Visual resolution with epi-retinal electrical stimulation estimated from activation profiles in cat visual cortex.

Blinds with receptor degeneration can perceive localized phosphenes in response to focal electrical epi-retinal stimuli. To avoid extensive basic stimulation tests in human patients, we developed techniques for estimating visual spatial resolution in anesthetized cats. Electrical epi-retinal and visual stimulation was combined with multiple-site retinal and cortical microelectrode recordings of local field potentials (LFPs) from visual areas 17 and 18. Classical visual receptive fields were characterized for retinal and cortical recording sites using multifocal visual stimulation combined with stimulus-response cross-correlation. We estimated visual spatial resolution from the size of the cortical activation profiles in response to single focal stimuli. For comparison, we determined activation profiles in response to visual stimuli at the same retinal location. Activation profiles were single peaked or multipeaked. In multipeaked profiles, the peak locations coincided with discontinuities in cortical retinotopy. Location and width of cortical activation profiles were distinct for retinal stimulation sites. On average, the activation profiles had a size of 1.28 +/- 0.03 mm cortex. Projected to visual space this corresponds to a spatial resolution of 1.49 deg +/- 0.04 deg visual angle. Best resolutions were 0.5 deg at low and medium stimulation currents corresponding to a visus of 1/30. Higher stimulation currents caused lower spatial, but higher temporal resolution (up to 70 stimuli/s). In analogy to the receptive-field concept in visual space, we defined and characterized electrical receptive fields. As our estimates of visual resolutions are conservative, we assume that a visual prosthesis will induce phosphenes at least at this resolution. This would enable visuomotor coordinations and object recognition in many indoor and outdoor situations of daily life.