Visual System


In the human visual system, signals travel from the retina to the lateral geniculate nucleus (LGN) to the primary visual cortex. In the retina, photoreceptors transduce light into electrical signals that are processed by the local neurons of the retina, which in turn provide input to the retinal ganglion cells, the output neurons of the retina. There are many types of retinal ganglion cells, including those sensitive to colour and form and motion and change, and retinal ganglion cells generally have receptive fields with a centreā€surround organisation. The LGN acts as a relay station between the retina and primary visual cortex. In the cortex, the luminance sensitive simple and complex cells respond to oriented bars or edges. Simple cells respond to properly oriented bars of light at particular locations, while complex cells respond to properly oriented bars at any location within their receptive fields. Double opponent cells in primary visual cortex allow the visual system to sense colour contrasts. Primary visual cortex has a complex horizontal organisation with overlapping maps of visual topography, ocular dominance and orientation tuning.

Key Concepts

  • A receptive field refers to the region of visual space in which stimuli elicit a response from the cell.
  • The retina is a laminated structure at the back of the eye that transmits visual information to the brain.
  • The retina contains two types of photoreceptors, called rods and cones, that are responsible for converting light into electrical signals.
  • Retinal ganglion cells are the output cells of the retina that send information to the lateral geniculate nucleus of the thalamus.
  • The lateral geniculate nucleus is a laminated structure divided into layers based on eye of origin and cellular properties.
  • Neurons in the primary visual cortex exhibit selectivity for stimulus orientation.
  • Doubleā€opponent cells in the primary visual cortex respond to colour patterns and contribute to colour perception.
  • The mammalian visual pathway is organised retinotopically so that spatial representation of the visual field is preserved.
  • The primary visual cortex contributes to the initial processing of all visual information that is essential for visual perception.

Keywords: vision; retina; lateral geniculate nucleus; primary visual cortex; early visual system

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 cross over so that axons representing the right half of visual space travel to the left lateral geniculate nucleus (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 radiation and project to primary visual cortex (V1), where LGN axons representing each eye ramify in an alternating fashion.
Figure 2. (a) The major cell types in the retina and their laminar organization. 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 panel b. Light in the centre hyperpolarizes 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. Based on Werblin, F.S. and Dowling, J.E. 1969 Organization of the retina of the mudpuppy, Necturus maculosus. II. Intracellular recording, Journal of Neurophysiology 32:339–355.
Figure 3. The lateral geniculate nucleus (LGN) of the rhesus monkey and selected synaptic connections in primary visual cortex (V1). Midget cells of the retina innervate layers 3–6 of the LGN, and the parasol cells innervate layers 1–2. Koniocellular cells innervate K1–K6 but are also diffusely present throughout the entire LGN. 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 lateral geniculate nucleus. Based on Casagrande, V.A. and Kaas, J.H. 1994. The afferent, intrinsic, and efferent connections of primary visual cortex in primates. In: Cerebral Cortex, Vol. 10, Primary Visual Cortex of Primates. Peters, A. and Rockland, K., Eds. Plenum Press, N.Y., pp. 201–259.
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 panel a responds to a bar rotated 45 degrees 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 in panel b. (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). (e) In Hubel and Wiesel's model for the formation of simple and complex receptive field properties, 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. (f) In this same model, 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. Note 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 panel a and the simple cells in panel b.
Figure 5. Horizontal organization of visual cortex. (a) The topographic projection of visual space in the right visual hemifield onto an idealized 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) Shows 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, while 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) Shows the same area of cortex as panel 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) From Gary G. Blasdel and Guy Salama, “Voltage‐sensitive dyes reveal a modular organization in monkey striate cortex”, Nature volume 321, pages 579–585 (1986). Reproduced with permission of 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.

Casagrande VA and Kaas JH (1994) The afferent, intrinsic, and efferent connections of primary visual cortex in primates. In: Peters A and Rockland K (eds) Cerebral Cortex, Vol. 10, Primary Visual Cortex of Primates, pp 201–259. Plenum Press: New York.

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

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. Thieme: Stuttgart.

Hubel DH (1995) Eye, Brain, and Vision. Scientific American Library: New York, NY.

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

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

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

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Stacy, Andrea K, Nelson, Sacha B, and Van Hooser, Stephen D(Jul 2020) Visual System. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000230.pub3]