Cortical Processing of Visual Signals

Abstract

This chapter describes how visual information is processed in the cerebral cortex. Major events that precede cortical processing include (1) phototransduction by retinal photoreceptors, (2) the establishment of On and Off pathways and center/surround receptive fields, (3) the transmission of visual signals from the retina to the lateral geniculate nucleus (LGN), and (4) the transmission of visual signals from the LGN to the primary visual cortex. Although other pathways transmit visual information to the visual cortex, the retino-geniculo-cortical pathway is the major route for visual signal conduction to the cerebral cortex. The retino-geniculo-cortical pathway carries information about the brightness, color, and contrast of objects within a visual scene, as well as information about the timing and motion of these objects. If the human brain used only this rudimentary composition of information to knit together a representation of the surrounding world, people would not be capable of seeing and interpreting the complex scenes of daily life: cars rushing by while waiting to cross the street, a soccer ball flying toward a goal, or even these words you are reading. Humans’ ability to process and react to these highly complicated images is made possible by the properties of the visual cortex.

Glossary

Binocular disparity

Difference in the image viewed by the right and left eye due to the space separating an animal’s two eyes

Blobs

Functional structures in layers 2 and 3 in V1 that contain color selective cells

Complex cell

One of two main cell types found in the visual cortex, orientation selective cells that have nonlinear receptive fields

Cortical column

An area of cortex spanning multiple layers and containing cells with a shared property, e.g., orientation

Dorsal pathway

One of the two main pathways in extrastriate cortex, processes information about spatial navigation

FFA

Fusiform face area, contains cells selective for faces

IT

Inferior temporal cortex, contains FFA

K

Koniocellular

LGN

Lateral geniculate nucleus

M

Magnocellular

MST

Medial superior temporal area

MT

Middle temporal area

ODC

Ocular dominance column

P

Parvocellular

PMLS

Posteromedial lateral suprasylvian area

Receptive field

An area of visual space to which a neuron responds

Retinotopy

The organization of cells in the retina such that adjacent cells in the retina have adjacent receptive fields in visual space

Simple cell

One of two main cell types found in the visual cortex, orientation selective cells that have linear receptive fields

Spatial frequency

Describes the spacing between identical portions of a periodic stimulus

Temporal frequency

Describes the velocity at which a periodic stimulus moves

V1

Primary visual cortex

V2

Secondary visual cortex

Ventral pathway

One of the two main pathways in extrastriate cortex, processes information about object recognition

Visuotopy

The organization of cells in the visual cortex such that adjacent cells have receptive fields in the same area of visual space