Biomedical subjects

D A Burkhardt

Publications and source records attributed to D A Burkhardt.

40 records · Page 3Linked to original sources

PubMedDate not supplied

The influence of center-surround antagonism on light adaptation in cones in the retina of the turtle.

The influence of center-surround antagonism on light adaptation in cone photoreceptors was investigated by intracellular recording from red-sensitive cones in the retina of the turtle, Pseudemys scripta elegans. Test flashes of 0.15-mm diameter were applied at the center of background fields of 0.25-mm or 2.2-mm diameter. Immediately upon expanding the background from 0.25 to 2.2 mm, the membrane potential depolarized by about 1-4 mV. The test flash response was enhanced if the depolarization was primarily due to synaptic feedback from horizontal cells, whereas the response was attenuated if the prolonged depolarization, an intrinsic response of the cone, was the dominant source of the depolarization. After several seconds, however, only the synaptic depolarization was maintained so maintained illumination of the large background field produced an enhancement of the cone's incremental sensitivity. The enhancement was examined in detail in steady-state conditions by obtaining amplitude-intensity measurements for centered test flashes on steady background fields over a large range of intensity. The effect of the large background field at any fixed intensity was fairly well described as a vertical (upward) shift of the amplitude-intensity curve obtained on the small field. This operation constitutes a quasi-subtractive mechanism of light adaptation and might provide a basis for the sort of subtractive mechanisms inferred from psychophysical studies of human vision. The enhancement was quantified by measuring the incremental sensitivity over four decades of background illumination. The magnitude of the enhancement increased with background intensity and then tended to stabilize at higher background intensities. The maximum difference in incremental sensitivity obtained on the large vs. small background field averaged 0.46 log unit (+/- 0.12 S.D.). At higher background intensities, incremental sensitivity conformed to Weber's Law behavior about equally well for flashes applied on either small or large background fields. In sum, the present results provide evidence for an additional mechanism of light adaptation in cone photoreceptors by showing that the incremental light sensitivity, initially set by mechanisms in the outer segment, can be modulated some three-fold by synaptic feedback at the inner segment of the cone.

Adaptation, Ocular↗

PubMedDate not supplied

Synaptic feedback, depolarization, and color opponency in cone photoreceptors.

For some 20 years, synaptic feedback from horizontal cells to cones has often been invoked, more or less convincingly, in discussions of retinal action and vision. However, feedback in cones has proved to be rather complex and difficult to study experimentally. The mechanisms and consequences of feedback are therefore still only partly understood. This review attempts to assess the knowns and unknowns. The limitations of the evidence for feedback are reviewed to support the position that unequivocal evidence still largely rests on intracellular recording from cones. Of the three distinct types of depolarization observed in cones, the graded depolarization is taken as the fundamental manifestation of feedback. The evidence for the hypothesis that GABA is the neurotransmitter for feedback appears reasonably strong but several complications will have to be resolved to make the hypothesis more secure. There is evidence that feedback contributes to aspects of light adaptation and spatiotemporal processing of visual information. The contributions seem modest in magnitude. The role of feedback in shaping the color-opponent responses of retinal neurons is evaluated with particular emphasis on pharmacological studies, spatial and temporal aspects of the response of chromatic horizontal cells, and the enigmatic nature of depolarizations in blue- and green-sensitive cones. On this and other evidence, it is suggested that feedback may impress some detectable wavelength dependency in some cones but the dominant mechanisms for color opponency probably reside beyond the photoreceptors.

Adaptation, Ocular↗

PubMedDate not supplied

Effects of light adaptation on contrast processing in bipolar cells in the retina.

Effects of light adaptation on contrast processing in the outer retina were investigated over nearly four decades of background illumination by analyzing the intracellular responses of 111 bipolar cells, 66 horizontal cells, and 22 cone photoreceptors in the superfused eyecup of the tiger salamander (Ambystoma tigrinum). Light adaptation had striking and similar effects on the average contrast responses of the hyperpolarizing (Bh) and depolarizing (Bd) classes of bipolar cells: Over the lower two decades of background illumination, the contrast gain increased 7-fold to reach values as high as 20-30, the dynamic range and the half-maximum contrast decreased by about 60%, the total voltage range increased some 40%, and contrast dominance changed from highly positive to more balanced. At higher levels of background, most aspects of the contrast response stabilized and Weber's Law then held closely. In this background range, the contrast gain of bipolar cells was amplified some 20X relative to that of cones whereas the corresponding amplification in horizontal cells was about 6X. Differences in the growth of contrast gain with the intensity of the background illumination for cones versus bipolar cells suggest that there are at least two adaptation-dependent mechanisms regulating contrast gain. One is evident in the cone photoresponse such that an approximately linear relation holds between the steady-state hyperpolarization and contrast gain. The other arises between the voltage responses of the cones and bipolar cells. It could be presynaptic (modulation of cone transmitter release by horizontal cell feedback or other mechanisms) and/or postsynaptic, that is, intrinsic to bipolar cells. Contrast gain grew with the background intensity by a larger factor in horizontal than in bipolar cells. This provides a basis for the widely held view that light adaptation increases the strength of surround antagonism in bipolar cells. On average, the effects of light adaptation and most quantitative indices of contrast processing were remarkably similar for Bd and Bh cells, implying that both classes of bipolar cells, despite possible differences in underlying mechanisms, are about equally capable of encoding all primary aspects of contrast at all levels of light adaptation.

Adaptation, Ocular↗

PubMedDate not supplied

Responses of ganglion cells to contrast steps in the light-adapted retina of the tiger salamander.

The impulse discharge of single ganglion cells was recorded extracellularly in superfused eyecup preparations of the tiger salamander (Ambystoma tigrinum). Contrast flashes (500 ms) were applied at the center of the receptive field while the retina was light adapted to a background field of 20 cd/m2. The incidence of cell types in a sample of 387 cells was: ON cells (4%), OFF cells (28%), and ON/OFF cells (68%). Quantitative contrast/response measurements were obtained for 83 cells. On the basis of C50, the contrast necessary to evoke a half-maximal response, ON/OFF cells fell into 3 groups: (1) Positive Dominant (26%), (2) Balanced (23%), and (3) Negative Dominant (51%). Positive Dominant cells tended to be relatively contrast insensitive. On the other hand, many Negative Dominant cells showed remarkably low C50 values and very steep contrast/response curves. Contrast gain to negative contrast averaged 8.5 impulses/s/% contrast, some four times greater than that evoked by positive contrast. In most ON/OFF cells, the latency of the first spike evoked by a negative contrast step was much shorter (40-100 ms) than that evoked by a positive contrast step of equal contrast. OFF cells typically showed higher C50 values, larger dynamic ranges, and longer latencies than those of Negative Dominant ON/OFF cells. Thus, different pathways or mechanism apparently mediate the off responses of OFF and ON/OFF cells. In sum, the light-adapted retina of the tiger salamander is strongly biased in favor of negative contrast, as shown by the remarkably high contrast sensitivity and faster response of Negative Dominant cells, the remarkably low incidence of ON cells, and the insensitivity of Positive Dominant cells. Some possible underlying influences of bipolar and amacrine cells are discussed.

Adaptation, Ocular↗