Toward the development of a cortically based visual neuroprosthesis

Abstract

Motivated by the success of cochlear implants for deaf patients, we are now facing the goal of creating a visual neuroprosthesis designed to interface with the occipital cortex as a means through which a limited but useful sense of vision could be restored in profoundly blind patients. We review the most important challenges regarding this neuroprosthetic approach and emphasize the need for basic human psychophysical research on the best way of presenting complex stimulating patterns through multiple microelectrodes. Continued research will hopefully lead to the development of and design specifications for the first generation of a cortically based visual prosthesis system.

Figure 1

A cortical visual prosthesis system would consist of a miniaturized video camera in a pair of eyeglasses, a retina-like signal processing encoder (worn in a pocket) and arrays of microelectrodes implanted in visual cortex.

Figure 2

(A) The signal processing of the retinal encoder/stimulator transforms an input video stream into a spatio-temporal stimulus pattern that is sent to the implanted electrode arrays. (B) Performance of the encoder/stimulator in transforming an image of a hand into an electrode stimulus pattern.

Figure 3

Left: the Utah Electrode Array (UEA) contains 100, 1.5 mm long penetrating microelectrodes. The UEA provides highly selective stimulation of neurons of the visual cortex and forms the basis for our approach to a cortical visual prosthesis. Right: higher magnification of electrode tip.

Figure 4

Left: single unit recordings from feline motor cortex, 8 months post-implant. Right: single units recordings recorded acutely from human temporal cortex, 5 days post-implant.

Figure 5

The wireless embodiment of the UEA contains an integrated circuit (VLSI), a ferrite film to enhance the magnetic coupling to the receiving coil and a surface mounted device (SMD, capacitor) all integrated, making a package only slightly larger than the UEA.

Figure 6

Example of behavioral stimulus as projected on video monitor (30° × 40° of visual angle). Experiments were conducted in a dark chamber with background luminance below 0.0001 cd m⁻². The dark adapted animal was trained to place its hands on proximity sensors and to fixate on a + in the center of CRT. Phosphene-like optic stimuli were displayed on the screen for 500 ms (Gaussian white object). Following an auditory cue, the animal would indicate its response to the stimuli. Eye positions were sampled at 1 kHz (cloud of white dots) and if position remained within 3° of fixation (white circle) throughout trial, trial was accepted.

Figure 7

Behavioral data during performance of the detection task by a nonhuman primate fitted to the Weibull function (solid lines) [26]. The stimulus was presented at eccentricities of 2°, 4° and 16° of visual angle along the horizontal meridian of the right hemisphere of the visual field. The shifts in the detection curves correlated with the retinal density of rod photoreceptors.

Figure 8

Results of one spot–two spot discrimination behavioral experiments performed by trained non-human primate over a 3 month period. Monkey performs at a high level of success.