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PubMed1989

Adaptation to stress increases the heart resistance to ischemic and reperfusion arrhythmias.

Adaptation to repeated stress prevents or limits ischemic and reperfusion arrhythmias in the whole organism. In studying mechanism of this phenomenon, we have investigated the effect of local ischemia and subsequent reperfusion on the function of isolated hearts of rats adapted to the stress of repeated immobilization. We established that such adaptation limited the depression of the amplitude and velocity of contraction and velocity of relaxation of the heart in ischemia and subsequent reperfusion. Simultaneously this adaptation limited reperfusion-induced arrhythmias to a considerable extent; in particular, the duration of reperfusion-induced fibrillation was reduced two-fold. Thus the cardioprotective antiarrhythmic effect of adaptation of the organism to stress exposure depends not only on adaptive alterations of central regulation, but to a considerable extent, is determined by processes occurring at the level of the heart itself.

Adaptation, Physiological↗
PubMed1981

Adaptation of tracheal stretch receptors.

The adapting properties of airways slowly adapting stretch receptors have been generally measured as their declining response to maintained inflation only within the first few seconds. We measured the adaptation of tracheal stretch receptors for periods of 30 min or more and related this property to the mechanics of the structure containing them, i.e. the trachealis muscle. We recorded action potentials from thin filaments of the right vagus originating from slowly adapting stretch receptors in the extra-thoracic trachea of spontaneously breathing dogs. A transversal elongation was applied at constant speed to the posterior region of the extra-thoracic trachea and maintained for at least 30 min. The receptor discharge declined rapidly at first and progressively more slowly afterwards reaching a steady value after approximately 3 min. A similar time course was measured for the decay in force developed by the trachealis muscle subjected to a comparable elongation. The rapid decay in receptor discharge was affected by the rate at which the elongation was introduced. The slower decrease in activity resembles the stress relaxation process of the trachealis muscle. Antidromic stimulation of the endings did not modify the time course of their adaption. From these results we infer that the adaptive processes of these tracheal receptors are essentially dependent on the visco-elastic properties of the trachealis muscle.

Adaptation, Physiological↗
PubMed1987

Adaptation, plant evolution, and the fossil record.

The importance of adaptation in determining patterns of evolution has become an important focus of debate in evolutionary biology. As it pertains to paleobotany, the issue is whether or not adaptive evolution mediated by natural selection is sufficient to explain the stratigraphic distributions of taxa and character states observed in the plant fossil record. One means of addressing this question is the functional evaluation of stratigraphic series of plant organs set in the context of paleoenvironmental change and temporal patterns of floral composition within environments. For certain organ systems, quantitative estimates of biophysical performance can be made on the basis of structures preserved in the fossil record. Performance estimates for plants separated in time or space can be compared directly. Implicit in different hypotheses of the forces that shape the evolutionary record (e.g. adaptation, mass extinction, rapid environmental change, chance) are predictions about stratigraphic and paleoenvironmental trends in the efficacy of functional performance. Existing data suggest that following the evolution of a significant structural innovation, adaptation for improved functional performance can be a major determinant of evolutionary changes in plants; however, there are structural and development limits to functional improvement, and once these are reached, the structure in question may no longer figure strongly in selection until and unless a new innovation evolves. The Silurian-Devonian paleobotanical record is consistent with the hypothesis that the succession of lowland floodplain dominants preserved in the fossil record of this interval was determined principally by the repeated evolution of new taxa that rose to ecological importance because of competitive advantages conferred by improved biophysical performance. This does not seem to be equally true for Carboniferous-Jurassic dominants of swamp and lowland floodplain environments. In these cases, environmental disruption appears to have been a major factor in shaping the fossil record. This does not mean that continuing adaptation was not important during this interval, but it may indicate that adaptive evolution was strongest in environments other than those best represented in the paleobotanical record.

Adaptation, Biological↗
PubMed1982

Low spatial-frequency channels in human vision: adaptation and masking.

Previous work showed that adapting to low spatial frequency gratings (below 1.5 cycles/degree) may cause maximal spatial adaptation at a significantly higher spatial frequency. It has been suggested that there are no adaptable spatial-frequency channels tuned to below 1.5 c/deg. Contrary to this view, we found that adaptation and masking with low spatial frequencies (0.12-1.0 c/deg) produced maximal threshold elevations when the test patterns were the same spatial frequency as the adapting or masking pattern. These results were obtained using test patterns that turned on and off gradually or sharply. The results suggest that there are form mechanisms optimally sensitive to very low spatial frequencies. Adaptation was selective to position (phase) and orientation at low spatial frequencies; masking was observed to be selective to orientation at a spatial frequency as low as 0.2 c/deg. A clear dichotomy between transient, motion channels and sustained, form channels at low spatial and temporal frequencies may represent an unrealistic simplification. There may exist directionally-selective motion mechanisms sensitive to very slow motion, and these may play a role in the discrimination of form. The discussion considers the bandwidths of the low spatial frequency mechanisms.

Adaptation, Ocular↗
PubMed1983

Dark adaptation of the long-wavelength sensitive cones.

Long-wavelength sensitive (L) cone dark adaptation curves were measured with a 10-msec, 20' v.a., 650 nm test flash and a moderate intensity adapting field. When the 650 nm and 500 nm fields were equated for the L cones, the dark adaptation curve fell faster for the 500 nm field than for the 650 nm field. Over the range of adapting lights used, no adjustment of the intensities of the two fields made their dark adaptation curves similar. When 500 nm light was added to the 650 nm field, dark adaptation was faster. Visual sensitivity is not regulated by any single photoreceptor type in this experiment.

Dark Adaptation↗
PubMed1984

Facilitatory and inhibitory after-effect of spatially localized grating adaptation.

Aftereffects of spatially localized grating adaptation were measured for different locations of the adaptation grating relative to test grating. When the adaptation grating was located on or near the retinal area occupied by the test grating, contrast sensitivity was markedly reduced. When the adaptation grating was spatially separated from the test grating, contrast sensitivity was significantly increased. This aftereffect of spatially localized grating adaptation suggests that spatial-frequency-selective detectors are not spatially independent, but tonically inhibited by spatially contiguous mechanisms. Thus the adaptation of these mechanisms might cause an increase in contrast sensitivity of detectors subserving the test grating.

Adaptation, Ocular↗
PubMed1985

The early phase of dark adaptation in human infants.

Two types of thresholds with 8 degrees circular test fields were measured in 7- and 13-week-old human infants and in adults. Increment thresholds were measured against adapting fields of 0.50 and 50 cd/m2. All ages showed Weber's Law for incremental sensitivity over this 2 log unit range of luminance. Thresholds during early dark adaptation were also measured for the 5 sec immediately following the offsets of these adapting fields. The reductions in threshold during the early phase of dark adaptation were quantitatively similar in all age groups at both adapting luminances, despite substantial developmental differences in the absolute values of these thresholds. These data do not reject the hypothesis that the neural processes underlying early dark adaptation are adult-like in early infancy.

Adult↗
PubMed1988

The functional role of contrast adaptation.

Prolonged inspection of high contrast sinewave gratings increases the contrast required to detect gratings having a similar spatial frequency and orientation. The functional role of such adaptation has, however, in the past, eluded disclosure. We here show that 5 min adaptation to a 2 c/deg sinewave grating of 0.8 contrast changes the observer's ability to discriminate the contrast level of a subsequently presented grating of the same spatial frequency and orientation. Similar to the threshold elevation effect, the observers required more incremental contrast for background contrast levels between 0.1 and 0.4 following adaptation. However, for contrast levels above 0.5, the observers required less delta contrast, following adaptation, to correctly discriminate which of two gratings was incremented in contrast. A simple model for adaptation is proposed to account for the findings which is based on a shift in the semi-saturation constant of the detector's contrast-response function. According to this model, adaptation acts to linearize the underlying mechanism's response in the region near the prevailing contrast level.

Adaptation, Ocular↗
PubMed1988

Evoked potential estimates of the time course of adaptation and recovery to counterphase gratings.

Scalp-recorded evoked potentials (VEP) were sequentially sampled in humans during adaptation to and recovery from prolonged viewing of counterphase sinusoidal grating targets. The sum of the power at the first and second harmonics of the Fourier-transformed VEP components was found to decrease during adaptation and increase during recovery. Time constants (T) for the adaptation and recovery processes as estimated from exponential functions ranged from 2.9 to 19 sec, varying non-monotonically with the spatial frequency and contrast of the stimulus. The observed T values are shorter than those reported in psychophysical studies of adaptation but overlap estimates derived from single cell studies. An unexpected finding was the occurrence of a 3-6 sec delay in the appearance of the maximum VEP response after the onset of the adaptation stimulus. The delay occurred in all subjects and at all spatial frequencies when moderate to high adapting contrasts (e.g. greater than 0.2) were used. The data support a feature-selective, multi-channel lateral inhibitory model of spatial vision and suggest the presence of tonic inhibition between the channels.

Adaptation, Ocular↗
PubMed1993

Forward pattern masking and adaptation: effects of duration, interstimulus interval, contrast, and spatial and temporal frequency.

Eight experiments are described that compare pattern adaptation and forward pattern masking by examining the effects of five variables on the contrast threshold of a target presented after an adapter or masker. The target is a Gabor pattern with a center frequency of 2 c/deg and a duration of 33 msec. Thresholds are determined using an adaptive spatial forced-choice method. Principal results are as follows. (1) An adapt-refresh regime with a 2 sec refresh and a 2 sec recovery period on each trial is shown to maintain constant performance. (2) Desensitization is very rapid, reaching near maximum in < 200 msec. (3) Recovery is very rapid during the first 100-200 msec and then very slow with the rate of slow recovery decreasing as adapter/masker duration increases. (4) Threshold vs contrast functions are step-like for certain frequency pairs. (5) Sensitivity vs frequency functions derived from adapting and masking are similar in form. (6) Masker temporal frequency (0-15 Hz) has very little effect. These results are described by a theory that postulates that the target is detected by a few mechanisms that are differentially tuned to spatial frequency. The effect of both a forward masker and an adapter is to desensitize the mechanisms that respond to it. Recovery is a weighted sum of two decay processes, one fast and one slow. The theory fits the data from both paradigms well with some differences in parameters.

Adaptation, Ocular↗
PubMed1995

The effect of contrast adaptation on briefly presented stimuli.

Wilson and Humanski (1993) have recently reported evidence that adapting to low temporal frequency sinewave gratings yields little threshold elevation for briefly presented test stimuli. We postulated that brief stimuli may be detected by a transient channel which would be minimally affected by a low temporal frequency adapting pattern. We therefore measured the effect of adaptation on briefly presented test stimuli for a wider range of adapting temporal frequencies. The results indicate that adaptation may yield threshold elevation for briefly presented stimuli and that threshold elevation is greater for high than low temporal frequency adapting patterns. These results are consistent with the hypothesis that briefly presented stimuli are detected by a transient channel.

Adaptation, Ocular↗
PubMed1995

Binocular interactions in rapid saccadic adaptation.

An adaptive mechanism controls the strength of innervation to the two eyes independently. However, under some circumstances an adjustment in strength of innervation to one eye is generalized to the other. The coupling and uncoupling of the two eyes during saccadic motor learning was studied using the technique of intrasaccadic target displacements to provide a precise visual-motor error proportional to the commanded movement. Early adaptive changes (saccade plus fast vergence) were measured within the saccadic interval and late adaptive changes (vergence error) were measured after the saccadic interval. When one viewing eye was retrained using intrasaccadic displacements, saccadic amplitude changes generalized to the other nonviewing eye. Thus, rapid adaptive changes trained monocularly were transferred to the nonviewing eye. But when two eyes were viewing and an adaptive stimulus was provided to only one eye (binocular viewing-monocular training), adaptive changes also occurred in both eyes. Experiments described here suggest that the recalibration of the saccade occurs quickly as a conjugate adjustment of gain which is used to balance innervation to the two eyes. Thereafter, disconjugate mechanisms provide a further recalibration to each eye independently.

Adaptation, Physiological↗
PubMed1996

Color perception with test and adapting lights perceived in different depth planes.

Adapting to a chromatic light can alter the color appearance of other lights in view. The chromatic adapting effect is measured here with the test and adapting field perceived in the same depth plane, or perceived in different depth planes (using stereo disparity). The measurements show only a weak, though consistent, shift in the appearance of the test when adapting field and test are perceived in different depth planes, compared to when they are in the same plane. Adding complexity to the adapting stimulus, in the form of a second chromatic light surrounding the background, alters the appearance of the test but shows no dependence on the depth relations. Overall, there is only a small difference in chromatic adaptation caused by introducing a three-dimensional representation of these stimuli.

Adaptation, Physiological↗
PubMed1995

Attentional modulation of adaptation to two-component transparent motion.

We have studied the effects of voluntary attention on the induction of motion aftereffects (MAEs). While adapting, observers paid attention to one of two transparently displayed random dot patterns, moving concurrently in opposite directions. Selective attention was found to modulate the susceptibility to motion adaptation very substantially. To measure the strength of the induced MAEs we modulated the signal-to-noise ratio of a real motion signal in a random dot pattern that was used to balance the aftereffect. Results obtained for adapting to single motion vectors show that the MAE can be represented as a shift of the psychometric function for motion direction discrimination. Selective attention to the different components of transparent motion altered the susceptibility to adaptation. Shifting attention from one component to the other caused a large shift of the psychometric curves, about 70-75% of the shift measured for the separate components of the transparent adapting stimulus. We conclude that attention can differentiate between spatially superimposed motion vectors and that attention modulates the activity of motion mechanisms before or at the level where adaptation gives rise to MAEs. The results are discussed in light of the role of attention in visual perception and the physiological site for attentional modulation of MAEs.

Adaptation, Physiological↗
PubMed1996

Adaptive changes in saccade amplitude: oculocentric or orbitocentric mapping?

The saccadic system rapidly adjusts the amplitude of refixation movements to visual targets when abnormal postsaccadic errors occur. This is called rapid saccadic adaptation. It is not yet clear whether this form of adaptation produces changes related to oculocentric mechanisms, such as retinal error or motor error, or orbitocentric mechanisms, such as eye or gaze position. These experiments were designed to test whether rapid saccadic adaptation was orbitocentric, oculocentric, or both by creating a precise sensory motor mismatch between the visual target and the required saccade. Measurements were made to determine adaptive changes as function of (1) saccade direction; (2) eye position; and (3) saccade amplitude. Changes were found to be amplitude- and direction-specific but changes were generalized across a broad range of orbital positions. Two conditions of adaptation: increasing and decreasing amplitude, produced quantitatively similar results, indicating that similar mechanisms underlie both processes. Thus, these data support the view that changes during rapid saccadic adaptation are organized principally in a retina-referenced (oculocentric) map, but only broadly, if at all, in a head-referenced (orbitocentric) map. The changes are consistent with a mechanism represented in a spatial mapping of either retinal or motor error.

Adaptation, Ocular↗
PubMed1981

Multiple receptor sites mediate sweetness: evidence from cross adaptation.

The method of cross adaptation was implemented to determine whether only one type of receptor site mediates the perception of sweetness, or whether more than one such type exists. Fourteen stimuli, seven artificial sweeteners varying widely in chemical structure as well as seven sugars, were cross adapted with one another. When a sugar was employed as the adapting stimulus, a consistent reduction in the intensity of the test solution's sweetness was found. However, the result of the cross adaptation when the adapting stimulus was an artificial sweetener was unpredictable; it led not only to a reduction but, in some cases, to an enhancement or no change in the test solution's intensity, depending on its identity. In previous investigations, enhancements have been explained through the existence of a water taste. Since this explanation is insufficient to account for the enhancement effects found in this study, it appears that cross adaptation does not always occur between sweet-tasting compounds. For this reason, it is concluded that more than one receptor mechanism may be responsible for the perception of the sweet quality.

Adaptation, Physiological↗
PubMed1995

Variability in adaptive response to low dose radiation in human blood lymphocytes: consistent results from chromosome aberrations and micronuclei.

The frequencies of chromosome aberrations and micronuclei were evaluated to assess the induction of adaptive response to low dose ionizing radiation in each of the blood samples collected from eight different individuals. Following stimulation with phytohemagglutinin, the cells were exposed to an adaptive dose of 1 cGy X-radiation at 24 hours and a challenge dose of 150 cGy gamma radiation at 48 hours. Lymphocytes were fixed at 54 hours to examine the incidence of chromosome aberrations and at 72 hours to examine the frequency of micronuclei in cytokinesis-blocked binucleated cells. Lymphocytes from five donors, i.e., "responders", exhibited the induction of adaptive response; their lymphocytes, which were pre-treated with 1 cGy had significantly fewer chromosome aberrations and micronuclei induced by the challenge dose of 150 cGy gamma radiation, as compared to the cells which did not receive the pre-treatment with 1 cGy. Such an induction of adaptive response was not observed in the remaining three donors, i.e., "non-responders"; the incidence of chromosome aberrations and micronuclei induced by the challenge dose of 150 cGy was not significantly different between the cells which were pre-exposed and un-exposed to 1 cGy. In all eight individuals, there was a strong positive correlation between the incidence of chromosome aberrations and micronuclei. Hence, whether or not an individual is a 'responder' or 'non-responder' could be assessed using either chromosome aberrations or micronuclei as the end-point. The overall pattern of response confirms the heterogeneity in adaptive response between individuals to ionizing radiation, which may in part be genetically controlled. Because of the simplicity of the technique and rapid assessment of the binucleated cells, we suggest the use of the micronucleus test as an alternative procedure in large scale population studies related to the adaptive response.

Adaptation, Physiological↗
PubMed1995

Oxidant resistance of cadmium-adapted human lung fibroblasts.

Metallothionein (MT) is a metal and thiol-rich protein readily induced by cadmium (Cd) exposure. In vitro experiments have demonstrated that MT is able to serve as a scavenger of hydroxyl radicals as well as superoxide anions, albeit to a lesser extent. The role of MT as a mediator in Cd induced oxidant resistance was investigated in a nontransformed human lung fibroblast cell line (IMR-90). Cells were passaged three times either in a Cd-containing medium (8.9 microM CdCl2) or in a medium which lacked Cd. Cellular MT content, as quantitated by a modification of the heme/109Cd binding assay, increased significantly with each passage in Cd. Immunocytochemistry studies revealed that all Cd-pretreated cells contained MT and that MT was localized in both cytoplasmic and nuclear compartments. Immunolabeling was more intense in some cells compared to others. Very slight immunolabeling was observed in physiological control cells, grown in the absence of Cd, and virtually no staining was observed in Cd-adapted or non-adapted cells when primary antibody was omitted. Using the xanthine/xanthine oxidase system as a generating system for active oxygen species, we found that the magnitude of cell injury for Cd-adapted and non-adapted fibroblasts was dependent upon oxidant concentration and duration of oxidant exposure. Cd-adapted fibroblasts, which were characterized by over-expression of MT, were significantly more resistant to injury by active oxygen species and also exhibited a greater ability to scavenge extracellular hydrogen peroxide compared to cells with no previous history of Cd exposure. Experiments with aminotriazole demonstrated that catalase was not a major contributor to the additional hydrogen peroxide scavenging capacity of Cd-adapted cells. The data presented in this report are consistent with involvement of MT in protecting critical cellular targets from reactive oxygen species.

Adaptation, Physiological↗