Using the anterograde degeneration technique, we examine the tangential organization of a thalamofugal axon population (class I of Frost and Caviness, '80) whose terminations are preferentially distributed to the middle tier (located in layers III and/or IV) of three radially separated tiers of thalamic projections to the neocortex. Less extensive data are also presented on the tangential organization of thalamofugal axon populations (class II of Frost and Caviness, '80) that do not terminate preferentially in the middle tier, but that are otherwise heterogeneous with respect to their radial pattern of intracortical termination. The projections of class I axons are distributed to all neocortical fields with the possible exception of fields 13,25, and 35. The class I projections to a given cortical field (with the possible exception of the cortex of the second somatosensory representation) originate in only one thalamic nucleus. The class I projections of an individual thalamic nucleus form a cortical representation of the nucleus that constitutes a "first order line-to-line" (topologic) transformation of the nuclear volume. The ensemble of class I projections forms a cortical representation of the corresponding thalamic regions that constitutes a "second order line-to-line" (non-topologic) transformation of the thalamic volume. Class II axons project to all neocortical fields. Classs II and class I projections contrast in that the class II projections of multiple thalamic nuclei overlap in the tangential plane of any given sector of the cortex. While the class II projections of the intralaminar nuclei and the widely projecting ventromedial nucleus are known to be topologically organized, the tangential organization of class II projections arising in other nuclei is incompletely understood.

Tangential organization of thalamic projections to the neocortex in the mouse.

Retinogeniculate terminations have been studied by fiber degeneration and microelectrode mapping methods in normal cats and in Siamese cats in which the retinogeniculate pathway is congenitally abnormal. The normal representation of the visual field, including the monocular temporal crescent, has been determined. It has also been shown that there is a cellular discontinuity in lamina A which corresponds to the blind spot or optic disc. In normal cats four geniculate layers receive retinogeniculate terminals; two layers (A and C) receive contralateral and two layers (A1 and C1) receive ipsilateral afferents.



In Siamese cats laminae A and C are normal. Lamina A1 is broken up into four main cell segments. Two of these receive a normal input from the ipsilateral temporal hemiretina; two receive an abnormal projection from the contralateral temporal hemiretina. The abnormal projection originates primarily from a vertical strip of retina about 20°–25° in width that lies just temporal to the area centralis. Within this abnormal projection a normal retinotopic sequence is maintained but, since this projection goes to the contralateral instead of the ipsilateral lamina A1, the abnormal segments of lamina A1 receive a mirror-image of the normal representation.



Lamina C1 and the medial interlaminar nucleus receive an abnormal contralateral input from the temporal hemiretina similar to that found in lamina A1.

A study of normal and congenitally abnormal retinogeniculate projections in cats

Patterns of synaptic interconnections in the dorsal lateral geniculate nucleus of cat and monkey: a brief review

Spike activities of optic tract fibers and corresponding relay cells were recorded simultaneously in layers A and A1 of the dorsal lateral geniculate nucleus of the cat. Light stimuli of various diameters were shone into the receptive field center of these unit pairs and their input/output ratios were determined. An increase of the stimulus size leads to an impairment of the input/output ratio in on-center and off-center relay cells. This suppressive effect has approximately the same latency as the excitatory response.

Inhibitory interaction in the cat's lateral geniculate nucleus.

Behavioral, anatomical and electrophysiological data are presented for three cats. One had complete removal of area 17 as evidenced by gross and histological examination of the remaining cortex and by the degeneration in the lateral geniculate body. Dorsal portions of areas 18 and 19 were essentially intact and much of the central and medial interlaminar nuclei of the lateral geniculate body survived. This cat had excellent pattern vision. So too did a cat in which all of the posterior suprasylvian gyrus, dorsal surface of the middle suprasylvian gyrus and much of anterior marginal gyrus had been extirpated. In contrast, an animal lacking area 18 and most of area 19 in addition to area 17, and thus having near “total” degeneration of the lateral geniculate body, was incapable of pattern discrimination. Photically evoked potentials could nevertheless be demonstrated throughout an extensive suprasylvian field in the latter animal. It is concluded that area 17 is not essential for pattern vision in the cat, but that in its absence, even in the case of ablations performed neonatally, cortex receiving major projection from the central and medial interlaminar nuclei, i.e., areas 18 and 19, is required.

Survival of pattern vision after removal of striate cortex in the adult cat.

The projection of the cat's retina on the lateral geniculate nucleus.

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The projection of the cat's retina on the lateral geniculate nucleus.