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Lecture 8

Lecture 8 - Feb 1.doc

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PSYC 211
Yogita Chudasama

PSYC211 Lecture 8 - Feb. 1 Receptive Fields: • The receptive field of a neuron is the area of the visual field in which the presence of a stimulus influences the firing rate of that neuron, i.e. The part of space in which the light must fall for the neuron to be stimulated • Microelectrodes are used to record electrical activities of single neurons. Invasive technique used on cats and monkeys • To find a receptive field, the investigator can shine light in various locations while recording from a neuron • If the light from a particular spot excites the neuron then that location is part of the neurons excitatory receptive field (ON firing) • If the light inhibits activity, the location is in an inhibitory receptive field (OFF firing) On and Off Firing: • Neurons respond with either ON firing or OFF firing depending on the location of the spot of light in the receptive field • Stimulation of the central field (center) or the surrounding field (surround) have contrary effects • ON cells are excited by light falling the center but inhibited by light falling in the surround • OFF cells are excited by light falling in the surround but inhibited by light falling in the center Seeing Edges (The Perception of Contrast): • A visual edge is a place where two different areas of a visual image meet The center-surround organization of receptive fields enhances our ability to detect • the outlines of objects when the contrast between the object and background is low (contrast enhancement) • Occurs in the retinal ganglia cells • When a receptor fires, it inhibits its neighbours via a lateral neural network. This is known as lateral inhibition because it spreads laterally across an array of recep- tors • The amount of lateral inhibition produced by a receptor is greatest when it is most intensely illuminated, and it has its greatest effect on its immediate neighbours Neural Basis of Contrast Enhancement: Lateral Geniculate Nucleus (LGN): •The retina-geniculate-striate-pathway conducts signals from the retina to the pri- mary visual cortex (striate cortex or area V1) via the lateral geniculate nucle- us (LGN) •The LGN comprises six layers of neurons • Layers 1 and 2 have large cell bodies and large receptive fields (inner magno- cellular layers) • Layers 3-6 have small cell bodies and small receptive fields (outer parvocellu- lar layers) The Retina-Geniculate-Striate Pathway (The Primary Visual Pathway): •The optic nerves join at the base of the brain to form an x-shaped optic chiasm • The axons from the ganglion cells serving the inner half of the retina (nasal sides), cross through the chiasm and ascend to the LGN of the opposite side of the brain • The axons of the ganglion cells serving the outer half of the retina (temporal sides) remain on the same side of the brain • Each hemisphere receives information from the contralateral visual scene/eye Anatomy of the Striate Cortex: Orientation-Sensitive Neurons (Hubel and Wiesel, 1960’s): • An orientation-sensitive neuron in the striate cortex will respond only when a line of a particular orientation appears within its receptive field Response Characteristics of a Orien- tation Neurons: • Simple cells • Neurons whose receptive field is organized in an opponent fashion • Complex cells Neurons which do not have an inhibitory surround; they respond when line • moves perpendicular to its angle of orientation • Hypercomplex cells • Neurons that have inhibitory regions at the end (or ends) of a
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