Retinal Encoding and Processing
Vision begins when light strikes the retina, a thin sheet of neural tissue in the back of the eye. The optics of the eye form a focused image in the plane of the retina, where light is absorbed by a layer of photoreceptor cells and converted into a neural signal. This signal flows through a layer of bipolar neurons to the final layer of ganglion cells, which send brief electrical pulses, known as action potentials or spikes, down the optic nerve to the central brain. In addition, classes of interneurons called horizontal cells and amacrine cells spread signals laterally within the plane of the retina.
(Figure 1) The eye, retina, and optic nerve. B. Anatomy of the retina, showing its five major cell classes (Golgi stain from Ramon y Cajal).
All visual information is encoded in the time of occurrence of the spikes produced by ganglion cells. The multi-electrode recording array allows spikes from many ganglion cells to be recorded while complex and realistic visual stimuli are presented to the retina with a computer monitor. Together, these techniques enable precise control of the input to the retinal circuit and measurement of the output relevant to the brain.
(Figure 2) The multi-electrode array records spikes from many retinal ganglion cells, while a computer monitor provides rich visual stimulation.
How does the activity of retinal neurons represent a visual scene? At the level of the primary light sensors of the retina, this representation is quite straightforward. One can think of the layer of photoreceptors as the biological equivalent of a CCD camera, where each photoreceptor is a pixel in the retinal image and the voltage across its cell membrane encodes the intensity of light falling on it at each moment. However, the layer of ganglion cells is quite different: each cell receives input from an overlapping set of many photoreceptors and delivers its output in the form of a sparse sequence of spikes. Furthermore, the human retina contains 100 million photoreceptors but only 1 million ganglion cells, so it must perform enormous data compression without degrading the content of the visual input. Research on retinal encoding is being conducted by Professor Michael Berry.
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Michael Berry