3.2 Experiment 3
3.2.3 Discussion
Only a handful of studies, until now, have attempted to determine whether hemispheric interaction efficiency is directly related to callosal transfer speed, but these have produced unclear or contradictory results. Two studies used evoked potential measures of IHTT and found that only faster left-to-right IHTT was associated with more efficient hemispheric interaction (Brown & Jeeves, 1993; Larson & Brown, 1997). Braun et al. (1997) found significant correlations between only four pairs (out of 16) of measures of IHTT and hemispheric interaction but since they used a measure of IHTT based on complex RT, which may or may not assess callosal transfer (see Bashore, 1981; Brysbaert, 1994b, for a discussion), the meaning of these results is unclear. A fourth study (Corballis, 2002) did not find a significant relationship between IHTT and hemispheric interaction.
The present study was able to demonstrate a direct relationship between IHTT and a widely used measure of hemispheric interaction.
IHTT was measured with the Poffenberger paradigm which yielded an average transfer time of 2.73 ms, which is in accord with past findings of 2-5 ms (Bashore, 1981; Brysbaert, 1994a; Iacoboni & Zaidel, 2000). The validity of the measure was demonstrated by a significant hand by visual field interaction confirming that in trials
where no inter-hemispheric transfer was required, RTs were shorter than in those that did require inter-hemispheric transfer.
IHI measured in the letter-matching task produced a negative latency BDA (- 34.7 ms) and a non-significant accuracy BDA (0.45%) at the group level. This means
that most participants performed better on the task when the two matching letters were presented either in the left or right visual fields compared to processing the matching letters across visual fields. However, 13.5% of participants responded faster, and 40% more accurately, when matching letters were presented to different hemispheres.
A correlation of .354 was found between IHTT and accuracy BDA suggesting that individuals with faster hemispheric transfer showed more efficient hemispheric interaction than those with slower hemispheric transfer. However, a significant correlation between IHTT and BDA could also be explained by other variables such as functional lateralisation, handedness, sex, age, or attention. First, with respect to functional lateralisation, hemispheric interaction relies on the sharing of processing between the two hemispheres. Consequently the relative capacity of each hemisphere to perform the different processes involved in each task might influence performance in both simple RT and complex RT tasks. If one hemisphere were less proficient at some sub-processes necessary to complete the task and the other hemisphere were over- loaded for these sub-processes, a less efficient hemispheric interaction would be expected. Second, handedness has been shown to be associated with functional lateralisation and callosal structural differences (Westerhausen et al., 2003) and might also affect hemispheric interaction and transfer. With regard to sex, males tend to be more lateralised than females morphologically and functionally (e.g., Aboitiz, Scheibel, & Zaidel, 1992; Allen et al., 2003; Siegel-Hinson & McKeever, 2002; Weiss et al., 2003). Consequently, men might differ in hemispheric interaction and be more affected by the efficiency of callosal transfer. With respect to age, as individuals get older, they tend to be less functionally lateralised compared to younger individuals (Cabeza et al., 2002) or, at least, present with a different pattern of hemispheric lateralisation compared to younger populations (Hausmann, Güntürkün, & Corballis, 2003), and different brain regions are affected differently by the ageing process (Driesen & Raz, 1995; Sullivan et al., 2002b). As a consequence, if functional lateralisation significantly influences hemispheric interaction, age could modulate this relationship. Finally, although there are no strong indications suggesting that attention should affect measures of IHTT and hemispheric interaction in a systematic way, nevertheless, because attentional processes seem to be more lateralised to the right hemisphere (Davidson, Cave, & Sellner, 2000; Doty, 2002; Ernest, 1998; Hausmann, Ergun, Yazgan, & Güntürkün, 2002; Siegel- Hinson & McKeever, 2002; Vogel, Bowers, & Vogel, 2003; Weekes & Zaidel, 1996), there is a possibility that IHTT and hemispheric interaction could be affected.
A multiple regression analysis showed that none of the above variables was a better predictor of the accuracy BDA than IHTT. Further analysis showed that varying performance levels between participants could not account for this finding either. Therefore, it is most likely that individuals with faster inter-hemispheric transfer have a more efficient (as measured by accuracy) hemispheric interaction, at least on this type of task.
It should be noted that in a recent study, Singh and O’Boyle (2004) found that mathematically gifted children (age 13 years) demonstrated increased hemispheric interaction compared to age-matched controls of average ability, and college students. They suggest that the increased hemispheric interaction in these children might be due to differences in brain organisation and possibly to increased callosal connectivity. This explanation would be consistent with the present findings. At this stage, however, it is not possible to discount the possibility that this relationship between cognitive abilities
and hemispheric interaction is unrelated to callosal connectivity. In my view, it is unlikely that the present findings are due to differences in cognitive ability since most participants were undergraduate students and thus the spread in cognitive ability would have been much smaller than that present in Singh and O’Boyle’s study. Nonetheless, since data on cognitive ability were not collected in the present study, an effect of this variable cannot be excluded and should be controlled for in future research.
Other studies, especially those based on behavioural measures of IHTT, have generally failed to demonstrate a direct relationship between callosal transfer speed and hemispheric interaction. This is likely to be due to small sample sizes and small numbers of trials used, or the use of a measure which may not be a reliable index of IHTT (Bashore, 1981; Brysbaert, 1994b). In this study, using 1200 trials for the Poffenberger task and 2304 trials for the letter task, the relationship between IHTT and hemispheric interaction stabilised once approximately 20 participants were tested, with correlations (always significant) oscillating between .3 and more than .5 until testing was completed.
Based on previous findings using a task very similar to that used in this study, (Weissman et al., 2000) an across visual field advantage for both RT and for accuracy was expected. It was therefore somewhat surprising to find a within-hemisphere advantage for RT. This discrepancy may be due to a practice effect that is more prominent in the within visual field condition and which has been described elsewhere (Liederman et al., 1985; Weissman & Compton, 2003). To determine whether an RT and accuracy BDA would be present if a number of trials similar to previous studies had been used, the data of the first session were analysed separately but neither an across nor a within-hemisphere advantage (or disadvantage) was found. It is unclear why this is the case. Although a display of different shape (square vs v-display) was used, results from Experiment 1 (Chapter 2), have shown no difference between these two types of displays. It may be that since Weissman, Banich, & Puente (2000) tested participants on a 3 item display and a 4 item display (although it is unclear whether these two tasks were mixed or counterbalanced) and since Pollman, Zaidel, and Cramon (2003) used a 4 item display but cued the matching letters, these differences improved across- hemisphere performance.
The lack of RT and accuracy BDA at group level could be interpreted as showing that there is no difference in the level of hemispheric interaction in the within- and across-hemisphere condition. If this were the case, the meaning of a correlation between IHTT and the BDA measures would be unclear. To address this concern, the data of a subgroup of participants (n = 21) who all showed a RT and accuracy advantage in the across-visual field condition were analysed. Even when only the first session of the letter task was considered, the correlation between accuracy BDA and IHTT was larger (r = .587) than that found in the whole group, and again larger when all four sessions were considered (r = .681) strengthening an interpretation of the present results as an association between IHTT and BDA.
A relationship between CUD and hemispheric interaction for accuracy but not for RT was somewhat surprising, particularly since accuracy and response time BDA were strongly correlated (r = -.527) which suggests that individuals who have higher accuracy BDAs also have more efficient latency BDAs. Other factors such as hemispheric organisation may underlie this relationship or, at least, mask the influence of IHTT. This hypothesis is supported, at least in part, by the finding that in participants who show both a latency and accuracy BDA during the first session, a significant relationship was found not only between the accuracy BDA and IHTT for the first and for all four sessions, but also between the overall latency BDA and IHTT (r = -.559). Therefore the latency measure might be “noisier” than the accuracy measure, and more
sensitive to uncontrolled experimental variables. Furthermore, in the entire participant group as well as in the sub-group who demonstrated positive BDAs (21 participants who responded faster and more accurately in the across hemisphere condition) a more efficient accuracy interaction was associated with a more efficient response time interaction. This is consistent with a developmental study showing that as the CC matures, both response time and accuracy BDAs increase (Hagelthorn et al., 2000).
A second CUD measure was computed using RT to letter-matches to determine whether complex RT CUD was a valid measure of IHTT. No significant correlation was found between simple RT CUD (3 ms, SD 3.83) and complex RT CUD (16 ms, SD 20.19). The difference between the two types of CUDs was not unexpected as it is probable that the type of information transferred between the cerebral hemispheres during the simple RT task (pre-motor) (Tettamanti et al., 2001) and during the complex RT task (high-level cognitive) is quite different and therefore mediated by different types of fibres with different conduction times. The complex RT transfer time of 16 ms found is consistent with the transfer speed of callosal fibres (19-25 ms) that transfer high-level cognitive information (Aboitiz et al., 2002). The lack of correlation between the two CUD measures can therefore be interpreted in two ways. There may be no relationship between the transfer speed of discrete callosal channels in different individuals, in which case the complex RT CUD might be a valid measure of IHTT for specific channels, and unrelated to those conveying simple RT information. Alternatively, since I have shown that the simple RT CUD is related to the hemispheric interaction measured by the complex RT task, which suggests a relationship between different callosal channels, the complex RT CUD might simply not be a ‘pure’ measure of IHTT but also reflect the effects of other variables such as functional lateralisation. The latter explanation seems more likely but others cannot be excluded at this stage.
In summary, these results demonstrate that individual speed of hemispheric transfer (a behavioural measure) is related to the efficiency of hemispheric interaction (accuracy) so that individuals with faster IHTT tend to have more efficient hemispheric interaction. This relationship cannot be explained better by other variables such as functional lateralisation, handedness, sex, age, or attention. This relationship seems to be strongest between IHTT and the accuracy BDA although a significant correlation was also found between IHTT and RT BDA.