Spectral profile of a Sg cell showing changes in discharge-rate and modulatory responses to sinusoidal drifting gratings.

<p>A. Raster plot of cell responses to stimulation with sinusoidal gratings drifting in the optimal direction with spatial and temporal frequency (SF and TF) that elicited a maximum discharge rate (F0). Thin vertical line indicates beginning of grating movement. Single response boxed with grey line was used to construct B, C and D. B. Representative peristimulus time histogram (PSTH) of the unit’s single trial response boxed in A. F0 is defined as the mean firing rate of the response averaged over the time of a single stimulus presentation (1 s of stimulus movement). C. The amplitude spectrum computed from the PSTH shown in B. F1 is the amplitude of the response at the TF of the stimulus (red line). The solid horizontal line depicts mean value of the amplitude spectrum (meanFFT), the dashed line indicates one standard deviation above the mean (sdFFT). zF1 is defined as the ratio of the difference between F1 and meanFFT over the standard deviation from the mean of the spectrum. A value of zF1>1 indicates the presence of modulations. D. Fundamental Fourier components of the neuron’s response to single stimulus trial. Each of the five dots on the complex plane represent fundamental Fourier component of the response to each single, out of five, stimulus cycle of the within one 1 s of grating movement (extracted from single response boxed in A). E–H. Raster plot, PSTH, amplitude spectrum and Fundamental Fourier components of responses to stimulation with a sinusoidal grating drifting in the optimal direction with a spatiotemporal frequency combination that elicited the strongest modulations. Conventions as in A–D. In G notice that zF1, indicating the strength of modulation, is higher than for the response shown in A–D despite a lower firing rate. Dot clustering in the areas distant from the 0 point in D and H indicates the phase sensitivity of the responses. I. Contour plot of the discharge-rate spectral receptive field (RF) of a Sg cell. The surface was fitted to the values of mean discharge rates (F0) obtained in response to grating moving with the SFs and TFs indicated by the black dots. Response strength is color coded according to the scale on the right. Note that the increased firing rate in response to the moving grating is present over a limited range of stimulus SFs and TFs. J. Analogous contour plot of the spectral RF for the same Sg cell constructed by surface fitting of the mean strength of modulations (zF1). Note that the strongest modulations are elicited by stimulus parameters that hardly evoke any increase in firing rate (I). Open circles in I and J represent stimulus parameters evoking highest discharge rate (raster plot of the response is shown in A). The cross indicates stimulus parameters inducing the strongest modulation (raster plot in E). K, L. Temporal (K) and spatial (L) frequency tuning curves obtained from cross-sections of the F0 and zF1 spectral RFs shown in I and J. Blue lines correspond to cross-sections of the discharge-rate RF (I) and the red lines correspond to cross-sections of the zF1 RF (J). Cross-sections were done through the point of the maximum discharge-rate response (marked with circle in I) and through the point of maximum modulation (cross in J).