Since its discovery at the end of the 19th century, functional cerebral lateralisation has attracted a lot of interest. Its study has led to a better understanding of the brain’s modular structure but it has also overshadowed the importance of the interactions taking place between the brain’s hemispheres during normal cognition. More recently, the importance of studying the joint contribution of the two cerebral hemispheres to behaviour has been recognised, and has produced results of great relevance to researchers interested in both hemispheric specialisation and hemispheric interaction.
Research into hemispheric interaction has focused mainly on the influence of factors external to the participants tested, such as the type of task and stimuli used, the length of trials, and the modality in which stimuli were presented. Past studies have shown that hemispheric interactions are particularly beneficial when complex and demanding tasks are being performed. Such benefits of hemispheric interactions have been shown to be present in the visual, but also in the auditory and in the tactile modalities, and can be generalised to a variety of tasks, and of stimuli such as words, objects, shapes, sounds and faces. The effect of variables that differ between individuals, or within individuals across time, have been largely disregarded. A better understanding of such effects is important because variables such as sex, age, handedness, functional lateralisation, practice, attention, and hemispheric activation have been shown to interact with each other, and with interhemispheric transfer and brain structure, and therefore are likely to influence hemispheric interactions. Moreover, hemispheric interaction deficits have been shown to be associated with attentional deficits and some neurological syndromes such as dyslexia, schizophrenia, and bipolar disorder, all clearly associated with substantial inter-individual variability.
The current research was aimed at assessing some of the influences of interhemispheric transfer, sex, age, handedness, functional lateralisation, practice, attention, and hemispheric activation on hemispheric interactions and to determine whether this influence was merely detectable or whether it might play a significant role in influencing behaviour. The present findings indicate that these variables do interact in significant and meaningful ways with hemispheric interaction and consequently, inter- individual variability in these factors should affect hemispheric interactions. Therefore, hemispheric interaction is likely to influence differently the performance of many complex cognitive tasks in different individuals, in different populations, in different environments, and across time. Furthermore, unless the effect of varying levels of hemispheric interaction in different individuals is taken into account, seemingly inconsistent results between experimental studies investigating hemispheric interactions may be produced.
The present findings are not only relevant to researchers interested in hemispheric interactions. Using designs that allow for the assessment of hemispheric interactions in typical experiments might also better explain inter-individual variability in other areas of cognition and provide information on task parameters such as complexity, rate of learning, and functional lateralisation. A better understanding of the dynamics of hemispheric interactions is also likely to shed light on the pathological mechanisms involved in disorders such as dyslexia and also on the normal mechanisms underpinning brain development and the ageing process.
Finally, the new measurement technique (fTMT) developed as part of this research relies on precise measurements of ear membrane temperatures as an index of lateralised hemispheric activation. This measure was shown to be more sensitive than originally expected and is likely to have a wide range of applications once its operating parameters have been further defined, and the measure has been validated against other physiological measures of brain activation such as fMRI and EEG.
These results are important because they show a need for testing participants over a greater number of sessions of potentially longer duration if a true picture of hemispheric interaction and functional lateralisation is to be painted. It may be that a complex task which appears to be less lateralised than a simpler task is in fact more lateralised. However, because initially more resources need to be recruited bilaterally in the complex task, results might suggest that it is not lateralised. With practice, less
interhemispheric resources will be recruited which might reveal a stronger functional lateralisation. Since little is known about the effect of task lateralisation on hemispheric interaction and the effect of practice in this context, these suggestions are speculative and should be tested in future research.
These results are important because they show a need for testing participants over more and longer testing sessions if a true picture of hemispheric interaction and functional lateralisation is to be painted. It may be that a complex task which appears to be less lateralised than a simpler task is in fact more lateralised. However, because initially more resources need to be recruited bilaterally in the complex task, results might suggest that it is not lateralised. With practice less interhemispheric resources will be recruited and might reveal a stronger functional lateralisation. Since little is known about the effect of task lateralisation on hemispheric interaction and the effect of practice in this context, these suggestions are speculative and should be tested in future research.