Retinal Processing and Outputs Lecture Notes PDF

Retinal Processing and Outputs Center/surround properties 1. Antagonistic… effect of light on the center is opposite to the effect of light on the surround, and if light covers both the center and the surround, the effects cancel out. 2. That our visual system is set up with centers and surrounds th...

Retinal Processing and Outputs Center/surround properties 1. Antagonistic… effect of light on the center is opposite to the effect of light on the surround, and if light covers both the center and the surround, the effects cancel out. 2. That our visual system is set up with centers and surrounds that are antagonistic to each other suggests that the visual system is more sensitive to contrasts in the intensity of illumination than to total brightness. a. For an On Center RGC, when the light completely fills the center and not the surround, the net effect is maximal increase in action potentials. c. For an On Center RGC, light falling only on the receptive field surround actually suppresses the ongoing rate of AP production. d. When light either fills both the center and the surround or it is dark there is little to no net effect on action potential production. The action potential rate is nearly the same as the baseline condition. Why is the visual system organized to detect contrasts? 1. The logical answer is that contrasts in light intensity are more informative than the overall illumination. Where are the contrasts in the figure below? 2. Another way to say this is that the amount of light reflected by an object can vary dramatically depending upon ambient illumination, but we do not want our perceptions to vary dramatically in different lighting conditions. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Mechanisms for the formation of an antagonistic surround A. The key mechanism in the formation of the antagonistic surround is the horizontal cell. B. The diagram below shows the role of a horizontal cell (HC) when light falls on a photoreceptor (PR) in a bipolar cell's (BP) and RGC’s surround: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Responses when light shines to center of ON-center RGC PR PR HC BP Responses to light onto surround of ON-center RGC. RGC C. How does light falling on the center and surround of a BC or RGC receptive field produce antagonistic effects? Light in the surround of a bipolar cell's receptive field produces the opposite effect on the membrane potential of a photoreceptor than light falling in the center. This is because of the horizontal cell’s synaptic connection between photoreceptors in the surround and photoreceptors in the center of a bipolar cell's and retinal ganglion cell’s receptive field. D. Figure 9.22 in Bear et al. is an excellent depiction of a single bipolar cell, with synaptic inputs from photoreceptors from its center, surround, and outside of its receptive field. What happens to the output from the retina? (Where do axons of retinal ganglion cells (RGCs) go?) © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. A. Below is a diagram illustrating general features of serial and parallel processing: Serial Processing Parallel processing + serial processing The next diagram is an overview conception of the organization of the visual systems. Note that there is massive parallel and serial processing of the retinal output. V3 Eye Lateral Geniculate Nucleus (LGN) V1 V2 V4 V5 Superior Colliculus (SC) Suprachiasmatic n. (SCn) These are all regions in visual (V) cortex B. In most of the components of the visual system, visual information is mapped in an orderly point-to-point fashion: These are visuotopic and retinotopic maps. 1. Visuotopic map: every point in the visual world is mapped in a point to point fashion onto various brain areas in the visual system (including the © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. retina). 2. Retinotopic map: every point in the retina in mapped in a point to point fashion onto various brain areas in the visual system. Visuotopic and retinotopic maps are essentially the same. What is different is whether the visual field or the retina is used as the initial point of reference. The visual world is repeatedly represented by visuotopic maps in different brain regions (to date there are 25-30 known visuotopic maps in the brain!!). C. How do these orderly representations (maps) occur? The short answer is that the orderly location of the RGC in the retina is maintained by orderly point-to-point neuronal projections from visual brain area to visual brain area. The diagram below shows how the retinas are divided into halves at the fovea and how the RGCs from the temporal hemiretinas project to the ipsilateral (same side) of the brain while the nasal hemiretinas project to the contralateral (opposite) side of the brain: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Nasal hemiretinas Temporal hemiretina Temporal hemiretina 1. The retinas are vertically divided in half through the fovea. 2. The two halves of the retina have different patterns of projections: a. Nasal hemiretinas project to the opposite side of the brain (contralateral projections). b. Temporal hemiretinas project to the same side of the brain (ipsilateral projections). D. Below is a diagram depicting how a visuotopic map is produced. However, the general answer is that the orderly representations of the visual field in the brain are produced by orderly patterns of anatomical connections. (Also, see book Figure 10.8). © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Your Visual Field A Eyes/Retinas B E C D A D C E F B F B E D D C E B F A C Superior Colliculus 1. This diagram represents a bird’s eye view. 2. Light from objects in the extreme periphery of the visual field (A and F) to the contralateral eyes is blocked by the nose, so they are detected only in the ipsilateral eye. 3. The superior colliculus on each side of the brain contains a complete visual representation of the contralateral visual field. 4. So, the mechanism for the formation of the visuotopic map is that the nasal hemiretina’s RGCs project in an orderly manner to the contralateral side © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. of the brain and the temporal hemiretina’s RGCs project in an orderly manner to the ipsilateral side of the brain. Also, the axons from RGCs in either eye that are activated by light from the same object converge on the same neurons in the superior colliculus. 5. Therefore, if you were to record from neurons at point B in the superior colliculus, the cells there could be driven by inputs from the right eye, the left eye, or both eyes. 6. At F or A locations in the superior colliculus the cells respond to light input only from the contralateral hemiretina. © Hongdian Yang. 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