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PubMed1963

NEURAL AND PHOTOCHEMICAL MECHANISMS OF VISUAL ADAPTATION IN THE RAT.

The effects of light adaptation on the increment threshold, rhodopsin content, and dark adaptation have been studied in the rat eye over a wide range of intensities. The electroretinogram threshold was used as a measure of eye sensitivity. With adapting intensities greater than 1.5 log units above the absolute ERG threshold, the increment threshold rises linearly with increasing adapting intensity. With 5 minutes of light adaptation, the rhodopsin content of the eye is not measurably reduced until the adapting intensity is greater than 5 log units above the ERG threshold. Dark adaptation is rapid (i.e., completed in 5 to 10 minutes) until the eye is adapted to lights strong enough to bleach a measurable fraction of the rhodopsin. After brighter light adaptations, dark adaptation consists of two parts, an initial rapid phase followed by a slow component. The extent of slow adaptation depends on the fraction of rhodopsin bleached. If all the rhodopsin in the eye is bleached, the slow fall of threshold extends over 5 log units and takes 2 to 3 hours to complete. The fall of ERG threshold during the slow phase of adaptation occurs in parallel with the regeneration of rhodopsin. The slow component of dark adaptation is related to the bleaching and resynthesis of rhodopsin; the fast component of adaptation is considered to be neural adaptation.

Adaptation, Ocular↗

PubMed1990

[Electrophysiological properties of fast- and slow-adapting units and their generator potentials of the frog tongue].

The purpose of this study was to determine the stimulus response properties of fast and slow adapting units in the fungiform papillae of the frog tongue. Secondly, fast- and slow- adapting generator potentials were recorded from fast and slow-adapting mechanoreceptors in the single fungiform papilla glossopharyngeal nerve preparations, respectively. Results I. Impulse response properties of fast- and slow-adapting units 1) Most of the fungiform papillae were innervated by both fast- and slow-adapting units. Fast-adapting units evoked 1-4 impulses to each stimulus and the adaptation time was less than 17.5 msec. Slow-adapting units evoked 27.8 +/- 8.0 impulses (range: 11-49 impulses n = 18) during a pressure stimulation of 3 sec, and the adaptation time was 1.64 +/- 0.73 sec (range: 0.53-2.86 sec, n = 18). 2) Threshold, latency and absolute refractory period for fast-adapting units were 7.0 +/- 1.9 microns (range: 3.0-11.8 microns, n = 189), 2.31 +/- 1.29 msec (range: 0.85-6.80 msec, n = 31) and 2.9 +/- 1.0 msec (range: 1.8-5.6 msec, n = 33), respectively. Those for slow-adapting units were 4.6 +/- 1.8 microns (range: 2.0-11.8 microns, n = 152), 13.54 +/- 11.29 msec (range: 2.00-54.00 msec, n = 35) and 6.5 +/- 3.6 msec (range: 1.9-19.6 msec, n = 35), respectively. 3) A fast-adapting unit innervated 5.1 +/- 2.5 fungiform papillae (range: 1-13 fungiform papillae, n = 58) and the receptive area was 0.342 +/- 0.312 mm2 (range: 0.005-1.548 mm2, n = 55). A slow-adapting unit innervated 3.3 +/- 2.0 fungiform papillae (range: 1-12 fungiform papillae, n = 50) and the receptive area was 0.158 +/- 0.144 mm2 (range: 0.006-0.616 mm2, n = 29). 4) Conduction velocity of the fast-adapting unit was 23.0 +/- 3.1 m/sec (range: 15.0-30.6 m/sec, n = 528) and that of the slow-adapting unit was 12.8 +/- 2.2 m/sec (range: 4.4-21.1 m/sec, n = 495). The conduction velocity was calculated from the time necessary to conduct at two different points of the nerve fiber. 5) The upper limits of fast- and slow-adapting units for vibratory stimulation were 62.7 +/- 10.5 Hz (range: 50-80 Hz, n = 15) and 34.5 +/- 9.6 Hz (range: 15-45 Hz, n = 10), respectively. II.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1973

Adaptation and dynamics of cat retinal ganglion cells.

1. The impulse/quantum (I/Q) ratio was measured as a function of background illumination for rod-dominated, pure central, linear square-wave responses of retinal ganglion cells in the cat.2. The I/Q ratio was constant at low backgrounds (dark adapted state) and inversely proportional to the 0.9 power of the background at high backgrounds (the light adapted state). There was an abrupt transition from the dark-adapted state to the light-adapted state.3. It was possible to define the adaptation level at a particular background as the ratio (I/Q ratio at that background)/(dark adapted I/Q ratio).4. The time course of the square-wave response was correlated with the adaptation level. The response was sustained in the dark-adapted state, partially transient at the transition level, and progressively more transient the lower the impulse/quantum ratio of the ganglion cell became. This was true both for on-centre and off-centre cells.5. The frequency response of the central response mechanism at different adaptation levels was measured. It was a low-pass characteristic in the dark-adapted state and became progressively more of a bandpass characteristic as the cell became more light-adapted.6. The rapidity of onset of adaptation was measured with a time-varying adapting light. The impulse/quantum ratio is reset within 100 msec of the onset of the conditioning light, and is kept at the new value throughout the time the conditioning light is on.7. These results can be explained by a nonlinear feedback model. In the model, it is postulated that the exponential function of the horizontal cell potential controls transmission from rods to bipolars. This model has an abrupt transition from dark- to light-adapted states, and its response dynamics are correlated with adaptation level.

Action Potentials↗

PubMed1997

The concept of an evolved adaptation.

A Darwinian adaptation is an organism's feature that was functionally designed by the process of evolution by selection acting in nature in the past. Functional design rules out explanations of drift, incidental effect, phylogenetic legacy and mutation. Elucidation of the functional design of an adaptation entails an implicit reconstruction of the selection that made the adaptation. Darwinian adaptations and other individual traits may be currently adaptive, maladaptive or neutral. One relatively recent meaning of adaptation is inconsistent with the Darwinian conception of adaptation. The inconsistent meaning characterizes much research on humans and non-human species in behavioural ecology. Its focus is on equating Darwinian adaptation with current adaptiveness. Current adaptiveness is not an actual scientific prediction of a hypothesis about Darwinian adaptation. Some aspects of the discussion in the evolutionary literature surrounding the current adaptiveness view of adaptation are evaluated. Contrary to claims by some who advocate current adaptiveness, the environment of evolutionary adaptedness of humans and other organisms is scientifically knowable through discovery of the functional design of Darwinian adaptations.

Adaptation, Psychological↗

PubMed2005

Cerebellar AMPA/KA receptor antagonism by CNQX inhibits vestibuloocular reflex adaptation.

Vestibuloocular reflex (VOR) performance and adaptation have been investigated during antagonism of cerebellar AMPA/quisqualate and kainate receptors (AMPA/KA) by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Injection of CNQX into the vestibulo-cerebellum of the goldfish before adaptation significantly inhibited and, at the highest dosage, completely prevented acquisition of adaptive reflex gain increases and decreases during a 3-h training period. Injection of CNQX before initiation of VOR adaptive training did not affect pre-adapted baseline performance of the reflex. Injection of CNQX, 1 to 2 h after the initiation of training did not alter the performance of adaptive gain increases that occurred before the injection. If injection of CNQX occurred at the end of adaptive training, there was an accelerated loss of the previously adapted gain changes during the retention period when the animal remained stationary in the dark. CNQX injection did not produce any permanent or long-term deficits, because goldfish could be retrained 48 h later to produce adaptive VOR gain changes similar to control animals. Thus, this work demonstrates that the AMPA/KA receptors located in the vestibulo-cerebellum of the goldfish are necessary for acquisition of short-term adaptive VOR gain increases and decreases. The deficit in adaptive capability was not the result of a deficit in performance, because CNQX did not inhibit an adaptive change that had already occurred as long as the adapting vestibular and visual stimulation continued. This adaptive performance could possibly be maintained by other glutamatergic (metabotropic) receptors located on the Purkinje cells. The retention of adapted gain increases and decreases after CNQX application was inhibited because AMPA/KA antagonism accelerated VOR gain loss after the completion of training when no vestibular or visual stimulation was present. Because the AMPA/KA receptors are located only in the molecular layer of the goldfish cerebellum, these results are, presumably, the result of AMPA/KA receptor antagonism at synapses located on the Purkinje cell dendrite tree.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2006

Bilateral basal ganglia activation associated with sensorimotor adaptation.

Sensorimotor adaptation tasks can be classified into two types. When subjects adapt movements to visual feedback perturbations such as in prism lens adaptation, they perform kinematic adaptations. When subjects adapt movements to force field perturbations such as with robotic manipulanda, they perform kinetic adaptations. Neuroimaging studies have shown basal ganglia involvement in kinetic adaptations, but have found little evidence of basal ganglia involvement in kinematic adaptations, despite reports of deficits in patients with diseases of the basal ganglia, such as Parkinson's and Huntington's disease, in these. In an effort to resolve such apparent discrepancy, we used FMRI to focus on the first few minutes of practice during kinematic adaptation. Human subjects adapted to visuomotor rotations in the context of a joystick aiming task while lying supine in a 3.0 T MRI scanner. As demonstrated previously, early adaptive processes were associated with BOLD activation in the cerebellum and the sensory and motor cortical regions. A novel finding of this study was bilateral basal ganglia activation. This suggests that, at least for early learning, the neural correlates of kinematic adaptation parallel those of other types of skill learning. We observed activation in the right globus pallidus and putamen, along with the right prefrontal, premotor and parietal cortex, which may support spatial cognitive processes of adaptation. We also observed activation in the left globus pallidus and caudate nucleus, along with the left premotor and supplementary motor cortex, which may support the sensorimotor processes of adaptation. These results are the first to demonstrate a clear involvement of basal ganglia activation in this type of kinematic motor adaptation.

Adaptation, Physiological↗

PubMed1976

The effect of parathyroidectomy and large doses of cholecalciferol on the ability of rats to adapt to changes in dietary intake of calcium.

1. Adaptation to different dietary levels of calcium was produced by feeding a low (0-2%) calcium diet or one of two high (1-6 or 0-8%) calcium diets for 4 or 6 weeks. Adaptive changes in true and apparent absorption of calcium, apparent absorption of phosphate and urinary excretion of calcium, were observed. 2. Parathyroidectomy performed prior to adaptation did not greatly impair the ability of rats to adapt to different levels of calcium in the diet. The response of the rats to parathyroidectomy was affected by their subsequent dietary history. 3. Six weeks after parathyroidectomy the plasma calcium was significantly higher than it had been immediately post-operatively. This rise in plasma calcium was seen in the rats adapted to the 1-6% calcium diet but not in those adapted to the 0-2% calcium diet. 4. Parathyroidectomy performed after adaptation had taken place did not abolish the adaptive changes. The response of rats to parathyroidectomy was affected by their previous dietary history. 5. Large doses of cholecalciferol given for 8 days after adaptation had taken place increased the absorption of calcium in rats adapted to the 0-8% calcium diet thereby abolishing or reducing the adaptive differences in absorption between these rats and rats adapted to the 0-2% calcium diet. The cholecalciferol increased urinary calcium excretion but did not abolish adaptive differences in urinary excretion of calcium between rats adapted to diets with different calcium levels. 6. It is concluded that parathyroid hormone does not play a major role in mediating adaptation to different dietary intakes of calcium. The possible role of 1-25 dihydroxycholecalciferol is discussed.

Adaptation, Physiological↗

PubMed1981

Longitudinal spread of adaptation in the rods of the frog's retina.

1. The stimulus-response function of the red rods in the retina of the common frog (Rana temporaria) was determined in different adaptational states by measuring aspartate-isolated receptor responses. 2. Flash stimuli, background adaptations and bleaches were delivered through the same optical channel forming an oblique light-beam striking the receptor side of the isolated and flat-mounted retina at an angle of 10 degrees. 3. When the light was blue-green and optimally polarized the absorbance of the receptor layer was about 2, from which follows that 70-80% of the light was absorbed in the distal third of the rod outer segments, i.e. the exposure was local. Homogeneous exposures of the whole rod outer segments were obtained with orange and red lights. 4. Combinations of homogeneous and local stimuli with homogeneous and local adaptations were used to investigate the longitudinal spread of background, intermediate and opsin adaptation, i.e. the sensitivity-reducing effect of a background light, and the transient and permanent sensitivity losses following a bleach isomerizing 3.5-26% (usually 10%) of the rhodopsin in the retina. 5. The results obtained were related to predictions based both on the assumption that the adaptation effects spread longitudinally within the rod outer segments and the assumption that they are strictly confined to the disks absorbing the adapting lights. 6. These comparisons reveal that all three types of adaptation spread longitudinally. It is for instance clear that the sensitivity loss observed with homogeneous stimuli and local adaptation (as compared to homogeneous adaptation) is larger than that predicted by the non-spreading hypothesis. 7. The longitudinal spread of background adaptation is largely finished within 10 sec after turning on the background light, while an efficient spread of the intermediate adaptation effect may require minutes. 8. A background light decreasing the sensitivity by about one log unit decreases the time from flash to response maximum from 5 to 1 sec (small responses). Corresponding opsin adaptation effects are accompanied by less dramatic changes in response kinetics. 9. Independent of adaptation type - homogeneous or local, background, intermediate or opsin - it was found that local stimuli are less efficient that homogeneous stimuli in light-adapted retinae. This effect can be explained assuming that the sensitivity-reducing effects are pronounced in the distal than in the proximal parts of the rod outer segments. 10. The opsin adaptation effect following 10% local bleaches decreases the sensitivity to both homogeneous and local stimuli 2-3 times more than corresponding homogeneous bleaches. This means that the strength of the opsin effect is not related to the average percentage bleached but to the fraction bleached in the distal part of the rod, or generally to the fraction bleached in the most affected region. 11...

Adaptation, Ocular↗

PubMed2005

Time-course of vibratory adaptation and recovery in cutaneous mechanoreceptive afferents.

Extended suprathreshold vibratory stimulation applied to the skin results in a desensitization of cutaneous mechanoreceptive afferents. In a companion paper, we describe the dependence of the threshold shift on the parameters of the adapting stimulus and discuss neural mechanisms underlying afferent adaptation. Here we describe the time-course of afferent adaptation and recovery. We found that absolute and entrainment thresholds rise and fall exponentially during adaptation and recovery with time constants that vary with fiber type. slowly adapting type I (SA1) afferents adapt most rapidly, and pacinian (PC) afferents adapt most slowly, whereas rapidly adapting (RA) afferents exhibit intermediate rates of adaptation; SA1 fibers also recover more rapidly from adaptation than RA and PC fibers. We also showed that threshold adaptation is accompanied by a shift in the timing of the spikes within individual cycles of the adapting stimulus (i.e., a shift in the impulse phase). We invoked an integrate-and-fire model to explore possible mechanisms underlying afferent adaptation. Finally, we found that the time-course of afferent adaptation is more rapid than that of its psychophysical counterpart, as is the time-course of recovery from adaptation, suggesting that central factors play a role in the psychophysical phenomenon.

Action Potentials↗

PubMed2005

Orientation-tuned FMRI adaptation in human visual cortex.

Adaptation is a general property of almost all neural systems and has been a longstanding tool of psychophysics because of its power to isolate and temporarily reduce the contribution of specific neural populations. Recently, adaptation designs have been extensively applied in functional MRI (fMRI) studies to infer neural selectivity in specific cortical areas. However, there has been considerable variability in the duration of adaptation used in these experiments. In particular, although long-term adaptation has been solidly established in psychophysical and neurophysiological studies, it has been incorporated into few fMRI studies. Furthermore, there has been little validation of fMRI adaptation using stimulus dimensions with well-known adaptive properties (e.g., orientation) and in better understood regions of cortex (e.g., primary visual cortex, V1). We used an event-related fMRI experiment to study long-term orientation adaptation in the human visual cortex. After long-term adaptation to an oriented pattern, the fMRI response in V1, V2, V3/VP, V3A, and V4 to a test stimulus was proportional to the angular difference between the adapting and test stimuli. However, only V3A and V4 showed this response pattern with short-term adaptation. In a separate experiment, we measured behavioral contrast detection thresholds after adaptation and found that the fMRI signal in V1 closely matched the psychophysically derived contrast detection thresholds. Similar to the fMRI results, adaptation induced threshold changes strongly depended on the duration of adaptation. In addition to supporting the existence of adaptable orientation-tuned neurons in human visual cortex, our results show the importance of considering timing parameters in fMRI adaptation experiments.

Adaptation, Physiological↗

PubMed1998

Visually induced adaptation in three-dimensional organization of primate vestibuloocular reflex.

The adaptive plasticity of the spatial organization of the vestibuloocular reflex (VOR) has been investigated in intact and canal-plugged primates using 2-h exposure to conflicting visual (optokinetic, OKN) and vestibular rotational stimuli about mutually orthogonal axes (generating torsional VOR + vertical OKN, torsional VOR + horizontal OKN, vertical VOR + horizontal OKN, and horizontal VOR + vertical OKN). Adaptation protocols with 0.5-Hz (+/-18 degrees ) head movements about either an earth-vertical or an earth-horizontal axis induced orthogonal response components as high as 40-70% of those required for ideal adaptation. Orthogonal response gains were highest at the adapting frequency with phase leads present at lower and phase lags present at higher frequencies. Furthermore, the time course of adaptation, as well as orthogonal response dynamics were similar and relatively independent of the particular visual/vestibular stimulus combination. Low-frequency (0. 05 Hz, vestibular stimulus: +/-60 degrees ; optokinetic stimulus: +/-180 degrees ) adaptation protocols with head movements about an earth-vertical axis induced smaller orthogonal response components that did not exceed 20-40% of the head velocity stimulus (i.e., approximately 10% of that required for ideal adaptation). At the same frequency, adaptation with head movements about an earth-horizontal axis generated large orthogonal responses that reached values as high as 100-120% of head velocity after 2 h of adaptation (i.e., approximately 40% of ideal adaptation gains). The particular spatial and temporal response characteristics after low-frequency, earth-horizontal axis adaptation in both intact and canal-plugged animals strongly suggests that the orienting (and perhaps translational) but not inertial (velocity storage) components of the primate otolith-ocular system exhibit spatial adaptability. Due to the particular nested arrangement of the visual and vestibular stimuli, the optic flow pattern exhibited a significant component about the third spatial axis (i.e., orthogonal to the axes of rotation of the head and visual surround) at twice the oscillation frequency. Accordingly, the adapted VOR was characterized consistently by a third response component (orthogonal to both the axes of head and optokinetic drum rotation) at twice the oscillation frequency after earth-horizontal but not after earth-vertical axis 0.05-Hz adaptation. This suggests that the otolith-ocular (but not the semicircular canal-ocular) system can adaptively change its spatial organization at frequencies different from those of the head movement.

Adaptation, Physiological↗

PubMed2005

Effect of rapid or gradual grain adaptation on subacute acidosis and feed intake by feedlot cattle.

The effects of grain adaptation protocol on subacute acidosis and feed intake by cattle were studied in a completely randomized experiment using 12 crossbred heifers (384 +/- 25 kg BW). The dietary proportion of concentrate was increased from 40 to 90% (DM basis) either by rapid adaptation (65% concentrate diet fed for 3 d) or by gradual adaptation (five intermediate diets containing 48.3, 56.7, 65.0, 73.3, and 81.7% concentrate, fed for 3 d each). Feed intake and ruminal pH (by indwelling ruminal electrodes) were monitored over 20 d. Mean daily pH variables did not differ (P > or = 0.10) between treatments on any of the 3 or 4 d that 65 or 90% concentrate was fed. Variances of a number of pH variables were greater (P < 0.05) for rapidly adapted heifers than for those on the gradual adaptation protocol during adaptation to 65 and 90% concentrate. Mean hourly pH did not differ over the first 24 h of adaptation to 65% concentrate, but variance of hourly pH tended (P < 0.10) to be greater for rapidly adapted than for gradually adapted heifers for eight of the first 24 h. On the first day of feeding 90% concentrate, ruminal pH tended (P = 0.07) to be less at 11 and 12 h after feeding with rapid adaptation than with gradual adaptation. Variance of hourly pH increased steadily in rapidly adapted heifers from 6 h after feeding onward. Ruminal VFA concentration and osmolality did not differ between treatments. Ruminal lactate concentration was < 1 mM, except in two rapidly adapted heifers and one gradually adapted heifer after introduction to 90% concentrate. Adaptation method did not affect DMI or day-to-day variation in DMI. Detection of acidosis was associated with increased variance in ruminal pH variables. A range of individual responses to grain challenge was observed, but current management strategies for preventing acidosis in pens of cattle are based on responses of the most susceptible individuals. A better understanding of factors governing individual responses to acidotic challenge may allow for the development of more effective acidosis prevention practices.

Acidosis↗

PubMed1997

Adaptation to altered visual-vestibular feedback: mechanisms of maintenance and recovery.

Adaptation of perceived movement during head motion (apparent concomitant motion, ACM) and the subsequent elimination of adaptation were studied in two experiments. During the adaptation phase of both experiments, subjects performed voluntary 1-Hz head oscillations for 6 min while fixating a stimulus moving either in the same (with) direction as or the opposite (against) direction of head movements. In Experiment 1, ACM adaptation was measured following either a 1- or a 4-min delay after the adaptation phase. Results indicated some loss of adaptation during the additional 3-min delay, demonstrating a tendency of the system linking head and image to return to its preadaptation state following removal of an adaptation stimulus. In Experiment 2, subjects viewed a stimulus after adaptation that appeared to move minimally in the same manner as the adaptation stimulus during 3 min of head oscillations. No loss of adaptation was measured in these subjects between the beginning and the end of the 3-min interval. In another condition, subjects viewed a stimulus that appeared to move alternately in the same direction as and in the opposite direction of the adaptation stimulus during a similar 3-min interval following adaptation. ACM adaptation was substantially reduced during this 3-min interval. These results implicate two mechanisms that operate to either maintain or eliminate the short-term adaptation. One is passive and operates in the absence of visual feedback to eliminate the short-term adapted state, and the other responds to postadaptation visual feedback.

Adaptation, Psychological↗

PubMed1991

[The adaptive response to mitomycin C exposure in the hyper-radioresistant mutant Escherichia coli Gamr444].

Adaptive response to mitomycin C (MC) (lethal effect and recovery of molecular mass of DNA) in hyper-radioresistant mutant Escherichia coli Gamr444 have been investigated. This mutant is more resistant to MC than parent strain E. coli K12 AB1157. Adaptation of Gamr444 mutant to MC in nonlethal concentrations increases its resistance to MC in lethal concentrations with dose modification factor (DMF) 2.4 at the LD90 level. During the adaptation of this mutant to methyl-methane sulfonate (MMS) its resistance to this agent increases with DMF by 2.2 and resistance to MC with DMF by 1.5 times. During the adaptation of Gamr444 mutant to MC its resistance to MMS increases with DMF by 1.5 times. Adaptive response to MC abolishes by chloroamphenicol treatment during the adaptation. Adaptive response to nitrogen mustard (HN2) in E. coli Gamr444 is absent (HN2 induces cross-links in DNA as MC). Degradation of DNA following the formation of cross-links in DNA takes place. Adaptation to MC in Gamr444 mutant leads to restoration of DNA molecular mass which is more quicker than in the case without adaptation. Adaptive restoration of DNA molecular mass after the MC treatment is absent in E. coli K12 AB1157. The repair of cross-links in DNA after the treatment of HN2 in Gamr444 mutant takes place with equal rate both in the case of adaptation to HN2 and in the case without adaptation. It is proposed, that under the treatment of MC in E. coli Gamr444 the ada-alkA-dependent adaptive response takes place. This adaptive response is connected with alkylation of O6-guanine and elimination of the product by O6-alkyl-DNA-alkyltransferase. Partial recA-dependency of the adaptive response to MC allows to suggest the participation of another inducible system. The nature of this system is unknown.

Adaptation, Physiological↗

PubMed1987

Corresponding chromaticities for different states of adaptation to complex visual fields.

While each of his or her two eyes was independently adapted to a different illuminant in viewing a complex visual field, each of a number of observers matched a series of test colors seen by one eye with a juxtaposed variable stimulus seen by the other eye. The 2 degrees test and matching stimuli were located centrally in the complex adapting field, which subtended an angle of 31 degrees X 24 degrees. In making the matches, the observer viewed the test and matching stimuli for a series of brief intervals (approximately 1 sec) while viewing the complex adapting field with normal eye movements. Nine experiments were performed with different pairs of illuminants and different illuminances ranging from that of an average living room to that of a scene illuminated with hazy sunlight. In three other experiments each of the observer's two eyes was adapted to a different illuminance of D55. The amount of adaptation was more nearly complete at high levels of illuminance than at low levels, and the proportional amount of adaptation was less for the "blue" receptors. When adaptation coefficients were determined from the actual adaptation differences (e.g., from corresponding tristimulus values for matching neutrals) rather than from the adapting illuminants, a linear von Kries transformation based on experimentally determined visual primaries gave corresponding chromaticities that were in good agreement with the results obtained in each of the chromatic-adaptation experiments, except at the lowest illuminances. The results of the experiments in which each eye was adapted to different levels of the same illuminant indicated again that adaptation to the different levels was incomplete, the proportional amount of adaptation being less at low illuminances and for the "blue" receptors. This caused a change in chromatic adaptation with the level of illuminance even when the chromaticities of the adapting lights were equal. The results of these experiments also indicated that higher purities are needed in order to produce the same absolute color appearances at low levels of illuminance.

Color↗

PubMed1997

The adaptational system as a dynamical feedback system.

The characteristics of biological systems of adaptation are developed from the principle that the manifestations of life are modified by and must conform with their environment in order to enable organismic persistence. The two roles of the environment, termed "modifying" and "adaptive", give rise to the distinction of three elementary and sequentially coupled subsystems characterizing the adaptational system: the modifying system, comparator system, and state regulation system. The third system determines which state alterations are required for adaptation of manifestations (responses) to the demands of the adaptive environment. Adaptational valuation is introduced as a measure of adaptedness of response to adaptive environment. The dynamical aspect of adaptation is shown to be completely described by the timing of feedback. Discrete and continuous forms of adaptation can thus be treated on the same conceptional basis. All steps of the generic system formulation are illustrated with the help of a simple model with additive effects and linear state regulation. The system representation is used to demonstrate how basic intuitive conceptions, such as adaptational lag or adaptational capacity, can be made amenable to precise analysis. Another demonstration concerns recognition of adaptational clues resulting from the distinction between the two environmental classes, modifying and adaptive. Among these clues are challenges of low correlation between the two classes to adaptational systems and the specification of questions of the evolution of phenotypic plasticity.

Adaptation, Biological↗

PubMed1992

Human ocular following responses are plastic: evidence for control by temporal frequency-dependent cortical adaptation.

Optokinetic nystagmus (OKN) induced by wide-field visual stimulation was measured with and without prior adaptation to moving sinusoidal gratings. Under unadapted conditions the mean gains of the slow phases of OKN in the first 500 ms were 0.5-0.8, and the eye velocities and amplitudes had rise times with time constants of 0.1-0.2 s. By contrast, following adaptation to as little as 1 s of image motion, the magnitude of the initial gains fell and the rise times of the velocities and amplitudes increased markedly. The degree of adaptation depended on the adapting temporal frequency, the optimum adaptive frequencies being 1.7-3.4 Hz. In this range of temporal frequencies, the initial gains fell to 0.1-0.3 and the rise times for velocity and amplitude ranged from 0.4 to 7.0 s, depending on the length of the adapting period. Thus the observed changes in the time constant were up to 70-fold. Neither spatial frequency or image velocity had any marked influence on the level of adaptation. The dependence on temporal frequency rather than image velocity suggests that the motion detectors feeding the adaptive system respond to local motion-related changes in luminance. The adaptive effects were direction-selective, showing that this must also be the case for the motion detectors. The adaptive effects were observed both when the drift temporal frequency on the retina was established by artificially maintaining a fixed gaze or when the adapting temporal frequency was induced by retinal slip during OKN. Time constants for recovery from adaptation were similar to motion aftereffects measured by psychophysical and physiological methods. The results suggest a link between cortical motion adaptation and adaptive mechanisms effecting the oculomotor system.

Adaptation, Physiological↗

PubMed1991

The time course of adaptation to spatial contrast.

We explored the buildup and decay of threshold elevation during and after adaptation to sinewave gratings in a series of experiments investigating the effects of adapting time, adapting contrast, spatial frequency and retinal eccentricity. Contrast thresholds for vertical sinewave gratings truncated in space by a one-dimensional Gaussian envelope were measured before and after adaptation to a full-field suprathreshold grating of the same spatial frequency and orientation. Thresholds were measured intermittently after adaptation in a "seen/not-seen" single presentation procedure until these thresholds returned to baseline values. The first test grating was presented 300 msec after the offset of the adapting stimulus, and thereafter at regular intervals. At different times after adaptation, contrast thresholds were estimated by off-line analysis of the data using the QUEST algorithm. Adapting time was either 1, 10, 100 or 1000 sec and adapting contrast was either 9, 19, 29 or 39 dB (re. 1%). The test gratings were presented centered either at the fixation point or at 5 and 10 deg eccentricity along the horizontal meridian. The results suggest that up to the saturation level the buildup and the decay of adaptation to contrast is well described by a power function of time. The slope of the best fitting line on log-log axes is fairly constant for the adaptation times tested. As reported earlier, thresholds increased with adapting contrast and these contrast-dependent differences were evident 300 msec after the termination of adaptation. Adaptation at 10 deg eccentricity yielded slightly higher threshold elevations than for central vision. Based on these results, a description is given of the dynamic response of the underlying neural mechanisms.

Adaptation, Ocular↗