hemispheric specialization and attention: effects of complete and partial callosal section and...

Neuropsycholoyio, Vol. 24, No. 4, pp. 483496, 1986. Printed in Great Bntain.

a)28-3932/86 S3.00f0.00 Pqamon Journals Ltd.

HEMISPHERIC SPECIALIZATION AND ATTENTION: EFFECTS OF COMPLETE AND PARTIAL CALLOSAL SECTION AND

HEMISPHERECTOMY ON DICHOTIC MONITORING*

JOCELYN WALE? and GINA GEFFEN~

Centre for Neuroscience and Psychology Discipline, The Flinders University of South Australia, Adelaide. South Australia

(Accepted 15 January 1986)

Abstract-Dichotic word monitoring tasks were given to four cases with complete forebrain comissurotomy, three cases of partial section of the corpus callosum, one case of right hemispherectomy, and control subjects. The task involved a unimanual response to target words in either ear (divided attention, Experiment 1) or to target words in one voice that changed ears unpredictably (focused attention, Experiment 2). Left-ear extinction, defined as chance level (25%) detection of left-ear targets and 75% right-ear target detection, varied both between and within subjects during divided attention. No examples of extinction were obtained with focused attention. These results highlight attentional aspects of dichotic listening. Neither the structural, hemispheric asymmetry model (Cortex 3, 163-178, 1967), nor the cognitive, attention priming model (Attenrion and Performance V, pp. 87-97, Academic Press, New York) could account for the obtained results. We propose an alternative account in terms of the different processing modes and attentional capabilities of the two cerebral hemispheres.

INTRODUCTION

DKHOTIC listening involves the presentation of different speech signals simultaneously to each ear. Right-ear signals are perceived more accurately than those on the left. Two models have been proposed to explain the right-ear advantage (REA). KIMURA [19-211 proposed that one hemisphere (typically the left) is specialized for language processing and that information on the stronger contralateral auditory pathways suppresses the inputs on the ipsilateral pathways during competitive stimulation. Thus, the stimuli are initially lateralized, and left ear-right hemisphere inputs must be transferred across the corpus callosum to the left hemisphere in order to be processed. K~NSBOURNE [22-241 argued that the assumption of ipsilateral suppression was unnecessary. According to his model, each hemisphere controls attention to the opposite side of space and the hemispheres are in a dynamic state of balance that is mediated by the corpus callosum. Thus, if one hemisphere becomes more active, e.g. with a verbal task such as dichotic listening, then attention will be biased to the opposite side of space (the right side with verbal tasks).

* This research was supported by an A.R.G.S. grant to Gina Get&n. t Present address: Psychiatric Unit, Royal Adelaide Hospital, Adelaide, Australia, 5000. 1 Addresscorrespondence to: Gina Geffen. School ofsocial Sciences, The Flinders University ofSouth Australia,

Bedford Park, Australia, 5042.

483

484 JOCELYN WALE and GINA GEFFEN

The demonstration of left-ear extinction on a dichotic digits recall task in subjects with complete commissurotomy 1291 was critical to the development of these models. Either interruption of callosal transfer or a lack of mediation of attention between the hemispheres could explain the lack of recall of left-ear items. Indeed, Kinsbourne claimed that without an intact corpus callosum, the right-sided bias in attention could not be overcome [24]. The evidence that a split-brain subject could process the left-ear input by selectively attending to it [33] runs counter to both the concept of suppression and to Kinsbourne’s claim. A recent review [lo] provided much evidence against the assumption of ipsilateral suppression, and indicated that with selective attention, subjects could overcome the right-sided bias provided that their information processing capacities were not exceeded.

Recent evidence indicates that the hemispheres differ in the nature and capacity of attention, Electrophysiological studies of both normal subjects [ 171 and cases with unilateral cortical lesions 1251 suggest that the left hemisphere attends to the right side of space while the right hemisphere attends to both sides. There is also suggestive evidence that the right hemisphere is specialized for attention to stimulus location, while the left is specialized for selectively attending to the type of information presented [ 167.

Dichotic monitoring, a task of attention [30] with no recall component, has not previously been investigated in subjects with complete commissurotomy. Kinsbourne’s model would predict left-ear extinction on this task, while the structural model would predict some left-ear responses if the response mode is appropriate to the right hemisphere, e.g. left-hand responses. Left-ear extinction has been described as a statistically larger than normal difference between the ears [S] but originally defined as a complete absence of response to left-ear items with responses to the right ear at a high level due to lack of interference from the left-ear input [I].

EXPERIMENT 1

In our first experiment we gave subjects with complete and partial callosal sections dichotic monitoring tasks that required manual responses to target detection. A right hemispherectomy case was also included. If target words on the right ear received responses, but those on the left ear were not responded to (left-ear extinction), this would indicate that (a) the ipsilateral input on the left ear was suppressed, and(b) target words reaching the right hemisphere from the contralateral left ear could not be distinguished from non-target words. Alternatively, this result could be explained by Kinsbourne’s model. If left-hand responses were made to left-ear targets and right-hand responses to right-ear targets but no crossed ear-hand responses occurred, then this would be evidence for suppression of ipsilateral input and right-hemisphere word processing. Larger than normal ear differences with this pattern of performance would be evidence for callosal transfer of information from the right to the left hemisphere in the intact brain. If crossed ear-hand responses to targets were obtained, this would be evidence against suppression of ipsilateral pathway information, since contralateral motor control of fine manual responses is assumed. Any ability to respond to left-ear targets at an above chance level by either hand would provide evidence that contradicts Kinsbourne’s contention that an intact corpus callosum is necessary to overcome the right- sided bias of attention with verbal stimulation.

DilTerent rates of stimulus presentation and voice combinations make different demands on processing capacity and have been shown to affect the size of the REA [2, I I]. These variables were manipulated in the present experiment. Fusable word pairs (see Table 2) were

HEMISPHERIC SPECIALIZATION AND ATTENTION 485

also included, since subjects with partial section of the corpus callosum have shown normal response rates to fusable word pairs despite left-ear extinction for targets [S], demonstrating that left-ear items were available for processing. The critical location of the partial callosal section that produces left-ear extinction is uncertain [IS, 12, 37). The interhemispheric auditory fibres cross the corpus callosum in the region of the anterior splenium [S]. If left-ear extinction occurred with a partial section that did not include the anterior splenium, then more complex attention factors would be implicated rather than modality-specific disconnection between the hemispheres.

METHODS Subjects

The subjects were L.B., N.G., A.A. and R.Y. with full commissure sections, D.W. with a right hemispherectomy, D.M., S.A. and W.Y. with partial commissure sections, and 10 control subjects. The case histories of all the clinical subjects have been published elsewhere [S, 9, 12-14, 28, 361, but relevant details for each subject are summarized in Table 1. The control subjects were volunteers covering a wide range ofeducational and occupational backgrounds. There were five females and five males ranging in age from 18-64 yr (female mean age = 37.4, male mean age=38.2 yr), with males and females being evenly distributed across the age range.

Table 1. Brief case histories of the eight patients giving also age at testing, and post-operative verbal and performance intelligence test scores

Age symptom Age

Patient Sex onset Operation tested VIQPIQ

L. B.

N.G.

A.A.

R.Y.

D.W.

D.M.

S.A.

W.Y.

M

F

M

M

M

M

M

M

18

5

17

6;

11

I4

59

At age 13 for intractable epilepsy. Complete section of forebrain commissures with retraction of right hemisphere, including anterior and hippocampal commissures. No radiological or neurological signs of localized brain damage preoperatively. Recovery rapid. At age 30, as above. Radiological evidence of 1 cm calcified lesion beneath right cortex and EEG signs of focus in left posterior temporal region pre-operatively. One year post-operative EEG normal and no neurological signs of localized cortical damage. At age 14, as above. Operation difficult. Residual cortical lesions in the left arm area and right leg area. At age 43, operation as above. Slight spasticity and poor motor control of left hand. Seizures attributed to car accident at age 13. At ages 7 and 9, underwent right hemispherectomy for intractable epilepsy. Presumed sparing of basal ganglia and thalamus. Frontal topectomy 10 months earlier. Left- handed prior to surgery, but shown to have left-language lateralization on Wada testing. At age 23, partial commissure section involving the anterior commissure, the anterior 5 cm of corpus callosum for relief of epilepsy. Section presumed to include hippocampal commissure, splenium intact. Rapid recovery. Seizures attributed to closed head injury at age 11. At age 14, 2 cm section of anterior splenium of corpus callosum for clipping of AVM extending into right hemisphere. Left homonymous hemianopia. At age 61 section of genu for exploration of III vent. Area of opacity on CT not identified. Quick recovery.

29 1 lO/loO

49 8317 1

30 77182

57 99179

24 80160

36 108176

18 9418 1 *

61 114199

* These IQ scores differ from those reported by Geffen et al. which were 3 months post-operative, while the above scores were obtained 2 years post-operative.


Stimuli

Lists of high-frequency monosyllabic words were constructed as follows. Each list consisted of specified target words, fusable distractor words, phonemic distractor words differing from the target word in the initial consonant only, and other distractor or irrelevant words having no more than one phoneme in common with the target word. One of four target words could be used per list with the corresponding fusable distractor and phonemic distractor words as shown in Table 2.

Table 2. Targets, fusable pairs, and distractor words, and their categorization, used in dichotic lists

Categorization List 1 Words used

List 2 List 3 List 4

Targets

Fusable words

Phonemic distracters

Black

Back Lack

Clack Slack

Clock

Cock Lock

Block Flock

Glow

Go Low

Blow Flow

Play

Pay Lay

Clay Slay

Where possible, all words used were classified as AA or A by the THOKNUIKE and LOKGF. [38 ] classification, although this was not possible with some of the distractor words. Each list contained an equal number of targets on each channel, plus the same number of fusable word pairs, and similarly word pairs containing a fusable distractor with a non&sable word. The words occurred in a random order with the constraint that any word pair containing a target or a fusable distractor was followed by a pair of irrelevant words. Target words were only paired with irrelevant words and the probability of successive targets occurring on the same ear was no more than 0.5. Of all word pairs, 50% were irrelevant pairs. A PDP 1 l/34 computer program was used to generate lists of digitized word pairs which were recorded onto tape at a specified rate, with word onsets being matched to within 1 msec. Each word was preceded by a 10 kHz, 1 msec tone burst occurring 50 msec before word onset, which could be heard. if its presence was pointed out, as a centrally located ‘click’ immediately preceding each word pair.

Lists were. constructed at two presentation rates; one word pair per set and one pair per 750 msec. The slower hsts contained 120 word pairs, 16 targets (eight on each channel), I6 fusable word pairs (e.g. eight pairs with BACK on Channel 1 and LACK on Channel 2, and eight pairs with the opposite presentation). and 16 fusablc distractor words paired with non-fusable words which were again equally counterbalanced between the two channels. Lists presented at one pair per 750 msec contained 20 of each of the above categories of word pairs, with these again being counterbalanced between the two channels.

There were also two types of voice combination used in the lists: different voices on each channel (male female or female-male on Channels 1 and 2 respectively) or the same voice on each channel (either male male or femaleefemale).

Matched TDH-39 headphones in Maico auraldomes were used for preaentatlon of stimuli to the aublect from a Uher two-channel tape recorder. Depressing a button on a response panel modulated an FM tone that was converted to a 5-V digital impulse and sent to an Apple II. At the same time, channel I of the stimulus tape wa relayed via an A D converter. which utilized the 10 kHz tone bursta, to the Apple Il. A computer program then assigned responses to target or non-target events presented to either the left or right ear.

Subjects were tirst presented with a 10 set binaural I KHz tone which should have been perceived centrally provided that there were no inequalities in hearing hetween the two cars. No subject reported any asymmetry in the perceived tones. A shaping procedure followed hy four monaural lists was then admtnistered to the subjects in order to familiarize them with the test requirements and to ensure that they could respond adequately to each ear on its own. Two dichotic practice lists were given heforc the test lists with instructions to respond to targets in either ear. Before each test list they were told whether the words would he spoken m a male voice, a female voice. or with different voices on each ear. They were also informed of the target word which was printed on a card and positioned centrally to ensure that they did not forget it. The visual target word also serves as a central fixation point. Headphones were reversed after each second list to control for any channel bias, and the order of the hand used for response was R L L R or L R R L, beginning with the preferred hand (excepting D.W.. who used only his right hand).


Since the subjects ability to cope with word-pairs every 750 msec was unknown, the lists at the slower rate were given first. The control subjects were not given the slow lists as previous experiments with intact subjects had shown ceiling performance at this presentation rate. Since short periods were available for testing S.A. and W.Y., they were given only the fast lists. All the other subjects completed the faster lists second, after a 10 min rest. At each rate of presentation, there were four lists: two had the male voice in one ear and the female voice in the other ear (different voice condition), with each ear-voice combination sampled. One of the other two lists had the male voice in each ear and the remaining list had the female voice in each ear (same voice condition).

RESULTS AND DISCUSSION

Monaural scores were all above 87%, indicating adequate performance with each ear separately stimulated. The presence of left-ear extinction on the dichotic tasks was defined according to chance level on the binomial test. This required a score below 25% for the disadvantaged ear (normally left) and above 75% for the dominant ear (normally right). Table 3 shows the percentage hit rate (HR) in each condition for each subject, together with means, standard deviations and confidence limits at the 0.05 level for the control group.

Table 3. Percentage of targets detected for each subject with commissure section (full and partial) and the right hemispherectomy subject, on each of the four dichotic tasks according to ear, together with the means, standard

deviations and confidence limits (0~ =0.05) for the control group

Subjects and extent of section

Rate l/set Rate l/750 msec

Same Different Same Different voice voice voice voice

LE RE LE RE LE RE LE RE

Complete L.B. N.G. A.A. R.Y. D.W.*

Partial D.M. S.A. W.Y.

Controls R SD.

Confidence limits - 59-83 72 -93 5&76 68-92

44 81 7s 50 45 80 50 75 44 94 75 88 100 100 90 100 25 81 38 56 20 55 0 35 - 50 31 38 15 50 25 20

0 56 0 81 0 65 10 55

88 50 19 63 80 45 35 40 - - 15 95 50 95 - - 20 80 45 85

- - 71 83 66 - 19 16 16

80 18

* Right hemispherectomy.

There were only four instances of left-ear extinction according to the above definition (A.A. l/set, same voice; D.W. l/set, different voice; S.A. and W.Y. l/750 msec, same voice). Nine left-ear scores ~25% were accompanied by right-ear scores that were significantly below normal (A.A., both l/750 msec; R.Y., D.W., three conditions each; D.M., l/set, different voice). Six pairs of scores consisted of normal or superior right-ear scores with left- ear HRs significantly below normal but above chance (L.B., three conditions; N.G., S.A., W.Y., one condition each). With the slower rate and different voices, A.A. and R.Y. showed smaller REAs, with both left- and right-ear HRs below normal. A left-ear advantage (LEA) was shown by L.B. (l/set) and R.Y. (l/750 msec) with different voices. D.M. showed an LEA


in the same voice conditions. Finally, N.G. showed a normal REA (l/set, different voice), with ceiling performance at the fast rate.

In summary, with respect to the normal 95% confidence limits at the faster presentation rate, significantly larger than normal REAs were obtained, implicating interhemispheric transfer of speech signals in intact subjects during dichotic listening. Although every subject showed an enhanced REA in at least one condition, and left-ear HR was significantly below normal in 22/28 pairs of scores, half of the 28 pairs contained significantly depressed right-ear HRs. Thus, the left-ear input probably provided interference. Indeed, while L.B. and N.G. with full callosal sections showed lower than normal left-ear scores in some conditions, none of these were at chance level, therefore they did not show left-ear extinction. Both of these subjects have less extra-callosal damage than the other full commissurotomy subjects, both have been extensively used in research exploring the language capabilities of the right hemisphere [40], and L.B. has developed compensatory skills 1353. The possibility of L.B. having bilateral language skills has also been raised [9, 271, although he does not appear to have right-hemisphere speech [39]. L.B. subjectively reported that the left ear was harder to hear and that he concentrated more on that ear. This suggests that he was attending to the ipsilateral input to the left hemisphere. He would not have been able to make such a statement had he been accessing his right hemisphere. Thus, L.B. would seem to have overcome left-ear extinction with a strategy of voluntary attention.

Left-ear HR exceeded the chance level in 15/28 pairs of scores. Only D.W., with no right hemisphere, consistently failed to respond to left-ear target words. However, in three conditions his right-ear HR was also below normal, suggesting that the ipsilateral input from his left ear was interfering with the dominant contralateral right-ear input. D.W.‘s response rate to fusable word pairs was 2845%, well within the normal range [IS]. The perception of such pairs as targets could only have occurred by the combination of the information on the contralateral and ipsilateral auditory pathways from his right and left ears, respectively. All of the subjects showed normal rates of response to fusable word pairs, confirming previous findings 181.

To determine whether the left-ear input was processed by the right hemisphere or the left via the ipsilateral pathway in the other subjects, the HR for each ear-hand combination was examined (see Table 4). L.B. and N.G. responded to left- and right-ear targets with either hand. Thus, their right hemispheres were capable of processing target words and the ipsilateral input from each ear to either hemisphere was not suppressed. Alternatively, bihemispheric control of the hands may be implicated [27]. The results of R.Y., who was not defined as showing left-ear extinction by the previous criteria, are of particular interest. Although Table 3 shows R.Y. as having a 50% HR to right-ear items (l/set, same voice), it can be seen in Table 4 that this results from a 100% right-ear HR with right-hand responses, but a zero score when the left hand responded. This is a clear example of left-ear extinction with each hand being controlled by the contralateral hemisphere and language being processed in the left hemisphere only. Hence only right-ear inputs could be processed and only the right hand was able to respond. However, this effect was not sustained by R.Y. across conditions. In the l/set condition with different voices, right-hand responses were made to the right-ear targets and the left hand responded to left-ear targets. At the faster speed both the right hand and left hand responded to right-ear items but only the left hand to left-ear input. This last effect was also demonstrated by A.A. and S.A. in the same voice conditions, and by D.M. in the different voice condition at the slower rate. It is possible that the right hemisphere (left hand) is better able to attend to more than one stimulus, whereas

Tab

le

4.

Perc

enta

ge

of t

arge

ts

dete

cted

fo

r ea

ch

clin

ical

su

bjec

t on

ea

ch

of f

our

dich

otic

co

nditi

ons

acco

rdin

g to

ha

nd

and

ear

resp

onse

Subj

ects

an

d ex

tent

of

se

ctio

n

Sam

e vo

ice

LH

R

H

LE

R

E

LE

R

E

Rat

e l/s

et

Rat

e l/7

50

mse

c

Dif

fere

nt

voic

e Sa

me

voic

e D

iffe

rent

vo

ice

LH

R

H

LH

R

H

LH

R

H

LE

R

E

LE

R

E

LE

R

E

LE

R

E

LE

R

E

LE

R

E

Com

plet

e

L.B

. N

.G.

A.A

.

R.Y

.

Part

ial

D.M

. S.

A.

W.Y

.

F ij 38

88

50

75

10

0 50

50

50

50

90

40

70

40

90

60

60

5

63

100

25

88

63

100

88

75

100

100

100

100

80

100

00

100

50

75

0 88

25

25

50

9

88

40

70

0 40

0

43

0 30

0 0

0 10

0 63

0

0 75

30

30

0

70

50

30

0 10

z 5 4

88

63

88

38

38

25

0 10

0 70

60

90

30

30

10

40

70

3

- _

_ _

30

100

0 90

30

90

70

10

0 -

- -

_ 20

70

20

90

40

10

0 50

70

2


the left hemisphere (right hand) is more suited to contralateral attention only. On this hypothesis, only the contralateral input to the left hemisphere would be processed. That input would be perceived clearly and the response rate would consequently be high. In contrast, the contralateral and ipsilateral inputs to the right hemisphere from the left and right ears, respectively, would be processed, but overall performance would be poor due to interference between the inputs.

We found that left-ear extinction was not dependent upon section of the auditory callosal fibres. S.A. and W.Y. yielded remarkably similar results although their partial sections were made at the opposite extremes of the corpus callosum, being splenium and genu, respectively. Both showed left-ear extinction in the same voice condition. As the genu section sustained by W.Y. should not have interrupted the auditory pathways [S], his results appear to be due to attentional factors. This subject did not show a large pre-operative ear difference (left ear = 65%, right ear = 75%). D.M., with the anterior 2/3 of the corpus callosum sectioned had previously not shown left-ear extinction on a recall task [13]. He showed a lef-ear advantage in the same voice conditions, possibly indicative of right-hemisphere superiority

for speech processing. The use of different voices led to reduced ear differences, as shown previously in intact

subjects [Z, 111. However, there was no reduction in ear differences at the slower presentation, contrary to previous findings [2, 111. Subjects completed the slow lists first and showed large ear differences on these lists. Perhaps familiarity with the task enabled processing of the left-ear input.

In summary, the results indicated that inputs on the ipsilateral auditory pathways were not suppressed. Reduced right-ear HR, responses to fusable pairs, and crossed ear-hand responses all point to this conclusion. Furthermore, the right hemisphere could distinguish target from non-target words (left hand, left ear) albeit at a lower level than the left hemisphere (right hand, right ear). Across subjects, their ability to control attention may have been an important determinant of performance. Four subjects (A.A., R-Y., D.W., S.A.) who produced the largest ear differences have right-hemisphere lesions, commonly associated with disorders of attention [7]. R.Y. and A.A. showed poor left ear performance which is suggestive of a right-hemisphere attention disorder. Since these two subjects have had full forebrain commissurotomy, subcortical mechanisms may play a role in mediating the functions of the two hemispheres (cf. [27, 343).

EXPERIMENT 2

If attention is crucial on dichotic tasks and there are hemispheric differences in attention, then the use of divided attention may prejudice results. Since divided attention is more difficult than focused attention [39], the perceptual biases obtained may be related to processing strategy rather than structural mechanisms. Either division of attention per se, or distribution of attention to both halves of space, could cause difficulty.

Focused attention should be more efficient when determined by type of input [39]. Thus, use ofa focused attention task when a particular voice requires attention should clarify issues of hemispheric asymmetries in processing as they relate to attentional selectivity and spatial bias.

Experiment 2 examined these issues using a task on which subjects were required to switch attention between left- and right-ear inputs. Since division of attention was not required, and the input to be selected was determined by the sex of the voices presented, it was predicted


that no subject would show left-ear extinction, as the left ipsilateral input would be accessible to processing by the left hemisphere. Lower performance on left-ear items was expected to be associated with switches in the attended voice from the right ear to the left. Targets placed well after the switch in voices should obtain more responses, due to additional time being available for attention to become focused on the less dominant ipsilateral input. Lower left- ear performance or a right-sided bias was expected to be more prominent in persons having extracallosal damage to the right hemisphere.

METHODS Subjects

The same eight clinical subjects as in Experiment 1 were used

.4pparatus

The equipment used for stimulus presentation and data collection was the same as that described previously.

Sfimuli

Two word lists with a presentation rate of l/750 msec were constructed according to the following list specifications. The target word in each list was “DOG” and distractor words varied by no more than one phoneme (e.g. LOG). These words were only paired with irrelevant words that were phonemically dissimilar (e.g. HOUSE) and never with each other. One word ofeach word pair was spoken in a male voice and one in a female voice. Target and distractor words comprised no more than 40% of the total list. Each list contained 32 targets and 32 distractor words, 16 of each being on channel 1 and 16 being on channel 2. Word pairs containing a target or distractor word (relevant words) were always followed by a pair ofirrelevant words. Each list contained 180 word pairs and lasted for 2 min 15 sec. Throughout each dichotic list the male and female voices periodically changed or switched channels, Some of these switches were not associated with the occurrence of target words, but some were, with an equal number ofswitches occurring for each. Four targets and four distracters in each voice occurred at the same time as the switch, four ofeach occurred one word pair after the switch, and eight ofeach were not associated with a switch of voice. The number of targets occurring at the switch and one pair after the switch were evenly divided between the voices (two ofeach in each position), but this was not possible in the no switch positions given other list constraints. Hence, the same list was always repeated with the headphones reversed to ensure equal counterbafancing of the input to each ear.

Procrdure

Subjects were asked to fixate a central point on a screen in front of them and listen for the target word “DOG” in only one voice, either male or female, with the selected voice being chosen by the subject. A brief practice list involving two voice switches was presented prior to presentation of the four test lists (two repeated). The hand of response was in a R-L-L-R order where both hands were functional.

RESULTS AND DISCUSSION

The same criteria for left-ear extinction were applied as in Experiment I (left ear < 25%, right ear > 75%). The majority of subjects made no responses to the unattended voice, and those who did no more than two or three responses were made.

Table 5 shows the HR scores for all subjects according to ear, plus comparative data obtained with normal subjects [2]. The results are shown according to the position of target occurrence in relation to voice switches, together with the overall mean scores. As predicted, no subject showed left-ear extinction overall on this task and all were able to respond to more than 25% of left ear targets.

Three subjects with full callosal sections showed large ear differences (N.G. 38%, A.A. 28% and R.Y. 53%), but this was not demonstrated by L.B. who responded equally well to left- and right-ear items. Large ear differences were also shown by D.W. (61 “A) and S.A. (64%) but not by D.M. (10%) and W.Y. (no ear difference). The normal data showed a high HR with only a small ear difference.

492 JKELYN WALE and GINA GEFFEN

Table 5. Percentage of targets detected for each subject according to ear and target position in relation to voice switches

Subjects and extent of Position 0 Position 1 No switch Overall B section LE RE LE RE LE RE LE RE

Complete L.B. N.G. A.A. R.Y. D.W.*

100 15 100 100 91 100 95 95 38 100 75 100 IO 100 62 100 25 50 25 50 40 70 33 61 38 88 38 15 20 80 28 81

0 100 0 75 60 100 33 94


Normals

100 100 100 0 100 38

100 100 100

93 (12H 97 (4) 96 (8)

88 95 100 50 100 100

96 (8) 93 (6)

82 91 87 100 36 100 100 100 100

95 (5) 94 (8) 96 (6)

* Right hemispherectomy. t Standard deviations in brackets.

Position 0 = target occurs at the same time as voice switches. Position 1 = target occurs one word pair following the switch. No Switch = target unrelated to voice switch.

Greater difficulty in directing attention to the left than the right ear was shown by five of the eight subjects. L.B. and W.Y. did not show this effect, due to ceiling performance, Both D.W. and S.A. were unable to respond to left-ear targets in the attended voice if they occurred at the same time as the voice switched from the right ear to the left ear (position 0). D.W. was also unable to respond if the appropriate target occured in the word pair immediately following the switch of voices, but he was able to respond to 60% of subsequent targets which were unrelated to the switch. By contrast, both D.W. and S.A. were able to respond to all right-ear items at a high level. A lower HR for left-ear items close to the switch of voices was also obtained by N.G. and A.A. Target position made little difference to response levels for right-ear items (N.G., A.A., R.Y., D.N., S.A.). The results of A.A., D. W. and S.A. in particular support the suggestion that a right-hemisphere lesion enhances the bias towards right-ear items. This was manifest in the difficulties these subjects demonstrated in rapidly switching attention from right- to left-sided inputs.

Left- and right-ear HR scores are shown in Table 6 according to hand of response for all but D.W. who could respond only with his right hand. All subjects were able to respond to items in either ear with either hand, except for A.A. and R.Y. The data ofsix subjects in Table 6 was analysed using a two-way ANOVA, ear x hand. W.Y. was left out due to his ceiling level of responding. Right-ear targets were responded to more often than those on the left, F( 1, 5) = 5.7, P=O.O6, and the interaction of ear x hand approached significance, F(I, 5)=5.92, P=O.OSS).

As in Experiment 1 the right hemisphere (left hand) could respond to both the left (71%) and right (78%) ear targets, but the left hemisphere (right hand) had a much greater response rate to the contralateral, right ear (96%) than the ipsilateral, left ear (46%). D.M. with proposed right-hemisphere language dominance (Experiment I, LEA) showed an opposite pattern: overall LEA and his left hemisphere (right hand) responded to either input, while his right hemisphere (left hand) responded predominantly to his left ear. These results indicate


Table 6. Percentage and targets detected according to hand and car of response, Experiment 2

Subjects and extent of section

LH RH LE RE LE RE

Complete L.B. N.G. A.A. R.Y

100 89 89 100 69 100 54 100 56 22 11 100 56 72 0 89


100 83 95 90 47 100 26 100

100 100 100 100

that the ipsilateral input is not suppressed, supporting previous claims to this effect [34, 373. The use of a voice cue differentiated the two inputs, facilitating processing of the weaker ipsilateral input by the left hemisphere, as well as right-hemisphere processing of these common words.

GENERAL DISCUSSION

The REA (or LEA for D.M.) was larger than normal with both divided (Experiment 1) and focused attention (Experiment 2), confirming previous reports using dichotic recall or identification (see [lo] for review). Thus, interhemispheric transfer of left-ear/right- hemisphere speech stimuli across the corpus callosum probably occurs in the intact brain. However, left-ear extinction was not a general finding, and the assumption of interruption of modality-specific transfer of sensory information due to a lesion of the callosal pathways interconnecting the temporal lobes [S] was not supported. Two subjects with partial section of the corpus callosum showed left-ear extinction. The case with section of the anterior splenium (S.A.) that presumably interrupted the auditory callosal fibres was expected to show this. The case with section of the genu (W.Y.) was not expected to show left-ear extinction, yet this occurred. The other case with anterior section of the corpus callosum (D.M.) also produced large ear differences with divided attention. During focused attention, however, both D.M. and W.Y. showed a high level of responding to either ear. The frontal lobes have been implicated in the control of attention [17, 251. It is feasible that anterior callosal sections could disrupt attention by interrupting communication between the left and right frontal lobes.

We found that the HR of left-ear target words increased when the left hand responded, confirming a previous report of a similar finding in two subjects with complete callosal section using manual responses to dichotic consonant-vowel syllables [33]. Thus, right- hemisphere speech recognition occurs, albeit less frequently and less accurately than that in the left.

Evidence against the assumption of suppression of information on the ipsilateral pathways was found: crossed ear-hand responses occurred in the subjects with complete commissurotomy in both experiments and right-hand responses were made to left-ear target words by D.W. with right hemispherectomy (Experiment 2). The ipsilateral input must have been


available for such responses to be possible. Further evidence was the reduced HR for the right ear in several instances (including D.W.) in Experiment 1, indicating interference from left- ear words. Moreover, normal response rates to fusable word pairs were obtained even in the hemispherectomized subject. This confirms the suggestion that the ipsi- and contralateral inputs fused to produce the perception of a target word [S]. Thus, an essential assumption of the structural model, that which claims initial lateralization of dichotic inputs must be rejected (cf. [lo]).

Support for an alternative model is provided by the evidence that several cognitive factors affected performance: the use of different voices reduced ear differences, practice improved left-ear performance despite a faster presentation rate and one subject (L.B.) devised a voluntary strategy of attention to the left-ear to improve HR. All subjects improved HR to left-ear targets with selective attention to a voice cue (Experiment 2) overcoming the strong attentional bias to the right half of space.

Contrary to Kinsbourne’s view that in cases of right hemispherectomy the ensuing left- hemisphere bias to the right side cannot be overcome when simultaneous stimuli are presented [24], D.W. responded to targets on the ipsilateral input from his left ear on a dichotic task, provided that the targets occurred 1.5 set after the voice switch. Right- hemisphere lesions (A.A., D.W., S.A) reduced subjects’ ability to switch attention rapidly to the left ear. Similarly, right-hand responding, controlled by the left hemisphere, produced a larger REA by decreasing responses to left-ear targets. In contrast, responses with the left hand (right hemisphere) were made to either ear. Clearly, callosal section disrupts attention, disturbing the balance of interaction between the hemispheres [24]. If hemispheric differences in attention are assumed [ 16, Ii’, 251, this suggests that left-ear extinction may result from a loss of the right hemisphere’s ability to process inputs from either hemispace. Differences in hemispheric processing modes [26] may have intrinsic effects on the attentional capabilities of the two hemispheres.

In relation to the underlying mechanisms which operate in dichotic listening, the following assumptions are made:

(1) The left hemisphere is more adept at selective processing, while the right hemisphere’s processing mode is holistic.

(2) These differences are reflected in the attentional capabilities of the two hemispheres, with the left being more suited to selective or focused attention while the right is more adept at divided attention or parallel processing of different stimuli.

(3) Contralateral stimuli are prepotent, as in both Kimura’s and Kinsbourne’s models, but ipsilateral information is available for processing and can become predominant given the appropriate salience to offset the contralateral bias of the left hemisphere.

(4) The corpus callosum enables the integration of information between the hemispheres and maintains a balance of both attention and processing modes, with subcortical structures also playing a role in these functions.

Thus, the REA reflects left-hemisphere processing in a broader sense than simply language specialization, as assumed in previous models. Although this formulation agrees with Kinsbourne regarding an attentional bias for right-sided information, this is a result of left- hemisphere selectivity in terms of both contralateral space and stronger contralateral inputs, rather than an orienting bias. The two hemispheres differ in both attention and processing modes and the corpus callosum integrates functioning at both levels rather than simply in lateral control of attention and transmission of modality specific information.


This formulation can cope with research findings not congruent with previous models. These include the equivocal results of studies looking for an LEA with non-verbal dichotic stimuli (e.g. [3, 4]), the phenomenon of stimulus dominance which transcends ear differences [15, 313, the effects of voluntary attention to the left ear [34], and the reduction in right- hemisphere language processing when ‘noise’ is introduced and there is a need for selectivity

c401. This model has implications for any research into hemispheric asymmetries and attention

disorders, e.g. hemi-neglect, as it predicts that not only the type of processing but also the attentional requirements of a task will affect the results obtained.

Acknowledgements-The authors are grateful to Dr R.W. Sperry, Hixon Professor of Psychobiology, California Institute of Technology, and Dr Eran Zaidel, Psychology Department. University of California, Los Angeles, for granting access to L.B., N.G., R.Y., A.A., D.W. and D.M. Dr Sperry also kindly provided laboratory facilities during May, 1981. Mr D. A. Simpson allowed access to S.A. Mr D. A. Simpson, Dr J. Willoughly and Mr. P. Reilly allowed access to W.Y. We are grateful for the cooperation of these doctors and to the subjects for their time.

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