Neuronal Pathways of Vision - Vision with Direction - page 9

further here; see [41, 223]. We do this to limit the scope but also because this path to
the visual cortex is much less studied than the one passing through the LGN.
1.5 Lateral Geniculate Nucleus (LGN)
The lateral geniculate 4 nucleus (LGN) is a laminated structure in the thalamus. Its
inputs are received from the ganglion cells coming from each eye (Fig. 1.4). The
input to the layers of LGN is organized in an orderly fashion, but the different eyes
remain segregated. That is there are no LGN cells that react to both eyes, and each
layer contains cells that respond to stimuli from a single eye. The left eye (L) and the
right (R) eye inputs interlace when passing from one layer to the next, as the figure
illustrates. Being R,L,L,R,L,R for the left LGN, the left-right alternation reverses
between layers 2 and 3 for reasons that are not well understood. Layer 1 starts with
the inputs coming from the eye on the other side of the LGN, the so called contralat-
eral 5 eye, so that for the right eye the sequence is L,R,R,L,R,L. Each LGN receives
signals representing a visual field corresponding to the side opposite their own, that
is a contralateral view . Accordingly, the left and right LGNs cope only with, the
right and left visual fields, respectively.
Like nearly all of the neural visual signal processing structures, LGN also has a
topographic organization . This implies a continuity (in the mathematical sense) of
the mapping between the retina and the LGN, i.e., the responses of ganglion cells
that are close to each other feed into LGN cells that are located close to each other. 6
The small ganglion cells (midget cells) project to the cells found in the parvocel-
lular layers of LGN. In Fig. 1.4 the parvocellular cells occupy the layers 3-6. The
larger cells (parasol cells) project onto the magnocellular layers of the LGN, layers
1-2 of the figure. The koniocellular outputs project onto the layers K1-K6. The ko-
niocellular cells, which are a type of cells found among the retinal ganglion cells,
have also been found scattered in the entire LGN. Besides the bottom-up feeding
from ganglion cells, the LGN receives significant direct and indirect feedback from
the V1 area, to be discussed in Sect. 1.6. The feedback signals can radically influence
the visual signal processing in LGN as well as in the rest of the brain. Yet the func-
tional details of these connections are not well understood. Experiments on LGN
cells have shown that they are functionally similar to those of the retinal ganglion
cells that feed into them. Accordingly, the LGN is frequently qualified as a relay
station between the retina and visual cortex, and its cells are also called relay cells .
The outputs from LGN cells form a wide band called optic radiations and travel to
the primary visual cortex (Fig. 1.1).
4 Geniculate means kneelike, describing its appearance.
5 The terms contralateral and ipsilateral are frequently used in neurobiology. They mean,
respectively, the “other” and the “same” in relation to the current side.
6 Retrospectively, even the ganglion cells are topographically organized in the retina because
these are placed “behind” the photoreceptors from which they receive their inputs.