Abstract
Abstract: :
Purpose: A crucial issue in optimizing the design of a retinal prosthesis for human use is the ability to accurately and reliably assess the quality of vision that the device is capable of producing. We have developed an animal model in which the efficacy of artificial (neurotransmitter–based or electrically–based) stimulation can be evaluated by contrasting it to that of natural (visual) retinal stimulation using multi–electrode recording arrays chronically implanted in the visual cortex of animals. An imperative factor for the success of this model is that the neurons’ receptive field (RF) locations and properties can be mapped rapidly before the artificial retinal stimulation period begins. Methods: We chronically implanted multi–electrode arrays in the primary visual cortex of adult cats. The cats were anesthetized and paralyzed during the experimental sessions. To rapidly and simultaneously map the location and spatial RF envelope of all recorded neurons, we employed a stimulus set comprised of 240 msec long motion impulses (MI). Each MI was a stationary–moving, direction–reversing Gabor patch. A complete RF mapping experiment consisted of a temporally–contiguous sequence of MI stimuli with 5 different orientations and 3 different spatial frequencies presented on a CRT. Gabor patch sizes were larger than the neurons’ classical RF size, but they were presented on a densely sampled spatial grid, resulting in high resolution mapping of the RF envelopes. Results: By combining (at each grid location) the neuronal responses to MIs of different spatial frequencies and orientations, we obtained high signal–to–noise ratio RF envelopes from all (10–30) recorded neurons within 10 minutes. This is considerably faster than other methods, such as spot mapping. Smooth estimates of other RF properties, such as orientation and spatial frequency tuning, were obtained by separately analyzing data collected using Gabor patches of different orientations and spatial frequencies, and were accomplished by increasing the stimulation period to 30–50 minutes. Conclusions: Our technique for rapid, simultaneous mapping of the RF envelopes of cortical neurons using multi–electrode arrays under natural (visual) stimulation conditions is a valuable tool for the assessment of retinal prostheses. We are using this technique in combination with artificial stimulation of the retina to compare cortical responses elicited by the two types of stimulation, thereby evaluating the effectiveness and spatial resolution of potential retinal prostheses.
Keywords: visual cortex • receptive fields • electrophysiology: non–clinical