chap 4.docx

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Department
Psychology
Course
PSY280H1
Professor
Christine Burton
Semester
Fall

Description
Psy280 chapter 4- Visual cortex and beyond Light is reflected from an object into the eye. This light is focused to form an image of that object on the retina. Light, in a pattern that illuminates some receptors intensely and some dimly, is absorbed by the visual pigment molecules that pack the rod and cone outer segments. Chemical reactions in the outer segments transduce the light into electrical signals. As these electrical signals travel through the retina, they interact, excite, and inhibit, eventually reaching the ganglion cells, which because of this processing have center-surround receptive fields on the retina. After being processed by the retina these electrical signals are sent out the back of the eye in fibers of the optic nerve. Lateral geniculate nucleus (LGN): most signals from the retina travel out of the eye in the optic nerve to this then to the thalamus. Signals travel to the primary visual receiving area in the occipital lobe of the cortex. The visual receiving area is also called the striate cortex because of the white stripe that are created within this area of cortex by nerve fibers that run through it. Superior colliculus: an area involved in controlling eye movements and other visual behaviours that receives about 10 percent of the fibers form the optic nerve. LGN neurons have the same center-surround configuration as retinal ganglion cells. Neurons in the LGN, like neurons in the optic nerve, responds best to small sports of light on the retina. Major function of LGN is to regulate neural information as it flows from the retina to the visual cortex and not to create new receptive field properties. 90% of the fibers in the optic nerve arrive at the LGN. The other 10% travels to the superior colliculus. The LGN also receives signals from the cortex from the brain stem, from other neurons in the thalamus and from other neurons in the LGN. The LGN receives information from many sources, including the cortex and then sends its output to the cortex. For every 10 nerve impulses the LGN receives from the retina, it sends only 4 to the cortex. Decrease in firing that occurs at the LGN is one reason for the that the purpose of the LGN is to regulate neural information as it flows from the retina to the cortex. LGN is not only regulates information flowing through it, it also organizes the information. Signals arriving at the LGN are sorted and organized based on the eye they came from, the receptors that generated them, and the type of environmental information that is represented in them. The LGN is a bilateral structure which means there is one LGN in the left and right hemisphere. Each layer receives signals from only one eye. Layers 2,3,5 receive signals from the ipsilateral eye: the eye on the same side of the body as the LGN. Layers 1,4,6 receive signals from the contralateral eye: the eye on the opposite side of the body from the LGN. Psy280 chapter 4- Visual cortex and beyond Each eye sends half of its neurons to the LGN that is located in the left hemisphere of the brain and half to the LGN that is located in the right hemisphere. Signals from each eye are sorted into different layers, the information from the left and right eyes is kept separated in the LGN. Correspondence between points on the LGN and points on the retina creates a retinotopic map on the LGN. Correspondence between locations on the retina and locations on the LGN means that neurons entering the LGN are arranged so that fibers carrying signals from the same area of the retina end up in the same area of the LGN. Retinotopic maps occurs not only in layer 6, but in each of the other layers as well, and the maps of each of the layers line up with one another. One million ganglion cell fibers travel to each LGN, and on arriving there, each fiber travel to each LGN and on arriving there, each fiber goes to the correct LGN layer, and finds its way to a location next to other fibers that left from the same place on the retina. More than 80% of the cortex responds to visual stimuli. Late 1950: idea that most of the cortex responds when the retina is stimulated. Hubel and Wiesel found cells in the striate cortex with receptive fields that like center- surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas. These areas are arranged side by side rather than in the center- surround configuration. Simple cortical cells: cells with side by side receptive fields. Respond best to vertical bars. A vertical bar illuminates only the excitatory area causes high firing, but as the bar is tilted so the inhibitory area is illuminated, firing decreases. Orientation tuning curve: determined by measuring the responses of a simple cortical cell to bars with different orientations. Tuning curve shows that cell responds with 25 nerve impulses per second to a vertically oriented bar. Cells response decreases as the bar is tilted away from the vertical thus begins stimulating inhibitory areas of the neurons receptive field. Bar tilted 20 degrees from the vertical elicits only a small response. Prefers a particular orientation. Complex cells:many cortical neurons responds best to moving barlike stimuli with specific orientations, responds best to bars of a particular orientation. Most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field. Particular direction of movement. End-stopped cells: fire to moving lines of a specific length or to moving corners or angles. Records to the right indicates that the neurons responds when the corner moves upward. Responds increases as the corner shaped stimulus gets longer, but then stops responding when the corner becomes too long. Some neurons in the cortex respond only to oriented lines was an extremely important discovery because it indicates that neurons in the cortex do not simply respond to light, they respond to some patterns of light and not to others. Neurons responds selectively to stationary and moving lines was an important step toward determining how neurons respond to more complex objects. Feature detectors: complex and end stopped cells fire in response to specific features of the stimulus such as orientation or direction of movement. retinal ganglion cells respond best to spots of light, cortical end stopped cells respond best to bars of a certain length that are moving in a particular direction.Psy280 chapter 4- Visual cortex and beyond Selectivity adaptation: if the neurons fire for long enough, they become fatigues or adapt. Causes 2 psychological effect: 1. Neurons firing rate decreases and 2.neuron fires less when that stimulus is immediately presented again. Adaptation is selective because only the neurons that respond to verticals or near- verticals adapt, and other neurons do not. If these adapated neurons have anything to do with perception, then adaptation of neurons that respond to verticals should result in the perceptual effect of becoming selectively less sensitive to verticals, but not to other orientations. Many selective adaptation experiments have used a stimulus called grating stimulus and behavioural measure called constant threshold. Measure sensitivity to range of one stimulus characteristic Adapt to that characteristic by extended exposure Re-measure the sensitivity to range of the stimulus characteristic Grating stimuli: alternating bars. High contrast gratings are on the left, and lower contrast gratings are on the right. Contrast thre
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