Although the lines composing the image are parallel, they do not appear to be.
Calcarine Sulcus
Location of V1 in the human occipital lobe. The central visual field is represented in hte psoterior calcarine sulcus. The peripheral visual field is represented in the anterior portion of the calcarine sulus.
Canonical Viewpoint
A particular viewpoint of an object that people are more likely to imagine, quicker to name and more likely to classify as the best view of an object. Suggests that object representation may not be viewpoint independent.
Cataracts
Cataracts are opacifications of the crystalline lens of the eye,
causing a loss of transparency. The crystalline lens is the
"focussing" mechanism of the human eye. The change in light
transmission is due to accumulation of water and/or denaturation of
the lens protein. A variety of factors cause cataracts, eg diabetes,
eye trauma, age related changes.
Center-Surround
A type of spatial receptive field structure in which the central region
of the receptive field and the Surround portion influence cell activities
in different ways e.g. ON-center, OFF-Surround
Chromatic Adaptation Technique
Used to identify receptor inputs. An adapting light is chosen such that it affects the response to one of the inputs to a cell far more than the other. The cells response is selectively desensitized so that the cell is largely driven by the other input. This way the response properties of the cell can now be studied in relative isolation.
Chromostereopsis
The optics of the eye are such that short wavelengths are refracted
slightly more than long wavelengths creating a positional disparity
on the retina. When viewed by two eyes a stereoscopic disparity is
obtained. This is termed Chromostereopsis or Chromatic Stereopsis
Chevreul Illusion
Ciliary Ganglion
Contains postganglionic neurons that innervate the smooth muscles of the pupillary sphincter, mediate pupillary reflexes. Receives input from the accessory oculomotor nucleus.
(See Figure).
Closed-Loop Control
A control system in which the output is continuously modified by feedback from the environment.
Color Opponency
When a cell is excited by one part of the visible spectrum and inhibited by another. First observed in ganglion cells in the retina.
Cells in the primary visual cortex (V1) that responding either to an edge, a bar or a slit stimulus of a particular orientation falling anywhere in its receptive field. The exact location of the stimulus within the receptive field is not as critical. Have receptive fields longer than those of simple cells. The cell behaves non-linearly, responding to a drifting cosine grating with a uniform increase in activity with no sinusiodal modulcation (i.e. insensitive to phase). For a historical perspective see Simple Cells.
(See also Hypercomplex Cells and Full-Wave Rectification).
Concentric Field
A receptive field divided into an inner circular region and an outer ring-shaped region. Light falling in each of the two regions has opposite effects on the response of the cell.
Cone
Photoreceptor for day vision. Higher acuity and temporal resolution than rods. Mediates color vision. Highly concentrated in fovea. Graded responses (no action potential). There are three classes of cones, short, medium and long wavelength cones.
(See also Rod).
Cone Opsin
Light absorbing molecule in cone cells
Constancy, Brightness
Often mistakenly used for Lightness contrast. However there is evidence
that luminescent objects can be perceived to be of the same
luminance despite changes in ambient illumination or background.
Constancy, Color
Despite changes in wavelength (such as at sunset, or on cloudy days)
objects are perceived to maintain their surface color.
Constancy, Lightness
Objects are perceived to maintain a relatively constant surface lightness
despite changes in illumination (Type I) or background color (Type II).
Contrast
The ratio between the maximum and minimum intensities in a light pattern.
Simple Contrast
Weber Contrast
Michelson Contrast
Peak-to-Peak contrast. Used for gratings.
Contrast-Reversing Grating
A space-time harmonic function.
Contrast Sensitivity Function
The reciprocal of the threshold contrast required to obtain a criterion response from a cell or a human subject as a function of spatial frequency. Falls off in sensitivity as the spatial frequency of the test pattern increases. (See also
Modulation Transfer Function)
An hypothesis by Gilchrist in which the visual system does
not use retinal contrasts to determine relative lightnesses but uses
depth information to determine which regions are at the same
depth (coplanar) and uses their contrasts to deteremine the lightness
perceived. This hypothesis has come under recent attack (see Gilchrist, 1994
for more details)
Correspondence Problem
In Stereopsis
The challenge of matching elements in one eye with elements in another eye.
Also known as the Matching Problem
In Motion
The challenge of matching moving elements in one frame with elements in a succeeding frame (especially true of random dot cinematograms).
Corridor Illusion
The cylinders of equal size in a picture are perceive to be of different sizes when the lie in distinct positions within a rectangular grid whose spatial scale diminishes toward a fixation point on the horizon which create depth perspective cues. (See also Ponzo Illusion).
Corticaltectal Pathway
Pathway from the cortex to the superior colliculi.
Cortical Magnification Factor
(Daniel & Whitteridge 1961)
Defined as millimeters of cortex per degree of visual angle. Described by the following relationship:
Where E is the retinal eccentricty in degress, E2, is the eccentricity at which magnification has changed by a factor of 2, and M2 is the foveal magnification (6-13 mm/deg). Integrating gives an expression for the distance, D, from the cortical mapping of the foveal center to a point at eccentricity E:
CPD
Cycles per degree of visual angle. The human limit is about 30cpd in the fovea for an individual with 20/20 vision.
Craik-O'Brien Cornsweet Effect
Two identically luminant rectangles except for a luminance cusp near the border appear to have different luminance levels.
Critical Flicker Frequency (CFF)
The frequency of flicker where the alternating levels of brightness
are seen as a single level. The frequency is dependent on brightness
levels and retinal location.
Cyclopean Separation
Cytochrome Oxidase
(Wong-Riley)
Histochemical method. Indicator of functional state of neurons as brain derives energy from oxidase metabolism. Used in the discovery of blobs in the visual cortex.
