Computational Vision CSCI 384, Spring 2004 Lecture 4 The Retina 1

Reading Assignment Read: Vision Science, Ch. 4.1, "The Eye", by Hubel Questions to think about: What is the importance of the neural processing in the retina? How does the processing in the frog retina (from the Journal club article) differ from that of the human retina? How does the visual system deal with changes in the total intensity of ambient light (e.g. bright daylight vs. dim evening light)?

The Eye

Major Cell Types in Retina Photoreceptors Bipolar Cells Horizontal Cells Amacrine Cells Retinal Ganglion Cells

Photoreceptors Photoreceptors sense light through phototransduction. They are located at the back of the eye, next to the pigment cells which prevent light scattering and are involved in metabolic maintenance of the photoreceptors. Two major types: Rods: Rod shaped, very sensitive to light, low acuity, slow temporal response. Cones: Cone shaped, lower light sensitivity, high acuity. Three types of pigments that are sensitive to three wavelengths of light (red, green, blue) for color vision.

The Fovea The fovea is a specialized region in the center of the eye. It measures about 0.5 cm in diameter Within this region, cones are closely packed together. This region gives us the greatest acuity, allowing reading and other fine visual judgments. There are no rods in the fovea. Outside the fovea there is a mixture of rods and cones. The ratio of rods to cones increases toward the periphery. Can see fainter light with your peripheral vision If fovea bleached (by looking at a bright light), you cannot read until it recovers.

Schematic of the Fovea

Other retinal cells Bipolar cells: Provide straight pathway from photoreceptors to retinal ganglion cells. Horizontal Cells: Present in the layer between the photoreceptors and the bipolar cells. Have long horizontal connections. Amacrine Cells: Present in the layer between the bipolar cells and the retinal ganglion cells. Also have horizontal connections. There are lots of different shapes. Retinal Ganglion Cells: Final cells in the retinal pathway. The first cells in the pathway to fire action potentials. They send signals to the LGN by way of their axons, which form the optic nerve.

Phototransduction In darkness, the photoreceptors are depolarized (-40 mV). There is a Na+ current flowing into the cell (the "dark" current). Light falling on a photoreceptor decreases the dark current, causing the photoreceptor to hyperpolarize. Light falls on the photopigment, Rhodopsin, causing a conformational change in the molecule. The conformational change leads to a chain of chemical events: Rhodopsin->activate transducin->cleave cGMP->Na+ channels close->decrease dark current. Amplification: 1 rhodopsin -> hundreds of transducin molecules Each transducin molecule-> cleaves over 1000 cGMP molecules This allows a single photon to have an effect. Saturation: When all Na+ channels are closed, increasing the light has no more effect on the photoreceptor.

Response of Retinal Ganglion Cells Retinal Ganglion cells and bipolar cells have receptive fields that exhibit a center-surround structure.

Circuitry for center-surround response Bipolar cells have a center surround response. They do not fire action potentials. An "off" response causes hyperpolarization. An "on" response causes depolarization. Bipolar cells connect directly to the retinal ganglion cells with an excitatory connection. The center response of the bipolar cell comes from photoreceptor input. An inhibitory connection from the photoreceptor to the bipolar cell causes an "on" response (because the photoreceptor hyperpolarizes with light). An excitatory connection causes an "off" response. Horizontal cells provide input for the surround response. Horizontal cells show a graded response to diffuse illumination over a spatial region that corresponds to the size of the surround. Horizontal cells have inputs from a wider range of photoreceptors than do the photoreceptors.

The Role of Center-Surround organization Discussion--what is the center-surround organization good for?