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

Abstract

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.

A cortical (V1) neurophysiological recording model for assessing the efficacy of retinal visual prostheses

“…Some investigators have used animal models to examine how electrical stimulation of the retina affects the responses of ganglion cells (Grumet et al, 2000;Stett et al, 2000;Jensen et al, 2003Jensen et al, , 2005a. Other groups have used animal models to examine how electrical stimulation of the retina affects the cortical response, either at a very gross scale, obtained by skull recordings of evoked potentials Schanze et al, 2005;Yamauchi et al, 2005;Chowdhury et al, 2005), at an intermediate scale, obtained by local-field potential recordings (Wilms et al, 2003) and by optical imaging of the cortex (Walter et al, 2005), and finally, at a cellular level, obtained by recording multi-unit spiking activity (Eger et al, 2005).…”

High-Resolution Electrical Stimulation of Primate Retina for Epiretinal Implant Design

“…To achieve near-normal vision, the diameter and density of stimulation electrodes will need to approach the size and density of the cells they are designed to stimulate. Testing in patients is currently limited to electrodes a few hundred micrometers in diameter (Humayun, 2003;Mahadevappa et al, 2005), but prototype arrays with much smaller electrodes placed in close proximity to the ganglion cell layer have been developed (Rizzo et al, 1996;Grumet et al, 2000;Stett et al, 2000;Litke et al, 2003;Wilms et al, 2003;Shah et al, 2007). These densely spaced arrays feature 10 -40 m electrodes and have traditionally been tested using rodent, rabbit, or chicken retinas.…”

Phase Boundaries as Electrically Induced Phosphenes

“…Furthermore, Wilms et al [23] have used direct electrical stimulation to estimate the spatial resolution of the retina as mapped onto the visual cortex. Unusual and abnormal stimuli have long been used as probes of the visual system, as they can often provide information about processing that is not available with more natural stimuli.…”