Behavioural Implications of Callosal Differences

Callosal differences would be of little practical interest were they not to involve behavioural consequences. Some relationships between callosal variability and behaviour (e.g. handedness, functional lateralisation) have been discussed in the previous section and will not be discussed here. Others are more relevant to topics treated in detail in future chapters and will be treated later. Therefore, a limited number of studies showing a relationship between callosal properties and behavioural consequences will be presented here.

1.4.6.1 Split-Brain and Acallosal Individuals

One of the more striking effect callosal differences can have on behaviour have been demonstrated in patients in whom the CC has been cut as a treatment for intractable epilepsy and in individuals with agenesis of the CC (ACC) in whom the CC

has developed only partially or not at all. Findings relating to these two groups will be discussed further later, therefore only one example will be given here.

In neurologically normal individuals who fixate their gaze in the middle of the visual field, stimuli appearing in the LVF are perceived by the right hemisphere whereas stimuli presented to the RVF are perceived by the left hemisphere. Since each hemisphere controls the contra-lateral hand, responding with one hand (by pressing a button) to a stimulus presented on the same side as the responding hand (direct route) is faster than if it is presented on the other side (crossed route) because information need not be transferred to the other hemisphere to elicit a motor response. The difference in response between the crossed and direct conditions is called the crossed-uncrossed difference (CUD). In normal individuals the CUD averages 2-6 ms. In split-brain individuals and ACCs, however, this measure is dramatically lengthened to around 20 ms in ACCs and 40 ms in split-brain patients (Berlucchi, Aglioti, Marzi, & Tassinari, 1995; Forster & Corballis, 1998; Iacoboni, Fried, & Zaidel, 1994). Thus the integrity of the CC is clearly essential for the fast transfer of information between the hemispheres even when this information can be transferred via other channels.

1.4.6.2 Functional Lateralisation

Most callosal fibres are excitatory, however the functional effect of the information relayed through these fibres can be excitatory or inhibitory in nature depending on the type of inter-neurons being activated (see Saron, Foxe, Simpson, & Vaughan, 2002, pp. 208-209). Consequently, larger CCs can potentially increase the cross-activation between the hemispheres or inhibit more the less active hemisphere. Yazgan, Wexler, Kinsbourne, Peterson, & Leckman (1995) conducted an experiment to clarify this issue. They were also interested in determining whether functional lateralisation was related to CC size. They hypothesised that if callosal function was mostly excitatory in nature, individuals with a larger CC should demonstrate a smaller lateralisation bias on task known to be functionally lateralised, than those with a smaller CC, because activation in one hemisphere would more easily spread to the other in the first group. Alternatively, if the CC had mostly an inhibitory function, individuals with a larger CC should show a greater lateralisation bias since inhibition would be spread more efficiently from the more active to the less active hemisphere. Yazgan and colleagues administered four tasks known to be lateralised to eleven individuals in whom the size of the CC was measured by MRI. They found that callosal size was significantly negatively correlated with lateralisation of a dichotic listening task (r = - .70), with a line bisection task (r = -.61), and with a leftward turning bias (r = -.70), suggesting that greater callosal size produced a greater spread of activation to the less active hemisphere and thus decreased the laterality bias in these tasks. Consistent with these findings, they also found that greater CC areas were associated with less interference in a dual task paradigm where the two tasks were presented to the same hemisphere (r = -.62) or not (r = -.73). Manually connecting sequential numbers was slowed less in individuals with larger CCs when a verbal task, consisting in conjugating irregular verbs in their past tense, was introduced. These results were interpreted as showing that interference was decreased in individuals with larger CCs because resources in the contralateral hemisphere were more efficiently recruited in these individuals.

1.4.6.3 Development and Age

In pre-term babies, the CC has been shown to be thinner than in full-term babies. Nosarti et al. (Nosarti et al., 2004) assessed seventy-seven individuals, aged 14-15 years, and born before 33 weeks of gestation, as well as age-matched controls. Total

callosal areas of the pre-term adolescents were 7.5% smaller than those of the controls. This size difference mostly affected posterior callosal regions and in boys was related to decrease in verbal IQ and verbal fluency.

In younger but not older adults the integrity of the CC (splenium) as measured by diffusion tensor imaging has been shown to be related to response time in a visual detection task consisting in pressing a button when a target (filled circle) was presented or another button when a familiar stimulus (filled square) or a novel stimulus (photograph of varying everyday objects) were presented (D. J. Madden et al., 2004). This suggests that behavioural performances related to CC properties are not uniformly affected across ages.

1.4.6.4 Sex

Davatzikos & Resnick (1998), mapped MRI images of the midsagittal callosal surface of 114 individuals aged 56-85 years onto templates of “typical” CC sections in order to quantify their deviation from the norm. They found that females had a larger and more bulbous CC (splenium) and that in females but not males the shape of all sections of the CC was significantly associated with cognitive performance, which was assessed with card rotation, object identification, letter fluency, and recognition memory tasks.

1.4.6.5 Plasticity of the Corpus Callosum: Morphology and Behaviour

In musicians, callosal connectivity appears to be affected by behaviour since a group of thirty professional pianists and string instruments players has been shown to have larger CCs, probably because the extensive practice of bimanual motor skills, which require interhemispheric transfer, has affected the development of their CC and their functional lateralisation (Schlaug, Jancke, Huang, Staiger, & Steinmetz, 1995). However, it has not been excluded yet that this effect might be at least partly due to a predisposition of certain individuals to play music because they possess a larger CC, and are functionally less lateralised. This view appears to be supported by twin studies showing that 94% of the variance in CC size can be explained by genetic factors (Scamvougeras, Kigar, Jones, Weinberger, & Witelson, 2003) and that this genetic influence remains strong throughout the lifespan (Pfefferbaum, Sullivan, & Carmelli, 2004). However, a study (Juraska & Kopcik, 1988) of rats reared in enriched environments has also shown that these rats develop a larger CC than rats reared in isolated environments. Other environmental effects on the size and shape of the CC have been demonstrated in relation to social interaction (Sanchez, Hearn, Do, Rilling, & Herndon, 1998), and pre-natal alcohol exposure (Bookstein, Sampson, Steissguth, & Connor, 2001; Riley et al., 1995). Furthermore, bilingual individuals have been shown to have a larger anterior midbody than non-bilinguals (Davatzikos & Resnick, 1998). Since none of the bilinguals tested in this study had a familial bilingual upbringing it appears that at least in certain conditions, callosal differences may be only due to environmental factors (bilingual education at school). It may be that under “normal” circumstances (e.g. typical environment, no intensive musical training) genetic influences prevail and that environmental factors only have a noticeable effect under atypical circumstances. It is more likely that genetic and environmental factors interact. Interestingly, the relationship between CC size and musical practice seems to be only present in males and not in females and might reflect the fact that women are functionally less lateralised and, as discussed previously, might also on average have a larger CC than males (Lee, Yi, & Schlaug, 2003).