Visual evoked potentials to infrared stimulation in normal cats and rats

The absence of effective treatments for retinal degenerative diseases has inspired several laboratories to pursue the development of a retinal prosthetic. In our laboratory, we have focused on the subretinal approach, using an array of photodiodes housed within a silicon chip. These photodiodes generate electrical current in response to wavelengths ranging from 500–1100 nm. Because the native retina is traditionally thought to be insensitive to wavelengths beyond ∼750 nm, we and others have attempted to isolate implant-mediated electrophysiological responses from those of the native retina by using longer wavelength stimuli in the near infrared range. Evoked potentials recorded over the visual cortex in response to infrared stimuli have been reported as evidence of a functional subretinal implant due to the typical physiological characteristics of the waveform: a direct relationship between amplitude and intensity, increased amplitude over the visual cortex, and repeatability of the response. However, these results should be interpreted with caution since here we report an unappreciated sensitivity of the native retina to infrared light under dark-adapted conditions.