Search

In addition to visually driven cells we found within the lateral suprasylvian visual cortex of cats a considerable number of auditory and/or bimodal cells. Most of the visually driven cells were direction and orientation selective with responses that were neither highly stimulus time locked nor very stable. Most of the auditory responses were also not very stable, had relatively high thresholds, and were readily habituated. Previous studies have suggested that populations of cells within the lateral suprasylvian area are specialized for the analysis of optic flow fields (Rauscheker et al., 1987; Sherk et al., 1995). Given that a remarkable proportion of cells within this area can be also driven by auditory stimuli, we hypothesize that the 'optic flow' model may be extended to the bimodal domain rather than restricted to visual clues only. This, however, remains to be corroborated experimentally.

In addition to visually driven cells we found within the lateral suprasylvian visual cortex of cats a considerable number of auditory and/or bimodal cells. Most of the visually driven cells were direction and orientation selective with responses that were neither highly stimulus time locked nor very stable. Most of the auditory responses were also not very stable, had relatively high thresholds and were readily habituated. Previous studies have suggested that populations of cells within the lateral suprasylvian area are specialized for the analysis of optic flow fields. Given that a remarkable proportion of cells within this area can be also driven by auditory stimuli we hypothesize that the "optic flow" model may be extended to the bimodal domain rather than restricted to visual clues only. This, however, remains to be corroborated experimentally.

Early blindness in humans and experimental visual deprivation in animal models are known to induce compensatory somatosensory and/or auditory activation of the visual cortex. An abnormal hydrocephalic cat with extreme malformation of the visual system, born in our breeding colony, rendered a good model system for investigating possible cross-modal compensation in such a pathological case. For comparison, we used normal and neonatally enucleated cats. When introduced to a novel environment, the abnormal cat behaved as if it was completely blind, yet it responded normally to auditory stimuli. As anticipated, single cells in the visual cortex of normal cats responded to visual, but not to auditory stimuli. In the visual cortex of enucleated cats, flashes of light did not elicit field-evoked potentials or single-unit responses. However, several cells did respond to various auditory stimuli. In the remnant visual cortex of the abnormal cat, auditory stimuli evoked field potentials and single-cell responses. Unexpectedly, however, unlike the enucleated cats, in the abnormal cat, flashes of light also elicited field-evoked potentials. Judging by its behavior, it is very likely that this deformed cat had completely lost its ability to perceive images, but had probably retained some sensitivity to light.

Auditory activation of the primary visual cortex (area 17) and two extrastriate visual cortical areas - the anterolateral lateral suprasylvian area (ALLS) and anteromedial lateral suprasylvian area (AMLS), was investigated in visually impaired cats. Impairment was accomplished shortly after birth by bilateral eyelid suturing (binocularly deprived cats, BD) or bilateral enucleation (binocularly enucleated cats, BE). In BE cats, the optic nerve and chiasm were entirely degenerated. No cortical atrophy or cytoarchitectural malformation was noticed in either BD or BE cats. In both normal and impaired cats we found auditory-responsive cells in the ALLS and AMLS, areas that are considered strictly visual. The most remarkable finding was an increase in the relative number of these auditory cells in the BD and BE cats, which was more prominent in the latter. Some auditory-responsive cells were also found in area 17 of BE cats. On the basis of formal calculation, it is tempting to suggest that the increase in relative number of auditory cells in these areas reflects the transformation of all the visual cells in the ALLS of BD and BE cats into auditory cells. In BE cats, all bimodal cells and an appreciable percentage of non-responsive cells also had transformed to auditory cells. In the AMLS of BD cats, it is primarily the bimodal cells that become auditory cells, whereas in BE cats all the visual and bimodal cells as well as non-responsive cells undergo this transformation. This assumption, however, is one possible interpretation of our results but not the only one. Other modes of neuronal plasticity that might yield similar results in the visually deprived cats can not be ruled out.