Function of the Dorsal Lateral Geniculate Nucleus of the Thalamus

The optic nerve fibers of the new visual system terminate in the dorsal lateral geniculate nucleus, located at the dorsal end of the thalamus and also called simply the lateral geniculate body, as shown in Figure 51-1. The dorsal lateral geniculate nucleus serves two principal functions: First, it relays visual information from the optic tract to the visual cortex by way of the optic radiation (also called the geniculocalcarine tract). This relay function is so accurate that there is exact point-to-point transmission with a high degree of spatial fidelity all the way from the retina to the visual cortex.

It will be recalled that half the fibers in each optic tract after passing the optic chiasm are derived from one eye and half from the other eye, representing corresponding points on the two retinas. However, the signals from the two eyes are kept apart in the dorsal lateral geniculate nucleus. This nucleus is composed of six nuclear layers. Layers II, III, and V (from ventral to dorsal) receive signals from the lateral half of the ipsilateral retina, whereas layers I, IV, and VI receive signals from the medial half of the retina of the opposite eye. The respective retinal areas of the two eyes connect with neurons that are superimposed over

Lateral Geniculate Bodies
Principal visual pathways from the eyes to the visual cortex. (Modified from Polyak SL: The Retina. Chicago: University of Chicago, 1941.)

one another in the paired layers, and similar parallel transmission is preserved all the way to the visual cortex.

The second major function of the dorsal lateral geniculate nucleus is to "gate" the transmission of signals to the visual cortex—that is, to control how much of the signal is allowed to pass to the cortex. The nucleus receives gating control signals from two major sources: (1) corticofugal fibers returning in a backward direction from the primary visual cortex to the lateral geniculate nucleus, and (2) reticular areas of the mesencephalon. Both of these are inhibitory and, when stimulated, can turn off transmission through selected portions of the dorsal lateral geniculate nucleus. It is assumed that both of these gating circuits help highlight the visual information that is allowed to pass.

Finally, the dorsal lateral geniculate nucleus is divided in another way: (1) Layers I and II are called magnocellular layers because they contain large neurons. These receive their input almost entirely from the large type Y retinal ganglion cells. This magnocel-lular system provides a rapidly conducting pathway to the visual cortex. However, this system is color blind, transmitting only black-and-white information. Also, its point-to-point transmission is poor because there are not many Y ganglion cells, and their dendrites spread widely in the retina. (2) Layers III through VI are called parvocellular layers because they contain large numbers of small to medium-sized neurons. These neurons receive their input almost entirely from the type X retinal ganglion cells that transmit color and convey accurate point-to-point spatial information, but at only a moderate velocity of conduction rather than at high velocity.

Calcarine Cortex
Visual cortex in the calcarine fissure area of the medial occipital cortex.
Medial Visual Area

Transmission of visual signals from the primary visual cortex into secondary visual areas on the lateral surfaces of the occipital and parietal cortices. Note that the signals representing form, third-dimensional position, and motion are transmitted mainly into the superior portions of the occipital lobe and posterior portions of the parietal lobe. By contrast, the signals for visual detail and color are transmitted mainly into the anteroventral portion of the occipital lobe and the ventral portion of the posterior temporal lobe.

Transmission of visual signals from the primary visual cortex into secondary visual areas on the lateral surfaces of the occipital and parietal cortices. Note that the signals representing form, third-dimensional position, and motion are transmitted mainly into the superior portions of the occipital lobe and posterior portions of the parietal lobe. By contrast, the signals for visual detail and color are transmitted mainly into the anteroventral portion of the occipital lobe and the ventral portion of the posterior temporal lobe.

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  • olle
    What is function of lateral geneculate body?
    9 months ago

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