Integration of visual information and the organization of receptive fields in V1 of the California ground squirrel

The research was an effort to provide a detailed characterization of the classical receptive field (CRF) and the non-classical receptive field (nCRF) in the primary visual cortex (V1) of the California ground squirrel, using conventional grating stimuli, as well as dynamic stimuli. The California ground squirrel (Spermophilus beecheyi) is a highly visual rodent whose V1 seems to lack a systematic map of orientation tuning (van Hooser 2005) - a unusual feature which makes it an attractive animal for the study of CRF/nCRF interaction. The spatial extent over which V1 neurons integrates visual information was estimated by four independent measures: the minimal response field ("hand-mappe" RF), the response to gratings of varying diameter, the response to annular gratings, and reverse correlation. For the majority of the recorded neurons, the size of the grating summation field was surprising large: 15deg in diameter in average, about twice the size of the subfield map produced by reverse correlation. The facilitation was found to originate from localized regions (the co-axial "end''-positions) outside the CRF (as defined by reverse correlation kernels), and was orientation selective. Surround gratings at ortho- orientation were found to be suppressive. Many neurons responded to annular gratings in a stimulus-feature selective manner, when the CRF was excluded from stimulation. This behavior is possibly related to the detection of illusory contours. A small population of neurons (10%) was found to be suppressed, regardless of the orientation of the surround gratings. The organization of the receptive field is most parsimoniously explained by large, elongate integration field, which is mostly sub- threshold except for a circular (or rectangular) CRF exposed above the threshold. This was based on reverse correlation experiments where the CRF and the nCRF were stimulated independently. The spatialtemporal structures of the two components were strikingly similar. The suppressive effect was explored by a dynamic stimulus where the orientation of the surround was updated at high refresh rate. A dissociation of facilitation and suppression was found in the temporal domain. Suppression was found to be orientation unspecific and had a different time course than that of facilitation