Feedback signals from cat's area 21a enhance orientation selectivity of area 17 neurons.

We have studied the contribution of feedback signals originating from one of the "form-processing" extrastriate cortical areas, area 21a (A21a), to orientation selectivity of single neurons in the ipsilateral area 17 (A17). Consistent with previous findings, reversible inactivation (cooling to 5-10 degrees C) of area 21a resulted in a substantial reduction in the magnitude of the maximum response (R (max)) of A17 cells accompanied by some changes in the half-width at half-height of the R (max) (HWHH). By fitting model functions to the neurons' response profiles we found that in the vast majority of orientation-tuned A17 cells tested (30/39, 77%), inactivation of A21a resulted in a "flattening" of their orientation-tuning curves. It is characterised by a substantial reduction in the R (max) associated with either a broadening of the orientation-tuning curves (17 cells) or a relatively small reduction (12 cells) or no change (1 cell) in the HWHH. When the "flattening" effect was quantified using a simple ratio index or R/W, defined as R (max)/HWHH, we found that R/W was significantly reduced during inactivation of A21a. The change in R/W is strongly correlated with the change in the maximum slope of the orientation-tuning curves. Furthermore, analysis of response variability indicates that "signal-to-noise" ratio of the responses of A17 neurons decreases during inactivation of A21a. Our results suggest that the predominately excitatory feedback signals originating from A21a play a role in enhancing orientation selectivity of A17 neurons and hence are likely to improve overall orientation discriminability.