Chapter 3: Spatial Vision Questions Flashcards
What is visual acuity and how can it be measured?
Visual acuity is the smallest spatial detail that can be seen accurately. It can be measured by doing a visual acuity test, which requires looking at figures from a distance and identifying them.
Explain what happens during the phenomenon of aliasing.
Aliasing is the misperception of a grating due to undersampling. When looking at gratings, the visual system “samples” the grating discretely via the array of receptors at the back of the retina. If the receptors are spaced such that the lightest and darkest parts of the grating fall on separate cones, the observer can detect the grating. However, if the lightest and darkest parts of the grating both fall on the same cones, then the grating will be aliased and appear gray.
Explain the meaning of being able to see 20/20.
Being able to see 20/20 means that the observer can identify an object at 20 feet as well as a “normal” observer would be able to identify it at 20 feet. If the observer’s vision is 20/40, that means that the observer can see at 20 feet what somebody with normal vision can see at 40 feet (meaning the observer needs glasses!).
What can we infer from the contrast sensitivity function?
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 this function that sensitivity to contrast depends on the spatial frequency of the stimulus.
Explain how retinal ganglion cells respond to stripes.
Each ganglion cell responds to certain types of stripes or gratings. For instance, an ON ganglion cell responds to gratings with spatial frequencies and phases that make the lightest part of the grating fall on the center of the cell and the darkest part of the grating fall on the surround. When the spatial frequency of the grating is too low, the ganglion cell responds weakly because part of the bar of the grating lands in the inhibitory surround, dampening the cell’s response. Similarly, when the grating’s spatial frequency is too high, the ganglion cell responds weakly because both dark and light stripes fall within the receptive field’s center and surround, washing out the response. When the frequency is just right, the cell responds vigorously.
What is the role of the lateral geniculate nucleus?
Primary Relay Center
What are the two types of layers in the LGN and how are they different from each other?
The two types of layers in the LGN are the magnocellular layers and the parvocellular layers. The magnocellular layers are the two bottom layers of the LGN, and 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.
Explain the notion of topographical mapping.
Topographical mapping is the orderly mapping of the world in the lateral geniculate nucleus and the visual cortex. Points of light that are near each other in the world fall on parts of the retina that are near each other and will be processed by neurons that are near each other in the brain. This orderly representation provides us with a neural basis of knowing where things are in space.
What are two important features of the visual cortex? Explain.
One important feature of the visual cortex is topographical mapping, which is the orderly mapping of the world in the brain. 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, whereas objects imaged in the periphery are allocated a much smaller portion of the striate cortex. This feature is known as cortical magnification.
What is orientation tuning?
In what way do striate cortex neurons function as filters?
Each striate cortex neuron responds to a particular location and is tuned to a particular spatial frequency, orientation, and phase. These narrow tuning functions mean that each striate cortex neuron functions as a filter for the portion of the image that excites the cell.
What is ocular dominance?
What is the difference between simple and complex cells?
Simple cells are cortical neurons with clearly defined excitatory and inhibitory regions, whereas complex cells are neurons whose receptive field responds to any properly oriented bar of light, regardless of whether it is light or dark.
What is the role of end stopping?
End stopping refers to a property of certain cortical neurons in which they respond vigorously when the end of a bar of light falls within their receptive field. It plays an important role in our ability to detect luminance boundaries and discontinuities.
What does a hypercolumn contain?
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.
