1 Introduction
1.1 The organization of the visual system
In primates, the path for visual information processing begins with light absorption in a dense mosaic of light-sensitive cells, the rod and the cone photoreceptors. The photoreceptors are in the deepest layers of the retina, next to the pigment epithelium, and the light must pass through rows of other, transparent cell layers (ganglion-, amacrine-, bipolar-, and horizontal-cells) before reaching the receptors (Figure 1.1). Functionally, the retina consists of these five main cell types, and in the retinal transmission of information the receptors are the first elements and ganglion cells are the last. The ganglion cells project to higher brain centers via their axons that form between 1-2 million nerve fibers. The optic nerve leaves the eye at about 17 deg to the nasal side of the fovea. These are the nerve bundles that form the connections between the ganglion cells in the retina and the cells in the lateral geniculate nucleus (LGN), the relay station in the pathway to higher brain centers.
1.1.2 Lateral Geniculate Nucleus (LGN)
The LGN constitutes the main relay station for visual information from retina to cortex.
Approximately 90% of the retinal ganglion cells project to the LGN, while the remainder connect to the superior colliculi and the pulvinar. The LGN in each thalamic hemisphere is distinguishable by its 6 distinct layers (Figure 1.1). The two ventral layers (layers 1 and 2) consist of relatively large magnocellular neurons, while the four dorsal layers (3 through 6) contain relatively small parvocellular neurons. In layers intercalated between the parvo- and magnocellular layers are some granular neurons, called koniocellular neurons. Because retinal fibers of the nasal retina cross over in the optic chiasm, each LGN receives visual information stemming from the contra-lateral visual field. For instance, the left LGN receives information from the left halves of each retina, which sample the right visual field. Each layer in LGN receives input from only one eye; layers 2, 3 and 5 from the eye on the same side as the LGN, and layers 1, 4 and 6 from the eye on the opposite side.
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Figure 1.1: The dominant visual pathways in primates
The left panel shows a schematic drawing of the pathway from the retina to the primary visual cortex (V1) through the dorsal lateral geniculate nucleus (LGN) of the thalamus. The right panels highlight the important anatomical structures. Light entering the eye passes through the ganglion cells and is imaged on the photoreceptor layer (rod photoreceptors, which are not active in color vision, are found between the cones).
Signals from photoreceptors pass through bipolar cells to ganglion cells, the axons of which form the optic nerve, which projects principally to the LGN. The horizontal and amacrine cell pathways within the retina allow spatial comparisons of cone signals. Ganglion cells from the temporal retina project to the ipsilateral LGN (red lines) and those from the nasal retina project to the contralateral LGN (green lines). Within the LGN, the projections from the two eyes are aligned, so the same topographic map (of the contralateral half of the visual field) is found in all layers. The axons of LGN neurons project almost exclusively to V1, where they terminate primarily in layer 4 and form ocular dominance columns (a small fraction of LGN cells project to extrastriate areas: see Sincich et al. (2004) and the references therein). The termination site within layer 4 depends on the layer in which the LGN neuron is found: parvocellular (PC) cells project mainly to layer 4Cβ, magnocellular (MC) to layer 4Cα, and koniocellular (KC) cells to layer 4A and lower layer 3. The shading depicts the distinct pattern that emerges when slices through V1 are stained for cytochrome oxidase (CO) activity. Reactivity is particularly high in layer 4 and in patches that dot the superficial layers 2 and 3. Source: Solomon & Lennie (2007).
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1.1.3 Visual cortex
An estimated 50-60% of the cortex is engaged in the processing of visual information. Thirty-two anatomically distinct visual areas have been identified in macaque cortex, of which 25 are primarily visual and 7 are involved in visually-guided motor control (Felleman & Essen, 1991; Merigan & Maunsell, 1993; Werner). The optic radiation forwards the signals from LGN towards the primary visual cortex (V1) (Figure 1.1), situated in the calcarine sulcus of the occipital lobe of primates, and from V1 the information is subsequently distributed across several visual areas. It is believed that visual information is segregated into two main cortical streams, “what” and “where” streams (Ungerleider & Mishkin, 1982; Goodale &
Milner, 1992), which seem to operate across distinct areas (Figure 1.2). The “what” stream (or temporal pathway) - traveling ventrally in the brain includes areas such as V2, VP, and V4 - is believed to be mainly associated with recognition of objects, a perceptual system, and thus with color-, pattern- and form-perception. The “where” stream (or parietal pathway) - encompassing mainly the dorsal cortical areas such as V3, medial temporal (MT) cortex, and parietal areas - is assumed to be mostly associated with spatio-temporal information processing.
Figure 1.2: Visual streams and areas
Location of select visual areas (left panel) and the major lobes (occipital, parietal, temporal, frontal) of the right hemisphere of macaque cortex (lateral view). Cortical area V3 cannot be seen from this view. The parietal and temporal pathways are shown in red and green dashed lines, respectively. The schematic on the right shows some of the major connecting circuits among visual areas that constitute the “where” and “what”
pathways. Areas V1, V2, V3, V4, ventral posterior (VP) and medial temporal (MT) are shown. Source: Modified after Werner J. S.; https://redwood.berkeley.edu/bruno/npb261b/werner-reading/Werner2.pdf.
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