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S Lehmkuhle

Publications and source records attributed to S Lehmkuhle.

35 records · Page 2Linked to original sources
PubMed1985

Temporal modulation of the background affects the sensitization response of X- and Y-cells in the dLGN of cat.

The responses of X- and Y-cells to a small flashing test probe modulated at 2 Hz were measured as a function of the diameter of a concentric circular background. The background was either a static homogeneous disk, a flickered homogeneous disk, a static radially-vaned disk, or a rotating vaned disk, all of equivalent space- and time-averaged luminance. Variation of background diameter produced systematic changes in response that reflected general X- and Y-cell receptive field differences. Initial increases in background diameter decrease the response to the test probe ("desensitization") of both X- and Y-cells to a minimum level at a diameter that approximates the size of the receptive field center. Further increases in the background diameter result in an increase of the response to the test probe ("sensitization") that is very large for X-cells and much less for Y-cells. Temporal modulation of the background does not alter the desensitization of X- or Y-cells, but strongly reduces the sensitization of X-cells. Temporal modulation of the background by flicker and by motion produced equivalent effects. These results are compared to similar human psychophysical tests used clinically. Implications concerning the mediation of psychophysical sensitization are also considered.

Action Potentials↗
PubMed1984

An electrophysiological assessment of X and Y cells as pattern and flicker detectors in the dorsal lateral geniculate nucleus of the cat.

We tested the hypothesis that geniculate X cells are the neural substrate of psychophysically identified pattern channels and that geniculate Y cells are the neural substrate of psychophysically identified flicker channels. The hypothesis was tested by measuring the relative sensitivity of isolated X and Y cells in the dorsal lateral geniculate nucleus of the cat to counterphase and on-off grating presentations. The fundamental and second harmonic responses of X and Y cells to sinusoidal counterphase and on-off temporal modulation were measured at a number of spatial frequencies using two contrasts, 0.1 and 0.4. The fundamental responses of both X and Y cells to sinusoidal counterphase were greater relative to on-off responses. The second harmonic responses of Y cells to counterphase were larger at high spatial frequencies. Contrast sensitivity also was measured. At all spatial frequencies, both X and Y cells were slightly more sensitive to counterphase than to on-off presentations. Since flicker sensitivity in humans is twice as high for counterphase as for on-off presentations across all spatial frequencies, whereas pattern sensitivity is equal for the two presentations, we conclude that X and Y cells do not subserve uniquely pattern and flicker sensitivity, respectively. This conclusion is based on the result that differences between X and Y cells to counterphase and on-off presentations were inconsistent with the differences observed for pattern and flicker sensitivity. We suggest then that a spatial/temporal dichotomy does not seem to accurately characterize the functional roles of X and Y cells.

Animals↗
PubMed1984

A comparison between Y-cells in A-laminae and lamina C of cat dorsal lateral geniculate nucleus.

Extracellular responses of Y-cells in the A-laminae and in lamina C of the cat dorsal lateral geniculate nucleus were recorded and compared for several sine-wave grating presentations. Both spatial and temporal contrast sensitivity functions were determined for these cells as well as suprathreshold response functions at 0.2 and 0.4 contrast. Qualitatively, the responses of the lamina C Y-cells were very similar to Y-cells of the A-laminae; differences were of a quantitative nature. At threshold, lamina C Y-cells were more sensitive at all spatial and temporal frequencies tested. Suprathreshold results showed no major differences in fundamental response amplitude between laminar Y-cells. Interlaminar differences were found with respect to second harmonic response amplitude. Lamina C Y-cells gave the largest overall second harmonic response for all stimulus conditions. A trend was observed for these laminar Y-cells such that the second harmonic responses were highest for Y-cells of lamina C, intermediate for lamina A Y-cells, and lowest for those of lamina A1. Based on differences in projection pattern and present electrophysiological results, we conclude that the lamina C Y-cells may represent a population of cells that is distinct from A-laminae Y-cells. These lamina C Y-cells provide a significant input to visual cortex.

Animals↗
PubMed1984

Spatial contrast sensitivity of dark-reared cats with striate cortex lesions.

We measured the spatial contrast sensitivity of two dark-reared cats before and after bilateral ablations of area 17 and parts of area 18. These lesions produced large deficits in the contrast sensitivity of both cats at intermediate and high spatial frequencies. This postoperative reduction in contrast sensitivity in dark-reared cats is relatively similar to the sensitivity deficits produced by the same cortical lesions in normally reared cats. However, both pre- and postoperatively, the normally reared cats exhibited contrast sensitivity that is clearly superior to that of the dark-reared cats. The results demonstrate both that striate cortex is significantly involved in the spatial contrast sensitivity of dark-reared cats and that dark rearing produces widespread deficits, independent of those in striate cortex, through much of the visual system.

Animals↗
PubMed1983

An oblique effect of spatial summation.

Contrast thresholds for sinewave gratings were measured as a function of the number of cycles in the grating for two different orientations and three spatial frequencies. At high spatial frequencies, where an oblique effect of contrast sensitivity was demonstrated, there was a greater increase in contrast sensitivity as a function of the number of cycles for the vertical gratings than for oblique gratings. At a lower spatial frequency, where the contrast sensitivity anisotropy was greatly diminished, both orientations displayed a similar increase in contrast sensitivity when the number of cycles in the gratings was increased.

Adult↗
PubMed1983

Spatial contrast sensitivity of monocularly deprived cats after removal of the non-deprived eye.

Spatial contrast sensitivity was behaviorally determined for two monocularly lid-sutured cats. The spatial contrast sensitivity function for the non-deprived eyes matched previously reported functions obtained from normally reared cats. In comparison, sensitivity was significantly lower at all spatial frequencies for the deprived eye of both cats. In one cat, subsequent removal of the non-deprived eye resulted in a two to threefold increase in sensitivity at all spatial frequencies. However, no improvement in sensitivity was observed for the second animal. Single-unit electrophysiological recording in the striate cortex of these two animals revealed a postenucleation difference in the percent of visually influenced cells. In the cat for which no behavioral improvement occurred, only 13% of striate cortex cells could be driven by visual stimulation of the previously deprived eye. In contrast 38% of striate cortex cells were similarly influenced in the cat for which visual improvement was observed. These data suggest a correlation between the physiological effectiveness of the postcritical period enucleation procedure and the visual capacity of monocularly lid-sutured cats.

Amblyopia↗
PubMed1982

The oblique effects of pattern and flicker sensitivity: implications for mixed physiological input.

The inferior contrast sensitivity for oblique gratings has been previously demonstrated at high spatial frequencies with an absolute criterion. In this study contrast sensitivity at oblique and main axis orientations was obtained under separate pattern and flicker threshold criteria across a range of both spatial and temporal frequencies. The anisotropy of contrast sensitivity was not observed at low spatial frequencies under any stimulus or criterion conditions. At high spatial frequencies the anisotropy was observed for both pattern and flicker thresholds, although the flicker anisotropy was less pronounced. If, as previously suggested, this anisotropy is mediated by X cells, then these psychophysical findings suggest a mixed model in which both pattern and flicker systems receive input from both X and Y-cell pathways.

Adult↗
PubMed1981

Cross-species correspondence of spatial contrast sensitivity functions.

Spatial contrast sensitivity functions (CSFs) have been obtained for at least 9 species, including man. In the present paper, the shapes and octave band widths of these functions are compared. For most species, the shape of the CSF was an inverted-U, and the full width at half amplitude of the CSFs varied less than one octave. These similarities suggest that there is a close correspondence of the CSFs of these diverse animals; the major difference is the location of each CSF in the spatial frequency domain.

Animals↗
PubMed1980

Effect of depth separation on metacontrast masking.

In most investigations of visual masking, spatially adjacent contours are located in the same depth plane, a practice derived from the assumption that depth is not a critical variable in contour formation. In nine experiments, the dependence of masking on the relative depth positions of the target and mask was examined. To permit facile manipulation of depth without introducing confounding changes in proximal stimulation, the stimuli were stereoscopic contours formed from dynamic random-element stereograms generated as anaglyphs on a projection color-television display. The target was a briefly presented Landolt C with gap position randomly varied; the mask was an annulus that surrounded the target. Masking with stereoscopic contours was robust and subject to the same spatial and temporal variables influential in masking with physical contours. Depth separation of the target and mask produced significant reductions in masking and interacted with temporal order and egocentric stimulus position. These results have implications for models of visual masking and indicate that perceived depth is important in contour formation.

Attention↗
PubMed1978

On the capacity of directionally selective mechanisms to encode different dimensions of moving stimuli.

Direction-specific losses of contrast sensitivity for sinusoidal test gratings as a function of the contrast of a sinusoidal adapting grating were found to be similar to those measured previously with square-wave gratings. Furthermore, both relationships were similar to that between motion aftereffect duration and the contrast of sinusoidal adapting gratings, and all three sets of data can be fit by a single function. The function shows that the magnitude of direction-specific adaptation effects increases linearly with the logarithm of adapting contrast in the low contrast region, but is essentially independent of contrast once the contrast exceeds threshold by more than a factor of five-six. In addition, it was found that direction-specific losses of contrast sensitivity are restricted to limited ranges of spatial frequency.

Adaptation, Ocular↗