FOCUSED AND DIVIDED ATIENTION IN EACH HALF OF SPACE WITH DISCONNECTED HEMISPHERES Jocelyn Wale 1 and Gina Geffen (Centre for Neuroscience and Psychology Discipline, The Flinders University of South Australia, Adelaide) INTRODUCfiON Speech stimuli are responded to faster and identified more accurately when presented to the right than to the left of a listener. While this right ear advantage (REA) is more reliably obtained with dichotic presentation, monaural stimula- tion also elicits the right side advantage (Geffen and Quinn, 1984). More efficient contra- than ipsi-lateral transmission of information, and left hemisphere spe- cialization for speech processing are necessary components of an explanatory account of the REA. However, these mechanisms do not explain all the findings. Voluntary direction of attention to the left half of space overcomes the right side advantage, thus an initial asymmetry in the spatial distribution of attention may contribute to the REA found with divided attention (Geffen and Quinn, 1984). Kinsboume ( 1970, 1973) attributes the REA to asymmetrical activation of the cerebral hemispheres during speech tasks, thereby biasing attention involuntarily to the right contralateral half of space. Moreover, recent evidence indicates that the left and right hemispheres may subserve different attentional processes (Wale and Geffen, 1986). Electrophysiological data from normal subjects (Brunia and Damen, 1988; Heilman and Van den Abell, 1980) and persons with unilateral lesions (Knight, Hillyard, Woods and Neville, 1981) itidicate that the right hemisphere is able to attend to both sides of space while the left can attend only to the right or contralateral side. A further differentiation between the hemispheres in attention processes was proposed by Harter, Aine and Schroeder (1982), with right hemisphere specialization for locating stimuli in space, and left hemisphere specialization for selective attention to the type of information presented. This is congruent with the view of left hemisphere superiority in sequential processing, while the right excels at parallel spatial processing (Bradshaw and Nettleton, 1983). It is possible that the type of attention that is engaged may depend upon the relative activation of the cerebral hemispheres due to the nature (e.g. verbal) and side of stimulation. While commissure sectioned subjects showed an enhanced REA with blocked rather than random presentation of monaural words to each 'Present address: Psychiatric Unit, Royal Adelaide Hospital, Adelaide, Australia, 5000. Cortex, (1989) 25, 33-45
Transcript
FOCUSED AND DIVIDED ATIENTION IN EACH HALF OF SPACE WITH DISCONNECTED HEMISPHERES
Jocelyn Wale1 and Gina Geffen
(Centre for Neuroscience and Psychology Discipline, The Flinders University of South Australia, Adelaide)
INTRODUCfiON
Speech stimuli are responded to faster and identified more accurately when presented to the right than to the left of a listener. While this right ear advantage (REA) is more reliably obtained with dichotic presentation, monaural stimulation also elicits the right side advantage (Geffen and Quinn, 1984). More efficient contra- than ipsi-lateral transmission of information, and left hemisphere specialization for speech processing are necessary components of an explanatory account of the REA. However, these mechanisms do not explain all the findings. Voluntary direction of attention to the left half of space overcomes the right side advantage, thus an initial asymmetry in the spatial distribution of attention may contribute to the REA found with divided attention (Geffen and Quinn, 1984).
Kinsboume ( 1970, 1973) attributes the REA to asymmetrical activation of the cerebral hemispheres during speech tasks, thereby biasing attention involuntarily to the right contralateral half of space. Moreover, recent evidence indicates that the left and right hemispheres may subserve different attentional processes (Wale and Geffen, 1986). Electrophysiological data from normal subjects (Brunia and Damen, 1988; Heilman and Van den Abell, 1980) and persons with unilateral lesions (Knight, Hillyard, Woods and Neville, 1981) itidicate that the right hemisphere is able to attend to both sides of space while the left can attend only to the right or contralateral side. A further differentiation between the hemispheres in attention processes was proposed by Harter, Aine and Schroeder (1982), with right hemisphere specialization for locating stimuli in space, and left hemisphere specialization for selective attention to the type of information presented. This is congruent with the view of left hemisphere superiority in sequential processing, while the right excels at parallel spatial processing (Bradshaw and Nettleton, 1983).
It is possible that the type of attention that is engaged may depend upon the relative activation of the cerebral hemispheres due to the nature (e.g. verbal) and side of stimulation. While commissure sectioned subjects showed an enhanced REA with blocked rather than random presentation of monaural words to each
'Present address: Psychiatric Unit, Royal Adelaide Hospital, Adelaide, Australia, 5000.
Cortex, (1989) 25, 33-45
34 Jocelyn Wale and Gina Geffen
ear (Gordon, 1975), intact subjects show an increased REA when the side of monaural presentation is randomized (Geffen and Quinn, 1984). With blocked presentation, focused attention to a particular side of space is required, whereas random ear of presentation requires spatially divided attention. The larger REA involving reduced left ear scores in commissure sectioned subjects with blocked presentation suggests some incapacity of the right hemisphere for focused attention. In contrast, the larger REA in intact subjects with divided attention may reflect the role of the corpus callosum in the maintenance of spatially distributed, divided attention.
The interplay between hemispheric processing and attention (both selectivity and spatial allocation) warrants further investigation. Tasks with comparable complexity to dichotic monitoring but avoiding left vs right spatial competition were required. Monitoring tasks using two voices were therefore administered to subjects in three different spatial locations, left, central and right, while they either focused attention on one voice, or divided attention between the voices. It was anticipated that the use of blocked presentation would reduce the possibility of a right side advantage occurring with normal control subjects. This was desirable if the task was to differentiate between clinical and control subjects on factors affected by hemispheric differences in attention. It was postulated that these clinical subjects (persons with callosal section or hemispherectomy) would show· improved performance with right sided stimuli and particularly in the condition of selective attention.
MATERIALS AND METHOD
Subjects
There were 10 control subjects (five male) covering a wide range of educational and occupational backgrounds, aged from 18-64 years (female mean age = 37.4 years, male mean = 38.2 years), with males and females being evenly distributed across the age range. The subjects were volunteers and had no known neurological or psychiatric illness.
There was also a group of clinical subjects. The subjects were L.B., N.G., A.A. and R. Y. with full commissure sections, and D. W. with a right hemispherectomy. The case histories of all subjects have been published extensively previously (Gott, 1973; Milner and Taylor, 1972; Sperry, Gazzaniga and Bogen, 1969). Relevant details for each subject, including any extra callosal damage, are summarized in Table I.
Stimuli
Lists of high frequency (classification AA or A by Thorndike and Lorge, 1944) monosyllabic words were constructed consisting of specified target words (e.g. black), phonemic dis tractor words different from the target word in the initial consonant only (e.g. slack) or with one of the first two consonants omitted (e.g. lack or back), and other distractor or irrelevant words having no more than one phoneme in common with target words. Each list contained 120 word pairs, which were presented simultaneously, with one word of each pair being spoken in a female voice and the other in a male voice. There were 24 targets and 24 distractor words in each list, with 12 of each being in one voice and 12 being in the other. Targets and distractor words were always paired with irrelevant words. The words occurred in a random order with the constraints that any word pair containing a target or a distractor word was followed by a pair of irrelevant words and successive targets
35 Focused and divided attention with disconnected hemispheres
TABLE I
Briefcase histories of the five clinical subjects, also giving age at testing andpost-operative test scores for verbal and performance IQ
Age Subject Sex Symptom
onset Operation Age
tested VIQ/PIQ
L.B. M 3V2 At age 13 for intractable epilepsy. 29 110/110 Complete section of forebrain commissures with retraction of right hemisphere, including anterior and hippocampal commissures. No radiological or neurological signs of
N.G. F 18
localized brain damage preoperatively. Recovery rapid. At age 30, as above. Radiological 49 83171 evidence of 1 em calcified lesion beneath right cortex and EEG signs of focus in left posterior temporal region pre-operatively. One year
A.A. M 5
post-operative EEG normal and no neurological signs of localized cortical damage. At age 14, as above. Operation difficult. Residual cortical lesions in
30 77/82
the left arm area and right leg area.
R.Y. M 17 At age 43, operation as above. Slight 57 99779 spasticity and poor motor control of left hand. Seizures attributed to car
D.W. M 6V2 accident at age 13. At age 7;9, underwent right hemispherectomy for intractable epilepsy.
24 80/60
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.
were equally likely to occur in either voice. Of all word pairs, 50% were irrelevant pairs. The rate of word pair presentation was 11750 msec. Construction of the word lists by a PDP 11/34 computer programme was the same as that described by Wale and Geffen (1986).
Apparatus
The equipment used was the same as that described by Wale and Geffen (1986) for two dichotic tasks, but the Uher 2-channel tape recorder (male voice on one channel and female on the other) was switched to monaural presentation rather than stereo so that the same stimuli (both voices or word pairs) went to each channel or headphone.
Procedure
Subjects had been tested previously in an experiment using a dichotic monitoring task (Wale and Geffen, 1986) and so were familiar with the procedure and task requirements. They had also been administered a 1 kHz binaural tone test to screen for gross hearing
36 Jocelyn Wale and Gina Geffen
deficits, plus single voice monaural tasks to ensure similar speech perception on each ear (see Wale and Geffen, 1986). As on the dichotic monitoring tasks, they were required to press a button on a response panel whenever a target word was heard. The hand of response was in a r/111/r/r/1 order over the six test lists.
Two conditions were administered: divided attention (six lists) and focused attention (six lists). Divided attention always preceded focused attention to minimize any possibility of priming effects to one voice interfering with the ability to divide attention equally between the voices. With divided attention subjects responded to targets in either voice, whereas with focused attention they responded only to targets in one of the two voices. This was the preferred voice, chosen by the subject at the end of the divided attention condition. All subjects~ except for NG and RY, chose the female voice.
In each condition, two lists were presented in each of three locations or presentation modes: to the left ear, to the right ear, and to both ears so that the stimuli were perceived centrally. With left or right sided presentation, one headphone was disconnected. There was a rest period of several minutes between the two conditions of attention while the tape was rewound. Each condition was preceded by a short practice list presented to both ears.
Individual performance was scored on the following measures:
(HR) the number of targets receiving responses 1 H. . tt rate , h b dt e num er presente
. . (FP) number of phonemic distractors receiving responses 2 F 1 . a se postttve rate , b d num er presente
3. Reaction time to targets in milliseconds (RT). 4. P(A) = (1 +HR-FP)0.5. P(A) scores of less than 0.5 indicate that the FP rate was
higher than the HR. The maximum P(A) = 1.0, with any score above 0.5 indicating target discrimination. P(A) measures item selection or target discrimination within a channel or input of information.
These dependent variables were then used in group analyses. Measure 4 was only used to compare the two conditions of attention as target discrimination was found to be a more sensitive measure of attention than HR or FP alone (Clark, Geffen and Geffen, 1987).
RESULTS AND DISCUSSION
Due to the number of measures and analyses used, possibly increasing the type I error rate, it was thought desirable for isolated effects to be significant at ex = 0.01 and for those at the 0.05 level to be reflected on more than one measure.
To examine whether one voice was perceptually more salient, and whether location of the input affected performance, the data for control subjects were analysed before comparison with the clinical group.
Divided Attention
Two-way ANOVAs, with the factors of Location (left, central, right) and Voice (male vs female), were done on the dependent variables HR, FP and RT. The means are presented in the final rows of Tables II and III.
Targets in the female voice were responded to more often (mean = 86.7, s.d. = 12.0l)thanthoseinthemalevoice(mean = 58.9,s.d. = 19.l),F = 11.51,d.f.
37 Focused and divided attention with disconnected hemispheres
TABLE II
Divided attention: Percentage targets detected (HR) and false positive responses (FP) for individual clinical subjects according to location of presentation (left, central, right) and voice (male vs. female).
* Lower than control mean by at least 1 s.d. t Higher FP rate than controls by at least 1 s.d.
TABLE III
Divided attention: Reaction time (msec) to targets for individual clinical subjects according to location of presentation (left, central, right) and voice (male vs. female). Control group means (s.d) are also
* Slower than control mean by at least 1 s.d. ** No response.
= 1, 9, p < 0.01. Location of the inputs did not affect HR, F < 1, and the interaction of Location and Voicewasnotsignificant(F = 2.67; d.f. = 2, 18; p > 0.05). There were no significant differences (main or interaction effects) one the FP measure. Faster responses occurred to the female voice (mean = 589, s.d. = 1 03) than to the male (mean = 673, s.d. = 93), F = 11.5, d.f. = 1, 9, p < 0.01, and the interaction of Location with Voice was not significant.
The female voice was perceptually more salient than the male voice: more female targets were detected and responses to female targets were faster. Loca
38 Jocelyn Wale and Gina Geffen
tion (left vs central vs right) had no significant effect on performance and did not interact with Voice. As location was confounded with order of presentation, this finding suggests that practice had little effect on performance and, as anticipated, control subjects did not demonstrate a right side advantage with blocked presentation.
Focused Attention
There were two few responses (<I%) to the unattended voice for analyses on RT to be meaningful. However, two-way ANOVAs (Location X Voice) on HR and FP showed a higher level or responding to the attended voice (female for all subjects). HR to the attended voice (mean= 94.3, s.d. = 7.3) was higher than to the unattended (mean= 1.4, s.d. = 2.3), F = 1325.0, d.f. = I, 9, p < O.OOI, as was FP to the attended voice (mean = I3.7, s.d. = I2.7) compared to the unattended voice (mean= 0.08, s.d. = 1.4), F = I9.7, d.f. = I, 9, p < 0.01. On none of these variables was there a significant effect of Location or a Location by Voice interaction.
As would be expected, there were more responses to items (both targets and dis tractors) in the attended voice. As with divided attention, the location of input presentation had no significant effect on performance.
Divided and Focused Attention Compared
Analyses with the factors, type of Attention (2) X Location (3), were carried out on all responses to the dominant voice in divided attention and the attended voice in the focused condition to determine whether the two strategies of attention differed according to location of input, despite the difference in salience of the two voices. There were no significant main effects or interactions on the analyses for HR, or FP. However, target discrimination, P(A), was better in focused attention compared to divided attention (F = 7.07; d.f. = I, 9; p < 0.05) as was response speed, RT (F = 7.22; d.f. = I, 9; p < 0.05). Thus P(A) for the selected voice in focused attention (mean = 0.88, s.d. = 0.06) was higher than for the dominant voice in divided attention (mean = 0.82, s.d. = 0.08). Similarly, RT to targets was faster for the selected voice in focused attention (mean = 540 msec, s.d. = 68.5) than for the dominant voice in divided attention (mean = 589 msec, s.d. = 108.9). There was also a tendency for the P(A) to be higher when inputs were presented centrally (mean = 0.88, s.d. = 0.08) compared to left ear (mean = 0.82, s.d. = 0.07) or right ear presentation (mean = 0.84, s.d. = 0.06), F = 3.53, d.f. = 2, I8, p = 0.05I. There were no other significant main or interaction effects.
In summary, the control data showed the female voice to be more salient than the male as has been found before (Denes and Pinson, 1973). As would be expected, focusing attention on that voice conferred advantages compared to measures for the same (dominant) voice in divided attention. Thus both target discrimination and speed of response were better in the condition of focused attention. The tendency, found on P(A), for centrally presented inputs to be
39 Focused and divided attention with disconnected hemispheres
better processed than those that w.ere laterally presented, was not reflected in other measures.
Clinical and Control Comparisons
Results for the 5 clinical subjects were compared to those of the control subjects. The individual and mean results are shown in Tables II-VII with marks (*or t) indicating where the individual patient's performance was at least one s.d. worse than that of the control group.
Divided Attention
Three-way ANOV As using the factors Group (2) X Location (3) X Voice (2) were performed on all dependent measures. Control subjects responded to more targets (mean = 72.8, s.d. = 9.4) than clinical subjects (mean = 57.0, s.d. = 16.1), F = 5.9, d.f. = 1, 13, p < 0.05; and targets in the female voice were responded to more often (F = 27.9, d.f. = 1, 13, p < 0.001). The effects of Location and the two and three-way interactions between Group, Location and Voice were not significant on the HR measure. When FP was analysed, the main effect of Voice was again significant (female mean = 29.1, s.d. = 16.6; male mean = 24.0, s.d. = 15.1), F = 5.1, d.f. · = 1, 13, p < 0.05, and there was a significant Group by Voice interaction. The dominance of the female voice shown on HR resulted in the clinical subjects giving more responses to dis tractor items in the female voice (mean = 41.6, s.d. = 19.1) than did control subjects (mean= 22.9, s.d. = 11.7), but only atthe0.05 significancelevel(F = 6.9, d.f. = 1, 13; p < 0.05). This was true for all subjects except L.B. (see Table II). No other interaction effects were significant. Interesting individual results were N.G.'s consistently high FP rate, L.B.'s lower overall performance in the central position, and the lack of responses to the less salient male voice by A.A. and D.W., when stimuli were presented on the right side. Only R.Y., like the control subjects, had relatively better performance to stimuli in the central location.
The superior performance of control subjects on the HR measure was also shown on RT where they were faster at responding to targets (mean = 631 msec, s.d. = 89.6) than were clinical subjects (mean = 766 msec. s.d. = 95.6), F = 7.3, d.f. = 1, 13, p < 0.05). However, the significant main effects of Group, Location and Voice were modified by a 3-way interaction of these factors (F = 6.1; d.f. = 2, 26; p < 0.01). This interaction was due to slower RT to left sided targets in the male voice which was more marked in the clinical (by 402 msec) than the control (74 msec) subjects. In addition the clinical subjects made very few responses to male voice targets in the right side of space, thereby producing an artifactually low mean RT (see Table III).
In summary, when both groups of subjects were considered, the female voice continued to be perceptually more salient than the male voice. Target detection (HR), and speed (RT) were better for control than clinical subjects. Furthermore, the clinical subjects tended to respond to more distractor items in the female voice. The location of stimulus input had an effect only on the RT measure, which
40 Jocelyn Wale and Gina Geffen
also reflected the apparent difficulty clinical subjects had in responding to the male voice. Two subjects (A.A. and D.W.) did not respond to male voice targets when these were presented on the right. This resulted in a fast group mean RT for right sided stimuli. This difficulty was also reflected in very slow RTs to male voice targets in other locations.
Focused Attention
Tables IV, VI and VII show the individual and mean results for the patients and controls on the condition of focused attention.
Analyses on HR and FP (Table IV) confirmed that performance on the attended voice was superior (p < 0.001). Clinical subjects also tended to respond to more distractor items (mean = 45.0, s.d. = 28.0) than did control subjects (mean = 21.7, s.d. = 15.6), F = 8.6, d.f. = 1, 13, p < 0.05). There was a significant interaction between Group X Voice for HR where clinical subjects detected fewer targets in the attended voice (mean = 74, s.d. = 22.4) and more in the unattended voice (mean= 10.5, s.d. = 9.0) than did controls (attended voice: mean = 94.3, s.d. = 7.3; unattended voice: mean= 1.4, s.d. = 2.3), F = 12.6, d.f. = 1, 13, p < 0.01. Except for A.A. and D.W. with right sided presentation, each clinical subject's performance on HR and/or FP was worse than that of controls. Thus, these effects were not restricted toN.G. and R.Y. who attended to the less salient male voice.
Clinical subjects were therefore less efficient in performance on focused attention. They were not as good at selecting only the attended voice and ignoring the unattended and they tended to make more responses to distractor items. All clinical subjects showed evidence of less efficient attention on these tasks.
TABLE IV
Focused attention: Percentage targets detected (HR) andfalse positive (FP) rates kor individual clinical subjects according to location ofpresentation (left, central and voice (male vs. fema e). Group means (s.d)
* Lower than control mean by at least I s.d. t Higher FP rate than controls by at least I s.d.
41 Focused and divided attention with disconnected hemispheres
Comparision of Div.ided and Fvcused Attention
The two conditions of attention were compared using only results for the dominant/ attended voices. Thus three-way ANOVAs, Group (2) X Attention (2) X Location (3), were carried out on all dependent variables. Better performance of controls compared to clinical subjects was obtained at the 0.05 level on HR, FP and RT. On HR and FP there were no other significant effects. Control subjects detected more targets (mean = 90.5, s.d. = 8.0), than did clinical subjects (mean = 76.0, s.d. = 15.5), F = 5.9, d.f. = 1, 13, p < 0.05, while clinical subjects responded to more distractor items (mean = 41.0, s.d. = 19.3) than did controls (mean = 20.8, s.d. = 11.3), F = 6.6, d.f. = 1, 13, p < 0.05.
Thus, control subjects (mean = 0.85, s.d. = 0.06) were better at target discrimination, P(A), than were clinical subjects (mean= 0.68, s.d. = 0.07), F = 22.3, d.f. = 1, 13, p < 0.001. For all subjects, targets tended to be discriminated better when presented centrally (mean = 0.82, s.d. = · 0.12) than in either of the lateral positions (left: mean= 0.76, s.d. = 0.12; right: mean= 0.80, s.d. = 0.10), F = 3.9, d.f. = 2, 26, p < 0.05. A tendency for control subjects to perform better with focused attention while the clinical subjects' target discrimination was similar with both divided (mean = 0.69, s.d. = 0.08) and focused attention (mean = 0.67, s.d. = 0.08) also failed to reach significance (F = 4.3; d.f. = 1, 13; p = 0.06). However, this finding may have resulted from two clinical subjects choosing to attend to the less dominant male voice.
The control subjects were faster at responding to targets than the clinical subjects (F = 6.4; d.f. = 1, 13; p < 0.05), and performance also tended to be faster with focused attention compared to divided attention for all subjects, as it had for the controls alone (F = 7.4; d.f. = 1, 13; p < 0.05) (see Tables III and VII).
In summary, the between group comparisons of divided and focused attention revealed that control subjects had an overall better level of responding to targets on measures of target detection (HR), target discrimination (P(A)), and
TABLE V
Divided attention: Target discrimination (P[A}) for individual clinical subjects according to location of presentation (left. central, right) and voice (male vs. female). Group means (s.d.) are also shown
Focused attention: Target discrimination (P[Aj) for individlllll clinical subjects according to location of presentation (left, central, right) and voice (attended vs. unattended). Group means (s.d) are also
L.B. .73* .51 .81* -t .75* .48 N.G. .63* .58 .65* 54 .54* .54 A.A. .63* -t .73* .52 .81 .48 R.Y. .52* .48 .58* .56 .63* .52 D.W. .58* .58 .65* .58 .81 .44 Patient mean (s.d.) .62 (.08) .53 (.06) .68 (.09) .55 (.03) .71 (.12) .49 (.04) Control mean (s.d.) .87 (.08) -t .91 (.07) -t .86 (.07) -t * Lower than control mean by at least 1 s.d. t Too few responses for meaningful scores.
TABLE VII
Focused attention: Reaction time (msec) to targets for individual clinical subjects according to location of presentation (left, central, right) and voice (attended vs. unattended). Control group means (s.d.) are also
L.B. 672* 504 644* 721* N.G. 521 390 674* 349 728* 423 A.A. 985* 795* ll61 845* R.Y. 706* 922 564 753 572 728 D.W. 508 597 552 1259 670* Patient mean (s.d.) 678 (193) 603 (228) 646 (98) 880(417) 707 (99) -t Control mean (s.d.) 542 (84) -t 517 (66) -t 562 (102) -t * Slower than control mean by at least I s.d. t Too few responses for meaningful scores.
response speed (RT), while the clinical group made more false positive responses (FP) to distractor items. Response times tended to be faster with focused attention, but focused attention did not improve performance on any other measure. There was also a tendency for targets to be discriminated better when presented centrally.
GENERAL DISCUSSION
The results suggest that subjects with separated hemispheres have less control over attention than intact subjects, being less efficient at both divided and focused attention. Compared to the control group, each lesioned subject (includ
43 Focused and divided attention with disconnected hemispheres
ing L.B. with minimal extracallosal damage) showed evidence of reduced efficiency on auditory attention tasks, with poorer target detection and discrimination, and slower responses to targets. Whereas central presentation of stimuli tended to produce the best performance in the control group, three clinical subjects showed best target discrimination with right sided presentation as predicted (L.B. divided attention; A.A. and D.W. focused attention). In situations with no lateral competition or uncertainty about stimulus location an intact corpus callosum may enable the cerebral hemispheres to function together and process centrally located information better than that which is laterally displaced. A disadvantage for midline depth-perception in the visual modality has been shown to occur after commissurotomy (Mitchell and Blakemore, 1970).
The results of the patient group implicate the corpus callosum and the unified functioning of both hemispheres as being critical for efficient attentional skills. Moreover, extracallosal hemispheric damage appeared to affect attention in specific ways. N.G. had a left temporal focus which would explain her consistently low target discrimination scores and better performance with left sided presentation. R.Y. has evidence of significant extracallosal damage to the right hemisphere, and his generally low level of performance on these tasks appears to reflect a reduced level of attentional resources. It is notable that both of these subjects' level of attentional skill was so low that they did not choose the more salient or dominant female voice (chosen by all other subjects) on which to focus attention. In particular, A.A. and D.W., both with right hemisphere disorders, were unable to divide attention between two voices when they were both presented to the right ear. In this condition they responded as if selectively attending to the dominant voice, with good performance for both target detection and discrimination in that voice. Right sided presentation appeared to potentiate selective processing with this effect occurring regardless of the specified task pemands on attention. The left hemisphere is specialized for both language processing (Levy, 1978) and for attention to the right contralateral side of space (Heilman and van den Abell, 1980). Furthermore the speech processing demands of the task would have activated the left hemisphere more than the right (Kinsbourne, 1970). The combination of right sided presentation with language processing appears to have induced selective processing of one voice only, despite instructions to divide attention. This result suggests an interaction between the nature of the task ( eg verbal vs visuo-spatial), direction of stimulation, and attention strategy, with a resultant mutual facilitation which may induce specialized processing. The results of D.W. and A.A. suggest that the right hemisphere is important for the spatial distribution of attention, and implicate mechanisms which moderate the functioning of the left hemisphere. If these mechanisms are dysfunctional, the left hemisphere's attentional abilities can be affected by the direction from which stimuli· are received. Hence, right sided stimulation appeared to activate the left hemisphere in such a way that divided attention was not possible. The fact that all subjects with full callosal sections did not show this effect supports previous claims that the right hemisphere can affect attention in the left hemisphere at a sub-cortical level (Sperry, 1974), and that attentional processes involve an interaction between the henuspheres (Levy, Trevarthen and Sperry, 1972).
44 Jocelyn Wale and Gina Geffen
These results also raise questions regarding the frequent assumption that left ear extinction on dichotic tasks reflects suppression of the ipsilateral auditory pathways by the contralateral (Kimura, 1967). It may be that with right hemispherectomy, or when there is right hemisphere damage as well as callosal section, only the dominant input will receive attention. With dichotic stimuli this is the right ear input. It is notable that D.W. has previously shown that he could respond to left ear items on a dichotic task of selective attention, despite showing left ear extinction on a task of divided attention (Wale and Geffen, 1986). Thus attentional factors may explain some of the discrepancies in the results of commissurotomy subjects on dichotic tasks.
The notion of the left hemisphere being more specialized for selective attention is congruent with findings showing the left hemisphere to be more specialized on tasks involving language and which require selective, sequential processing (Levy, 1978). Differential attentional processes for the two hemispheres has also been postulated for spatial location (Heilman and van den Abell, 1980) and for selection according to type of stimuli (Harter et al., 1982). The proposal of hemispheric specialization for selective attention to account for the findings in the present paper does not conflict with those studies.
ABSTRACT
The effects of spatial location and strategy of attention on the processing of speech messages were investigated in ten right-handed subjects and four persons with complete forebrain commissurotomy and one case of right hemispherectomy. Lists of simultaneous but different words, with one of each pair spoken by a male and the other by a female, were monitored for target words. The lists were presented to the left ear, the right ear and to both ears (central) in separate conditions. Unimanual responses were made to targets in either voice (divided attention) or to only one of the voices (focused attention). The performance of the clinical subjects was generally less efficient than that of the controls. In contrast to intact subjects, they responded to more distractor and unattended items. They also showed a nonsignificant tendency for better processing of right rather than centrally-presented words. With right-sided presentation two clinical subjects, with extracallosal damage to the right cortex, were unable to divide attention and responded to only one of the voices. These results implicate an intact corpus callosum for efficient use of attention strategies as well as hemispheric differences in the control of attention.
Acknowledgements. This research was supported by an A.R.G.S. grant to Gina Geffen.
We 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. and D.W. Dr. Sperry also kindly provided laboratory facilities during May, 1981.
REFERENCES
BRADSHAW, J.L., and NETILETON, N.C. Human Cerebral Asymmetry. New Jersey: Prentice-Hall Inc., 1983.
BRUNIA, G.H.M., and DAMEN, E.J.P. Distribution of slow brain potentials related to motor preparation and stimulus anticipation in a time estimation task. Electroencephalography and Clinical Neurophysiology, 69: 234-243, 1988.
45 Focused and divided attention with disconnected hemispheres
CLARK, C.R., GEFFEN, G.M., and GEFFEN, L.B. Catecholamines and attention II: studies in normal humans. Neuroscience and Biobehavioural Reviews, JJ: 353-364, 1987.
DENES, P.B., and PINSON, E.N. The Speech Chain: The Physics and Biology of Spoken Language. New York: Anchor Books, 1973.
GEFFEN, G., and QUINN, K. Hemispheric specialization and ear advantages in processing speech. Psychological Bulletin, 96: 273-291, 1984.
GORDON, H.W. Comparison of ipsilateral and contralateral auditory pathways in callosum-sectioned patients by use of a response-time technique. Neuropsychologia, 13: 9-18, 1975.
Gorr, P.S. Cognitive abilities following right and left hemispherectomy. Cortex, 9: 266-274, 1973.
HARTER, M.R., AINE, C., and ScHROEDER, C. Hemispheric differences in the neural processing of stimulus location and type: Effects of selective attention on visual evoked potentials. Neuropsychologia, 20: 421-438, 1982.
HEILMAN, K.M., and VAN DEN ABELL, T. Right hemisphere dominance for attention: The mechanism underlying hemispheric asymmetries of inattention. Neurology, 30: 327-330, 1980.
KIMURA, D. Functional asymmetry of the brain in dichotic listening. Cortex, 3: 163-178, 1967. KINSBOURNE, M. The cerebral basis of lateral asymmetries in attention. Acta Psychologica, 33:
193-201, 1970. KINSBOURNE, M. The control of attention by interaction between the cerebral hemispheres. In S.
Kornblum (Ed.), Attention and Performance IV. New York: Academic Press, 1973, pp. 239256.
KNIGHT, R.T., HILLYARD, S.A., WOODS, D.L., and NEVILLE, H.J. The effects of frontal cortex lesions on event-related potentials during auditory selective attention. Electroencephalography and Clinical Neurophysiology, 51: 571-582, 1981. ·
LEVY, J. Lateral differences in the human brain in cognition and behavioural control. In P. Buser and A. Rougeul-Buser (Eds.), Cerebral Correlates of Conscious Experience. Amsterdam: Elsevier, 1978, pp. 285-298.
LEVY, J., TREVARTHEN, C., and SPERRY, R.W. Perception of bilateral chimeric figures following hemispheric deconnection. Brain, 95: 61-78, 1972.
MILNER, B., and TAYLOR, L. Right hemisphere superiority in tactile pattern-recognition after cerebral commissurotomy: Evidence for nonverbal memory. Neuropsychologia, 10: 1-15, 1972.
MITCHELL, D.E., and BLAKEMORE, C. Binocular depht perception and the corpus callosum. Vision Research, 10: 49-54, 1970.
SPERRY, R.W. Lateral specialization in the surgically separated hemispheres. In F.O. Schmitt and F.G. Worden (Eds.), The Neurosciences Third Study Programm, 5-19. Cambridge, Mass.: The M.I.T. Press, 1974.
SPERRY, R.W., GAZZANIGA, M.S., and BOGEN, J.E. Interhemispheric relationships: The neocortical commissures; syndromes of hemisphere disconnection. In P.J. Vinken and G.W. Bruyn (Eds.), Handbook of Clinical Neurology, 4. Amsterdam: North-Holland Publishing Co., 1969, pp. 274290.
WALE, J., and GEFFEN, G.M. Hemispheric specialization and attention. Effects of complete and partial callosal section and hemispherectomy on dichotic monitoring. Neuropsychologia, 24: 483-496, 1986
Gina Geffen, School of Social Sciences, The Flinders University of South Australia, Bedford Park, Australia, 5042.
