Spatial properties of ganglion cell activity in the turtle retina

Ganglion cell activity in response to drifting or alternating stimulus patterns was recorded in an eyecup preparation of the turtle. The stimuli were sinusoidal gratings with various spatial frequencies and spatial phases. Spike histograms of the activity were Fourier-analyzed. The results indicated that the retinal circuitry always functioned in a nonlinear manner. The average, first, and second harmonics of the activity were all a function of stimulus conditions, including spatial phase. Similar outcomes were obtained with full-field (retina-wide) and a small-field (1 mm diameter) stimulation. In the middle of the spatial-frequency range, the ratio of second to first harmonic activity was higher in response to static, contrast-reversing gratings than for drifting, constant-contrast ones. The findings suggest that, as distinct from the usual X, Y, and W segregation of ganglion cell properties in other retinas, there is a single class of cells whose responses seem to have attributes of one or another of the usual categories depending on stimulus characteristics. This may be the result of a degree of stimulus-dependent functional modification in the retinal input circuitry to the turtle ganglion cells.