Crossmodal effects appear in regions once thought to be unisensory
Traditionally, visual cortex was thought to handle only vision, auditory cortex only hearing, and so on. But recent studies find crossmodal influences in these areas. For example, haptic shape information—gathered by touch—can be decoded from early visual cortex even when participants are not seeing anything [1]. In auditory cortex, visual stimuli can alter how auditory neurons respond, and anatomical evidence shows multisensory convergence within putatively unisensory fields [2]. These results indicate that sensory-specific regions are not isolated processors but can be influenced by other senses.
The idea that low-level cortical areas integrate multiple senses is further supported by evidence that multisensory processing occurs in areas previously considered unisensory, requiring a revision of how we think about unisensory processing [3]. This challenges the clear divide between unisensory and multisensory cortex, suggesting instead a more graded or distributed arrangement.
Crossmodal influences can occur without feedback from higher areas
One might assume that crossmodal effects in sensory regions are simply the result of feedback from higher, multisensory association areas. However, studies of multisensory integration for timing show that interactions can occur in unisensory areas and may not depend on feedback from higher cortical regions [4]. This implies that sensory-specific cortex itself has the machinery for some crossmodal integration.
In addition, crossmodal plasticity can be induced by sensory experience. For instance, visual deprivation leads to potentiation of thalamocortical synapses in auditory cortex, enhancing information processing [5]. This suggests that crossmodal effects are not fixed but can be shaped by experience, further blurring the line between unisensory and multisensory cortex.
The brain may be better described as a continuum of multisensory influence
Taken together, these findings challenge the traditional unisensory/multisensory cortex divide. Instead of strict categories, the evidence points to a continuum where even low-level sensory areas receive and integrate information from other senses [1][2][3]. This does not mean that all areas are equally multisensory; rather, the degree of crossmodal influence may vary, and the distinction may be more about the extent of integration than a binary classification.
The studies also suggest that crossmodal effects can be selective and experience-dependent, as seen in crossmodal plasticity [5]. Thus, the divide is not only blurred but also dynamic, with the potential for crossmodal interactions to be strengthened or weakened by sensory experience.
Sources used in this answer
Visual and Haptic Shape Processing in the Human Brain: Unisensory Processing, Multisensory Convergence, and Top-Down Influences.
Haptic shape representations were found in early visual cortex, and ventrolateral occipito-temporal cortex decoded shape across modalities, indicating crossmodal effects in visual areas.
Physiological and anatomical evidence for multisensory interactions in auditory cortex
Physiological and anatomical evidence shows multisensory interactions in auditory cortex, with visual stimuli influencing auditory processing.
Multisensory contributions to low-level,'unisensory'processing
Multisensory integration occurs in low-level cortical areas previously considered unisensory, challenging the traditional view of unisensory processing.
Multisensory integration for timing engages different brain networks
Multisensory interactions for timing occur in unisensory areas and may not depend on feedback from higher cortical regions.
Crossmodal induction of thalamocortical potentiation leads to enhanced information processing in the auditory cortex
Visual deprivation induces crossmodal potentiation in auditory cortex, enhancing synaptic strength and information processing, suggesting crossmodal plasticity in adult sensory cortex.