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20 Cards in this Set
- Front
- Back
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What is acuity and how can it be measured?
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Acuity is the smallest spatial detail that can be resolved. It can be measured by doing a visual acuity test, which requires looking at figures from a distance and identifying them.
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Explain what happens during the phenomenon of aliasing.
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Aliasing is the misperception of a grating due to undersampling. When looking at gratings, the visual system "samples" the grating discretely, through the array of receptors at the back of the retina. If the receptors are spaced such that the whitest and darkest parts of the grating fall on separate cones, the observer can make out the grating.
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Explain the meaning of being able to see 20/20.
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Being able to see 20/20 means that the observer can just identify an object at 20 feet as well as a "normal" observer would be able to just identify it at 20 feet.
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What can we infer from the contrast sensitivity function?
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The contrast sensitivity function describes our window of visibility. Any object whose spatial frequencies and contrast fall within the region specified by the contrast sensitivity function will be visible. Those objects outside the region are outside our window of visibility. We can infer from it that sensitivity to contrast depends on the spatial frequency of the stimulus.
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Explain how retinal ganglion cells respond to stripes.
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Retinal ganglion cells respond to a variety of stripes. Each ganglion cell responds to certain types of stripes or gratings. For instance, an ON ganglion cell responds to gratings of different spatial frequencies. When the grating is too low, the ganglion cell responds weakly because part of the bar of the grating lands in the inhibitory surround, damping the cell's response. Similarly, when the grating spatial frequency is too high, the ganglion cell responds weakly because both dark and bright stripes fall within the receptive-field center, washing out the response. When the frequency is just right, the cell responds vigorously.
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What is the role of the lateral geniculate nucleus?
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The lateral geniculate nucleus is a nucleus in the midbrain that serves as a relay station between the retina and visual cortex.
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What are the two types of layers of the LGN and how are they different from each other?
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The two types of layers of the LGN are the magnocellular layers and the parvocellular layers. The magnocellular layers are the two bottom layers of the LGN, and they contain neurons that are physically larger than those in the parvocellular layers. Neurons in these layers respond to large, fast-moving objects. The parvocellular layers are the top four layers of the LGN. They contain neurons that respond to details of stationary objects.
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Explain the notion of topographical mapping.
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Topographical mapping is the orderly mapping of the world in the lateral geniculate nucleus and the visual cortex. It provides us with a neural basis of knowing where things are in space.
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What are two important features of the visual cortex? Explain.
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One important feature of the visual cortex is topographical mapping, which is the orderly mapping of the world. The second feature is the dramatic scaling of information from different parts of the visual field. Objects on or near the fovea are processed by neurons in a large part of the striate cortex, but objects imaged on the far right or left periphery are allocated a tiny portion of the striate cortex. This feature is known as cortical magnification.
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What is orientation tuning?
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Orientation tuning is the tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others.
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In what way do striate cortex neurons function as filters?
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Each striate cortex neuron is tuned to a particular spatial frequency, which corresponds to a particular line width. These narrow tuning functions mean that each striate cortex neuron functions as a filter for the portion of the image that excites the cell.
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What is ocular dominance?
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Ocular dominance is the property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other.
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What is the difference between simple and complex cells?
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Simple cells are cortical neurons with clearly defined excitatory and inhibitory regions, while complex cells are neurons whose receptive field characteristics cannot be easily predicted by mapping with spots of light.
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What is the role of end stopping?
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End stopping is the process by which cells in the cortex first increase their firing rate as the bar length increases to fill up its receptive field, and then decrease their firing rate as the bar is lengthened further. It plays an important role in our ability to detect luminance boundaries and discontinuities.
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What does a hypercolumn contain?
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A hypercolumn contains at least two sets of columns, each covering every possible orientation, with one set preferring input from the left eye and one set preferring input from the right eye.
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What is the enzyme cytochrome oxidase used for?
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This enzyme is used to reveal the regular array of "CO blobs," which are spaced about 0.5 mm apart in the primary visual cortex. These blobs have been implicated in processing color, motion, and spatial structure.
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How can adaptation provide insights into the properties of cortical neurons?
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Adaptation is the diminishing response of a sense organ to a sustained stimulus. It is helpful in learning about the properties of cortical neurons because by exposing an observer to an extended period of time of viewing particular stimuli, the experimenter can make inferences about the visual system due to the observer's changing responses.
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What idea does the tilt aftereffect support?
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The tilt aftereffect supports the idea that the human visual system contains individual neurons selective for different orientations.
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What are spatial frequency channels?
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Spatial frequency channels are pattern analyzers, implemented by ensembles of cortical neurons, with each set of neurons tuned to a limited range of spatial frequencies.
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Why is it critical to find visual impairments early in life?
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It is critical to find visual impairments early because there is a critical period of development for the visual system (between 3 and 8 years of age), during which cortical neurons are still being wired up to their inputs from the two eyes. Many impairments found during this time can be medically fixed, so that vision will develop normally.
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