ReviewSensory modalities are not separate modalities: plasticity and interactions
Introduction
Cross-modal integration is performed on a vast level in the brain and contributes significantly to adaptive behavior in our daily life. Very little is known about how integration is achieved or its underlying neural mechanisms, however, because the overwhelming majority of studies on perception have focused on one sensory modality. Studying perception in an isolated single modality would be justifiable if different modalities processed sensory inputs independently of each other, as separate ‘modules’. But are sensory modalities really separate modules? A variety of evidence seems to counter this notion of modularity. In this review, we summarize the evidence for vigorous interaction among sensory modalities.
Section snippets
Plasticity across sensory modalities
Both animal and human studies suggest that sensory modalities in early stages of development are not as inherently distinct and independent as was previously once thought. For example, in a study of cross-modal plasticity Sur et al. [1] removed the superior colliculus of both the ferret and the hamster on the day of birth by direct ablation. They also deprived the medial geniculate nucleus or the ventrobasal nucleus from their normal sensory input by sectioning the major input pathways. The
Interaction across modalities
In the phenomenological and psychophysical literature, a wealth of literature on mature perceptual systems indicates that there is vigorous interaction and integration among the sensory modalities—a far greater interaction than what is expected from a ‘naive modularity’ point of view. One of the most intriguing examples of cross-modal interaction is synesthesia, in which an involuntary conscious sensation (such as color) is induced by a stimulus in another modality (such as sound). Synesthesia
Neural mechanisms
At what point along the perceptual processing pathway do these cross-modal interactions take place? Recent data from brain imaging studies suggest that they occur at brain sites that used to be considered as modality-specific. For example, Calvert et al. [35] carried out a functional magnetic resonance imaging (fMRI) study that showed that the primary auditory cortex is activated when a talking face is viewed in the absence of sound. The activation was observed specifically in speech or
Conclusions
We have discussed a wide variety of evidence against the notion of strict modularity of sensory modalities. Both animal studies and human-deprivation cases provide evidence for a surprising degree of cross-modal plasticity in cortical processing. Psychophysical data indicate that interaction between modalities is the rule as opposed to the exception in brain function, and brain imaging and recording studies provide evidence against modularity and for interaction in areas traditionally thought
Acknowledgements
We apologize to the many researchers whose work was omitted owing to space constraints. This work was supported in part by NIH grant HD08506. We thank the section editors for their insightful comments.
References and recommended reading
Papers of particular interest, published within the annual period of review, ave been highlighted as:
• of special interest
•• of outstanding interest
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