Early anatomical, electrophysiological and voltage-sensitive dye imaging studies of the cerebral cortex of slider turtles indicate that the dorsal regions of the three-layered cortex are involved in the processing of visual information. Some of these studies revealed that relatively unspecific visual stimulation evokes oscillations in the local field potential with complex spatio-temporal gradients. Anatomical and physiological studies further suggest that cortical neurons possess extensive receptive fields in the absence of a clear retinotopic map of visual space in dorsal cortex. Therefore, it is still not clear whether and if so, how, spatio-temporal activity patterns correlate with specific features of visual stimulus space.Here, visually-evoked responses were recorded from awake red-eared sliders (Trachemys scripta elegans) using chronically implanted, high-density surface electrode grids covering large areas of dorsal cortex. Stimuli were delivered using a projection system and a semi-spherical screen, enabling the stimulation of extensive parts of visual space. The stimulus set consisted of complex natural visual stimuli (scenes and videos) that differed in global features such as frequency content, spatial and/or temporal structure.Evoked activity was consistently located in the same region of dorsal cortex and contained evoked, coherent oscillations in the 15-35Hz range forming a rostrocaudal phase gradient, confirming and extending earlier results. Furthermore, natural stimuli (static and dynamic) evoked significantly stronger and more coherent oscillations than stimuli with reduced spatial or temporal integrity. Responses to stimulation with natural movies contained additional high-frequency components (up to 45Hz). These results indicate that cortical dynamics are influenced by spatio-temporal features of visual stimuli. Using multilinear regression it was possible to explain approximately 32% of the variance in the recorded oscillatory power using a set of global stimulus features based on the intensity histogram, the distribution of local contrast elements and spatial frequency composition. These features may therefore be considered to be relevant for the processing of natural scenes in turtle dorsal cortex.The increased cortical responses to natural vs. matched control stimulation and the absence of retinotopy suggest a similarity of turtle dorsal cortex to non-retinotopic extrastriate cortex in the mammalian object recognition pathway.
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