Psychology 304: Brain and Behaviour Lecture 28

The Visual System 1. What are the major areas of the brain that are associated with the perception of light? 2. What features of the visual system facilitate edge perception?

What are the major areas of the brain that are associated with the perception of light? The thalamic neurons that receive visual information subsequently project the information to the primary visual cortex.

Primary Visual Cortex

Primary Visual Cortex

The primary visual cortex is organized into functional The primary visual cortex is organized into functional vertical columns. Information received by the primary visual cortex is segregated into distinct pathways that project to areas of the secondary visual cortex and, then, the association visual cortex. Two main pathways from the primary visual cortex have been identified: The dorsal stream and the ventral stream. The dorsal stream is associated with movement; the ventral stream is associated with colour and form.

The Dorsal and Ventral Streams

Secondary Visual Cortex

What features of the visual system facilitate edge perception? Three features of the visual system facilitate the perception of edges 1. Lateral Inhibition  The impact of lateral inhibition is illustrated by the following image:

Contrast Enhancement

. Contrast enhancement results from lateral inhibition  Contrast enhancement results from lateral inhibition among the receptors of the retina.  The axons of adjacent receptors form a lateral neural network.  Each receptor in the network fires at a rate proportional to the intensity of the light striking it. Moreover, when a given receptor fires, it inhibits the firing of adjacent cells.

. A receptor that fires rapidly produces greater  A receptor that fires rapidly produces greater inhibition of adjacent cells than a receptor that fires slowly.  The differential rates of firing and, hence, differential rates of lateral inhibition produced by receptors surrounding an edge account for contrast enhancement:

Lateral Inhibition and Contrast Enhancement

. Receptor D receives less lateral inhibition than  Receptor D receives less lateral inhibition than Receptors A, B, and C and, thus, fires more rapidly than these receptors.  Receptor E receives more lateral inhibition than Receptors F, G, and H and, thus, fires less rapidly than these receptors.

2. On-center and off-center cells of the retina- 2. On-center and off-center cells of the retina- geniculate-striate system  The neurons that comprise the retina-geniculate- striate system (i.e., retinal ganglion cells, lateral geniculate neurons, and neurons of lower layer IV of the primary visual cortex) are characterized by circular receptive fields.  These fields are comprised of an excitatory area and an inhibitory area separated by a circular boundary.

. On-center cells respond to illumination in the  On-center cells respond to illumination in the central region of its receptive field by firing more rapidly, and illumination in the peripheral region of its receptive field by firing less rapidly.  Off-center cells respond to illumination in the peripheral region of its receptive field by firing more rapidly, and illumination in the central region of its receptive field by firing less rapidly.

The Receptive Fields of On-Center and Off-Center Cells

. The firing of on-center and off-center cells is most  The firing of on-center and off-center cells is most affected by contrasting levels of illumination between the “on” and “off” regions of the receptive field.  Diffuse light of a constant intensity shone on both the on and off regions does not effect the firing rate of on-center and off-center cells.  Thus, on-center and off-center cells facilitate edge perception.

On-Center Cells, Off-Center Cells, and Edge Perception

3. Simple Cortical Cells  Comprise all layers of the primary visual cortex, except lower layer IV.  Characterized by rectangular receptive fields.  These fields are comprised of excitatory areas and inhibitory areas separated by straight lines.

The Receptive Fields of Simple Cortical Cells Light bar in dark field. Dark bar in light field. Straight edge between dark and light areas. The Receptive Fields of Simple Cortical Cells

. The firing of simple cortical cells is most affected  The firing of simple cortical cells is most affected by contrasting levels of illumination between the “on” and “off” regions of the receptive field.  Diffuse light of a constant intensity shone on both the on and off regions does not effect the firing rate of simple cortical cells.  Thus, simple cortical cells facilitate edge perception.

 Simple cortical cells are distinct from complex cortical cells.  Like simple cortical cells, complex cortical cells comprise all layers of the primary visual cortex, except lower layer IV. They are also found in the secondary visual cortex.

. Like simple cortical cells, complex cortical cells  Like simple cortical cells, complex cortical cells are characterized by rectangular receptive fields. However, these fields are larger and have no inhibitory areas; thus, the firing of complex cortical cells is affected by illumination in any region of the receptive field.  Complex cortical cells respond to movement of stimuli across the receptive field and, thus, are referred to as “movement detectors.”

The Visual System 1. What are the major areas of the brain that are associated with the perception of light? 2. What features of the visual system facilitate edge perception?