In Reply

The questions about the duality and hemispheric functional specializations of the human brain have occupied in interested scientists for decades. Experimental findings from split-brain and binocular rivalry research strongly suggest that the two hemispheres of the brain are both actively engaged in constructing and representing different aspects of the content of human consciousness. The seminal research by Sperry¹  and Gazzaniga²  in the 1960s on split-brain patients led to a novel understanding of functional lateralization of the brain. It is well known that under special circumstances, the two hemispheres can show an amazing amount of autonomy and cognitive functions associated with different contents of internal and external realities. In binocular rivalry experiments with normal participants, visual perception of each eye alternates to become the content of consciousness, which is mediated primarily by one of the hemispheres. Although evidence for functional lateralization is measurable, broad generalization of lateral dominance has yet to be treated carefully. Evidence from various cognitive experiments collectively suggests that hemispheric dominance is related to task,³  rather than to handedness or the sites for representation of conscious contents. For example, as in the majority of population, when we are speaking and reading, the left brain privileged in language and verbal reporting is dominant, but when we are navigating or appreciating music or an artwork, the right brain is dominant.

The handedness of our study participants was unfortunately not recorded at the time when the experiments were conducted.⁴  Nonetheless, we know that the left hemisphere is dominant for speech in approximately 95% of right-handed people and in approximately 70% of left-handed people.⁵  This raises the question whether handedness can be regarded as a reliable, authentic representation of hemispherical functional specialization. Moreover, working memory and semantics have been reported to involve mainly left-lateralized brain networks.^6,7  This may explain why in our experiments the task-induced brain activity and connectivity patterns showed pronounced lateralization to the left in the wakeful baseline condition and the absence of this lateralization during deep sedation.⁸  In our experiments, there was no motor action performed during imaging scan; behavioral evaluations were conducted only during the interval between scans to assess the level of sedation. However, auditory stimuli were continuously presented during scanning. Thus, it is conceivable that during deep sedation, nonspecific thalamocortical connectivity had to be more suppressed in the left than in the right hemisphere, preventing the incoming stimuli to become the dominant content of consciousness. In other situations, where the nature of task demands a predominant involvement of the right hemisphere, the results could be dramatically different (e.g., a prominent suppression of right hemisphere activity or connectivity). Thus, we consider our observations of more pronounced suppression of nonspecific thalamocortical connectivity in the left hemisphere, as reported in our publication, to be task related. We thank Dr. Derakhshan for commenting on our work and offering interesting interpretations of the results.