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At least 955 records · Page 53Linked to original sources
PubMed1983

Intracellular recording from identified photoreceptors and horizontal cells of the Xenopus retina.

Intracellular recordings were made from rods, cones and horizontal cells of the Xenopus retina. The cells under study were identified by injection of the fluorescent dye, Lucifer yellow. Rod spectral sensitivity peaked near 524 nm, that of cones near 612 nm whereas horizontal cells reflected input from both these classes of photoreceptors. No intracellular recordings were made from blue-sensitive rods (lambda max = 445 nm) nor did this rod appear to provide an input to the horizontal cell. Under dark-adapted conditions, horizontal cells had a slow waveform, a Vmax less than or equal to 18 mV and were driven by 524 nm rods only. When light-adapted, horizontal cell responses were fast, Vmax was 30-40 mV and the responses reflected only 612 nm cone input. In the mesopic state rod and cone inputs to the horizontal cell interacted non-linearly: weak green backgrounds greatly enhanced the response to a superimposed red flash compared to the red flash response on a dark field. The length constant of the horizontal cell exceeded its dendritic arbor by 2-15 fold. All of the stained horizontal cells, however, possessed a long slender axon without a terminal but which emitted periodic short branches that appeared to contact receptors.

Action Potentials↗
PubMed1984

Rhodopsin and visual adaptation: analysis of photoreceptor thresholds in the isolated skate retina.

Photoreceptor thresholds in the isolated retina of the skate, determined by extracellular measurement of the photoreceptor potential during periods of light and dark adaptation, were analyzed in relationship to prevailing states of the visual pigment. The starting assumption of the analysis is that relative levels of three forms of the pigment molecule [native rhodopsin (R), a photoactivated intermediate (R*), and bleached pigment (B)] govern (quasi-) stable levels of threshold measured (a) during exposure of the retina to background light of fixed incident intensity (Ib), and (b) after irradiation that bleaches a defined fraction (B) of the rhodopsin. It is shown that experimental data are described well by the equation It/ It0 = (1 - B)-1 X F X (1 + 0(3)B), where F = [1 + 0(1)Ib(1 - B) + 0(2)B]. In this equation, It/ It0 is the relative threshold for detection of a test flash; (1 - B) approximates the relative efficiency of quantum capture; and 0(1) - 0(3) are constants. For values of 0(1) - 0(3) yielding an optimal fit to experimental data, log (It/ It0 ) approximately log F over a broad range of values of Ib and B. It is further shown that the algebraic form of the term F in the above equation is consistent with the predictions of a (steady-state) model for the role of the pigment molecule in photoreceptor adaptation. The model proposes that R* and B desensitize the photoreceptor by acting (in qualitatively similar fashion) to reduce the availability of E, an intracellular substance whose activation supports generation of the flash response. Results of the analysis are discussed in relation to the Dowling- Rushton equation (Dowling, 1960, 1963; Rushton , 1961), and to the results of more recent studies examining light and dark adaptation.

Adaptation, Ocular↗
PubMed1985

Temporal information processing in cones: effects of light adaptation on temporal summation and modulation.

We have studied the temporal information processing of turtle cones in steady states of light adaptation using intracellular recording techniques. We measured the linear range incremental sensitivity of cones as a function of the stimulus duration. Linear range incremental sensitivity is a function of the background intensity. It is also proportional to the duration of short duration stimuli but is independent of duration for long duration stimuli. The plot of log sensitivity versus log stimulus duration displays two straight line asymptotes; a slope of one for short durations and a slope of zero for long durations. These asymptotes intersect at a time, the critical duration, which decreases with increasing background intensity. Linear systems theory was used to predict these results in addition to the interdependence of critical duration, response kinetics, and sensitivity for any state of adaptation. We have also calculated cone sensitivity as a function of sinusoidal frequency for a variety of background intensities. Correlations between these results and psychophysical studies suggest that the limits on temporal summation established by the cones appear not to be substantially altered by the rest of the retina.

Adaptation, Ocular↗
PubMed1986

Rod influence on cone flicker detection: variation with retinal eccentricity.

Previous studies have shown that cone flicker thresholds are influenced by the adaptation state of the rod system. We have examined how the properties of this rod-cone flicker interaction differ with retinal eccentricity. The threshold for detecting 25 Hz flicker was measured in the dark-adapted eye, against a rod-saturating Ganzfeld background and following a Ganzfeld bleach. The magnitude of the interaction, defined as the difference between light- and dark-adapted flicker thresholds, covaries with changes in rod absolute threshold across the visual field. At a given eccentricity, the magnitude of the interaction is constant for test diameters ranging from 7' to 1.7 degrees, indicating that variations in magnitude with eccentricity do not result from changes in spatial summation properties. The measurement of cone flicker thresholds during bleaching recovery provides evidence that variations in the magnitude of the rod-cone flicker interaction with retinal eccentricity may result from differences in the channel capacity of the pathway carrying the threshold-elevating signal from rods.

Adaptation, Ocular↗
PubMed1987

Light adaptation of red cones and L1-horizontal cells in the turtle retina: effect of the background spatial pattern.

Electrical coupling allow red cones and L1-horizontal cells to respond to light stimuli illuminating remote retinal loci. The contribution of the spatial pattern of background light to flash sensitivity of red cones and L1-horizontal cells was studied intracellularly in the turtle retina. Lateral spread of background adaptation was observed in the cones. The sensitivity of the cones was related to the steady hyperpolarization of the cells irrespective of the background's spatial pattern. Light sensitivity of L1-horizontal cells mainly depended upon the background illuminating their dendritic field. Unlike red cones, the horizontal cells responded differently to spot and annular backgrounds regardless of the steady hyperpolarization induced.

Adaptation, Ocular↗
PubMed1987

Light adaptation and responses to contrast flashes in cones of the walleye retina.

The effects of light adaptation on the intracellular responses of cones to incremental and decremental light flashes were investigated in the superfused walleye retina. The time course of light adaptation was relatively slow, some 10-20 min being required to reach steady-state conditions. Under steady-state conditions: (1) both dark-adapted and light-adapted amplitude/intensity data were well described by the Michaelis-Menten relation, (2) the incremental response did not saturate on intense backgrounds and (3) the cone sensitivity conformed closely to Weber's law. Effects of the polarity and magnitude of the flash contrast were analyzed by deriving "equivalent contrast" relations for cones and comparing the results to those of recent psychophysical experiments. Results based on the response amplitude suggest that responses to negative contrast flashes may be enhanced by post-receptor mechanisms. Results based on cone latency suggest that equivalent contrast relations for visual latency may be largely shaped in the very early, quasi-linear phase of the cone response.

Action Potentials↗
PubMed1991

Electroretinographic assessment of background adaptation in 10-week-old human infants.

Full field, scotopic b-wave stimulus/response functions of 10-week-old infants and adults were measured in the dark-adapted condition, and in the presence of steady backgrounds. Dark adapted b-wave sensitivity (log sigma) differed significantly between infants and adults; the median dark adapted sensitivity of infants was 0.50 log unit less than that of adults. The median eigengraus of infants (-1.32 log scot. td) and adults (-1.55 log scot. td) did not differ significantly. The median slope of the linear portion of the background adaptation function was about 0.9 for infants and adults. These results argue for post-receptoral immaturities, but do not rule out receptoral immaturities.

Adaptation, Ocular↗
PubMed1995

Adaptation mechanisms in spatial vision--I. Bleaches and backgrounds.

To examine how the mechanisms of bleaching and background adaptation affect spatial pattern vision, contrast detection thresholds were measured in the fovea for sinusoidal (increment-Gabor) targets, during long-term dark adaptation following full bleaches, and against steady adapting backgrounds of various intensities. The dark-adaptation curves were found to be invariant in shape over the range of spatial frequencies tested (1-15 c/deg); in other words, the amplitude sensitivity functions were invariant during dark adaptation. These results support the hypothesis that bleaching adaptation is local and multiplicative. On the other hand, the background-adaptation curves measured for different spatial frequencies were found to converge as background intensity increased; the amplitude sensitivity functions became flatter. These results reject the equivalent-background hypothesis.

Adaptation, Ocular↗
PubMed1996

Matched filtering by a photoreceptor membrane.

This study demonstrates how phototransduction cascades and membranes tune photoreceptor response dynamics to image quality, and eliminate noise introduced in cell signalling. Intracellular recordings from intact retina confirm that the light-adapted photoreceptors of the crane fly Tipula paludosa (Diptera; Tipulidae) have a slow response, appropriate for their visual ecology. To provide a slow response, the phototransduction cascade's impulse response fails to narrow with light-adaptation, despite reductions in the timescales of latency and quantum bumps. The photoreceptor membrane acts as a passive RC-filter, because light induced depolarization inactivates voltage-gated potassium currents. The frequency response of the membrane equals the cascade's and, as a result, the membrane is a matched filter that suppresses photon shot noise. This type of broad-band filter, matched to the predictable dynamics of preceding processes to remove noise, could be widely employed in vision and in many other chains of cellular communication.

Adaptation, Ocular↗