Response properties of neurons in area 17 projecting to the striate-recipient zone of the cat's lateralis posterior-pulvinar complex: comparison with cortico-tectal cells

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Abstract
The main input of the lateral part of the cat's lateralis posterior-pulvinar complex (LP-P) comes from the primary visual cortex. We investigated the response properties of cells in area 17 projecting to the striate-recipient zone (LPl) of the cat's LP-P complex. The cells' receptive fields were stimulated with drifting sine-wave gratings. Cells whose fibres terminate in the superior colliculus were also recorded, to determine how their properties compare with those of cortico-LPl cells and to investigate the possibility that LPl is innervated by collaterals of cortico-tectal units. A total of 26 cells in the striate cortex were identified by antidromic activation from the LPl (mean latency 2.2 ms) and 22 from the colliculus (mean latency 2.5 ms). Only six cortical cells could be activated from the LPl and the colliculus. All cortico-LPl cells except for two responded to drifting sinusoidal gratings with unmodulated discharges (AC/DC ratios <1). On the basis of their modulation index, these units were classified as complex cells. All cortico-LPl cells were selective for the orientation of gratings (mean bandwidth of 28°). There was a tendency for cortico-LPl cells to prefer vertical and horizontal orientations. More than half of these cells (57%) were direction selective. Strong orientation anisotropies were also found in the receptive fields of cortico-tectal cells, since almost all units responded preferentially to horizontally oriented gratings. The mean preferred spatial and temporal frequencies of cortico-LPl cells were 0.74 c/deg (bandwidth 2.03 octaves) and 2.7 Hz (bandwidth 2.5 octaves), respectively. These properties did not differ significantly from those of cortico-tectal cells. Most cortico-LPl cells (72%) exhibited contrast-response curves with saturation at low contrast (mean half-saturation 0.2). For the remaining units, the responses increased linearly with contrast without clear saturation. For more than half of cortico-tectal cells (60%), the contrast function was also characterised by a response saturation. Almost all cortico-LPl cells responded to moving random dot patterns with mean tuning functions of 43.6°. “Standard” as well as “special” complex cells were found to be equally responsive to the motion of visual noise. Similar properties were recorded for cortico-tectal cells (mean bandwidth of 44.2°). Cortico-LPl and cortico-tectal cells were either binocularly or monocularly driven by the contralateral eye and their mean spontaneous firing rates were 11.7 and 10.9 spikes/s, respectively. These cells were presumably located in layer V. Stimulation of LPl and colliculus also evoked trans-synaptic responses in area 17. The average latency of the orthodromic responses from LPl was much shorter than that from the colliculus (medians 3.5 and 50 ms, respectively). The findings indicate that almost all cortico-LPl units have complex receptive fields and that their overall properties differ from those of recipient cells in LPl. These results also indicate that LPl is not likely to be innervated by collaterals of fibres of cortico-tectal cells. While cortico-LPl and cortico-tectal cells appear to form two distinct populations, there is no significant difference between the overall properties of these two cell groups.

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