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PSYB51 ch.3
| Question | Answer |
|---|---|
| Contrast | difference in luminance b/w lighter & darker parts of the same object |
| Acuity | smallest spatial detail that can be resolved; acuity specified using terms 20/20 (eye doctors, scientists talk abt smallest visual angle of a cycle of the grating that can be perceived |
| Cycle | for a grating, a pair consisting of one dark bar and one bright bar |
| Visual angle | the angle subtended by an object at the retina; angle that would be formed by lines going from top to bottom of a cycle on the page through the center of the lens & on to the retina |
| Sine wave grating | a grating w/ a sinusoidal luminance profile |
| Aliasing | misperception of a grating due to undersampling; perceiving the gratings to be longer than they actually are |
| Problem w/ resolution acuity to spatial vision | finest high-contrast detail that can be resolved; limit determined primarily by spacing of photoreceptors in the retina (p.53) |
| Herman Snellen | constructed a set of block letters for which the letter as a whole was 5 times as large as the strokes that formed the letter |
| Snellen defined visual acuity as | the distance at which a person can just identify the letters over the distance at which a person w/ “normal” vision can just identify the letters (20/20 vision) |
| Spatial frequency | number of cycles of a grating per unit of visual angle (usually specified in degrees |
| Cycles per degree | number of dark & bright bars per degree of visual angle |
| Contrast sensitivity function | function describing how the sensitivity to contrast (defined as the reciprocal of constant threshold) depends on the spatial frequency (size) of the stimulus shaped like an upside down U |
| Otto Schade | showed ppl sine wave gratings w/different spatial frequencies & had them adjust the contrast of the gratings until they could be detected |
| Contrast Threshold | smallest amount of contrast required to detect a pattern; eg. For a 1-cycle/degree grating to be distinguishable from uniform gray, the dark stripes must be 1% darker than the light stripes |
| Retinal ganglion cells | tuned to spatial frequency: each cell responds best to a specific spatial frequency that matches its receptive field size & it responds less to both higher & lower spatial frequencies |
| Response of a ganglion cell | depends on the phase of the grating |
| Lateral geniculate nucleus (LGN) | a structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells & has input & output connections to the visual cortex |
| magnocellular layers | neurons in the bottom 2 layers of the LGN which are physically larger than those in the top 4 layers; receives input from the M ganglion cells |
| parvocellular layers | neurons in the top 4 layers of the LGN, which are physically smaller than those in the bottom 2 layers; receives input from the P ganglion cells |
| magnocellular pathway | responds to large fast-moving objects |
| parvocellular pathway | is responsible for processing details of stationary targets |
| contralateral | refers to the opposite side of the body and/or brain |
| LGN | layers 1,4,6 of right LGN listen to the left eye (contralateral) |
| LGN | layers 2,3,5 of the right LGN receive input from the right eye (ipsilateral) |
| ipsilateral | refers to the same side of the body and/or brain |
| topographical mapping | orderly mapping of the world in the LGN & the visual cortex |
| primary visual cortex | area of the cerebral cortex of the brain that receives direct inputs from the LGN, as well as feedback from other brain areas, & is responsible for processing visual info. (also known as area 17 or striate cortex) has 6 major layers |
| Corticol Magnification | amount of corticol area (usually specified in milimeters) devoted to specific region (eg. 1 degree in the visual field) |
| Corticol Magnification 2 | objects imaged on or near the fovea are processed by neurons in large part of striate cortex, but objects imaged in the far right or left periphery are allocated only a tiny portion of the striate cortex |
| corticol representation of the fovea is greatly magnified compared to the corticol representation of peripheral vision | |
| consequence of corticol magnification | is that visual acuity declines in an orderly fashion w/eccentricity (distance from the fovea) |
| orientation tuning | tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others |
| filter | acoustic, electrical, electronic, or optical device, instrument, computer program, or neuron that allows the passage of some frequencies or digital elements & blocks the passage of others |
| ocular dominance | 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 |
| simple cells: phase-sensitive | a corticol neuron w/ CLEARLY DEFINED excitatory & inhibitory regions; |
| complex cell: phase insensitive | neuron whose receptive field characteristics cannot be easily predicted by mapping w/spots of light; attuned to particular orientation & spatial frequency & shows ocular preference |
| end stopping: play an imp. role in human ability to detect luminance boundaries & discontinuities | process by which a cell in a cortex 1st INCREASES its firing rate as the bar length increases to fill up its receptive field, & then DECREASES its firing rate as the bar is lengthened further- subclass of simple & complex cells |
| column | vertical arrangement of neurons in the striate cortex, arranged by having similar orientation preferences |
| hypercolumn: eg. hypercolumn in part of cortex that represents fovea may "see" portion of visual field that is 0.05 degrees of visual angle across | 1-mm block of striate cortex containing two sets of columns, each covering every possible orientation (0-180 degrees), w/1 set preferring input frm the left eye & 1 set preferring input from the right eye |
| cytochrome oxidase (CO) | enzyme used to reveal the regular array of “CO blobs,” which are spaced abt 0.5mm apart in the primary visual cortex |
| adaptation | reduction in response caused by prior or continuing stimulation (eg. Looking @ 20 degree lines then look @ 0 degree lines, they'll appear to be -20 degree b/c the neurons most sensitive to the adapting stimulus is fatigued |
| tilt aftereffect | perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation; strongly supports idea that the visual system contains individual neurons selective for different orientations |
| spatial frequency channel | pattern analyzer, implemented by an ensemble of corticol neurons, in which each set of neurons is tuned to a limited range of spatial frequencies (Campbell & Robson 1968) |
| strabismus | misalignment of the 2 eyes such that a single object in space is imaged on the fovea of 1 eye, & on a nonfoveal area of the other (turned) eye |
| amblyopia | developmental disorder, characterized by reduced spatial vision in an otherwise healthy eye even w/ proper correction for refractive error. Often referred to as “lazy eye” |
| Hubel & Wiesel | neurons in the striate cortex respond to stripes not stars. Receptive fields of the striate cortex neurons not circular (i.e in retina or LGN); they are elongated & respond more vigorously to bars, lines, edges, & gratings than to round circles of light |
| more cells in the striate cortex are responsive to horizontal & vertical orientations than to obliques | |
| Neurons that share the same eye preference also have a columnar arrangement; eye preference switches every 0.5mm or so | |
| cytochrome oxidase (CO) "blobs" | spaced @ 0.5 mm apart, CO blobs have been implicated in processing colour w/interblob regions & processing motion & spatial structure |
| info. frm 2 eyes kept completely separate in retinas and the 2 LGNs; no single neuron receives input frm both eyes until the striate cortex | transfer of adaptation effects from 1 eye to another implies that selective adaptation occurs in corticol neurons |
| multiple spatial frequency model of vision | implies that spatial frequencies that stimulate different pattern analyzers will be detected independently, even if different frequencies are combined in the same image |
| figure 3.30 p. 74 | images show that low frequencies emphasize the broad outlines of the face & high frequencies carry info. abt fine details |
| Robert Fantz research on vision in infants figure 3.32 (a) p.75 | forced-choice preferential-looking stimuli & the experimental setup |
| (b) | visual evoked potential (VEP)setup |
| (c) | results of a sweep VEP experiment in which the spatial frequency of the stimulus is swept (continuously varied frm low to high spatial frequency), illustrating the extrapolated acuity |
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Ugly.Beauty
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