Chapter 3- Spatial Vision: From Spots to Stripes Flashcards
Contrast
The difference in luminance between an object and the background, or between lighter and darker parts of the same object.
Acuity
The smallest spatial detail that can be resolved at 100% contrast.
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.
Sine Wave Grating
A grating with a sinusoidal luminance profile.
- The light intensity in such gratings varies smoothly and continuously across each cycle.
Vertical Meridian Asymmetry
Visual acuity in peripheral vision is not uniform- it falls off more rapidly along the vertical midline of the visual field than along the horizonal midline. We also have better acuity a fixed distance below the midline of the visual field than above.
Visual Acuity
Distance at which a person can just identify the letters/ Distance at which a person with “normal” vision can just identify the letters.
Amblyopia
A developmental disorder characterized by reduced spatial vision in an otherwise healthy eye, even with proper correction for refractive error. Also known as lazy eye.
Minimum Visible Acuity
Refers to the smallest object that one can detect.
What are the types of acuity?
1) Minimum Visible Acuity
2) Minimum Resolvable Acuity
3) Minimum Recognizable Acuity
4) Minimum Discriminable Acuity
Minimum Resolvable Acuity
Refers to the smallest angular separation between neighboring objects that one can resolve.
Minimum Recognizable Acuity
Refers to the angular size of the smallest feature that one can recognize or identify.
Minimum Discriminable Acuity
Refers to the angular size of the smallest change in a feature (e.g., a change in size, position, or orientation) that one can discriminate.
Spatial Frequency
The number of grating cycles (e.g., changes in light and dark) per unit of visual angle (usually specified in degree) in a given unit of space.
Cycles per Degree
The number of grating cycles per degree of visual angle.
Contrast Sensitivity Function (CSF)
A function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.
Contrast Threshold
The smallest amount of contrast required to direct a pattern.
Fourier Analysis
A mathematical procedure by which any signal can be separated into component sine waves at different frequencies. Combining the sine waves (Fourier synthesis) will reproduce the original signal.
Phase
The position of grating relative to a fixed position measured in degrees, where one complete cycle is 360 degrees.
Filter
An acoustic, electric, electronic, or optical device, instrument, computer program, or neuron that allows the passage of some range of parameters (e.g., orientations, frequencies( and blocks the passage of others.
Lateral Geniculate Nucleus (LGN)
A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.
Magnocellular Layer
Either of the bottom two neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically larger than those in the top four layers.
Parvocellular Layer
Any of the top four neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically smaller than those in the bottom two layers.
Koniocellular Layer
A neuron located between the magnocellular and parvocellular layers of the lateral geniculate nucleus. This layer is known as the koniocellular layer.
Contralateral
Referring to the opposite side of the body or brain.
Ipsilateral
Referring to the same side of the body or brain.
Topographical Mapping
The orderly mapping of the world in the lateral geniculate nucleus and the visual cortex.
Primary Visual Cortex, (VI), Area 17, or Striate Cortex
The area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus, as well as feedback from other brain areas.
Cortical Magnification
The amount of cortical area (usually specified in millimeters) developed to a specific region (e.g., 1 degree) in the visual field.
Eccentricity
The angular distance from the fovea (the region of highest visual acuity).
Perceptual Consequences of Cortical Magnification
1) Visual acuity declines in an orderly fashion with eccentricity.
Why is the foveal representation in the cortex so highly magnified?
The visual system must make a trade-off. High resolution requires a great number of resources: a dense array of photoreceptors, one-to-one lines from photoreceptors to retinal ganglion cells, and a large chunk of striate cortex. Thus, we have evolved a visual system that provides high resolution in the center and lower resolution in the periphery.
Visual Crowding
The deleterious effect of clutter on peripheral object recognition.
What was Hubel and Wiesel’s most fundamental discovery?
Discovered that the receptive fields of striate cortex neurons are not circular, as they are in the retina and LGN. Rather, they are elongated. As a result, they respond much more vigorously to bars, lines, edges, and gratings than to round spots of light.
Orientation Tuning
The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others.
How are the circular receptive fields in the LGN transformed into the elongated receptive fields in the striate cortex?
… studies have shown that the arrangement of LGN inputs is indeed crucial for establishing the orientation selectivity of striate cortex cells. However, other evidence suggests that neural interactions (e.g., lateral inhabitation) within the cortex also play an important role in the dynamics of orientation tuning.
Ocular Dominance
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.
Simple Cells
A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions.
Complex Cells
A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions.
End-Stopping
The process by which a cell in the cortex increases its firing rate as the length of a bar increases until the bar fills up its receptive field, and then it decreases its firing rate as the bar is lengthened further.
Columns
A vertical arrangement of neurons. Neurons within a single column tend to have similar receptive fields and similar orientation preferences.
Hypercolumn
A 1-millimeter block of striate cortex containing two sets of columns, each covering every possible orientation (0-180 degrees), with one set preferring input form the left eye and one set preferring input from the right eye.
CO Blobs
Regular arrays of “blobs” spaced about 0.5 millimeter apart in the striate cortex (VI), so named because their presence is visualized by staining with the enzyme cytochrome oxidase (CO). They may function in color perception.
Adaptation
A reduction in response caused by prior or continuing stimulation.
Tilt Aftereffect
The perceptual illusion of tilt, produced by adaptation to a pattern of given orientation.
What does the transfer effect of adaptation effects imply?
Implies that selective adaptation occurs in cortical neurons…
Spatial-Frequency Channels
A pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies.
What does the multiple spatial-frequency model of vision imply?
Implies that spatial frequencies that stimulate different pattern analyzers will be detected independently, even if the different frequencies are combined in the same image.
Why would the visual system use spatial frequency filters to analyze images?
One important reason may be that different spatial frequencies emphasize different types of information.
Critical Period
A phase in the life span during which abnormal early experience can alter normal neuronal development.
Strabismus
A misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and on a nonfoveal area of the other (turned) eye.
Anisometropia
A condition in which the two eyes have different refractive errors (e.g., one eye is farsighted and the other not).
What limits the development of acuity and contrast sensitivity?
The primary postnatal changes in the retina concern differentiation of the macular region. (See page 90)
