Single Neuron Studies

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It might reasonably be assumed that whenever different sensory inputs converge on a single neuron, that neuron is immediately rendered capable of integrating those inputs. In this way, any of the various cross-modal convergence patterns that are possible among the more than 40 subcortical and multiple cortical inputs to the SC would be effective in creating the substrate for multisensory integration. However, this assumption proved to be incorrect. In cat, one of the primary models used for understanding the neural bases of multisensory integration, it has been estimated that 20% of the multisensory neurons in the SC are incapable of synthesizing their cross-modal inputs (i.e., "nonintegrative"). These neurons respond quite well to different modality-specific inputs and come in all varieties (visual-auditory, visual-somato-sensory, auditory-somatosensory, and trimodal), but their responses to cross-modal combinations of cues are no greater or less than their responses to the most effective of these stimuli alone. A key component of the SC circuitry underlying multisensory integration appears to be the inputs from association cortex.

Inputs from two regions of association cortex have been found to be critical in this regard: the anterior ectosylvian sulcus (AES) and the rostral portion of the lateral suprasylvian sulcus (rLS). This conclusion is based on studies in which the modality-specific and multisensory responses of cat SC neurons were studied before, during, and after reversibly blocking (via cryogenic deactivation) the neuronal activity in these areas. The profound effect of blocking these cortical inputs is illustrated in Fig. 4. In this example, blocking AES activity eliminated the multisensory response enhancement in the SC. However, as was typically

Spatial Coincidence Spatial Disparity

Figure 5 Orientation behaviors are altered by multisensory stimuli such that cross-modal stimulus combinations at the same location enhance performance while those at different locations impair performance. Cats were trained in a perimetry device (top) to attend straight ahead and then to orient to and approach a flashed light-emitting diode (LED) or a brief noise burst (see text for details). When the stimuli were of low intensity, they were difficult to notice and the animal responded correctly to them in less than 50% of the trials (left). However, when they were presented simultaneously and at the same location (AV) performance was enhanced more than predicted by their sum. Another animal was trained to respond only to the visual stimulus (it was never presented with the auditory stimulus during training). Its responses to the LED were markedly impaired when the neutral auditory stimulus was presented 60° medial to the LED, and this was most evident when correct responses were high (right). In this training paradigm performance could also be enhanced when the LED was made dimmer and the two stimuli were presented at the same location (not shown here) (reprinted from Stein et al., J. Cogn. Neurosci. 1,12-24, copyright 1989, with permission from The MIT Press).

Figure 5 Orientation behaviors are altered by multisensory stimuli such that cross-modal stimulus combinations at the same location enhance performance while those at different locations impair performance. Cats were trained in a perimetry device (top) to attend straight ahead and then to orient to and approach a flashed light-emitting diode (LED) or a brief noise burst (see text for details). When the stimuli were of low intensity, they were difficult to notice and the animal responded correctly to them in less than 50% of the trials (left). However, when they were presented simultaneously and at the same location (AV) performance was enhanced more than predicted by their sum. Another animal was trained to respond only to the visual stimulus (it was never presented with the auditory stimulus during training). Its responses to the LED were markedly impaired when the neutral auditory stimulus was presented 60° medial to the LED, and this was most evident when correct responses were high (right). In this training paradigm performance could also be enhanced when the LED was made dimmer and the two stimuli were presented at the same location (not shown here) (reprinted from Stein et al., J. Cogn. Neurosci. 1,12-24, copyright 1989, with permission from The MIT Press).

found, the influences from the AES (or rLS) were quite selective; they were critical only for multisensory integration. The neuron's responses to modality-specific stimuli were not significantly altered.

Some SC neurons depend only on influences from AES to exhibit multisensory integration, and others depend only on influences from rLS. However, many receive combined influences from these two cortical areas, and both cortices influence the multisensory integration of such SC neurons. In most of these cases, it is necessary to maintain the influences from both cortices; if either one is removed, the SC neuron loses its capacity for multisensory integration. It is not clear why these different patterns of dependence on AES and rLS among multisensory SC neurons have emerged, but it does raise the possibility that one cortex could compensate for early damage to the other.

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