Visual System


Humans and many other animals obtain much of their information about the world through their eyes. Patterns of light are transformed into nerve impulses in the retina and visual information is processed by nerve cells in the primary visual cortex. In the human brain, about oneā€half of the cerebral cortex is dedicated in some way to the processing of visual information.

Key Concepts:

  • Humans and many other animals obtain much of their information about the world through their eyes.

  • In the human brain, about one half of the cerebral cortex is dedicated in some way to the processing of visual information.

  • Patterns of light are transformed into nerve impulses in the retina.

  • Visual information is processed by nerve cells in the primary visual cortex.

  • The human visual cortex exhibits functional organisation.

Keywords: vision; retina; lateral geniculate nucleus; primary visual cortex; LGN; eye; blobs; V1

Figure 1.

Anatomy of the visual system. Light arrives at the eye and is focused by the lens onto the retina, where photoreceptors transduce the light into electrical signals that are processed by local retinal neurons. Axons of retinal ganglion cells, the output cells of the retina, leave the retina in a bundle called the optic nerve. At the optic chiasm, some axons crossover so that axons representing the right half of visual space travel to the left LGN and axons representing the left half of visual space travel to the right LGN (not shown). In the LGN, the axons segregate into layers according to eye of origin and other properties. LGN relay cell axons form a band called the optic radiations and project to primary visual cortex, where LGN axons representing each eye ramify in an alternating fashion.

Figure 2.

(a) The major cell types in the retina and their laminar organisation. See the text for laminar abbreviations. (b) A centre‐surround retinal ganglion cell that responds to light in the centre of its receptive field and is inhibited by light in the surrounding region. The stimulus is shown on the left, and action potentials in the cell relative to the onset and offset of the stimulus are shown on the right. Note that the cell responds most vigorously to a light spot in the centre surrounded by a dark annulus. These properties are often denoted symbolically with the notation at bottom, with ‘+’ indicating a preference for more light relative to background and ‘–’ indicating less light. (c) Schematic of retinal circuitry that mediates the centre‐surround cell depicted in (b). Light in the centre hyperpolarises a cone, which excites a bipolar cell, which in turn excites the retinal ganglion cell. Horizontal cells mediate the effect of the surround, providing inhibition to the bipolar cell in the centre when there is light in the surround. This figure is adapted from Werblin and Dowling (), who studied the salamander Necturus maculosus, and similar circuitry has been found in other vertebrates.

Figure 3.

The LGN of the rhesus monkey. Midget cells of the retina innervate layers 3–6, and the parasol cells innervate layers 1–2. Koniocellular cells innervate K1–K6 but are also diffusely present throughout the entire LGN.

Figure 4.

(a) Many neurons in the primary visual cortex respond to bars or edges at a particular orientation. The stimulus is shown at the left, and action potentials in the cell relative to the onset and offset of the stimulus are shown at the right. The neuron in (a) responds to a bar rotated 45° clockwise from vertical, but responds poorly to bars with other orientations. (b) Simple cells have separate regions of their receptive fields that respond to light increments and light decrements and thus respond to bars at specific locations. One example of such a receptive field pattern is shown. (c) In contrast to simple cells, complex cells respond to a properly oriented bar anywhere in their receptive fields. (d) Many cells in V1 respond to moving oriented bars. The arrows in the stimulus (left) indicate direction of bar movement. Most cells in V1 are orientation selective and not direction selective, responding to movement in both directions (top), but some cells are direction selective and only respond to bars moving in a particular direction (bottom).

Figure 5.

Hubel and Wiesel's model for the formation of simple and complex receptive field properties. (a) A simple cell (at right) preferring a dark oriented bar on a light background could receive input from several adjacent centre‐surround cells that respond to darkness in their receptive field centres. (b) A complex cell (at right) could obtain its indifference to bar position by receiving input from several adjacent simple cells (at left) sharing one orientation preference. Notice that the complex cell's receptive field properties cannot be represented in the same form as the schematics for the LGN cells and simple cell in (a) and the simple cells in (b).

Figure 6.

Selected synaptic connections in primary visual cortex. In each ocular dominance band, parvocellular LGN neurons from one eye project to cortical layer 4B, magnocellular LGN neurons project to layer 4A and koniocellular LGN neurons project to the CO blobs in layers 1–3. Neurons in layer 4 make connections with neurons in layers 2–3, both in the blobs and in between the blobs, and these cells in turn project to layers 5–6. Cells in layers 2, 3 and 5 make connections with adjacent cortical areas, and cells in layer 5 also make connections with subcortical structures such as the superior colliculus. Finally, cells in layer 6 project back to the LGN. Adapted from Casagrande and Kaas ().

Figure 7.

Horizontal organisation of visual cortex. (a) The topographic projection of visual space in the right visual hemifield onto an idealised, unfolded primary visual cortex (adapted from Hubel, ). Note the large representation of the central region in V1, and the relatively small representation of the periphery. Within this topographic map is an alternating map of input from the two eyes. (b) A small section of V1 imaged optically using voltage sensitive dyes by Blasdel and colleagues. Regions that respond to visual stimulation of the left eye are coloured black, whereas regions that respond to stimulation of the right eye are white. Woven into the topographic map and ocular dominance bands is a semi‐regular map of orientation preference. (c) The same area of cortex as (b), except that the eyes are being stimulated with bars of different orientation. Each pixel in the image is colour coded according to the bar orientation that evoked the largest response (see legend at right). For example, red regions in the image showed greatest activation by horizontal bars. (b) and (c) are reproduced from Blasdel and Salama (). © Nature Publishing Group.



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Further Reading

Burns ME and Baylor DA (2001) Activation, deactivation, and adaptation in vertebrate photoreceptor cells. Annual Review of Neuroscience 24: 779–805.

Dowling JE (1987) The Retina: An Approachable Part of the Brain. Cambridge: Harvard University Press.

Ferster D and Miller KD (2000) Neural mechanisms of orientation selectivity in the visual cortex. Annual Reviews of Neuroscience 23: 441–471.

Hassler R (1966) Comparative anatomy of the central visual systems in day‐ and night‐active primates. In: Hassler R and Stephen H (eds) Evolution of the Forebrain, pp. 419–434. Stuttgart: Thieme.

Leventhal AG (1991) The Neural Basis of Visual Function. London: Macmillan Press Ltd.

McIlwain JT (1996) An Introduction to the Biology of Vision. Cambridge: Cambridge University Press.

Wandell BA (1995) Foundations of Vision. Sunderland, MA: Sinauer Associates.

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Van Hooser, Stephen D, and Nelson, Sacha B(Nov 2014) Visual System. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000230.pub2]