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Physiology of Photoreceptors • Vertebrate photoreceptors hyperpolarize and produce graded potentials • Photoreceptors use glutamate as transmitter • Bipolar cells can both hyperpolarize and depolarize producing both ON and OFF responses • ON bipolar – glutamate is inhibitory • OFF bipolar – glutamate is excitatory Receptive Fields • Record from a single ganglion cell in the retina • Using small spots of light activate the portion of the visual field that activates the neuron Schematic Retina Showing a Receptive Field Orange are excitatory inputs into the receptive field. Blue are inhibitory inputs into the receptive field. Receptors Horizontal Cells Bipolar Cells Amacrine Cells Ganglion Cells + - The excitatory connection stimulates and increase in the firing rate in the ganglion cell. Light strikes a cone with an excitatory connection to the ganglion cell Number of Action Potentials: 1 Light now fills the excitatory region of the receptive field. Number of Action Potentials: 12 If light falls in the inhibitory region, the firing rate of cell is reduced. Number of Action Potentials: 8 Lateral Inhibition – Variations in the On/Off Structure • Lights on the edge of the field cause a reduction in the background activity of the cell • On and off neurons • Center-surround structure – need to examine in light of different channels of information direct to the cerebral cortex Receptive Fields in the Retina • Two types of ganglion cells: – on and off dependent upon the bipolar neurons • Center Surround structure of the receptive field described by Kuffler • Best activated by central illumination • Best inhibited by annular illumination Different View of Center-Surround Organization: Parallel Pathways • Transformation of visual information is evident in the ganglion cells of the retina • X cells – sustained linear responses • Y cells – transient, excitatory non-linear responses P and M Projections to LGN: Different Physiologic Channels • P cells in the retina (also known as midget ganglion cells) project to the parvocellular layers (3-6) of LGN • M cells in the retina (also known as parasol cells) project to the magnocellular (ventral most) layers (1-2) of the LGN • Intercalated layers are termed koniocellular (dustlike or tiny cells) Physiology of V1 • Originally studied by Hubel and Wiesel who demonstrated two types of neurons – Simple cells – constructed from LGN on/off cells – Complex cells – constructed from simple cells Cortical Simple Cell Cortical Complex Cells: Example of Hierarchy of V1 • Strong orientation selectivity in cells • Moving bars in a specific direction • NO on/off areas like in simple cells • Receptive fields were not elongated • Located in layers 2,3, and 5 which receive input from layer 4 (from ? simple cells)