The Mind's Machine: Chapter 7

accommodation

The process by which the ciliary muscles adjust the lens to focus a sharp image on the retina.

amacrine cell

A specialized retinal cell that contacts both bipolar cells and ganglion cells and is especially significant in inhibitory interactions within the retina. Compare horizontal cell.

Amblyopia

Reduced visual acuity that is not caused by optical or retinal impairments.

Binocular

Referring to two-eyed processes.

bipolar cell

An interneuron in the retina that receives information from rods and cones and passes the information to retinal ganglion cells. See Figure 7.3. Compare amacrine cell and horizontal cell.

blind spot

The portion of the visual field from which light falls on the optic disc.

Blindsight

The paradoxical phenomenon whereby, within a scotoma, a person cannot consciously perceive visual cues but may still be able to make some visual discrimination.

Brightness

One of three basic dimensions of light perception, varying from dark to light. Compare hue and saturation.

ciliary muscle

One of the muscles that control the shape of the lens inside the eye, focusing an image on the retina. See Figure 7.1.

complex cortical cell

A cell in the visual cortex that responds best to a bar of a particular size and orientation anywhere within a particular area of the visual field and that needs movement to make it respond actively. Compare simple cortical cell.

Cone

Any of several classes of photoreceptor cells in the retina that are responsible for color vision. See Figure 7.3. Compare rod.

Convergence

The phenomenon of neural connections in which many cells send signals to a single cell. Compare divergence.

Cornea

The transparent outer layer of the eye, whose curvature is fixed. The cornea bends light rays and is primarily responsible for forming the image on the retina. See Figure 7.1.

extraocular muscle

One of the muscles attached to the eyeball that control its position and movements.

extrastriate cortex

Visual cortex outside of the primary visual (striate) cortex.

Fovea

The central portion of the retina, which is packed with the highest density of photoreceptors and is the center of our gaze. See Figure 7.1.

ganglion cell

Any of a class of cells in the retina whose axons form the optic nerve. See Figure 7.15. Compare amacrine cell, bipolar cell, and horizontal cell.

horizontal cell

A specialized retinal cell that contacts both photoreceptors and bipolar cells. Compare amacrine cell and ganglion cell.

Hue

One of three basic dimensions of light perception, varying through the spectrum from blue to red. Compare brightness and saturation.

iris (pl. irides)

The circular structure of the eye that provides an opening to form the pupil.

lateral geniculate nucleus (LGN)

The part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex.

lateral inhibition

The phenomenon by which interconnected neurons inhibit their neighbors, producing contrast at the edges of regions.

Lens

A structure in the eye that helps focus an image on the retina.

Myopia

Nearsightedness; the inability to focus the retinal image of objects that are far away.

occipital cortex

Also called visual cortex. Cortex of the occipital lobe of the brain, corresponding to the primary visual area of the cortex. See Figure 7.10.

off-center bipolar cell

A retinal bipolar cell that is inhibited by light in the center of its receptive field. See Figures 7.13, 7.14. Compare on-center bipolar cell.

off-center ganglion cell

A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell's receptive field. See Figures 7.13, 7.14. Compare on-center ganglion cell.

off-center/on-surround

Referring to a concentric receptive field in which stimulation of the center inhibits the cell of interest while stimulation of the surround excites it. See Figure 7.14. Compare on-center/off-surround.

on-center bipolar cell

A retinal bipolar cell that is excited by light in the center of its receptive field. See Figures 7.13 7.14. Compare off-center bipolar cell.

on-center ganglion cell

A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell's receptive field. See Figures 7.13, 7.14. Compare off-center ganglion cell.

on-center/off-surround

Referring to a concentric receptive field in which stimulation of the center excites the cell of interest while stimulation of the surround inhibits it. See Figure 7.14. Compare off-center/on-surround.

opponent-process hypothesis

A hypothesis of color perception stating that different systems produce opposite responses to light of different wavelengths. See Figures 7.23, 7.24.

optic ataxia

Spatial disorientation in which the patient is unable to accurately reach for objects using visual guidance.

optic chiasm

The point at which parts of the two optic nerves cross the midline. See Figure 7.10.

optic disc

The region of the retina that is devoid of receptor cells because ganglion cell axons and blood vessels exit the eyeball there. See Figure 7.7.

optic nerve

Cranial nerve II; the collection of ganglion cell axons that extend from the retina to the brain. See Figures 2.7, 7.10.

optic radiation

Axons from the lateral geniculate nucleus that terminate in the primary visual areas of the occipital cortex. See Figure 7.10.

optic tract

The axons of retinal ganglion cells after they have passed the optic chiasm. Most of these axons terminate in the lateral geniculate nucleus. See Figure 7.10.

photopic system

A system in the retina that operates at high levels of light, shows sensitivity to color, and involves the cones. See Table 7.1. Compare scotopic system.

photoreceptor adaptation

The tendency of rods and cones to adjust their light sensitivity to match current levels of illumination.

Photoreceptor

A neural cell in the retina that responds to light.

primary visual cortex (V1)

Also called striate cortex or area 17. The region of the occipital cortex where most visual information first arrives. See Figures 7.10, 7.11, 7.20.

pupil

The opening, formed by the iris, that allows light to enter the eye. See Figures 7.1, 7.6.

range fractionation

The means by which sensory systems cover a wide range of intensity values, as each sensory receptor cell specializes in just one part of the overall range of intensities.

receptive field

The stimulus region and features that affect the activity of a cell in a sensory system.

Refraction

The bending of light rays by a change in the density of a medium, such as the cornea and the lens of the eyes.

Retina

The receptive surface inside the eye that contains photoreceptors and other neurons. See Figures 7.1, 7.3.

Rhodopsin

The photopigment in rods that responds to light.

Rod

A photoreceptor cell in the retina that is most active at low levels of light. See Figure 7.3. Compare cone.

Saturation

One of three basic dimensions of light perception, varying from rich to pale. Compare brightness and hue.

Scotoma

A region of blindness within the visual fields, caused by injury to the visual pathway or brain.

scotopic system

A system in the retina that operates at low levels of light and involves the rods. See Table 7.1. Compare photopic system.

simple cortical cell

Also called bar detector or edge detector. A cell in the visual cortex that responds best to an edge or a bar that has a particular width, as well as a particular orientation and location in the visual field. Compare complex cortical cell.

spatial-frequency model

A model of vision that emphasizes the analysis of different spatial frequencies, of various orientations and in various parts of the visual field, as the basis of visual perception of form.

spectrally opponent cell

Also called color-opponent cell. A visual receptor neuron that has opposite firing responses to different regions of the spectrum. See Figures 7.26, 7.27.

topographic projection

A mapping that preserves the point-to-point correspondence between neighboring parts of space. For example, the retina extends a topographic projection onto the cortex.

Transduction

The conversion of one form of energy to another, as converting light into neuronal activity.

trichromatic hypothesis

A hypothesis of color perception stating that there are three different types of cones, each excited by a different region of the spectrum and each having a separate pathway to the brain.

visual acuity

Sharpness of vision.

visual field

The whole area that you can see without moving your head or eyes.

Wavelength

The length between two peaks in a repeated stimulus such as a wave, light, or sound.