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Chapter 4

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Department
Psychology
Course
PS262
Professor
Laurie Manwell
Semester
Winter

Description
Chapter 4 : The visual cortex and beyond Light is refl ected 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 fi elds on the retina. After being processed by the retina these electrical signals are sent out the back of the eye in fi bers of the optic nerve. Followng signals form the retina cortex - One way researchers have approached this problem is by determining how neurons at various places in the visual system respond to stimuli presented to the retina. - The visual system Most of the signals from the retina travel out of the eye in the optic nerve to the lateral geniculate nucleus (LGN) in the thalamus. From here, 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 stripes (striate _ striped) that are created within this area of cortex by nerve fi bers that run through it (Glickstein, 1988). From the striate cortex, signals are transmitted along two pathways, one to the temporal lobe and the other to the parietal lobe (blue arrows). Visual signals also reach areas in the frontal lobe of VL1 the brain. - superior colliculus, an area involved in controlling eye movements and other visual behaviors that receives about 10 percent of the fi bers from the optic nerve. This view also shows how signals from half of each retina cross over to the opposite side of the brain Processing in the lateral-geniculate nuclei Receptive field of LGN Neurons - neurons in the LGN, like neurons in the optic nerve, respond best to small spots of light on the retina. - But further investigation reveals that a major function of the LGN is apparently not to create new receptive fi eld properties, but to regulate neural information as it fl ows from the retina to the visual cortex Information flow in the LGN - Thus, the LGN receives information from many sources, including the cortex, and then sends its output to the cortex. - Notice that (1) the LGN receives more input back from the cortex than it receives from the retina and (2) the smallest signal of all is from the LGN to the cortex. For every 10 nerve impulses the LGN receives from the retina, it sends only 4 to the cortex. This decrease in fi ring that occurs at the LGN is one reason for the suggestion that one of the purposes of the LGN is to regulate neural information as it fl ows from the retina to the cortex. - But the LGN not only regulates information fl owing through it; it also organizes the information - We will see that the 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. Organizing by left and right eyes - The lateral geniculate nucleus (LGN) is a bilateral structure, which means there is one LGN in the left hemisphere and one in the right hemisphere - ipsilateral eye, the eye on the same side of the body as the LGN. - contralateral eye, the eye on the opposite side of the body from the LGN. Thus, 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. Organization as a spatial map - When the man looks at the cup, points A, B, and C on the cup are imaged on points A, B, and C of the retina, and each place on the retina corresponds to a specifi c place on the lateral geniculate nucleus (LGN). - This correspondence between points on the LGN and points on the retina creates a retinotopic map on the LGN—a map in which each point on the LGN corresponds to a point on the retina. - We can determine what this map looks like by recording from neurons in the LGN. - The correspondence between locations on the retina and locations on the LGN means that neurons entering the LGN are arranged so that fi bers carrying signals from the same area of the retina end up in the same area of the LGN, each location on the LGN corresponds to a location on the retina, and neighboring locations on the LGN correspond to neighboring locations on the retina. - Thus, the receptive fi elds of neurons that are near each other in the LGN, such as neurons A, B, and C, in layer 6 (Figure 4.5), are adjacent to each other at A _ , B_, and C _ on the retina. Receptive fields of neurons in the striate cortex - In fact, more than 80 percent of the cortex responds to visual stimuli (Felleman & Van Essen, 1991). The idea that most of the cortex responds when the retina is stimulated is the result of research that began in the late 1950s. - found cells in the striate cortex with receptive fi elds that, like center-surround receptive fi elds of neurons in the retina and LGN, have excitatory and inhibitory areas. However, these areas are arranged side by side rather than in the center-surround confi guration (Figure 4.6a). Cells with these side- by- VL 2 side receptive fi elds are called simple cortical cells. - We can tell from the layout of the excitatory and inhibitory areas of the simple cell shown in Figure 4.6a that a cell with this receptive fi eld would respond best to vertical bars - The relationship between orientation and fi ring is indicated by a neuron’s orientation tuning curve, which is determined by measuring the responses of a simple cortical cell to bars with different orientations. - impulses per second to a vertically oriented bar and that the cell’s response decreases as the bar is tilted away from the vertical, and begins stimulating inhibitory areas of the neuron’s receptive fi eld - there are neurons that respond to all of VL 3 the orientations that exist in the environment - they found that many of the cells they encountered in the cortex refused to respond to small spots of light. - Wiesel changed their stimuli from small spots to moving lines and were then able to fi nd cells that responded to oriented moving bars. As with simple cells, a particular VL 4 neuron had a preferred orientation - Complex cells, like simple cells, respond best to bars of a particular orientation. - However, unlike simple cells, which respond to small spots of light or to stationary stimuli, most complex cells respond only when a correctly oriented bar of light moves across the entire receptive fi eld. - Further, many complex cells respond best to a particular direction of movement - Another type of cell, called end-stopped cells, fi re to moving lines of a specifi c length or to moving corners or angles - 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. - Hubel and Wiesel’s discovery that neurons respond selectively to stationary and moving lines was an important step toward determining how neurons respond to more complex objects. - Because simple, complex, and end-stopped cells fi re in response to specifi c features of the stimulus, such as orientation or direction of movement, they are sometimes called feature detectors. - Retinal ganglion cells respond best to spots of light, whereas cortical end-stopped cells respond best to bars of a certain length that are moving in a particular direction. De feature detectors play a role in perception Selective adaptation and feature detectors - The idea behind selective adaptation is that if the neurons fi re for long enough, they become fatigued, or adapt. This adaptation causes two physiological effects: (1) the neuron’s fi ring rate decreases, and (2) the neuron fi res less when that stimulus is immediately presented again. According to this idea, presenting a vertical line causes neurons that respond to vertical lines to respond, but as these presentations continue, these neurons eventually begin to fi re less to vertical lines. Adaptation is selective because only the neurons that respond to verticals or near- verticals adapt, and other neurons do not. - if these adapted 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. Grating stimuli and the contrast threshold - Grating stimuli are alternating bars. - A grating’s contrast threshold is the difference in intensity at which the bars can just barely be seen. - The important result of this experiment is that our psychophysical curve shows that adaptation selectively affects only some orientations, just as neurons selectively respond to only some orientations. - The psychophysical curve is slightly wider because the adapting grating affects not only neurons that respond best to verticals, but also more weakly affects some neurons that respond to nearby orientations. - The near-match between the orientation selectivity of neurons and the perceptual effect of selective adaptation supports the idea that orientation detectors play a role in perception. Selective rearing and feature detectors - The idea behind selective rearing is that if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent. T - his follows from a phenomenon called neural plasticity or experience-dependent plasticity—the idea that the response properties of neurons can be shaped by perceptual experience. - According to this idea, rearing an animal i
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