visually guided actions

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The Physiology of the SensesTransformations For Perception and Action

Lecture 6 Visually Guided ActionsTutis Vilis http://www.physpharm.fmd.uwo.ca/undergrad/sensesweb/

Recall that visual information from V1 divides along two streams:

1) a dorsal "Action" or "Where" stream which is concerned with the representations of space forthe guidance movements.2) a ventral "What" stream which is concerned with object recognition and perception.

These representations of spaceare used to guide a variety of movements: saccades, grasps, reaches, and feeding.

The activation of these areas direct one's attention to locations but the selection of the appropriateeffector (e.g. which arm to reach with) can occur unconsciously.

One distinguishing feature between the dorsal and ventral streams is the frame of reference. Theventral stream uses an object-centered frame. The dorsal stream uses various forms of egocentric

frames; retinotopic (for saccades), and head centered (for feeding).

Patients with lesions here have difficulty in making saccades, grasps, reaches, and feedingaccurately.

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Whe

re


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These multiple representations of space surround the Intra Parietalsulcus (IPS) .

The IPS is ideally located to integrate the representations of space that are derived from several

modalities of sensory information: visual, somatosensory, and auditory. Locations can be seen,felt, or heard.

PEF, the Parietal Eye Field, represents the retinotopic locations of objects that you intend to lookat. Neurons respond to visual and auditory stimuli.

PAF, the Parietal Arm Fields, represents retinotopic locations in the immediate peripersonalspace, the region of space one can reach to. PAF is used to guide arm movements. Neuronsreceive both visual and somatosensory information. Lesions here cause optic ataxia, (visuallydirected misreaching).

PGF the Parietal Grasp Field, represents not the location of objects, but the shape informationrequired to grasp objects.

Some of these egocentric regions, like PFF, map space that is near while others, like PEF, mapspace that is both near and far.

Presumably PGF receives input from the ventral stream of an object's allocentric co-ordinates e.g.in order to grasp a knife. by the handle and not the blade.

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An example of activity of one specific neuron in area the Parietal FaceField (PFF).

It is also activated by the sight of anobject approaching the mouth.

This neuron is activated by touchingthe mouth.

But it is not activated by the sight ofan object approaching the brow.That's the job of some other PFF

neuron.

And by the sight of an objectapproaching the mouth from any

direction.

And by the sight of an objectapproaching the mouth when

looking up or down.

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Neurons in PFF appear tocode specific locations with

respect to the head.


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Which areas of the brain are involved in generating visually directedsaccadic eye movements?

a)

Saccades to a novel peripheral stimulus (e.g.flashing/moving) involve the superior colliculus(SC).

This stimulus generates short latency or "express"

Saccades are rapid eye movements.

b) When voluntarily looking to one of many or aremembered target, the Prefrontal association areaholds several target locations in working memory.

The PEF selects the one of interest.

And the Frontal Eye Fields (FEF) generate alonger latency saccade.

stimulusworking

memory

attention


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You may recall that the retinal ganglion cellsthat project to the SC have large receptive fields.Because of this, the activity is not localized to apoint but to an area. The activity of cells in thecenter of the area have the highest activity.

This can be viewed as a hill of activity.

As we have seen, the superior colliculusmediates the visual grasp reflex. A visualstimulus in location A produces activity in a

corresponding location A in the SC. Activity atlocation A in the SC generates a motorcommand which turns the eye to A and activityagain returns to the foveal area of the SC.

But before a saccade can begin, a hill of activityat the center must be removed. This hill ofactivity projects to neurons in the brain stem andkeeps the eyes fixating at the current location byinhibiting the generation of saccades. Thus notmoving your eyes, fixation, is an active process.

To remove the activity at the center, the PEFmust disengage ones attention from the centerand then shift it to location A.

It must achieve a balance betweeni) not allowing us to be distracted by every

stimulus andii) not neglecting important cues.

A

Center

disengage and shift

The activity of the superior colliculus reflects the engaging anddisengaging of attention.

SuperiorColliculus


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a) T

Like the retina, V1, PEF, and FEF are topographic map-like structures where the location of agroup of active cells indicates the relative location of a target.

For example if one electrically stimulates a group of cells in the superior colliculus or FEF atlocation A, the eye would orient towards A. Stimulation at B causes an orientation to B.

If you electrically stimulate at A and B at the same time, the commands would cancel and nomovement occurs.

Note also that the figure shows both sides of the CNS with the midline running through the center.Half the foveal representation is on one side and half is on the other side.

opographic maps

A comparison of activity in the superior colliculus, frontal eye fields,parietal eye fields, and the primary visual cortex.

midline

retina visualcortex

PEF

A

B

turn eye to A

turn eye to B

FEF

centre fovea

peripheral

peripheral

SC


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i) A visual stimulus appears at A but no motorresponse is required.Enhanced activity is restricted to location A ofthe retina and visual cortex.

The foveal activity in the FEF and SCmaintains fixation (i.e. prevents saccades).

ii) A visual stimulus appears at A and thesubject is required to saccade to A.Same as i), plus enhanced activity in PEF,FEF, and SC at location A.

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Compare the activity in each area under the following conditions.

In each case one is recording from a region that is activated by a light stimulus at location A orB, and the eye is initially at the center.


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iii) A visual stimulus appears at A & B but thesubject is required to saccade to B.Enhanced activity is observed in all layers atlocation B. At location A, enhanced activityonly occurs in the retina and visual cortex.

iv) A visual stimulus appears at A and thesubject is required to make an arm movementto A while still fixating the center.

Little enhanced activity is observed in the FEFor SC. Enhanced activity at A in area PAFleads to activation of appropriate limb motorareas to direct limb movement to A.

Conclusion:Retina and Visual cortex: activity requires

a stimulus.PEF: activity requires a stimulus and

attention which selects the target for amovement.

FEF and SC: activity requires an orientingresponse of the eyes.

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Parietal and Prefrontal areas are activiated by the memory of targetlocations.

Here a target is shown briefly at location A, and one is asked to attend to it but not look at it.

After the visual stimulus disappears, activity is maintained in the parietal eye fields (PEF) andin working memory in the prefrontal (PF) cortex.

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A


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Parietal spatial representations are updated after saccades.

Suppose two visual targets, A and B, are briefly flashed in sequence. The instruction is to saccadeto A and then to B.

Immediately after the visual stimuli arepresented.

After a saccade to A, the stimuli are no longervisible but activity persists in PEF and theprefrontal cortex. The activity in parietal andprefrontal cortex has shifted with A appearingin the representation of the fovea.

It is thought that activity is shifted from oneset of neurons to another by a copy of thesaccadic eye movement. The copy commandis known as the corollary discharge. Thiscorollary discharge originates in the SC and isdirected to the FEF. The corollary dischargeshifts the activity to the group of neurons thatwould have been activated if A and B werevisible.

In the normal world in which visual targetsare continuously visible, the image of objectsshift during each eye movement. Yet we sensethat these objects are stationary. This isbecause the images land where our corollarydischarge tells the PEF to expect them.

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B A


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The frontal eye fields (FEF) and the parietal eye fields (PEF) areinvolved in covert shifts in attention.

As we have seen both the PEF and FEF directsaccades to locations of interest. These are alsoknown as overt shifts of attention. These twoareas are also involved in covert shifts ofattention.Covert shifts can, very rapidly, redirectingattention to a spatial locationwithout moving one's eyes.

These covert shifts of attentionselectively

enhance the neural activity of the correspondingretinotopic locations in early visual areas.

This in turn enhances the visual contrast atthose spatial locations .

This in turn helps locate potential targets for asaccade to Waldo.

Where is Waldo?

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In Conclusion

We will learn in the next session that thesensation of touch from your skin is mapped onto astrip of cortex behind the central gyrus. As in any

map, adjacent points on the skin are mapped toadjacent points on this cortical strip.

Another strip, just in front of this one, mapsthe locations of all your body parts.

We have seen in this session that theparietal cortex contains many more maps. Thesemap locations that one can act on.

One of these maps the locations one canreach either with the arms, legs, or even your head.This is called the peripersonal space.

These maps are elastic. They expand whenone uses tools such as a stick, a baseball bat, oreven a car. Over time one's map of the exteriorsurface of one's car becomes almost as familiar asone's skin and one can park in the tightest of spotswithout a scratch.

Toes

Knee

Hip

Trun

k

Hand

Fing

ers

Face

Lips

Tongue

S1

face

leg

central sulcus

visually guided actions

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