Browsing by Subject "retinal ganglion cell"

Visual prostheses are an emerging technology to restore vision in blind

individuals. The level of vision currently attainable with these prostheses is crude and

far from the level of normal vision though. Epiretinal prostheses work by using a multi-

electrode array implanted within the eye on the inner layer of the retina to electrically

stimulate the neural elements beneath the electrodes and produce punctate visual

percepts of light called phosphenes. Stimulation by serially delivering a cathodic

monopolar pulse of current with each electrode in the array would require the least

power to construct pixilated images of the visual scene. There is the possibility of

complex stimulation schemes that may be able to preferentially stimulate the neural

elements between the electrodes of the multi-electrode array by utilizing multiple

electrodes of the array at once though. Although this would require more power, this

would effectively increase the resolution capabilities of the epiretinal prosthesis without

the need to increase the number of electrodes on the multi-electrode array. To

investigate the possibility of such a stimulation scheme, a computational model of the

inner layers of the human retina including the nerve fiber layer and ganglion cells was

constructed. The model response was validated against studies of biological ganglion

cells, and under comparable conditions reproduced features of epiretinal stimulation

seen clinically. The response of the computational model of the inner retinal layers to

stimulation by up to two electrodes at once in the multi-electrode array was then

determined to evaluate the possibility of producing phosphenes between the electrodes.

The investigation found that disk electrodes using rectangular pulses of equal

magnitude could not produce a distinct phosphene between the electrodes of the model.

The retina signals visual information to the brain with the parallel channels of different retinal ganglion cell (RGC) types, whose signals ultimately lead to visual perception. Between cloudy nights and sunny days, the retina must combat the trillion-fold change in mean light intensity to successfully convey visual information. Critically, the nature of both signal and noise in RGC populations is altered across this broad range of light levels, creating a rich problem of how visual messages are encoded by the retina and transmitted to the brain. This thesis addresses these topics using large-scale multielectrode array recordings of RGC populations in different light conditions. In Chapter 2, I characterize how retinal signaling is altered over a wide range of light intensities. Chapter 3 investigates how adaptation impacts visual encoding of different RGC types. My results suggest that although retinal computations change substantially over light conditions, there are some elements of visual encoding that are invariant to light adaptation. Finally, Chapter 4 examines adaptation-induced changes in the structure of correlated activity and the subsequent impact on processing retinal output. The findings of this chapter clarify the nature of RGC responses crucial for downstream readout across light levels. Overall, this work identifies aspects of RGC activity that are important for encoding visual information and decoding retinal output from starlight to sunlight.