Visual Processing PDF

Neurophysiology PH5208 Visual Processing Dr. Moira Jenkins Each photoreceptor encodes the light intensity by varying its transmitter release in a graded fashion, relative to the level of illumination transmitter release from the photoreceptors varies inverse to changes in illumination: • decreasi...

Neurophysiology PH5208 Visual Processing Dr. Moira Jenkins Each photoreceptor encodes the light intensity by varying its transmitter release in a graded fashion, relative to the level of illumination transmitter release from the photoreceptors varies inverse to changes in illumination: • decreasing intensity (darker) → increased neurotransmitter release • increasing intensity (brighter) → decreased neurotransmitter release Kandel & Schwartz Fig. 26-7 now recall that transmitter released from photoreceptors serves to “control” bipolar cell stimulation of ganglion cells … Photoreceptors Adapt • Illumination decreases cGMP-→ Na channels close→ hyperpolarize→ less Glutamate • How do photoreceptors adapt when the illumination remains the same? • Calcium normally inhibits cGMP • Ca is “pumped” out of cell during all phases of illumination • As calcium is pumped out, less inhibition of cGMP, Na channels can open, depolarization of photoreceptor → through this mechanism photoreceptors adapt to continuous bright light, resulting in a reduced sensitivity to sustained illumination Direct responsiveness to amount of illumination since photoreceptors have Generator potentials Generator potential Action potential Processing begins in the retina ON & OFF Bipolar Cells • Photoreceptors release Glutamate neurotransmitter at synapse with bipolar cells • However, Bipolar cells respond differently to the glutamate **receptor on the ON & OFF bipolar cells are different • OFF Bipolar cells- channel is ionotropic, glutamate excites, EPSP The OFF bipolar cells will be excited when lights are OFF • ON Bipolar cells- channel is metabotropic G-protein, glutamate inhibits, IPSP The ON bipolar cells will be excited when the lights are ON (less glutamate) • These bipolar cells have opposite responses to illumination and to glutamate ON & OFF Bipolar Cells • ON-type bipolar cells: →postsynaptic response to Glu from the photoreceptor is an IPSP • OFF-type bipolar cells: →postsynaptic response to Glu from the photoreceptor is an EPSP • Ganglion Cells response to glutamate is always excitatory • signaling from a ganglion cell to the brain will vary relative to the light intensity sensed by the photoreceptor in that cell’s column … light dark Neuroscience Fig. 11-18  illumination ON OFF   illumination ON  OFF  Contrast, incremental changes in illumination, delineate boundaries, increase resolution  OFF and ON columns of retinal cells respond to both increments and decrements in the intensity of the light, but in opposite directions increase illumination decrease illumination note that the light stimulus is applied to just the central portion of the receptive field (“on/off center”) the membrane potential of photoreceptor and bipolar cells varies with light stimulation; these cells do not generate action potentials  opposite signaling arising from the ganglion cells to ON/OFF processing Neuroscience, Fig. 11-18 Receptive Field for Ganglion Cells The surface area of a bundle of photoreceptors converging on a ganglion cell Very small ganglion receptive fields at fovea Large in periphery There is overlap of receptive fields and a mix of ON and OFF bipolar cells Kandel & Schwartz Fig. 25.8 http://hubel.med.harvard.edu/book/b10.htm Adjacent ganglion cells “sample” almost equal regions of the image projected upon the retina, and because of the homogeneous intermixing of the two types of ganglion cells, each “pixel” of the image is effectively sampled through two channels, ON and OFF ON vs. OFF columns allow accurate detection of changing illumination: with increasing illumination, • OFF columns generate reduced signaling • ON columns generate increased signaling with decreasing illumination, • OFF columns generate increased signaling • ON columns generate reduced signaling Opposing OFF and ON columns also provides a structure for: • differentiating between areas of brighter and dimmer illumination upon the surface of the retina • locating the boundary between brighter and dimmer illumination Johnson, Essential Medical Physiology, Fig. 51-6 Each receptive field for a retinal ganglion cell demonstrates a “center-surround” functional organization Kandel & Schwartz Fig. 25.8 Receptive field is divided into two separate regions: • “center” – a circular cluster of photoreceptors forming the center of the receptive field • “surround” – those photoreceptors that encircle (surrounds) the central photoreceptors Center-Surround Receptive Field for Ganglion Cells The central photoreceptors release neurotransmitter onto the bipolar cell and are in the cell column that control the ganglion cell The surround photoreceptors DO NOT release neurotransmitter on the bipolar cells but the horizontal cells Horizontal cells are inhibitory interneurons that release GABA as the neurotransmitter onto the terminal portion of the central photoreceptor and cause hyperpolarization (inhibition) Horizontal cell activity therefore modulates neurotransmitter release from the central photoreceptors to the bipolar cell Neuroscience, Fig. 11-21 Center-Surround Receptive Field for Ganglion Cells What does this do? Begin neural processing No stimulus- baseline firing Stimulus in the center- maximum firing Stimulus entire field – similar to baseline Stimulus in surround but not in center – baseline is suppressed Mainly reporting boundaries and movement a maximum change in signaling frequency only occurs when the center and surround are oppositely illuminated This also shows adaptation but because the eyes are moving (saccadic) it is usually irrelevant. However, focus eyes and don’t move - Processing • This functional setup of the ganglion cell receptive fields gives information about motion and contrasting illumination in the visual space • This information is sent to the visual cortex and associated cortex to be processed and interpreted • Parvo pathway- stationary objects, color, spatial resolution • Magnopathway- motion, poor detail • These 2 centers processing together=parallel processing • Combined with convergence, binocular vs monocular cues- depth, shadows, relative size, color constancy, shape constancy, size constancy