Perceptual Coherence : Hearing and Seeing

Transformation of Sensory Information Into Perceptual Information 43
At the Primary Visual Cortex
The next point in the visual pathways at which the receptive fields have
changed qualitatively is the primary visual cortex (area V1; the lateral
geniculate physically is the next step in the pathway, but the receptive fields
are similar to those at the retinal ganglion cells). Almost all the visual input
to higher visual cortical areas passes through V1. The number of neurons in
the primary visual cortex is 200 to 500 times greater than the number in the
lateral geniculate, demonstrating the remarkable explosion in the number
of visual cells. The receptive fields of cells in V1 typically respond to a
narrower range of stimuli than those in the retina and lateral geniculate
and often are simultaneously selective with respect to spatial position,
orientation, spatial and temporal frequency, contrast, direction of movement,
and color. There is a retinotopic map with the cells locked to eye
movements. Much of this pioneering work was performed by Hubel and
Weisel (1962, 1968), which resulted in their receiving the Nobel Prize
in 1981.
Most cells in the primary visual cortex respond strongly to flashing and
moving bars and gratings, but not to static patterns. Hubel and Weisel, using
flashing dots, identified two types of cells. For simple cells, one spatial
region responds either to the onset or offset of light. For complex cells, all
spatial regions respond both to the onset or offset of light; complex cells
signal change. In the past 10 years it has become clear that a complete
description of both simple and complex cortical cells must involve the
analysis of the receptive field over time. The simple and complex cells can
be split into two major classes. For separable cells, the spatial organization
of the receptive field can be analyzed into independent x and y orientations.
The receptive field may be static, but if it does transform, the on- and offregions
do not shift spatially over time (although the on- and off-regions
may reverse). Approximately 50–70% of cells in the primary visual cortex
are separable; the response is unaffected by the velocity and direction of
movement of the stimulus. For nonseparable cells, the spatial organization
of the receptive field cannot be analyzed into independent x and y orientations
because the receptive field transforms over time; the response is affected
by the velocity and direction of movement of the stimulus (e.g., a
bar of light or black-and-white grating). To represent both separable and
nonseparable cells, the x axis will depict the receptive field along the dimension
perpendicular to the preferred orientation (i.e., the horizontal dimension
for cells with a vertical orientation). If the receptive field does not
transform over time, the y axis will depict the receptive field parallel to the
preferred orientation. If the receptive field does transform over time, the y
axis will depict the time from a spike. (The receptive field parallel to the
preferred orientation is not displayed.)