Neuronal Pathways of Vision - Vision with Direction - page 8

equip the brain with a rich set of signal processing tools, for, among others, color,
texture, motion, depth, and shape analysis, when the rest of the brain has no access to
the original signal. The exact qualities that establish each type and the role of these
are still debated. The most commonly discussed types are the small midget cells , and
the large parasol cells . There is a less-studied third type, frequently referred to when
discussing the lateral geniculate nucleus connections, the koniocelullar cells .
The midget cells are presumed to process high spatial frequency and color. They
have, accordingly, small receptive fields and total about 80% of all retinal ganglion
cells. The large majority of midget cells are color-opponent, being excited by red
in the center and inhibited by green in the surround, or vice versa. Parasol cells,
on the other hand, are mainly responsible for motion analysis. Being color indif-
ferent, they total about 10% of ganglion cells, and have larger receptive fields than
the midget cells. There are few parasol cells in the fovea. The ratio of parasol to
midget cells increases with eccentricity. Parasol cells are insensitive to colour, i.e.,
they are luminance-opponent. This is a general tendency; the receptive fields of gan-
glion cells increase with eccentricity. This means that bandpass filtering is achieved
at the level of retina. Accordingly, the number of ganglion cells decreases with ec-
centricity. Since ganglion cells are the only providers of signals to the brain, the
cerebral visual areas also follow such a spatial organization.
The koniocelullar cells are much fewer and more poorly understood than midget
and parasol cells. They are not as heterogenous as these either, although a few com-
mon properties have been identified. Their receptive fields lack surround and they are
color sensitive! In the center, they are excited by blue, whereas they are inhibited (in
the center) by red or green [104]. Presumably, they are involved in object/background
segregation.
1.4 The Optic Chiasm
The optic nerve is logically organized in two bundles of nerves, carrying visual sig-
nals responsible for the nasal and temporal views, respectively. The two optic nerves
coming from both eyes meet at the optic chiasm , where one bundle of each sort trav-
els farther towards the left and the right brain halves. The temporal retina bundle
crosses the midline, whereas the nasal retina bundle remains on the same side for
both eyes. The bundle pair leaving the chiasm is called the optic tract . Because of
the midline crossing arrangement of only the temporal retina outputs, the optical tract
that leaves the chiasm to travel to the left brain contains only visual signal carriers
that encode the patterns appearing on the right hemifield. Similarly, the one reach-
ing the right brain carries visual signals of the left hemifield. The optic tract travels
chiefly to reach the lateral geniculate nucleus, LGN to be discussed below. However,
some 10% of the connections in the bundle feed an area called superior colliculus, 3
(SC). From the SC there are outputs feeding the primary visual cortex at the back of
the brain, which we will discuss further below. By contrast, SC will not be discussed
3 This area is involved in visual signal processing controlling the eye movements.