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Keywords:

  • cortico-thalamic projections;
  • extrageniculate pathways;
  • striate cortex;
  • thalamus

Abstract

The cat's lateral posterior-pulvinar complex (LP-pulvinar) contains three main representations of the visual field. The lateral part of the LP nucleus (LPI or striate-recipient zone) is the only region of these extrageniculate nuclei which receives afferents from the primary visual cortex. We investigated the contribution of area 17 to the response properties (orientation and spatial frequency tuning functions) of LPI neurons by cooling or lesioning the visual cortex. Responses of 40 LPI cells were studied before, during and after the reversible cooling of the striate cortex. When tested for orientation, a total of 10 units out of 28 was affected (36%). For most of these cells (eight of lo), cooling the visual cortex yielded a reduction of the cells’ visual responses without altering their orientation-selectivity (there was no significant change in the orientation tuning width). For only two cells, inactivation led to an increase in the response amplitude. Also, blocking the visual cortex never modified the direction-selectivity of LPI cells. When tested for spatial frequency, 12 neurons out of 33 were affected (36%) by the experimental protocol. In most cases, we observed a reduction in the responses at each spatial frequency tested, with no change in tuning bandwidth. For only three LPI cells, the effects of inactivation of the visual cortex were restricted to specific spatial frequencies, altering the profile of the spatial frequency tuning function. In five cats, removing area 17 reduced the proportion of visual neurons in LPI and the spared visually evoked responses were noticeably depressed. Despite the reduction in responsiveness, a few LPI receptive fields within the cortical scotoma were still sensitive to the orientation and/or direction of a moving stimulus. This last observation suggests that some properties in LPI could be generated either by circuits intrinsic to the LPI or by afferents from extrastriate cortical areas. Overall, these results indicate that projections from the visual cortex to the striate-recipient zone of the LP-pulvinar complex are mainly excitatory. Despite the strong impact of the area 17 projections, our data suggest that the extrastriate cortex could also play a role in the establishment of response properties in the cat's LPI.