
ther (Milner & Goodale, 1995, pp. 25–66). Different uses of visualnformation impose quite different computational demands on therain, so there are compelling computational and information-rocessing considerations that speak in favor of a dual (or at anyate, multiple) visual systems architecture. But just how all this inurn lines up with issues concerning conscious and non-consciousision remains open to question, as we shall see.
. Complications
The very strongest claim in this area, that the contents of con-cious visual experience are exclusively determined by the codingnd processing operations carried out by the ventral stream, nowooks to be empirically suspect.3 Schenk and Milner (2006) showhat form discrimination, in the visual form agnosic DF, can bemproved if she engages in a concurrent visuomotor task. Specifi-ally, when asked to name the shape of a visually presented objecthile reaching for the object, performance on the naming task was
ignificantly improved. Further experiments ruled out the sugges-ion that this effect is due to DF using proprioceptive or efferentues, suggesting instead that she is able directly to access shape-elevant information (in fact, it turns out to be information aboutidth) being processed by the dorsal stream. This means that DF’s
hape judgements can be influenced by ongoing dorsal streamctivity. Does this mean that processing in the dorsal stream canontribute to the contents of DF’s visual experience? It is not clear.or the possibility remains that the dorsal influence provides onlyome kind of elusive, non-visual cue. As the authors put it:
“better ‘discrimination’ does not necessarily imply better ‘per-ception’, and instead DF could have been employing someindefinable ‘implicit’ cues of the kind that enable blindsightpatients to perform above chance in discrimination tasks.”(Schenk & Milner, 2006, p. 1502)
It has also been suggested (Matthen, 2005) that dorsal process-ng may contribute a kind of non-specific ‘feeling of presence’ tour contact with the visual scene. To bring this idea into focus,t helps first to distinguish, following Matthen, ‘descriptive’ frommotion-guiding’ vision. Descriptive vision, as Matthen uses theerm, corresponds rather closely to the kind of vision supportedy ventral stream coding. It is the kind of vision that supportsensory classification, allowing us to experience the scene as com-osed of objects and elements that might be classified as similarr different in such-and-such sensory (visual) respects. Descrip-ive vision is thus, as we might say, ‘epistemically pregnant’: itresents the visual world in ways that are apt for reasoning abouthat world (see Clark, 2001). Matthen contrasts descriptive visionnd ‘motion-guiding vision’, identifying the latter as supported byoth non-cortical routes and the dorsal (cortical) stream. Accordingo Matthen (2005, p. 296) “the link between motion-guiding visionnd bodily motion is direct; it is not routed through consciousness”.onscious seeing may, of course, provide the information that leadss to choose a certain target and a certain type of action. But itoes not provide moment-by-moment guidance of fine visuomotorction. This picture thus comports nicely with Milner and Goodale’siew concerning the non-conscious nature of the processing and
ontrol operations carried out by the dorsal stream. Nonetheless,atthen suggests, there is at least one way in which that pictureay be incomplete. For it fails to recognize the dorsal processingrigins of the ‘feeling of presence’ that accompanies many of our
3 In addition, some aspects of online object engagement may require ongoingentral stream effort (grip force (Jackson and Shaw), functionally informed gripsGoodale and Milner, 2004), complex object engagements (McIntosh et al., 2004).
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isual encounters with the world. The idea here is that the kinds ofetrically accurate depth and location information computed by
he dorsal stream allow us to feel that we are really in the presencef, say, a cup and not merely seeing a 2D drawing or photographf a cup. This feeling of presence (which can be at least partiallyuped by some kinds of two dimensional depiction) is said to formgenuine part of our visual experience. Even though it is not partf ‘descriptive vision’, it “makes a difference to the quality of one’sisual awareness of an object” (Matthen, 2005, p. 301).
Rizzolatti and Matelli (2003) and Gallese (2007), suggest thathe dorsal/ventral distinction itself is too coarse-grained, andhould be replaced by a tripartite distinction between dorso-dorsal,entro-dorsal, and ventral streams (for the anatomical details, seeallese, 2007, Sections 2–4). The dorso-dorsal stream is said toupport fast, non-conscious visuomotor transformations, just asnvisaged by Milner and Goodale. While the ventral stream, agains Milner and Goodale suggest, supports object identification andlassification. But the ventro-dorsal stream, though itself anatom-cally part of the dorsal stream, nonetheless contributes, theseuthors suggest, to the determination of conscious experience. Oneajor way it does so, it is argued, is by supporting the conscious
isual experience of others’ intentional actions. Such a sugges-ion gains plausibility if one way in which we come to grasp theature and goals of others’ observed actions is by activating ourwn motor systems, perhaps under the influence of mirror sys-em (Rizzolatti, Fadiga, Gallese, & Fogassi, 1996) processing. Theentro-dorsal stream is also said to be essential for the consciousisual awareness of space as an arena for our own motor activity,s when we see objects as reachable, or as manipulable in such anduch ways. Holding both these strands together is the guiding ideahat parieto pre-motor circuits support various forms of ‘embodiedimulation’ (Gallese, 2005) such that “side-by-side with the sensoryescription of the observed phenomena, the motor schemata asso-iated with these actions, or objects, are evoked in the observer”Gallese, 2007, p. 14). These active motor schemas are said to be the
echanism by which “conscious awareness of actions and spatialocations are generated” (Gallese, 2007, p. 14).
In this section we have scouted what are perhaps best seens a series of complications: refinements and additions to thetrong dual visual systems view. Though outstanding questionsemain,4 accommodating these refinements and additions leavesntact many of the central tenets of the account. Other challenges,owever, go deeper than this, some of them threatening even thelaims concerning the non-conscious status of the basic (dorso-orsal) visuomotor transformation itself. It is to these challengeshat we now turn.
. The ‘Narrow Vision of Conscious Vision’ worry
Milner and Goodale’s bold proposal is, as we have already beguno see, subject to an interesting and important (though initiallyomewhat elusive) kind of worry. We can dub it the ‘Narrow Visionf Conscious Vision’ worry. The worry takes many shapes and forms,
xperience with one or more of the components, forms, or styles, of
4 One residual issue hereabouts, which I won’t attempt to resolve in this treat-ent, concerns the nature of these projected additional elements of conscious
xperience. Is the ‘feeling of presence’ truly part of my visual experience? Is theidth information available to DF for form discrimination really given to her as
ome (perhaps weak or indeterminate?) form of visual experience? Does the mean-ngfulness of a visually observed action really belong to the visual experience itself?.
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onscious visual experience. That is to say, conscious visual expe-ience may involve many different ‘things’, supported by manyifferent processing streams and neural coalitions, and it woulde a mistake to identify conscious vision with just one of them.
An analogy5 may be helpful here. Visual and auditory expe-iences are different phenomena, and depend on differentechanisms and forms of internal processing. In the light of thisundane fact, we would be wrong to identify what we perceptually
xperience (in general) with what we visually experience. Correl-tively, we would be wrong to simply identify the mechanisms oferceptual experience with the mechanisms of visual experience.ow suppose that within the realm of visual experience itself therexist multiple quite different varieties of experiential phenomena,ach supported by different types or forms of mechanism and pro-essing. Suppose too that one or more such elements turns out toepend preferentially, just as Milner and Goodale suggest, uponrocessing in the ventral visual stream. Still it would not followhat conscious visual experience itself (in all its varieties) dependsreferentially upon processing in that stream. To do so would be to
all prey to what I am calling a “Narrow Vision of Conscious Vision”rror.
Here are two (distinct but related) versions of the ‘narrow vision’orry drawn from the recent philosophical and cognitive neurosci-
ntific literature.
.1. Comparing like with like
Jeannerod and Jacob (2005) make a strong case that simpleomparisons between the contributions of the dorsal and ventraltreams are misleading insofar as they fail to control for three fac-ors. The factors concern direction of fit, direction of causation,nd level of processing. Let’s take the first two first. The under-ying model here is the philosophical distinction (Anscombe, 1957;earle, 1983) between states that are belief-like and states thatre desire-like. Beliefs have a mind-to-world direction of fit, andworld-to-mind direction of causation. That is, a belief is a mental
tate that aims to fit the way the world is (thus, it exhibits a mind-to-orld direction of fit). One good way to ensure that kind of fit is for
t to be caused by the way the world is (thus also exhibiting world-o-mind direction of causation). Desires, by contrast describe wayshe world should be (they exhibit world-to-mind direction of fit),nd they may function so as to help bring it about that the world behat way (thus displaying a mind-to-world direction of causation).otice, then, that different attitudes (belief versus desire) are hereistinguished in part in terms of their direction of fit. Thus an agentay be said to be in states that share a content (e.g. that the shops
e open) with differences in attitude (one may believe it, anotherope it, another fear it).
Apart from this question about direction of fit, there is a ques-ion concerning nature and level of processing. Thus consider nexthe various ways an active agent may need to use visually presentednformation. The information may be used to ensure that they knowow things are out in the world. Jeannerod and Jacob (2005) call thishe ‘semantic processing of visual information’ Or (or in addition) it
ay be used to help act upon, and alter, that world. They call this the
pragmatic processing of visual information’. Semantic processingas a belief-like profile, exhibiting a mind-to-world direction of fitnd a world-to-mind direction of causation. Pragmatic processingas a more desire-like profile,6 exhibiting a world-to-mind direc-5 This analogy was originally suggested to me by an anonymous referee for Clark2007).
6 The actual story is a little more complex, since ‘pragmatic’ visuomotor repre-entations are said (op cit p. 3) to have a hybrid direction of fit, insofar as they
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ion of fit and a mind-to-world direction of causation. Moreover,ach kind of processing involves many levels of abstraction. Earlytages of semantic processing yield representations with ‘pictorialon-conceptual contents’ (for present purposes, we can think ofhese as essentially ‘iconic’, non-propositionally structured con-ents, in the sense of Coltheart (1983), but see Clark (2001) forurther discussion). Later stages yield representations whose con-ents are more structured and highly processed, presenting a worldf objects, relations, and attributes, in ways apt to inform processesf explicit reasoning and planning. Similarly, early stages of prag-atic processing are said to represent basic geometrical properties
width, as it figures in Schenk and Milner’s (2006) study – see Sec-ion 2 – would be one such property) of objects in ways apt forhe guidance of actions such as fluent reaching and grasping, whileater stages yield more abstract, or ‘conceptual’ representations.rucially, according to Jeannerod and Jacob:
“The scope of pragmatic processing. . . is not limited tothe visuomotor transformation, since pragmatic processing isinvolved in conceptually more complex operations like evalu-ating the feasibility of an action, anticipating its consequences,planning further steps, and learning the skilled use of tools byobservation” (Jeannerod & Jacob, 2005, p. 5)
The upshot is that:
“The visuomotor transformation is but a first, lower level com-ponent, of the human ‘pragmatic processing’ of objects. [Weshould] contrast this lower level pragmatic processing with ahigher level of pragmatic processing of objects involved in theskilled use and manipulation of complex cultural tools and arti-facts” (Jacob & Jeannerod, 2003, p. xviii)
These higher levels of pragmatic processing reach, the authorsrgue, all the way up to representations with consciously accessibleontents, as in the case where we experience motor imagery involv-ng the manipulation of these complex tools and artifacts. Suchses go well beyond simple grasping and involve the retrieval ofhat Jacob and Jeannerod (2003, p. 216) call ‘stored scripts’ for theanipulation and use of cultural objects. At this point, the authors
rgue “the distinction between action and perception loses muchf its significance” (Jacob & Jeannerod, 2003, p. 253).
Let’s return now to Milner and Goodale’s bold proposal. Theeepest worry raised by Jeannerod and Jacob may be put like this:e should be wary of conclusions (concerning the functional roles
f the dorsal and ventral streams, and the alignments of those rolesith conscious visual experience) that do not compare like with
ike. In particular, we should be wary of conclusions reached byomparing early levels of pragmatic processing with later levels ofemantic processing. Early stages of pragmatic processing (thoseevoted to the basic visuomotor transform) involve representa-ions with little or no conceptual content, and these are indeedot the kind of content that normally figure in our own experi-nce. But early levels of semantic processing, which fall short ofdentifying or classifying objects, are similarly silent, phenome-ally speaking. While later stages of both semantic and pragmaticrocessing involve contents that are not silent: that are present to
henomenal awareness. If we keep the nature and levels of process-ng matched, what we find are thus not differences in phenomenaltatus but rather differences in direction of causation and of fit.hat is to say, we find that there are indeed (at least7) two distinct
lso provide motor intentions with information about what actions the worldffords.7 Jeannerod and Jacob go on to suggest a further decomposition of function within
he parietal lobe/dorsal pathway, with varying admixtures of directions of fit and
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in the present treatment. On this model, DF (to take one strikingexample) may have visual experiences with rich conscious directivecontents that help explain her successful and often self-initiated
464 A. Clark / Neuropsycho
oles that visual information may be serving in the agent econ-my. But neither role lines up exclusively with conscious visualxperience.
.2. Directive content?
But what about the basic visuomotor transformation itself?ere, Jeannerod and Jacob seem to accept that the processing (that
nvolved in, e.g. transforming visual input into fluent acts of basiceaching and grasping) fails to reach or inform conscious visualxperience. But even this may be called into question.
Thus consider once again the case of DF, the ventrally com-romised carbon-monoxide-poisoned subject studied by Milnernd Goodale. In an interesting paper, Wallhagen (2007) suggestshat DF may really experience visually presented shape, but benable to report that experience due to some problem with bring-
ng her experience ‘under concepts’. The idea is that the intactorsal stream processing associated with the basic visuomotorransformation has its own attendant phenomenology, and that it ishis attendant phenomenology that explains DF’s remarkably fluentisuomotor behaviour. Thus DF sees (visually experiences) shapesnd orientations, but when asked to report on what she sees, she isnable (due to her compromised ventral processing) to do so. Notnly can she not report what she sees, in a real sense she does notnow what she sees. Just as a non-human animal, lacking the con-ept of ‘chair’, might well visually experience a chair yet not in anyense know it is experiencing a chair, so too DF might experienceworld of oriented lines, shapes and forms but not know (neither
ecognize nor be able to report) that she is doing so. According toallhagen:
“Aspects of form may well be phenomenally present to D.F., shemay well consciously sense, and hence represent, the shapes,sizes, and orientations of things, even if she cannot properlyconceptualize these aspects of form” (Wallhagen, 2007, p. 550)
O’Regan and Noë seem tempted by a similar thought. Theyescribe DF as a case of ‘partial awareness’ in which ‘she is unableo describe what she sees but is otherwise able to use it for theurpose of guiding action’ (O’Regan & Noë, 2001, p. 969).
These views are potentially more radical than that defended byeannerod and Jacob, since they call into question the identificationapparently accepted by Jeannerod and Jacob) of conscious percep-ual contents with the more highly conceptualized products of latertages of (both semantic and pragmatic) processing. One immediateroblem is that these less highly processed (more ‘unconceptu-lized’) contents, if they are indeed consciously experienced, willonetheless be hard if not impossible to bring into focus for reportnd description. For whenever we do so, we in effect move up therocessing ladder, calling on our grasp of the scene as a structured,ttended entity populated by nameable objects, shapes, and rela-ions. But for the moment, let’s bracket that worry and try to put aittle more flesh on the suggestion itself.
One way to think about this kind of proposal is further devel-ped in Nudds (2007). Nudds, like Jeannerod and Jacob, urges us
o distinguish between two kinds of content that visual perceptualxperiences may possess. He dubs these the ‘presentational’ and thedirective’ contents of visual experience. Presentational contentsorrespond to what Matthen (2005) termed ‘descriptive vision’ and
ausation associated with the various functions. The superior partietal lobule, theyuggest, is concerned with visuomotor processing, while the right inferior parietalobule deals with the perception of spatial relationships, and the left inferior lobule
ith representations of goal-directed actions. Both these latter roles, they argue, arelausibly associated with certain contents of conscious visual experience.
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hat Jeannerod and Jacob (2005) termed ‘semantic processing’:hey are contents that depict how things are in the world. Direc-ive contents correspond to what Matthen termed ‘motion-guidingision’ and what Jeannerod and Jacob called ‘pragmatic process-ng’: they are concerned with how to guide actions so as to bringbout desired results. Nudds suggestion, rather like that of Wall-agen, is that these contents figure in visual experience and helpo determine the shape and nature of our world-engaging activity.t is this latter claim that marks the point of departure from both
ilner and Goodale’s model and (at least for the basic visuomotorransformation) that of Jeannerod and Jacob).8
Why should we believe that directive contents, associated withasic visuomotor transformations, form part of our conscious visualxperience? Nudds reasoning is best appreciated by contrastingis picture with a more standard model that depicts the consciousisually guidance of action as exhausted by conscious intentionse.g. Peacocke, 1992). On this model visual experience presents aorld relative to which I may form an intention (e.g. to pick up
he coffee cup I see over there) that then determines the action.uch a model is consistent with (though it does not commit youo) the tele-assistence image favoured by Milner and Goodale (seeection 1). For it leaves room for the detailed kinematics of thection to be determined by something other than the content ofhe visual experience (which need only allow us to form apt inten-ions). But such a model, Nudds argues, is inadequate to explainisually guided action. Instead, Nudds argues that there is a dis-inctive kinematics (as Milner and Goodale showed) to reachingnd grasping performed while in visual contact with the scene.emove the contact and the kinematics alter. This alteration – andhis is crucial for his argument – is not exhaustively accounted fory an agent’s intentions (e.g. to pick up the cup, or even, movingo the higher levels of pragmatic processing, to use the screwdrivero as to screw in the screw), which may often be the same in bothases. But whereas Milner and Goodale depict these further preciseinematic details as determined by a non-conscious stream of low-evel pragmatic processing, Nudds thinks this fails fully to capturehe phenomenon. The reason he gives is that the precise way weerform the action seems to be something for which we (the agent,ather than just some subsystem of the agent’s brain) are responsi-le. The precise way the action unfolds is, he suggests, somethinge do. That I tie my shoelaces like this is not something that just hap-ens to me, or that I just find myself doing. The explanation, Nuddslaims, is that (e.g.) the detailed lace-tying kinematics are driven byonscious but directive contents actually given in visual experience.onetheless we will not necessarily be well-placed to report on
hose contents, and it will not ‘seem like’ anything very specific to beuided by them. This is because both reporting and (more generally)nowing what we are experiencing depend on content-monitoringapacities informed by the other (‘presentational’) dimensions ofonscious visual content.
This is perhaps the most difficult suggestion we will examine
ehaviours9 even though [she] “will not be under the impres-
8 Similarly Mathen’s comments on the ‘feeling of presence’ are probably best seens a kind of restricted version of Nudds’ claim (restricted insofar as the only con-ribution the directive contents make to experience is, on Matthen’s account, theddition of that sense of presence).9 It is worth noting here that DF retains descriptive visual experience of colour
nd texture, and thus knows when there is an object out there, and what kind ofurface (shiny, dull, etc.) it has. This is what is usually taken to explain her abilityo self-initiate actions, and thus already distinguishes her quite sharply (in terms ofractical action) from blindsight patients.
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A. Clark / Neuropsycho
ion that anything is any way, nor have any basis for judging thatnything is any way” (Nudds, 2007, p. 15). This also opens up anlternative interpretation of the Aglioti et al. experiments rehearsedn Section 1. For on Nudds’ account visual experience guides bothhe action (the well-calibrated grasp) and the illusion-prone ver-al response, but it is the directive content of the experience thatuides the visuomotor action and the presentational or descriptiveontent that guides the verbal response. Both the judgment and thection are thus guided by (different forms of) visual experience.10
oreover there is no inconsistency in the content of visual experi-nce here, since these different kinds of content share no ‘commonurrency’ in which to frame a disagreement.
. The argument from agency
In this section I aim to challenge the claim that these directiveontents (both in DF and in neurally uncompromised subjects) areroperly depicted as forming part of our conscious visual experi-nce. As already noted, however, we cannot challenge such a claimimply by pointing to the agent’s honest reports, since these willnly reveal what is present or absent to descriptive vision. What wean do, however, is attempt to gain some leverage from a simple,ut I think plausible, observation. The observation (which may ofourse be doubted, as we will later see) is that conscious experi-nce must always be the experience of some agent. The putativeirective contents of visual experience, being available only for oneind of purpose, and then only in highly transient form (that is,nly while in actual contact with the visual scene), seem to fail toeet this requirement. They are more like encapsulated pockets of
rocessing than genuine contents of visual (or any) experience. Inery much this vein the philosopher Gareth Evans once argued thatn informational state may underpin a conscious experience onlyf it (the informational state) is in some sense input to a reasoningubject. According to Evans, to count as a conscious experience annformational state must:
“[serve] as the input to a thinking, concept-applying andreasoning system: so that the subject’s thoughts, plans, anddeliberations are also systematically dependent on the infor-mational properties of the input. When there is such a link wecan say that the person, rather than some part of his or her brain,receives and processes the information” (Evans, 1982, p. 158)
I think this is almost right. But the real point here is (orhould be) quite independent of Evan’s appeal to the subject asoncept-using. What matters, rather, is that the information muste available to the agent qua ‘reasoning subject’, where this maye unpacked in many different ways, not all of them requiring full-lown concept-use on the part of the agent (see, e.g. Bermúdez
Macpherson, 1998; Hurley, 1997). As long as an animal canorm (non-conceptualized) goals, and can become directly andon-inferentially aware of specific environmental opportunitieshat allow, or that block, the fulfillment of those (limited) goals
nd projects, then transduced information can be, or fail to be,nput to this kind of minimal reasoning subject (for more onhis slightly less demanding conception, see Clark, 2007; Dretske,006; Hurley, 1998). A positive suggestion11 thus emerges accord-10 Tim Bayne (personal communication) suggests that the directive contentescribed by Nudds, insofar as it is indeed consciously experienced, may be besthought of as ‘motor intentional’ rather than genuinely visual. This is an interestinguggestion, but one that I shall not pursue in the present context.11 Versions of this suggestion are found in Evans (1982), Marcel (1983), Milner andoodale (1995), Goodale and Milner (2004), Hurley (1998), Clark (2001), Jacob and
eannerod (2003), Dretske (2006), and Clark (2007).
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ng to which conscious perceptual experience occurs when, andnly when, information is poised, however briefly, for direct andon-inferential use in the guidance of (at least minimal) rationalction. By contrast, the sensory transduction of visual informa-ion can sometimes (as in the case of blindsight) simply channelnformation so as to guide response, without providing the agenterself with any reasons, justifications, or rationales, for her actionsee Campbell, 2002; Dretske, 2006). In such cases, behaviourhose success depends on that very information will (all other
hings being equal) surprise the agent herself. In such cases, infor-ation is transduced, impacts behaviour, but is never poised so
s to provide me with a reason (visible to myself as an agent)or my actions or choices. At other times, however, somethingbout the form or nature of the processing poises specific itemsf transduced information in a way apt (if my attention is soirected) to provide me with reasons or motivations for my ownctions and choices (to provide what Dretske (2006, p. 168) calls
justifying reasons’ for my actions. Importantly, it may be (seeiscussion below) that even elements that we don’t yet attendo or notice in our own visual experience can be thus poised,s might elements that are not yet fully conceptualized (in thisay, as Fodor (2007) notes, even the contents of ‘iconic’ encodingsay provide justifying reasons for a subsequent act or judge-ent).For present purposes, what matters is that even these kinds of
weak but important) links between experience and agency do noteem to be present in the case of (merely) directive contents. Nuddsimself comments, as we saw, that “since directive content doesn’tresent an object as being some way, in having a visual experienceith directive content the subject will not be under the impression
hat anything is any way, nor have any basis for judging that any-hing is any way” (Nudds, 2007, p. 15). As a result, this informationill not be available for use in any form of practical reasoning, andill not provide the agent herself with any reasons (visible to her-
elf) for her own actions. But such total fractionation, of the putativeonscious content from what the agent knows or (more generally)as reason to do, seems in conflict (I suggest) with the image ofenuine experience being experience of the agent.
. The most radical challenges
Block (2007) asks a question that reaches to the very roots ofurrent scientific attempts to study conscious experience. He asksow we can know whether some content actually forms part of ouronscious experience. The answer, he notes, is only straightforwards long as we take reportability of content (for example, the broad-asting of content to a ‘global workspace’12) as a requirement ononscious presence.
But suppose we do not. Suppose we ask, instead, whetheronscious experience might go beyond that which we can accessr report? A natural worry is that such a question is simply unan-
Landman, Spekreijse, & Lamme, 2003; Sperling, 1960) arguinghat these studies suggest that “in a certain sense phenomenalonsciousness overflows cognitive accessibility” (Block, 2007, p.
12 Thus according to ‘global workspace’ theory (Dehaene, Kerszberg, Changeux,998; Dennett, 2001; Metzinger, 2003, etc.) information becomes conscious whent is poised for dissemination to many cortical areas (perhaps via long-range
hite matter pathways linking cortical areas—see Dehaene & Naccache, 2001).nformation poised for such widespread dissemination (information ‘in the globalorkspace’) will ipso facto be poised for the control of an open-ended variety
f rational responses, including report where available, so the global workspaceodel can be seen as providing one mechanism by means of which a strong link to
ersonal-level agency may be implemented in the brain.
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81). In the Sperling experiments13 subjects are briefly (50 ms)hown a 3 × 3 grid of letters, such as
D AR NZ B
The stimulus grid is removed after 50 ms. Many subjects thenlaim to have had and to have briefly retained a conscious expe-ience of all the letters, even though they could reliably reportin the so-called ‘full report’ condition) only about 4 of them. It
ay seem that there is no way to empirically test whether theubjects actually saw (properly saw, in distinct individual detail)ore letters than they can report. But Sperling then tested subjects
n a further ‘partial report’ condition. This showed that if rapidlysked instead for the letters in any given row (the top, middle,r bottom) subjects could quite accurately respond, regardless ofhich row was chosen. What this (at least taken in the context of
he subjects’ own experiential reports) suggests to Block (and seelso Dretske, 2006; Fodor, 2007) is that detailed and consciouslyncoded information about each and every letter was temporarilyvailable to drive report and noticing (if attention were rapidlyo be so directed) even though the subsequent selection of someetters to thus report renders the rest unavailable. That is to say,he initial experience contained more phenomenal informationhan (perhaps for reasons having to do with limitations on working
emory) any full report can subsequently display.In more recent studies (Landman et al., 2003) subjects were
hown 8 oriented rectangles for half a second, then a gray screen,hen the array of 8 but were informed that one rectangle mayave changed orientation. Subjects were able to keep track of therientation of about four rectangles from the group of eight (soheir ‘capacity measure’ was 4). Yet, much like the Sperling sub-ects, many of them reported seeing the specific orientation of allight rectangles. Once again, a partial report condition seems toear out the subject’s claim. If the experimenter adds a pointern the gray screen to ask the orientation question of any givenectangle, subjects can track almost all the rectangles (they dis-lay a capacity measure up to 6 or 7). The explanation oncegain, according to Block, is that the initial phenomenal experi-nce contains more information than the full report condition canisplay.14
With these results in mind, Block’s (2007) strategy is then toisplay a neuroscientific story concerning strong back-of-the-headeural coalitions (involving pockets of recurrent processing—seeamme, 2006) that nonetheless just fail to win a winner-take-allompetition for broadcast to the ‘global workspace’ (see footnote1) and hence for reportability. Such a story (which I shall notttempt further to rehearse here) is meant to make sense of thelaim that in these (and in many other) cases phenomenal con-ciousness ‘overflows’ cognitive accessibility and thus that we canand do) have experiences even in cases where we lack the kind ofccess that would yield some form of report that such and such anxperience had occurred.
The point I want to notice is that this argument (which is actu-lly a form of inference to the best explanation) takes as its startingoint the assertion that the only grounds we have for treating the
ust-losing coalitions as non-conscious is the unreportability of theutative perceptual experiences. But perhaps this is premature. Fornderlying the appeal to reportability is, I suggest, a deeper anderhaps more compelling access-oriented concern. It is the con-
13 These experiments are also discussed in Dretske (2006) and Fodor (2007).14 For some interesting worries, see Byrne, Hilbert, and Siegel (2008).
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ern, raised in the previous section, that any putative consciousxperience should be the experience of an agent.
Can we really make sense of the image of free-floating expe-iences, of little isolated islets of experience that are not evenotentially more widely available to act as fodder for a creature’sational choices and considered actions? Evans’ insight was thathe notions of conscious experience and reasoned agency (hereery broadly construed) are deeply intertwined: that there areon-negotiable links between what is given in conscious awarenessnd the enabled sweep of deliberate actions and choices availableo a reasoning subject. One way to begin to flesh this out (see,.g. Dretske, 2006) is, as we saw, to depict conscious perceptualxperience as providing an agent with self-transparent reasons forer own actions. Such a story opens up a different way of inter-reting the Sperling and the Landman et al. results. In these casesubjects report phenomenally registering all the items becausenformation concerning each item was, at that moment, availableo be deployed in the service of deliberate, reasoned, goal-directedction. Responses selected on the basis of this information wouldeet the key condition of being self-transparently grounded in the
gent’s perceptual connection to the world (for more on this idea,ee Clark, 2001). Such momentary potentiality is not underminedy the (interesting and important) fact that the selection of a fewtems to actually play that role then precludes the selection of theest. Contrariwise, Block argues that a subject such as GK (Rees etl., 2002) suffering from visuo spatial extinction may be having anxperience of a face and yet it be impossible for him, qua agent, tonow anything of this experience. This is because Block takes GK’shenomenal experience to consist in recurrent processing in theusiform face area. My suggestion, following Evans’ would be thatK can be consciously experiencing a face only if the informationiven in the putative experience is at least momentarily poisedn a way that makes it apt for use (though it need not actually besed) in the agent’s personal level reasoning, planning and for theeliberate and goal-driven selection action. In that way the link togency is maintained. Recurrent processing in the fusiform areaill no doubt prove to be among the many conditions necessary
but not sufficient) for realizing a state that plays this distinctiveausal role. Block’s just-losing coalitions fail to trigger winningrontal coalitions and hence fail to be in a position to contributeheir contents in this manner to the full sweep of the agent’seliberate acts and choices. It is this fact (rather than the relatedut admittedly more superficial and unreliable indicator of mereon-reportability) that should motivate our treating the contentsf the just-losing coalitions as non-conscious.
Perhaps, though, the very idea of clean facts hereabouts (factsoncerning what does and does not form part of our currenterceptual experience) is itself a deeply mistaken one. For it
ncreasingly seems (and see Schwitzgebel, 2008 for some com-elling arguments to this effect) that our grip on what it meansor something to form part of our current conscious experience isenuous at best. It is a grip compromised by our congenital inabilityo know what we are experiencing without turning attention tot (by active introspection) or attempting to recollect it at a later
oment. Each such act alters the set of cognitive mechanisms inlay, yet to eschew reliance on such methods tout court is to leaves with no anchor points at all. It leaves us with no means by whicho decide, for example, what subsets of behaviour and responseo look for in other (e.g. non-verbal, non-human, or impaired)ases. In other words, it seems we must make some antecedent
ecisions to get the experimental balls rolling, but that theseecisions themselves cannot be checked because the experimentshat would do so must be interpreted according to some closelyelated, equally unverified, set of assumptions. From this one mightonclude that there are facts here that are terminally resistant to
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cientific resolution. But a better conclusion, it seems to me, is thathere are no such finer-grained facts here at all.15
Thus take, for example, the currently extremely ‘live’ questionf the phenomenal status of currently unattended visual stuff. Isuch unattended stuff phenomenally experienced at the time, or ist at best poised to feed phenomenal experience (or perhaps merelyo inform memory) at some later moment? Wright (2006) argues,onvincingly it seems to me, that this question is unanswerablen the present state of the science. But more importantly, he alsouggests that the question itself is relatively scientifically uninter-sting. What matters, Wright argues, is getting a firm grip on whatontribution is made by various systems and sub-systems, and howhose contributions enable us to maintain fluent contact with, andnteract successfully with, the world. In getting such a grip, we areot forced to resolve questions concerning the phenomenal statusf unattended visual stuff.
The final view that I want to display, then, is one that we mayall the Mere Motley model of conscious perceptual experience.ccording to this model the phrase ‘conscious visual experience’
s just a rough and ready label for a typically integrated, butotentially highly dissociable, complex of capacities. Some of these
nvolve recall and report, some involve attention and noticing, oth-rs (if Block and Lamme are right) involve only various forms ofecurrent-processing amplified neural activity. Such a model woulde an instance of what Sloman (2007) calls a “‘labyrinthine’ theoryccording to which visual experience is itself highly structured andultiply layered, such that different combinations of the many bits
f the labyrinth determine different (often dissociable) aspects anduances of what we have come to think of as ‘our visual experience’.uch the same picture, again based upon theoretical apparatus and
nsights from work in artificial intelligence and robotics, is endorsedn Ballard (2007). Conscious visual experience, if such views areorrect, is not usefully understood via the metaphor of a singlenner light that is either on or off but consists instead in a mot-ey swathe of surprisingly dissociable elements and effects, relativeo which pressing the simple binary question (“is conscious visualxperience occurring or not?”) is just a recipe for trouble and confu-ion. The most famous defence of such a view is probably Dennett’s1991) ‘multiple drafts’ model of conscious experience, according tohich the only real facts hereabouts concern the ways the systemould respond to various kinds of probe made at various points
n the ongoing cycle of processing. But the essential core of theiew (which I take to be the assertion of motley processing witho simple facts of the matter concerning the presence or absencef conscious experience in many cases) may be developed in manyifferent ways.
. Conclusions: still revealing after all these years?
In the light of all this, what is most clearly right and importantn the strong dual visual systems model is the claim that ven-ral stream processing (along with some of the highest levels ofhat Jeannerod and Jacob call ‘pragmatic processing’) preferen-
ially determine what might be called our ‘reflective take on our
wn visual experience’. That is to say, such processing is preferen-ially involved in the way our own visual experience presents itselfo us: to rational, reflective agents motivated by a variety of plans,oals, and projects. But step outside that self-reflective arena, how-15 Schwitgebel’s position (personal communication) is that although he can seeow the arguments might be taken this way, he himself finds it compelling thathere must be a clean fact of the matter concerning e.g. the richness, or lack of it, ofngoing visual experience, and that this will be so even if that fact turns out to beermanently resistant to scientific resolution.
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ver, and the landscape changes dramatically. If experience, or somearieties of experience, outrun report, and dissociate from pro-esses of top-down attention and consolidation into agent-memoryperhaps occurring simply courtesy of recurrent processing inncapsulated pockets of the cognitive economy) then all bets areff. Worse still, we may be forced to embrace what I have calledMere Motley model of conscious experience, according to which
here is simply no answer to questions concerning (for example) thehenomenal status (‘seen or unseen’) of what Wright (2006) calls
unattended visual stuff’. Instead, ‘visual experience’ would dependn a messy, multi-faceted web of processing that links us variouslyo the world: a web of processing that we probe in various ways andn various time-scales, some of which inevitably recruit processingn ways tied up with report, memory, attention, and noticing, andthers of which do not.
Milner and Goodale have done more than just about anyonelse to bring these foundational issues into focus, involving neu-oscientists, cognitive psychologists, AI researchers, philosophers,nd many others in what has become one of the most exciting,mportant, and productive debates in recent decades. On thosencreasingly elusive questions concerning the nature and neuralnderpinnings of conscious visual experience itself, the jury (iteems to me) remains out. But whatever the outcome, there is nooubting the value and impact of the dual visual systems model
tself: still revealing after all these years.
cknowledgements
Thanks to Robert McIntosh, Tim Bayne, Thomas Schenk, Michaelye, Matthew Nudds, Mog Stapleton, Julian Kiverstein, Simon Gar-od, Steve.
Draper, Liz Irvine, John Henderson, Ned Block, and Tillmannierkant for stimulating exchanges on these topics. Thanks too to
he two referees, to all the participants in the Perception, Action andonsciousness: Sensorimotor Dynamics and Dual Vision conferenceeld at Bristol in July 2007, and to all the members of the Edin-urgh Philosophy, Psychology and Informatics Reading Group fortimulating discussions. This paper was prepared thanks to sup-ort from the AHRC, under the ESF Eurocores CNCC scheme, for theONTACT (Consciousness in Interaction) project, AH/E511139/1.
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