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At least 685 records · Page 38Linked to original sources
PubMed1992

The effect of light adaptation on scotopic spatial summation in 10-week-old infants.

Psychophysical area-intensity functions of individual 10-week-old human infants and adults were obtained in the dark adapted state, and in the presence of a steady background that elevated threshold 1 log unit above the dark adapted level. For dark adapted infants, the mean diameter for complete spatial summation (4.42 degrees; SD: 1.67 degrees) was significantly larger than that of adults (2.32 degrees; SD: 0.09 degrees). The background reduced the mean critical diameter to 2.67 degrees for infants (SD: 0.64 degrees) and to 1.16 degrees for adults (SD: 0.08 degrees). Spatial probability summation has similar effects on infant and adult thresholds, and, therefore, does not appear to account for the developmental decrease in critical diameters. Rather, decreases in receptive field size are suspected.

Adaptation, Ocular↗
PubMed1993

Light adaptation of human rod receptors: the leading edge of the human a-wave and models of rod receptor activity.

The human rod receptors can be studied by measuring the leading edge of the rod a-wave of the ERG. Computational models, previously shown to fit the recordings from single rods, are fitted to dark-adapted a-wave responses. A model proposed by Lamb and Pugh [(1992) Journal of Physiology, 499, 719-758] fits slightly better than the traditional models based upon n-stage exponential filters. To test alternative models of rod light adaptation, a-waves were recorded to flashes presented upon steady adapting lights. Steady adapting lights decrease the rods' sensitivity. Human rods must adapt as response compression alone predicts far greater decreases in sensitivity. The evidence suggests that the mechanism(s) of adaptation include a change in the time-course of the rod's response. Human rods appear to adapt in much the same manner as do the rods of other vertebrates.

Adaptation, Ocular↗
PubMed1993

Properties of the visual channels that underlie adaptation to gradual change of luminance.

Following adaptation to a spatially uniform patch of light that is gradually brightening (or dimming), a steady test patch appears to be gradually dimming (or brightening). We measured this ramp aftereffect with a nulling method, as a function of the amplitude and temporal repetition rate of the adapting sawtooth waveform and at various retinal eccentricities and levels of dark adaptation. We conclude that the underlying visual channels respond best to large-amplitude sweeps in luminance of at least 20 dB (1 log unit); but they are fairly insensitive to the temporal rate of this sweep. The channels are present out to an eccentricity of at least 40 degrees but they almost disappear during dark adaptation. The ramp aftereffects were asymmetrical: the subjectively darkening aftereffect produced by a brightening adapting ramp was slightly stronger than vice versa.

Adaptation, Ocular↗
PubMed1996

Summation of rod and S cone signals at threshold in human observers.

We examined whether signals from rods and S cones can combine to produce a threshold response. Test flashes of specific wavelengths superposed on a long wavelength adapting field were used to isolate threshold responses from the two receptor systems, simultaneously and at the same retinal location. Dark adaptation experiments and spectral sensitivity determinations indicated that, in the adaptational range from about 1.6 to 2.8 log scot td, 530 nm and 440 nm flashes were detected by rod and S cone photoreceptors, respectively. The intensities of the 530 nm and 440 nm flashes were mixed in various ratios and the increment threshold was then measured with these mixture flashes using the method of constant stimuli. The effects of rod and S cone excitation were found to summate linearly at threshold, under these experimental conditions. Summation occurred presumably at an early stage of the visual process.

Adaptation, Ocular↗
PubMed1992

Changes in sensitivity of the dark-adapted eye during concurrent light adaptation of the other eye.

Thresholds for detection of light by a dark-adapted test eye were measured while the other, non-test eye was either similarly dark adapted or while it was exposed to an intense red adapting field. An interocular effect that depends on the retinal location of the test was found: compared to the threshold during binocular dark adaptation, sensitivity decreased during contralateral light adaptation when the test was presented to the foveola and up to 4 deg above it; but sensitivity increased when the test was between 7 and 12 deg, showing a reversal at 5 deg.

Adaptation, Ocular↗
PubMed1961

The duplex nature of the retina of the nocturnal gecko as reflected in the electroretinogram.

The effect of light and dark adaptation on the electrical activity in two species of nocturnal gecko, Hemidactylus turcicus and Tarentola mauritanica was studied. The electroretinogram of both species changes from the scotopic type in the dark-adapted state to the photopic type after strong light adaptation. For the scotopic response fusion frequencies up to 18 flashes per sec. are obtained in both species. For the photopic response fusion frequencies up to 50 flashes per sec. are seen in Tarentola, and up to 25 flashes per sec. in Hemidactylus. Proceeding from dark to light adaptation the increment threshold (dI) is measured at different levels of adaptive illumination (I). At low levels of illumination the dI/I ratio is found to be small and at high levels of illumination to be large. No difference in the dI/I ratio is obtained for test lights of 462 and 605 mmicro. During dark adaptation the change of threshold after exposure to moderate and weak lights (up to 10(3) times dark threshold) is rather fast. After light adaptation to strong light (10(6) times dark threshold) duplex dark adaptation curves are seen with a break separating a fast and a slow phase of dark adaptation. The significance of these results from a retina which possesses sense cells of only one type is discussed.

Adaptation, Ocular↗
PubMedDate not supplied

Changes in lesion detectability caused by light adaptation in retinal photoreceptors.

This paper deals with specific physiologic mechanisms in the retina that can decrease apparent contrast in dark parts of an image. A new signal processing model based on these mechanisms is described which can be used to predict observer contrast thresholds for circular test patches superimposed on both uniform and nonuniform backgrounds. Experiments conducted using this model suggest that several types of viewing conditions commonly encountered in clinical practice may interfere with the radiologist's ability to detect low-contrast lesions. The authors' long-term objective is to use this model to predict observer performance when searching for marginally visible anatomic and pathologic detail in radiographic examinations.

Adaptation, Ocular↗
PubMed1975

Critical flicker frequency and the Pulfrich phenomenon.

The critical flicker frequency and the Pulfrich stereophenomenon were both used to investigate the long term effects on temporal resolution and visual latency of wearing a tinted lens in front of one eye for a period of five days. The results obtained from three subjects showed that in every case the critical flicker frequency was depressed by the tinted lens and displayed no adaptive recovery, but the Pulfrich effect displayed a reduction of apparent depth in the median plane indicating the presence of an adaptive recovery. It is postulated that the CFF indicates that the physiological transmission of the visual signal is unaltered over the the five days and the Pulfrich adaptation occurs due to an alteration of the interpretation of the unchanged visual response. That is, the adaptation is a perceptual rather than a physiological one.

Adaptation, Ocular↗
PubMed2001

Time course of the flash response of dark- and light-adapted human rod photoreceptors derived from the electroretinogram.

1. The a-wave of the electroretinogram was recorded from human subjects with normal vision, using a corneal electrode and ganzfeld stimulation. We applied the paired-flash technique, in which an intense 'probe' flash was delivered at different times after a 'test' flash. The amplitude of the probe-flash response provided a measure of the circulating current remaining at the appropriate time after the test flash. 2. We extended previous methods by measuring not at a fixed time, but at a range of times after the probe flash, and then calculating the ratio of the 'test-plus-probe' response to the 'probe-alone' response, as a function of time. 3. Under dark-adapted conditions the rod response derived by the paired-flash technique (in response to a relatively dim test flash) peaked at ca 120 ms, with a fractional sensitivity at the peak of ca 0.1 Td(-1) s(-1). 4. As reported previously, background illumination reduced the maximal response, reflecting a reduction in rod circulating current. In addition, it shortened the time to peak (to ca 70 ms at an intensity of 170 Td), and reduced the flash sensitivity measured at the peak. The flash sensitivity declined approximately according to Weber's Law, with a 10-fold reduction occurring at an intensity of 100-200 Td. We could not reliably measure responses at significantly higher background intensities because the circulating current became so small. 5. In order to investigate the phototransduction process after correction for response compression, we expressed the derived response as a fraction of the maximal response that could be elicited in the presence of the background. The earliest rising phase of this 'fractional response per unit intensity' was little affected by background illumination, suggesting that the amplification constant of transduction was unaltered by light adaptation.

Adaptation, Ocular↗