Biomedical subjects

A G Leventhal

Publications and source records attributed to A G Leventhal.

7 recordsLinked to original sources

PubMed1989

Organized arrangement of orientation-sensitive relay cells in the cat's dorsal lateral geniculate nucleus.

We studied the physiological orientation biases of over 700 relay cells in the cat's dorsal lateral geniculate nucleus (LGNd). Relay cells were sampled at regular intervals along horizontally as well as vertically oriented electrode penetrations in a fashion analogous to that used previously in studies of visual cortex (Hubel and Wiesel, 1962). The strengths of the orientation biases and the distributions of the preferred orientations were determined for different classes of relay cells, relay cells in different layers of the LGNd, and relay cells subserving different parts of the visual field. We find that, at the population level, LGNd cells exhibit about the same degree of orientation bias as do the retinal ganglion cells providing their inputs (see also Soodak et al., 1987). Also, as in the retina (Levick and Thibos, 1982; Leventhal and Schall, 1983), most LGNd cells tend to prefer stimuli oriented radially, i.e., parallel to the line connecting their receptive fields to the area centralis projection. However, the radial bias in the LGNd is weaker than in the retina. Moreover, there is a relative overrepresentation of cells preferring tangentially oriented stimuli in the LGNd but not in the retina. As a result of the overrepresentation of cells preferring radial and tangential stimuli, the overall distribution of preferred orientations varies in regions of the LGNd subserving different parts of the visual field. Reconstructions of our electrode penetrations provide evidence that, unlike in the retina, cells having similar preferred orientations are clustered in the LGNd. This clustering is apparent for all cell types and in all parts of laminae A and A1. The tendency to cluster according to preferred orientation is evident for cells preferring radially, intermediately, and tangentially oriented stimuli and thus is not simply a reflection of the radial bias evident among retinal ganglion cells at the population level. It is already known that cells having inputs from different eyes, on-center, off-center, X-, Y-, W-type, and color-sensitive ganglion cells are distributed nonrandomly in the LGNd of cats and monkeys (for review, see Rodieck, 1979; Stone et al., 1979; Lennie, 1981; Stone, 1983). The finding that relay cells having similar preferred orientations are also distributed nonrandomly suggests that the initial sorting of virtually all properties segregated in visual cortex may begin in the LGNd.

Animals↗

PubMed1979

Evidence that the different classes of relay cells of the cat's lateral geniculate nucleus terminate in different layers of the striate cortex.

Small electrophoretic injections of horseradish peroxidase (HRP) were made with recording microelectrodes in different layers of area 17 of the cat. The sizes and locations of labeled relay cells in the lateral geniculate nucleus (LGNd) were then studied. Layer I was found to receive inputs mainly from small LGNd cells (13--21 micrometer). These cells were located in the parvocellular C laminae and the MIN. Layer II and the upper portion of layer III were not found to receive LGNd afferents. Lower layer III was found to receive inputs mainly from small to medium-sized relay cells. These cells were located in the A laminae and the parvocellular C laminae. Layer IVab and the layer III--IV border region were found to receive afferents from predominantly large LGNd neurons (greater than 20 micrometers). These cells were located in alminae A, A1, C, and the MIN. Layer IVc was found to receive inputs from mostly small to medium-sized cells (15--23 micrometer). Nearly all of these cells were in the A laminae although some small cells in laminae C were observed following layer IVc injections which involved the layer IV--V border region. Layer V, away from the layer IV--V border region, was not found to receive LGNd afferents. Finally, layer VI was found to receive inputs from both large and small relay cells. These cells were all located in the A laminae. In the light of previous findings, these results suggest that the terminals of W, X, and Y cell axons are segregated within the cat's striate cortex: W cells projecting to layer 1, the deeper portion of layer III and the layer IV--V border region; X cells projecting heavily to layer IVc with a minor projection to layer VI and possibly also to lower layer III; Y cells projecting heavily to layer IVab and the layer III-IV border region with a smaller projection to layer VI.

Animals↗

PubMed1979

Brain stem afferents to visual cortical areas 17, 18 and 19 in the cat, demonstrated by horseradish peroxidase.

The origins of brain stem projections to the cytoarchitectonically different areas 17, 18 and 19 of the cat's visual cortex were studied following small horseradish peroxidase (HRP) injections. Labelled cells were counted in a dopaminergic nucleus (nucleus linearis rostralis (NLR)), other catecholaminergic nuclei (locus coeruleus, parabrachialis nuclei and nucleus subcoeruleus) and serotonergic nuclei (nucleus raphe dorsalis (NRD) and nucleus centralis superior (NCS)). Area 18 receives afferents from more locus coeruleus cells than either of areas 17 or 19. The number of labelled cells in the catecholaminergic nuclei far exceeds that in the serotonergic nuclei.

Afferent Pathways↗

PubMed1978

Physiological consequences for the cat's visual cortex of effectively restricting early visual experience with oriented contours.

1. The early visual experience of nine cats was restricted to viewing horizontal or vertical lines inside opaque goggles. 2. When the kittens were 3-4 mo old, extracellular recordings were made in the primary visual cortex. To obtain a representative sample of cortical cells, units were studied at regularly spaced intervals along the course of electrode penetrations traveling oblique to the cortical surface. An automated assessment of preferred orientation using a computer-driven optical display was employed, and during the recording session the experimenters did not know which orientation(s) each animal had viewed in early life. 3. In the cats that viewed horizontal lines with one eye and vertical lines with the other during rearing, two major findings of previous workers (14) were confirmed. First, a majority of units were not selective for orientation. Second, units with preferred orientations near vertical tended to be activated exclusively by the eye that had viewed vertical, and likewise for horizontal. 4. In cats that viewed lines of the same orientation with both eyes during rearing, a substantially smaller proportion of units were selective for orientation; the preferred orientations of these units also tended to match the orientation to which the cats had been exposed. 5. Portions of some electrode penetrations showed an orderly arrangement of cells according to preferred orientation similar to that seen in normal cats, but with regions over which only nonselective cells were found. Many penetrations appeared less orderly. 6. The results are consistent with a role for early visual experience in maintaining the responsiveness and innate selectivity of cortical neurons, although they cannot entirely rule out the possibility that experience may alter or determine the preferred orientation of some cells.

Animals↗

PubMed1978

Receptive-field properties of neurons in different laminae of visual cortex of the cat.

1. Receptive-field properties of neurons in the different layers of the visual cortex of normal adult cats were analyzed quantitatively. Neurons were classified into one of two groups: 1) S-cells, which have discrete on- and/or off-regions in their receptive fields and possess inhibitory side bands; 2) C-cells, which do not have discrete on- and off-regions in their receptive fields but display an on-off response to flashing stimuli. Neurons of this type rarely display side-band inhibition. 2. As a group, S-cells display lower relative degrees of binocularity and are more selective for stimulus orientation than C-cells. In addition, within a given lamina the S-cells have smaller receptive fields, lower cutoff velocities, lower peak responses to visual stimulation, and lower spontaneous activity than do the C-cells. 3. S-cells in all layers of the cortex display similar orientation sensitivities, mean spontaneous discharge rates, peak response to visual stimulation, and degrees of binocularity. 4. Many of the receptive-field properties of cortical cells vary with laminar location. Receptive-field sizes and cutoff velocities of S-cells and of C-cells are greater in layers V and VI than in layers II-IV. For S-cells, preferred velocities are also greater in layers V and VI than in layers II-IV. Furthermore, C-cells in layers V and VI display high mean spontaneous discharge rates, weak orientation preferences, high relative degrees of binocularity, and higher peak responses to visual stimulation when compared to C-cells in layers II and III. 5. The receptive-field properties of cells in layers V-VI of the striate cortex suggest that most neurons that have their somata in these laminae receive afferents from LGNd Y-cells. Hence, our results suggest that afferents from LGNd Y-cells may play a major part in the cortical control of subcortical visual functions.

Afferent Pathways↗

PubMed1977

Effects of early experience upon orientation sensitivity and binocularity of neurons in visual cortex of cats.

The class of neurons within the visual cortex of normal adult cats that has the smallest receptive fields (less than or equal to 2.25 degrees2) and that responds only to low rates of stimulus motion (less than or equal to 50 degrees / sec) responds preferentially to lines oriented about either the horizontal axis (+/-22.5 degrees) or the vertical axis (+/-22.5 degrees). In animals reared without exposure to patterned visual stimulation, many of these cells display orientation preferences but are activated monocularly. In contrast, in normal animals, neurons that have larger receptive fields or that respond to higher rates of stimulus motion do not exhibit a similar bias in the distribution of their orientation preferences. Cells of this type, studied in animals reared without exposure to patterned visual stimuli, are activated binocularly but do not display orientation preferences.

Animals↗

PubMed1975

Cortical effect of early selective exposure to diagonal lines.

Neurons in the visual cortex that respond preferentially to diagonal contours are present only in cats exposed to diagonal lines early in life. In contrast, cells that prefer horizontal or vertical contours are found following exposure to horizontal, to vertical, and to diagonal lines. Such cells do not require a specific visual input for maintenance or for development; neurons responding preferentially to diagonal lines do.

Age Factors↗