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Neural Processing and Perception.docx

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Elizabeth Olds

Neural Processing and Perception Introduction ­ Neural processing—the interaction of the signals in many neurons. Lateral Inhibition and Perception ­ Lateral inhibition —inhibition that is transmitted across the retina Lateral Inhibition in the Limulus  ­ They chose the Limulus because the structure of its eye makes it possible to stimulate individual receptors. The Limulus eye is made up of hundreds of tiny structures called ommatidia , and each ommatidium has a small lens on the eye’s surface that is located directly over a single receptor. ­ Nerve fiber of receptorA, they found that illumination of that receptor caused a large response ­ But when they added illumination to the three nearby receptors at B, the response of receptorAdecreased ­ They also found that further increasing the illumination of B decreasedA’s response even more ­ Thus, illumination of the neighboring receptors at B inhibited the firing caused by stimulation of receptorA. This decrease in the firing of receptor Ais caused by lateral inhibition that is transmitted from B toAacross the Limulus’s eye by the fibers of the lateral plexus Lateral Inhibition and Lightness Perception ­ Lightness —the perception of shades ranging from white to gray to black. The Hermann Grid: Seeing Spots at Intersections ­ You can prove that this grayness is not physically present by noticing that it is reduced or vanishes when you look directly at an intersection or, better yet, when you cover two rows of black squares with white paper. ­ Each bipolar cell has an initial response of 100. Bipolar cells B, C, D, and E each send 10 units of inhibition to bipolar cell A, as indicated by the red arrows. Because the total inhibition is 40, the final response of bipolarAis 60. ­ The final response of bipolarAis determined by starting with its initial response and subtracting any decrease caused by lateral inhibition, which is indicated by the red arrows. ­ Although the initial responses of bipolarsAand D are the same, their final responses are different, because D receives less lateral inhibition than A. Lateral inhibition, therefore, explains the dark images at the intersection. Mach Bands: Seeing Borders More Sharply  ­ Mach bands, illusory light and dark bands near a light-dark border. ­ The lightness is high as we begin moving to the right across the lighter stripe, but then, near the border at B, the lightness becomes even higher. The upward bump at B represents this slight increase in lightness we see just to the left of the border. Once across the border, we encounter the dark Mach band, indicated by the downward bump at C that represents the slight decrease in lightness we see just to the right of the border ­ Each of the six receptors in this circuit sends signals to bipolar cells, and each bipolar cell sends lateral inhibition to its neighbors on both sides ­ An example of simultaneous contrast: The receptors under the two small squares receive the same illumination. However, the light area surrounding the square on the left causes receptors under that area to respond rapidly and to send large amounts of inhibition to the neurons below the center square (large arrows). The dark area surrounding the square on the right causes the receptors under that area to fire less rapidly, so they send less inhibition to the neurons under the right square (small arrows). Because the cells under the left square receive more inhibition than the cells under the right square, their response is decreased more. This smaller response compared to the response of the neurons under the right square causes the left square to appear darker. A Display That Can’t Be Explained by Lateral Inhibition ­ White’s illusion: RectangleA, on the left, which appears to be resting on the white area under the black bars, looks much darker than rectangle B, on the right, which appears to be located on the black bars. However, rectanglesAand B reflect the same amount of light. ­ Our perception of lightness in influenced by a principle called belongingness , which states that an area’s appearance is influenced by the part of the surroundings to which the area appears to belong. Processing From Retina to Visual Cortex and Beyond Responding of Single Fibers in the Optic Nerve ­ The optic nerve, which leaves the back of the eye, contains about one million optic nerve fibers in the human. ­ While recording from this teased-out fiber, Hartline illuminated different areas of the retina and found that the fiber he was recording from responded only when a small area of the retina was illuminated. He called the area that caused the neuron to fire the nerve fiber’s receptive field, which he defined as follows: The region of the retina that must receive illumination in order to obtain a response in any given fiber ­ The cat receptive fields, it turns out, are arranged in a center-surround organization , in which the area in the “center” of the receptive field responds differently to light than the area in the “surround” of the receptive field ­ For the receptive field in Figure 3.21a, presenting a spot of light to the center increases firing, so it is called the excitatory area of the receptive field. In contrast, stimulation of the surround causes a decrease in firing, so it is called the inhibitory area of the receptive field. This receptive field is called an excitatory-center, inhibitory-surround receptive field . The receptive field in Figure 3.21b, which responds with inhibition when the center is stimulated and excitation when the surround is stimulated, is an inhibitory-center, excitatory-surround receptive field . ­ The discovery that receptive fields can have oppositely responding areas made it necessary to modify Hartline’s definition of receptive field to the retinal region over which a cell in the visual system can be influenced (excited or inhibited) by light ­ Center-surround antagonism , illustrated in Figure 3.22.Asmall spot of light presented to the excitatory center of the receptive field causes a small increase in the rate of nerve firing (a), and increasing the light’s size so that it covers the entire center of the receptive field increases the cell’s response, as shown in (b). Increasing stimulus size further causes a decrease in firing due to center-surround antagonism. Hubel and Wiesel’s Rationale for Studying Receptive Fields ­ Recording electrical signals from a fiber in the optic nerve of an anesthetized cat. Each point on the screen corresponds to a point on the cat’s retina. ­ The receptive field is always on the retina, because that is where the stimuli are received. ­ Shows how signals leaving the eye in the optic nerve travel to the lateral geniculate nucleus (LGN) , and then from the LGN to the occipital lobe of the cerebral cortex , the 2–4 mm thick covering of the brain that plays a central role in determining perception and cognition ­ The occipital lobe is the visual receiving area —the place where signals from the retina and LGN first reach the cortex. Viewing the underside of the brain shows the pathway from eye to cortex, plus the superior colliculus , which receives some signals from the eye. This structure plays an important role in controlling movements of the eyes. ­ The visual receiving area is also called the striate cortex , because it has a striped appearance when viewed in cross section, or area V1 to indicate that it is the first visual area in the cortex ­ One proposal of LGN function is based on the observation that the signal sent from the LGN to the cortex is smaller than the input the LGN receives from the retina
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