SPATIAL-FREQUENCY TUNING OF ORIENTATION-DISCONTINUITY-SENSITIVE CORTICOFUGAL FEEDBACK TO THE CAT LATERAL GENICULATE-NUCLEUS

1. The influence of spatial frequency on the inhibitory component of t he effects mediated by feedback from the visual cortex has been examin ed in X and Y cells in the A laminae of the feline dorsal lateral geni culate nucleus (dLGN). Experiments utilized a concentric, bipartite vi sual stimulus centred over the receptive fields of the cells studied. The responses of dLGN cells to selective stimulation of receptive fiel d centre (with the inner window) were compared with those to stimulati on of centre and surround mechanisms (both inner and outer window), wi th the stimuli either in or out of orientation alignment. 2. With thes e same stimuli, layer VI cells in the visual cortex showed a marked in crease in response magnitude when the inner and outer components of ti le stimulus were in orientation alignment, and presented at the prefer red orientation. In the case of dLGN X and Y cells we observed an enha ncement of the surround antagonism of the centre response when the inn er and outer sections of tile stimulus were in orientation alignment. 3. The effects of varying spatial frequency on these responses were ex amined in dLGN cells in the presence of corticofugal feedback. With th e stimulus sections in orientation alignment, surround stimulation pro duced a powerful and significant reduction in tile response to stimula tion of centre mechanism alone with the most marked effects for stimul i in tile range 0.1-0.85 cycles per degree (c.p.d.). The reduction pro duced by surround stimulation in the range 0.1-0.5 c.p.d. was notably more potent in X cells than in Y cells. 4. The responses to the same s timuli were examined in dLGN cells with the corticofugal feedback inac tivated. Comparison of data from cells studied with and without feedba ck revealed a significant decrease in surround-mediated attenuation of the centre response in Y cells for spatial frequencies in the range 0 .1-0.85 c.p.d. For X cells the decrease in strength of the surround an tagonism was also clear and significant but only seen in the range 0.1 -0.5 c.p.d. 5. The influence of the orientation alignment of inner and outer stimulus sections revealed a marked difference between cells st udied with and without feedback. In the presence of feedback fully ali gned stimuli enhanced surround antagonism of centre responses for spat ial frequencies in the range 0.1-0.5 c.p.d., in X and Y cells. In the absence of corticofugal feedback this alignment effect was essentially eliminated. 6. These data show that surround antagonism of the centre response is influenced by orientation alignment of tile stimulus sect ions at low spatial frequencies and in the presence of corticofugal fe edback. They support a cortically driven enhancement of the inhibitory mechanisms reinforcing surround mechanisms in the dLGN. The propose t hat feedback enhances a low spatial frequency cut-off in tile dLGN, th at this effect is maximal for a continuous iso-orientated contour, but diminished whenever there is an orientation discontinuity. The hyperp olarizing influence underlying this effect may contribute to the recen tly described synchronizing influence of the direct corticofugal conta cts onto relay cells. We suggest feedback of the cortical level of ana lysis refines the transfer of the visual input at geniculate level in a stimulus-context-dependent fashion.