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Figure 4.6
Receptive fields in the retina. Visual cells’ receptive fields in the retina
are often circular with a center-surround arrangement, so that light
striking the center of the field produces the opposite result of light
striking the surround. In the receptive field depicted here, light in the
center produces increased firing in the visual cell, whereas light in the
surround produces decreased firing; the arrangement in other
receptive fields may be just the opposite.
Figure 4.7
Visual pathways to the brain.
(a) Input from the right half of the
visual field strikes the left side of each
retina and is transmitted to the left
hemisphere (shown in red). Input from
the left half of the visual field strikes
the right side of each retina and is transmitted to the right hemisphere (shown in
green). The nerve fibers from each eye meet at the optic chiasm, where fibers
from the inside half of each retina cross over to the opposite side of the brain.
After reaching the optic chiasm, the major visual pathway projects through the
lateral geniculate nucleus in the thalamus and onto the primary visual cortex
(shown with solid lines). A second pathway detours through the superior colliculus
and then projects through another area of the thalamus and onto slightly different
areas of the primary visual cortex (shown with dotted lines). (b) This inset shows
a vertical view of how the optic pathways project through the thalamus and onto
the visual cortex in the back of the brain [the two pathways mapped out in
Early 1960’s: Hubel and Wiesel
• Studied activity in primary visual cortex of
cats
– Identified 3 major types of cells in the visual
cortex
• Simple
• Complex
• Hypercomplex
• Feature Detectors
Figure 4.8
Hubel and Wiesel’s procedure for studying the activity of neurons in the
visual cortex. As the cat is shown various stimuli, a microelectrode records
the firing of a neuron in the cat’s visual cortex. The figure shows the
electrical responses of a simple cell apparently “programmed” to respond
to lines oriented vertically.
Sensory organs and brain are the
most sophisticated signalprocessing devices known to man
Joseph Fourier and his most
remarkable theorem
• Proposed in 1807 that any function could
be decomposed into a sum of sinusoidal
elements when the functions exhibit
discontinuities
Figure 1.4. Left. The first 8 components of
the Fourier expansion of the square wave
shown in Figure 1.2. These 8 components
consists of a DC component and 7 sinusoids of
decreasing amplitude. Right. The waveform
obtained by summing successive components
of the Fourier expansion.