Psych 129 - Sensory processes
The outputs of the retina (ganglion cell axons) target two main structures
in the brain: the lateral geniculate nucleus of the thalamus (or LGN),
and the superior colliculus. The LGN is the main conduit to the visual
cortex and conscious visual perception. The superior colliculus is involved
in guiding eye movements and other automatic visuo-motor responses.
The outputs of the LGN terminate in the visual cortex (area V1). There
are about as many output fibers exiting the LGN as there are input fibers
from the retina. However, there are about ten times as many fibers descending
from the visual cortex to the LGN. The function of these feedback pathways
is largely unknown.
The receptive field properties of neurons in the LGN are largely similar
to those of ganglion cells in the retina. For this reason, the LGN has
been dubbed a "relay nucleus," although this is probably a vast understatement
of its actual function.
Primary visual cortex (area V1)
The visual cortex is just one part of the cerebral cortex, which is a two-dimensional
sheet of neurons about 2-3 mm thick that is highly convoluted. In macaque
monkeys, the visual cortex constitutes about 50% of the surface area of
the entire cerebral cortex. In humans, the fraction is about 20%. Structurally,
the cortex can be subdivided into six layers.
The primary visual cortex, or area V1 (also known as striate cortex), is
the primary target of fibers (axons) coming from the LGN. These fibers
terminate in layer IV.
Area V1 contains a topographic map of visual space, meaning that neighboring
neurons have receptive fields in neighboring parts of visual space. If
each neuron in V1 were to light up as it became active, the image of activity
you would see on the cortex would look something like a warped television
image of the world currently being viewed by a person.
Neurons in the primary visual cortex come in two major varieties: simple-cells
and complex cells. Both of these cells exhibit a property known as orientation
selectivity, meaning that they respond selectivity to luminance edges of
a particular orientation. Simple-cells are quite choosy about the position
of the edge, whereas complex-cells will respond to the edge as long as
it is in some general region of the image.
Neurons in the primary visual cortex also exhibit spatial-frequency selectivity,
meaning that they respond preferentially to luminance changes at different
spatial scales. One way of measuring this is to use a sinewave grating
as the stimulus and plot the cellÕs activity as a function of the
spatial-frequency of the grating.
The visual cortex contains a columnar organization, meaning that neurons
within a column perpendicular to the surface of the cortex have similar
response properties - i.e., similar position, orientation, and spatial-frequency