Observing the World Around Us

Cornea

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

Lens

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

Retina

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

Photoreceptors

Cells on the retina that are sensitive to light and that respond when they are stimulated by light

Rods

Photoreceptors that are sensitive to very low light levels but that are unable to discriminate hues and that have relatively poor acuity

Cones

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

Acuity

The ability to see fine detail

Fovea

The centre 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 it

Bipolar Cells

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

Ganglion Cells

A type of neuron in the eye. They receive their input from the bipolar cells and then their axons gather together to form the optic nerve, carrying info back from the lateral geniculate nucleus

Lateral Geniculate Nucleus

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

Lateral Inhibition

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

Mach Bands

A type of illusion in which one perceives a region to be slightly darker if it’s adjacent to a bright region, and perceives a region to be brighter of it’s adjacent to a dark region. This illusion is created by paternal inhibition

Single-Cell Recording

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

Receptive Field

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

Center-Surround Cells

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

Area V1

The cite on the occipital lobe where axons from the lateral geniculate nucleus first reach the cerebral cortex. This site is the location at which info about the visual world first reaches the brain

Parallel Processing

A system in which many steps are going on at the same time

Serial Processing

A system in which only one step happens at a time

What System

The system of visual circuits and pathways leading from the visual cortex to the temporal lobe and especially involved in object recognition

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

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

Neural Synchrony

A pattern of firing by neurons in which neurons in one brain area fire at the same time and with the same rhythm 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

Conjunction Errors

An error in perception in which a person correctly perceives what features are present but misperceives how the features are joined

Necker Cube

One of the classic reversible figures; the figure is a 2-D drawing that can be perceived as a cube viewed from above or as a cube viewed from below

Reversible Figure

Drawings that can be readily perceived in more than one way

Figure/Ground Organization

The processing step in which the perceiver determines which aspects of the stimulus belong to the central object and which aspect belongs to the background

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 independent of one another

Visual Features

The elements of a visual pattern that together form the overall pattern

Perceptual Constancy

The achievement of perceiving the constant properties ob objects in the world despite changes in the sensory info we receive that are caused by changes in our viewing circumstances

Size Constancy

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

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

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

Unconscious Inference

The hypothesized steps that perceivers follow in order to take one aspect of the visual scene into account in judging another aspect

Distance Cues

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

Binocular Disparity

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

Monocular Distance Cues

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

Pictorial Cues

Pattens that can be represented on a flat surface to create the sense of a 3-D object or scene

Interposition

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

Linear Perspective

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

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 ob objects farther away