Geniculate

Abstract: 1. Cat retinal ganglion cells may be subdivided into sustained and transient response-types by the application of a battery of simple tests based on responses to standing contrast, fine grating patterns, size and speed of contrasting targets, and on the presence or absence of the periphery effect. The classification is equivalent to the ;X'/;Y' (linear/nonlinear) subdivision of Enroth-Cugell & Robson which is thus confirmed and extended.2. The sustained/transient classification applied to both on-centre and off-centre cells.3. Lateral geniculate neurones may be similarly classified by the same tests. Occasional concentrically organized cells had a mixture of sustained and transient properties.4. A technique for simultaneous recording from a geniculate neurone and one or more retinal ganglion cells providing its excitatory input showed that the connexions were specific with respect to the sustained/transient classification as well as the on-centre/off-centre classification. Most geniculate neurones are excitatorily driven only by retinal ganglion cells of the same functional type. In a few cases the inputs were mixed but only with respect to the sustained/transient classification.5. Sustained retinal ganglion cells had slower-conducting axons than the transient type. The same was true for lateral geniculate neurones but in this case the distributions showed considerable overlap.6. The sustained/transient classification is the functional correlate for the well-known segregation of optic nerve fibres into two conduction groups.7. The pathways carrying sustained and transient information remain essentially separate from retina through the lateral geniculate nucleus to the striate cortex.

Abstract: Single cell recordings in monkey striate cortex have shown differences in response properties from one cell layer to the next and have also shown that the IVth layer, which receives most of its input from the geniculate, is subdivided into a mosaic of regions, some connected to the left eye, others to the right. In the present study small lesions were made in single layers or pairs of layers in the lateral geniculate body, and the striate cortex was later examined with a Fink-Heimer modification of the Nauta method. We hoped to correlate the laminar distribution of axon terminals in the cortex with functional differences between layers, and to demonstrate the IVth-layer mosaic anatomically. After lesions in either of the two most dorsal (parvocellular) layers, terminal degeneration was found mainly in layer IVc, with a second minor input to a narrow band in the upper part of IVa. A very few degenerating fibers ascended to layer I. In contrast, lesions in either of the two ventral (magnocellular) layers were followed by terminal degeneration confined, apparently, to IVb, or at times extending for a short distance into the upper part of IVc; no degeneration was seen in layer IVa or in layer I. After a lesion confined to a single geniculate layer, a section through the corresponding region of striate cortex showed discrete areas or bands of degeneration in layer IV, usually 0.5–1.0 mm long, separated by interbands of about the same extent in which there was no terminal degeneration. When serial sections were reconstructed to obtain a face-on view of the layer-IV mosaic, it appeared as a series of regular, parallel, alternating degeneration-rich and degeneration-poor stripes. When a geniculate lesion involved both layer VI (the most dorsal, with input from the contralateral eye) and the part of layer V directly below (ipsilateral eye), the cortical degeneration, as expected, occupied a virtually continuous strip in layer IVc and the reconstructed face-on view of this layer showed a large confluent region of degeneration. In some of the reconstructions the cortical stripes seemed highly regular; in others there was a variable amount of cross connection between stripes. The stripes varied in width from 0.25 to 0.50 mm, and width did not seem to correlate with region of retinal representation. It is concluded that the long narrow stripes of alternating left-eye and right-eye input to layer IV are an anatomical counterpart of the physiologically observed ocular-dominance columns. Because of this segregation of inputs, cells of layer IV are almost invariably influenced by one eye only. A cell above or below layer IV will be dominated by the eye supplying the nearest IVth layer stripe, but will generally, though not always, receive a subsidiary input from the other eye, presumably by diagonal connections from the nearest stripes supplied by that eye.

Abstract: In cortical area 17 of the cat, simple receptive fields are arranged in elongated subregions that respond best to bright (on) or dark (off) oriented contours, whereas the receptive fields of their thalamic inputs have a concentric on and off organization. This dramatic transformation suggests that there are specific rules governing the connections made between thalamic and cortical neurons (see ref. 4). Here we report a study of these rules in which we recorded from thalamic (lateral geniculate nucleus; LGN) and cortical neurons simultaneously and related their receptive fields to their connectivity, as measured by cross-correlation analysis. The probability of finding a monosynaptic connection was high when a geniculate receptive field was superimposed anywhere over an elongated simple-cell subregion of the same signature (on or off). However, 'inappropriate' connections from geniculate cells of the opposite receptive field signature were extremely rare. Together, these findings imply that the outline of the elongated, simple receptive field, and thus of cortical orientation selectivity, is laid down at the level of the first synapse from the thalamic afferents.

Abstract: 1 Many lines of evidence suggest that signals relayed by the magnocellular and parvocellular subdivisions of the primate lateral geniculate nucleus (LGN) maintain their segregation in cortical processing We have examined two response properties of units in the striate cortex of macaque monkeys, latency and transience, with the goal of assessing whether they might be used to infer specific geniculate contributions Recordings were made from 298 isolated units and 1,129 multiunit sites in the striate cortex in four monkeys Excitotoxin lesions that selectively affected one or the other LGN subdivision were made in three animals to demonstrate directly the magnocellular and parvocellular contributions An additional 435 single units and 551 multiunit sites were recorded after the ablations 2 Most units in striate cortex had visual response latencies in the range of 30-50 ms under the stimulus conditions used The earliest neuronal responses in striate cortex differed appreciably between individuals The shortest latency recorded in the four animals ranged from 20 to 31 ms Comparable values were obtained from both single unit and multiunit sites After lesions were made in the magnocellular subdivision of the LGN in two animals, the shortest response latencies were 7 and 10 ms later than before the ablations A larger lesion in the parvocellular subdivision of another animal produced no such shift Thus it appears that the first 7-10 ms of cortical activation can be attributed to activation relayed by the magnocellular layers of the LGN 3 The units with the shortest latencies were all found in layers 4C or 6 and their responses were among the most transient in striate cortex Furthermore, their responses all showed a pronounced periodicity at a frequency of 50-100 Hz This periodicity was stimulus locked, and the responses of all short-latency units oscillated in phase 4 An index of response transience was computed for the units recorded in striate cortex The distribution of this index was unimodal and gave no suggestion of distinct contributions from the geniculate subdivisions Magnocellular and the parvocellular lesions affected the overall transience of responses in striate cortex The changes, however, were very small; extremely transient responses and extremely sustained responses survived both types of lesions 5 A characteristic profile was observed in the response latencies in superficial layers Latencies appeared to increase monotonically from layer 4 toward the surface of cortex, with the most superficial neurons not becoming active until 15 ms after responses were observed in layer 4C(ABSTRACT TRUNCATED AT 400 WORDS)