Abstract
Abstract: :
Purpose: Ganglion cell responses to stimuli designed to simulate the Westheimer paradigm were compared to psychophysical results produced by human subjects viewing similar stimuli. Potential candidates for physiological mechanisms include local adaptation pool summation as well as center-surround receptive field mechanisms. Methods: Data were obtained from in vivo extracellular recording from macaque retinal ganglion cells. In the physiology experiments, stimuli consisted of a 4 min. of arc probe, modulated temporally at 5 Hz, centered on the ganglion cells' receptive fields and superimposed on circular background pedestals of diameters varying between 6 to 192 min. of arc. In the psychophysics, stimuli also consisted of a probe, modulated temporally at 5 Hz on background pedestals of diameters of various sizes. Sawtooth and pulse waveforms were also tested. Results: Responsivity of ON-center magnocellular (MC) cells gradually decreased as pedestal area increased, until a plateau was reached with little recovery. OFF-center cells demonstrated a vigorous response with small pedestals, but as size increased, responsivity dramatically decreased and subsequently recovered as pedestals encroached into the surround. The depression of responsivity observed in the OFF cells was associated with suppression of maintained activity. Psychophysically using a sinusoidal probe we obtained the classical Westheimer curve. Similar curves were obtained with positive and negative pulse and sawtooth stimuli. Conclusions: Physiological results indicate that the classical psychophysical Westheimer curve may have a complex physiological substrate involving an interaction between local adaptation pools in the retina and changes in maintained firing rate due to center-surround receptive field mechanisms. We have not yet detected the difference between ON and OFF center pathways at a psychophysical level.
Keywords: retina: proximal(bipolar, amacrine, and gangli • electrophysiology: non-clinical • ganglion cells