Studies Of Multisensory Integration In Human Cortex

In the past few years there have been a number of studies exploring the neural bases of multisensory integration in human cortex. Initially, such studies utilized event-related potential (ERP) recording techniques in which the averaged responses of thousands of neurons are recorded from surface electrodes on the scalp. The temporal resolution of ERP recordings is excellent (events can be measured in milliseconds) and can easily be combined with conventional behavioral or perceptual measures in order to examine the covariance between changes in behavior/perception and neural activity. One of the surprising results of these studies is the finding that activity in primary sensory cortex (e.g., auditory) can be modulated by

"nonappropriate" sensory stimuli (e.g., visual), an observation that questions the accepted dogma that each primary sensory cortex is responsive only to a single sensory modality. It also confirms some early observations (that were largely ignored) from singleneuron studies in animals suggesting that some of the neurons in primary sensory cortex are not modality specific and can, in fact, be activated by stimuli from other sensory modalities.

ERP studies have also identified presumptive poly-sensory cortices in humans. The changes in human brain activity that are induced by multisensory stimuli have also been associated with an increased response accuracy and reaction speed to these stimuli. Unfortunately, the spatial resolution of the ERP technique is not sufficient to allow researchers to identify the specific sulci and gyri that are involved in these processes.

It is in specifying the locus of evoked activity in the human brain that functional imaging techniques have had their greatest impact, because they have centimeter or subcentimeter resolution. Two imaging techniques, both of which utilize changes in cerebral blood flow to assess neural activity, have become quite popular in studies of multisensory integration: positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These techniques have confirmed the ERP observation that the activity in primary visual and auditory cortices can be modulated by other sensory stimuli, and they have also identified specific regions of cortex that are devoted to processing multisensory stimuli, including right insulaclaustrum, left basal posterior temporal lobe, and the medial inferior parietal lobe. Recently, fMRI techniques have strongly suggested that multi-sensory integration in human cortex is subject to some of the same rules of multisensory integration that have been demonstrated at the single-neuron level in the midbrain and cortex of animals. Thus, superadditive response enhancement (Fig. 7) was obtained in the left superior temporal sulcus when the visual signal (lip movements) matched the auditory signal (heard speech), whereas responses became subadditive when the lips and the heard signal were mismatched.

Surprisingly, these imaging studies have demonstrated that primary sensory cortex is subject to a remarkable degree of developmental plasticity. Blind or deaf individuals have their denervated cortices "taken over" by other sensory modalities. Thus, for example, disrupting the activity of visual cortex in a blind subject via transcranial magnetic stimulation

Figure 7 (Top) A magnetic resonance image (MRI) of multi-sensory enhancement in the human brain. An illustration of a horizontal section through the brain is shown. The white area (double-headed arrow) is a region of activation as measured by functional MRI (i.e., increased blood flow). The region is located in the ventral bank of the left superior temporal sulcus and is known to be involved in speech perception. Its response to a cross-modal (visual-auditory) stimulus combination exceeded that predicted by the sum of the individual modality-specific responses. The area is also illustrated on an image of the whole brain. (reprinted from Curr. Biol. 10, Calvert et al., 649-657, copyright 2000, with permission from Elsevier Science).

Figure 7 (Top) A magnetic resonance image (MRI) of multi-sensory enhancement in the human brain. An illustration of a horizontal section through the brain is shown. The white area (double-headed arrow) is a region of activation as measured by functional MRI (i.e., increased blood flow). The region is located in the ventral bank of the left superior temporal sulcus and is known to be involved in speech perception. Its response to a cross-modal (visual-auditory) stimulus combination exceeded that predicted by the sum of the individual modality-specific responses. The area is also illustrated on an image of the whole brain. (reprinted from Curr. Biol. 10, Calvert et al., 649-657, copyright 2000, with permission from Elsevier Science).

interferes with the subject's ability to read Braille. Apparently, the absence of visual input allowed visual cortex to capture some of the normal functions of somatosensory cortex, thereby expanding the brain tissue devoted to processing information from the body. Similar changes in the brain are likely to accompany damage to each of the senses. Nevertheless, whether blind or deaf people are actually better at using the information they process in their remaining senses than are normal individuals remains a controversial issue.

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