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T T Norton

Publications and source records attributed to T T Norton.

50 records · Page 3Linked to original sources
PubMed1979

Comparison of receptive-field properties of X and Y ganglion cells with X and Y lateral geniculate cells in the cat.

1. To examine the transmission of visual information through the lateral geniculate nucleus, we have studied the receptive-field properties of 65 X and Y optic tract axons and compared them with the receptive-field properties of X and Y LGN cells in paralyzed cats anesthetized with N2O/O2 (70/30%). The same experimental conditions and quantitative methods have been used as in the preceding study of LGN cells (2). 2. The spatiotemporal organization of the receptive fields of X and Y retinal axons are similar to those of X and Y LGN cells. X ganglion cell receptive fields show a simple center-surround organization, whereas Y ganglion cell receptive fields show a more complex organization with three concentric regions: a central region of center-type response, a region of mixed center-type and surround-type responses, and a region of surround-type response. 3. The inhibitory strengh of the surrounding region was tested with a centrally located flashing light spot of successively increased diameter. As in the LGN, the inhibitory strenght of the surrounding region was stronger in retinal X-cells than in retinal Y-cells, and the strength of the inhibition decreased as the diameter of the receptive-field center increased. 4. The decrease of the inhibitory strength of the surrounding region with increasing distance from the receptive-field center was similar in the retina and in the LGN for cells belonging to the same class (X or Y) and having the same receptive-field center size. 5. The differences in properties in the LGN between small-field X-, large-field X-, and Y-cells are best explained by assuming that they are driven, respectively, by small-field X, large-field X, and Y retinal ganglion cells. There does not appear to be a significant mixing of properties either between cells having different receptive-field center sizes. 6. The principal transformation we found between retinal and LGN units is that X LGN cells have sharply lower spontaneous activities and driven activities, as compared with X ganglion cells. Y LGN units show only a small decrease in spontaneous activity in comparison with Y ganglion cells. 7. We conclude that there is a significant alteration in the LGN only in the properties of X-cells, possibly by way of a strong inhibitory pool converging on X LGN units. We further suggest that this inhibitory pool plays a role in the modulation of transmission of information through the LGN only in the X channel, while the Y channel appears to be relatively unaffected.

Animals↗
PubMed1977

Loss of Y-cells in the lateral geniculate nucleus of monocularly deprived tree shrews.

In tree shrews (Tupaia glis) reared with one eye closed, Y-cells were almost entirely absent in the binocular segment of the lateral geniculate laminae receiving input from the deprived eye. Y-cells were found in the monocular segment of these laminae, and in the binocular segment of the laminae with input from the normal eye. X-cells were present in both the deprived and normal laminae and appeared unaffected by the deprivation. A number of abnormal cells were also found, and these were located primarily in the binocular segment where Y-cells were absent.

Animals↗
PubMedDate not supplied

Center/surround relationships of magnocellular, parvocellular, and koniocellular relay cells in primate lateral geniculate nucleus.

As in other primates, the lateral geniculate nucleus (LGN) of the prosimian primate, bush baby (Galago crassicaudatus), contains three morphologically and physiologically distinct cell classes [magnocellular (M), parvocellular (P), and koniocellular (K)] (Norton & Casagrande, 1982; Casagrande & Norton, 1991). The present study examined quantitatively the center/surround relationships of cells in all three classes. Estimates of receptive-field center size (Rc) and sensitivity (Kc) and of surround size (Rs) and sensitivity (Ks) were obtained from 47 LGN relay cells by fitting a difference of Gaussians function to contrast-sensitivity data. For M and P cells, center size (Rc) increases with eccentricity but is about two times larger for M than for P cells at a given eccentricity. Surround size (Rs) increases with eccentricity for P but not for M or K cells. The center sensitivity (Kc) is inversely related to center size (Rc) and surround sensitivity (Ks) is inversely related to surround size (Rs) for cells in all classes, a result consistent with the sensitivity regulation that is produced by light adaptation. High spatial-frequency cutoff (acuity) is inversely related to center size (Rc). However, the peak contrast sensitivity is relatively independent of Rc. The ratio of the integrated strength (volume) of the surround to the volume of the center remains relatively constant (median, 0.87) across all three cell classes. This ratio is an excellent predictor of a cell's rolloff in contrast sensitivity at low spatial frequencies: cells with a low surround/center ratio have less low-frequency rolloff. Although M, P, and K cells generally display similar center/surround relationships, differences in center size and the other parameters between the classes distinguish most M, P, and K cells. These findings demonstrate that both similarities and differences in the visual-response properties of primate LGN cells in these three parallel afferent pathways can be explained by basic center/surround relationships.

Afferent Pathways↗
PubMedDate not supplied

Lid-suture myopia in tree shrews with retinal ganglion cell blockade.

To determine whether central communication of retinal signals is necessary for the development of an experimentally induced myopia, tree shrews were exposed to monocular deprivation (MD) while the action potentials of retinal cells in the deprived eye were blocked with intravitreally injected tetrodotoxin (TTX-MD animals). TTX injections (0.6 microgram in 3 microL) and MD began about 15 days after eye opening, at the start of the susceptible period for the development of lid-suture myopia. Six injections were given, one every second day to produce 12 days of MD and TTX-blockade. Control TTX animals (TTX-open) received TTX in one eye, but not MD, on the same injection schedule and were always found to be behaviorally unresponsive to visual stimuli through the injected eye indicating that TTX blocked central communication of action potentials. Other control animals received intravitreally injected saline in either an open eye (saline-open), or an MD eye (saline-MD). A sham-injected group (sham-inj-MD) received MD and all anesthetic and surgical manipulations except for penetration of the sclera. In all groups, one eye in each animal was an untreated control. Two effects were found. All MD groups, including the TTX-MD animals, developed a significant vitreous chamber elongation in the deprived eye, indicating that an experimental myopia developed despite ganglion cell blockade. Thus, retinal mechanisms in tree shrew can detect the presence of a degraded visual image and produce an experimental myopia that does not depend on the receipt of visual messages by central neural structures. In addition, eyes in which the sclera was punctured had smaller vitreous chamber depths than comparable uninjected eyes, indicating that puncturing the sclera reduced the normal elongation. These data suggest that forces within the eye normally contribute to its expansion and may be resisted by the choroid and/or the sclera.

Action Potentials↗