Ch. 3: Visual Perception

Acuity

The ability to see fine detail

Area V1

The site on the occipital lobe where axons from the lateral geniculate nucleus first reach the cerebral cortex. This site is (for one neural pathway) the location at which information about the visual world first reaches the brain

Binding problem

The problem of reuniting the various elements of a scene, given that these elements are initially dealt with by different systems in the brain

Binocular disparity

A distance cue based on the differences between the two eyes’ views of the world. This difference becomes less pronounced the farther away an object is from the observer

Bipolar cells

A type of neuron in the eye. Bipolar cells receive their input from the photoreceptors and transmit their output to the retinal ganglion cells

Brightness constancy

The achievement of perceiving the constant brightness of objects despite changes in the light reaching the eye that result from variations in illumination

Center-surround cells

A type of neuron in the visual system that has a “donut-shaped” receptive field. Stimulation in the center of the receptive field has one effect on the cell; stimulation in the surrounding ring has the opposite effect

Cones

Photoreceptors that are able to discriminate hues and that have high acuity. Cones are concentrated in the retina’s fovea and become less frequent in the visual periphery. Often contrasted with rods

Conjunction error

An error in perception in which a person correctly perceives what features are present but misperceives how the features are joined, so that (for example) a red circle and a green square might be misperceived as a red square and a green circle

Cornea

The transparent tissue at the front of each eye that plays an important role in focusing the incoming light

Distance cues

Information available to the perceiver that allows the perceiver to judge how far off a target object is

Edge enhancement

A process created by lateral inhibition in which the neurons in the visual system give exaggerated responses to edges of surfaces

Figure/ground organization

The processing step in which the perceiver determines which aspects of the stimulus belong to the central object (or “figure”) and which aspects belong to the background (or “ground”).

Fovea

The center of the retina and the region on the eye in which acuity is best; when a person looks at an object, they are lining up that object with the fovea

Ganglion cells

A type of neuron in the eye. The ganglion cells receive their input from the bipolar cells, and then the axons of the ganglion cellsgather together to from the optic nerve, carrying information back to the lateral geniculate nucleus

Gestalt principles

A small number of rules that seem to govern how observers organize the visual input, grouping some elements together but perceiving other elements to be indendent of one another

Interposition

A monocular distance cue that relies on the fact that objects farhter away are blocked from view by closer objects that happen to be in the viewer’s line of sight

Lateral geniculate nucleus (LGN)

An important way station in the thalamus that is the first destination for visual information sent from the eyeball to the brain

Lateral inhibition

A pattern in which cells, when stimulated, inhibit the activity of neighboring cells. In the visual system, lateral inhibition in the optic nerve creates edge enhancement

Lens

The transparent tissue located near the front of each eye that (together with the cornea) plays an important role in focusing incoming light. Muscles control the degree of curvature of the lens, allowing the eye to form a sharp image on the retina

Linear perspective

A cue for distance based on the fact that parallel lines seem to converge as they get farther away from the viewer

Mach band

A type of illusion in which one perceives a region to be slightly darker if it is adjacent to a bright region, and also perceives a region to be slightly brighter if it is adjecent to a dark region. This illusion, created by lateral inhibition, contributes to edge enhancement

Monocular distance cues

Features of the visual stimulus that indicate distance even if the stimulus is viewed with only one eye

Motion parallax

A distance cue based on the fact that as an observer moves, the retinal images of nearby objects move more rapidly than do the retinal images of objects farther away

Necker cube

One of the classic reversible (or ambigious) figures; the figure is a two-dimensional drawing that can be perceived as a cube viewed from above or as a cube viewed from below

Neural synchrony

A pattern of firing by neurons in which neurons in one brain area fire at the same time as neurons in another area; the brain seems to use this pattern as an indication that the neurons in different areas are firing in response to the same stimulus

Optic flow

The pattern of change in retinal image in which the image grows larger as the viewer approaches an object and shrinks as the viewer retreats from it

Optic nerve

The bundle of nerve fibers, formed from the retina’s ganglion cells, that carries information from the eyeball to the brain

Parallel processing

A system in which many steps are going on at the same time. Usually contrasted with serial processing

Perceptual constancy

The acheivement of perceiving the constant properties of objects in the world (e.g., their size, shape, and color) despite changes in the sensory information we receive that are caused by changes in our viewing circumstances

Photoreceptors

Cells on the retina that are sensitive to light and that respond (i.e., send a signal to adjacent cells) when they are stimulated by light

Pictorial cues

Patterns that can be represented on a flat surface to create the sense of a three-dimensional object or scene

Receptive field

The portion of the visual field to which a cell within the visual system responds. If the appropriately shaped stimulus appears int he appropriate position, the cell’s firing rate will change. The firing rate will not change if the stimulus is of the wrong form or is in the wrong position

Retina

The light-sensitive tissue that lines the back of the eyeball

Reversible (or ambigious) figure

Drawings that can be readily perceived in more than one way. Classic examples include the vase/profiles, the duck/rabbit, and the Necker cube

Rods

Photoreceptors that are sensitive to very low light levels but that are unable to discriminate hues and that have relatively poor acuity. Often contrasted with cones

Serial processing

A system in which only one step happens at a time (and so the steps occur in a series). Usually contrasted with parallel processing

Shape constancy

The achievement of perceiving the constant shape of objects despite changes in the shape of the retinal image that result from variations in viewing angle

Single-cell recording

A technique for recording the moment-by-moment activation level of an individual neuron within a healthy, normally functioning brain

Size constancy

The achievement of perceiving the constant size of objects despite changes in the size of the retinal image that result from variations in viewing distance

Unconscious interference

The hypothesized steps that perceivers follow in order to take one aspect of the visual scene (e.g., viewing distance) into account in judging another aspect (e.g., size).

Visual features

The elements of a visual pattern—veritical lines, curves, diagonals, and so on—that, together, form the overall pattern

What system

The system of visual circuits and pathways leading from the visual cortex to the temporal lobe and especially involved in object recognition. Often contrasted with the where system

Where system

The system of visual circuits and pathways leading from the visual cortex to the parietal lobe and especially involved in the spatial localization of objects and in the coordination of movements. Often contrasted with the what system