Resumo

OBJECTIVE: The visual wulst is the avian forebrain target of the retinothalamofugal pathway and, in owls, bears striking functional analogy with the mammalian primary visual cortex. In this study, we characterize the response of owl wulst neurons to stationary grating patterns and assess how orientation selectivity unfolds within the time course of the response. We also compare the aforementioned results with those derived from moving grating stimulation. METHODS: Using standard extracellular recording techniques, we first isolated single units (n = 48) and characterized their receptive fields in terms of position, size, orientation selectivity, and spatiotemporal frequency tuning. Visual stimuli consisted of drifting and stationary sinusoidal gratings presented at different contrast levels and in several directions / orientations (22.5 degree steps). To quantify the decrease in response rate as a function of stimulus presentation time, we fitted spike-density functions from the initial response peak to the end of the stimulation period with an exponential decay function R(t) = (Rmax - Rmin) x exp(t/ô) + Rmin, where Rmax is the peak response rate, Rmin is the asymptotic response rate, and ô is the exponential decay time constant that captures the rate of response adaptation. The degree of sustained response (DSR) was calculated with an index given by (Rmin - Rspont)/(Rmax - Rspont), where Rspont is the spontaneous discharge rate. DSR ranges from 0 to 1, with lower values indicating strong adaptation. Von Mises functions were used to assess the temporal evolution of orientation/direction tuning. RESULTS: For stationary gratings, mean ô was 94 ±24 ms. For drifting gratings, mean ô was 192.9 ± 34 ms. Average DSR was 10% and 30% in response to stationary and drifting gratings, respectively. With regard to stimulus selectivity, preferred orientation/direction changed little between transient and sustained response periods. Nevertheless, for stationary gratings, we found that orientation-tuning bandwidths derived from the sustained period were significantly narrower than those computed within the transient period (Wilcoxon rank sum P < 0.001). Contrast invariance, a phenomenon observed with drifting stimuli, was also noted using stationary gratings. CONCLUSION: Neuronal responses in the visual wulst decline quickly to low sustained rates revealing an intense and rapid adaptation to static stimuli. Adaptation was comparatively slower and less intense in response to drifting gratings. With respect to orientation tuning, no significant differences in preferred orientation was noted between transient or sustained response periods. In contrast, tuning precision was found to improve during response adaptation. FINANCIAL SUPPORT: Fapemig, Capes.