Hemispheric Interaction and Attention

The role of attention in hemispheric interaction and the role of hemispheric interaction in attention have produced challenging questions ever since split-brain research demonstrated that attention was not a process as unitary as was originally thought (see Corballis, 1995; M. Gazzaniga, 2000, for a review). Performance of split- brain individuals showed that each hemisphere could concurrently attend to separate stimuli in certain circumstances (see McMains & Somers, 2004, for related findings in intact individuals), while in others they could not. For instance, split-brain individuals can conduct a simple visual search task in parallel in the two visual fields whereas intact individuals cannot, thus suggesting a role of the CC in attention. However, it has also been demonstrated that split-brain individuals are affected by negative priming (Lambert, 1993). Thus, split-brain individuals who were asked to categorise digits presented to the RVF as odd or even responded significantly slower, than did the controls, when a to-be-ignored digit was presented to the LVF. This shows that some attentional effects are not mediated by the CC (see also Lambert & Naikar, 2000). Overall, however, split-brain individuals demonstrate clear deficits in attention which may partly be related to the underlying pathology (epilepsy) but which are more likely to be the product of the section of the CC (Afraz, Montaser-Kouhsari, Vaziri-Pashkam, & Moradi, 2003; Corballis, 1995; Ellenberg & Sperry, 1979).

Levy, Trevarthen, and Sperry (1972) demonstrated that lateralisation of attentional control could be manipulated. They tested split-brain participants in a chimeric face recognition task in which halves of two different faces were presented to separate visual fields. The task consisted in reporting which face had been seen either by pointing in free vision to an array of faces positioned in front of participants or by naming the previously memorised identity of a face. When pointing, split-brain individuals reported significantly more faces (82%) viewed in the LVF than in the RVF. Remarkably, this was also the case when they used their right hand. However, when they were asked to voice the identity of the face, the pattern was reversed. This happened even when the responding instructions were switched just before a response was given. This suggests that faces presented to each visual field were attended concurrently and recognised, but that the attentional focus was shifted depending on response instructions. These findings also speak against a unified model of attention. Since such behaviours are not seen in intact individuals it is probable that some modulation of attentional control in each hemisphere is mediated by the CC.

A study (Hines, Paul, & Brown, 2002) investigating individuals with agenesis of the CC also supports this proposition. Ten individuals with agenesis of the CC and nine controls were tested in a cued target detection task in which cues were valid 80% of the

time. In invalid trials, attention needed to be redirected to the target location which occurred within visual field for half of the trials and across visual field for the other half. It was found that although individuals with agenesis of the CC did not differ from controls in the cost of shifting attention within visual fields, shifting attention between visual fields was significantly slower in these individuals.

In neurologically intact individuals, a role of the CC and hemispheric interaction in attention has also been demonstrated. Rueckert, Baboorian, Stavropoulos, & Yasutake (1999) assessed the influence of callosal efficiency on attention in forty-two healthy adults. They used two tasks to assess the efficiency of callosal transfer. A bi- manual coordination task specifically relying on the transfer of motor signals was conducted to assess the efficiency of the anterior CC (where pre-motor and motor fibres cross). A line comparison task, relying on the transfer of perceptual information was conducted to assess the efficiency of the posterior CC (where perceptual and associative fibres cross). Another two tasks measured sustained and focused attention. Sustained attention was measured with a simple vigilance task lasting 20 minutes, in which participants were asked to press a button when an “X” appeared on screen at inter- stimulus intervals (ISI) varying between 2 and 18 seconds. Focused attention was measured in a tachistocopic letter task in which participants had to determine whether the middle-letter of three horizontally presented letters matched any of the other two letters lateralised to a separate visual field. Results suggested that the efficiency of the anterior CC was related to the capacity to sustain attention over the entire 20 minute of the vigilance task, while the efficiency of the posterior CC was related to the ability of maintaining attentional focus for longer ISIs of the vigilance task. A general attention measure did not correlate with either of the tasks assessing callosal efficiency.

While the studies reported so far necessarily involved hemispheric interaction, they were more focused on the integrity and efficiency of callosal transfer than interaction of the two hemispheres per se. Studies reviewed previously and interested in assessing hemispheric interaction by comparing within and across hemisphere performance, such as the shape and identity letter-matching tasks, suggest that hemispheric interaction might have an attentional effect by reallocating resources based on task demands (low/high) and on available hemispheric resources. Another line of evidence also suggests that hemispheric interaction might modulate attentional processes by acting as a filter or a gate, thus avoiding interference from irrelevant information (see Banich, 1998 for a discussion of the role of hemispheric interaction in attentional processes based on different models of attention). Banich & Passarotti (unpublished but reported in Banich, 1998) tested the role of hemispheric interaction in attentional filtering. They presented participants with coloured geometrical shapes. Participants had to determine whether two of three shapes, in a display similar to that of the letter-matching task, matched on their form while colour was ignored. The match occurred either within VF or across VF and the degree of selective attention required to perform the task was varied by changing the colour of the matching and non-matching shapes. There were four conditions with increasing attentional demands: the probe had the same colour as the target while the non-matching shape did not, thus providing redundant information congruent with the task; the probe had the same colour as the target but so did the non-matching shape therefore providing irrelevant information; the probe had a colour different from the target and so did the non-matching shape, thus like the last condition, also providing irrelevant information; or the probe had a colour different from the target while the non-matching shape had the same colour as the target, therefore providing conflicting information. Across hemisphere performance was found to be significantly better in the last condition which required the highest degree of attentional filtering since in this condition colour was completely misinformative. These

findings, and consistent results using a global-local paradigm (Weissman & Banich, 1999), conflicting letter-colour matches (Sohn, Liederman, & Reinitz, 1996), or dual tasks (Merola & Liederman, 1990; Mikels & Reuter-Lorenz, 2004) demonstrate that hemispheric interaction plays a role in attentional processes, not only by allocating resources based on task demands and resources available, but also by filtering conflicting information.