| Resumo: |
So far, there is no evidence of columnar orientation preference maps in rodent primary visual cortex, such as commonly observed in carnivores and primates. Nevertheless, orientation selective neurons have been found in all rodent species investigated, though interspersed. This opens up the question whether the connectivity underlying the emergence of selective cortical response properties in animals with interspersed as compared to columnar maps follows a different blueprint. Rodent data are so far mainly available for species with nocturnal or crepuscular habits and small brain size, two factors that could also contribute to develop a different functional architecture. Therefore, we set out to compare the functional architecture of the primary visual cortex of carnivores with that of a big rodent with diurnal habits, and a V1 size comparable to cats and small primates. We performed multi-site electrophysiological recordings using spatial arrays from both anesthetized cats’ (Felis catus) and agoutis’ (Dasyprocta aguti) visual cortex. Visual stimuli consisted of contrast reversing checkerboards and oriented gratings of several spatial and temporal frequencies. Agoutis presented smaller orientation selectivity indices (median OSI = 0.10) than cats (median OSI = 0.19), and lower proportions of orientation (~45% for agouti V1 and ~75% for cat A18) and direction (~35% for agouti V1 and ~65% for cat A18) selective neurons. In order to describe the functional architecture based on the electrophysiological data, we quantified the orientation preference similarity between neurons according to the cortical distance between them. This analysis revealed a characteristic slow decrease in neuronal orientation preference similarity for cats. No such “classical” modularity was found for agoutis, but a clustering of neurons with similar orientation preference was observed for short ranges (< 250 μm). Overall, our results are consistent with recent literature reporting ‘mini-columns’ of orientation preference in mice, and therefore further prove that the rodents’ interspersed maps are not random, as previously assumed. We refute, however, recent theoretical literature suggesting that agoutis might have “classical” columnar orientation preference maps. Future research should focus on understanding the circuits, which lead to small selective receptive fields in agoutis and great visual performance while adopting a different functional architecture. |
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