Distinct mechanisms control contrast adaptation over different timescales

Daily dose-response from short-term monocular deprivation in adult humans

Short-term monocular deprivation (MD) shifts sensory eye balance in favour of the previously deprived eye. The effect of MD on eye balance is significant but brief in adult humans. Recently, researchers and clinicians have attempted to implement MD in clinical settings for adults with impaired binocular vision. Although the effect of MD has been studied in detail in single-session protocols, what is not known is whether the effect of MD on eye balance deteriorates after repeated periods of MD (termed 'perceptual deterioration'). An answer to this question is relevant for two reasons. Firstly, the effect of MD (i.e., dose-response) should not decrease with repeated use if MD is to be used therapeutically (e.g., daily for weeks). Second, it bears upon the question of whether the neural basis of the effects of MD and contrast adaptation, a closely related phenomenon, is the same. The sensory change from contrast adaptation depends on recent experience. If the observer has recently experienced the same adaptation multiple times for consecutive days, then the adaptation effect will be smaller because contrast adaptation exhibits perceptual deterioration, so it is of interest to know if the effects of MD follow suit. This study measured the effect of 2-h MD for seven consecutive days on binocular balance of 15 normally sighted adults. We found that the shift in eye balance from MD stayed consistent, showing no signs of deterioration after subjects experienced multiple periods of MD. This finding shows no loss of effectiveness of repeated daily doses of MD if used therapeutically to rebalance binocular vision in otherwise normal individuals. Furthermore, ocular dominance plasticity, which is the basis of the effects of short-term MD, does not seem to share the property of 'perceptual deterioration' with contrast adaptation, suggesting different neural bases for these two related phenomena.

Adaptation-induced sharpening of orientation tuning curves in the mouse visual cortex

OBJECTIVE

Orientation selectivity is an emergent property of visual neurons across species with columnar and noncolumnar organization of the visual cortex. The emergence of orientation selectivity is more established in columnar cortical areas than in noncolumnar ones. Thus, how does orientation selectivity emerge in noncolumnar cortical areas after an adaptation protocol? Adaptation refers to the constant presentation of a nonoptimal stimulus (adapter) to a neuron under observation for a specific time. Previously, it had been shown that adaptation has varying effects on the tuning properties of neurons, such as orientation, spatial frequency, motion and so on.

BASIC METHODS

We recorded the mouse primary visual neurons (V1) at different orientations in the control (preadaptation) condition. This was followed by adapting neurons uninterruptedly for 12 min and then recording the same neurons postadaptation. An orientation selectivity index (OSI) for neurons was computed to compare them pre- and post-adaptation.

MAIN RESULTS

We show that 12-min adaptation increases the OSI of visual neurons ( n  = 113), that is, sharpens their tuning. Moreover, the OSI postadaptation increases linearly as a function of the OSI preadaptation.

CONCLUSION

The increased OSI postadaptation may result from a specific dendritic neural mechanism, potentially facilitating the rapid learning of novel features.

Under the sun: adaptation effects to changes in facial complexion

BACKGROUND

Many Western people enjoy sunshine, and through the sun's stimulated increase in melanin, the skin tone or skin complexion will darken (and lighten again during winter). Although the initial salience of such a new look is remarkable - especially in the face - we seem to adapt to this new look relatively quickly. Research on face adaptation in general repeatedly showed that the inspection of manipulated versions of faces (so-called adaptor faces) leads to a change of the perception of subsequently presented faces. The present study investigates face adaptation to very natural changes in faces such as changes in complexion.

METHODS

During the adaptation phase in the present study, participants saw faces with either strongly increased or decreased complexion. After a pause of 5 min, participants had to identify the veridical (non-manipulated) face out of two faces (a face slightly manipulated in complexion combined with the non-manipulated face) during a test phase.

RESULTS

Results show strong adaptation effects to decreased complexion intensities.

DISCUSSION

It seems that we are updating our facial representations in memory quite quickly (i.e., optimizing our processing through adaptation) and seem to sustain those new representations over a certain timespan (at least 5 min). Our results demonstrate that changes in complexion draw our attention for deeper analysis (at least with decreased complexion). However, it loses its informative quality quickly via fast and relatively sustainable adaptation.

Is ocular dominance plasticity a special case of contrast adaptation?

The visual system can regulate its sensitivity depending on the prevailing contrast conditions. This is known as contrast adaptation and reflects contrast gain changes at different stages along the visual pathway. Recently, it has been shown that depriving an eye of visual stimulation for a short period of time can lead to neuroplastic changes in ocular dominance as the result of homeostatic changes in contrast gain. Here we examine, on the basis of previously published results, whether the neuroplastic ocular dominance changes are just manifestation of the mechanism responsible for contrast adaptation. The evidence suggests that these two visual processes are separate and do not have a common neural substrate.

Learning to see again: Perceptual learning of simulated abnormal on- off-cell population responses in sighted individuals

Many forms of artificial sight recovery, such as electronic implants and optogenetic proteins, generally cause simultaneous, rather than complementary firing of on- and off-center retinal cells. Here, using virtual patients—sighted individuals viewing distorted input—we examine whether plasticity might compensate for abnormal neuronal population responses. Five participants were dichoptically presented with a combination of original and contrast-reversed images. Each image (I) and its contrast-reverse (Iʹ) was filtered using a radial checkerboard (F) in Fourier space and its inverse (Fʹ). [I * F′] + [Iʹ * F] was presented to one eye, and [I * F] + [Iʹ * F′] was presented to the other, such that regions of the image that produced on-center responses in one eye produced off-center responses in the other eye, and vice versa. Participants continuously improved in a naturalistic object discrimination task over 20 one-hour sessions. Pre-training and post-training tests suggest that performance improvements were due to two learning processes: learning to recognize objects with reduced visual information and learning to suppress contrast-reversed image information in a non–eye-selective manner. These results suggest that, with training, it may be possible to adapt to the unnatural on- and off-cell population responses produced by electronic and optogenetic sight recovery technologies.

Time Course of Perceived Visual Distortion and Axial Length Growth in Myopic Children Undergoing Orthokeratology

Purpose: To establish the time course of the subjective visual function changes during the first month of orthokeratology treatment in myopic children, and to investigate how the time course variations are associated with the objective optical quality changes and the axial length growth (ALG) after 1 year of treatment.

Methods: A total of 58 myopic children aged from 8 to 16 years participated in this self-controlled prospective study. All subjects were fitted with designed spherical four-zone orthokeratology lenses. Subjective visual function was evaluated with orientation discrimination threshold (ODT), and objective optical quality was quantified with the high-order aberration root-mean-square (HOA-RMS) and the changing speed of HOA. The measurements were done before the lens fitting and 1 day, 1-, 2-, and 4-weeks after lens wear. Axial length was obtained at baseline and 1-year follow-up, and ALG was defined as the difference. One-way ANOVA was conducted to compare the difference for statistical analysis.

Results: After lens fitting, the ODT time courses peaked on day 1 in 28 children, 1 week in 15 children, 2 weeks in 11 children, and 4 weeks in 4 children. In contrast, the HOA-RMS steadily rose during the first month, and the changing speed of HOA was only transiently elevated on day 1 after the initial lens wear. The ALG was 0.12 ± 0.20 mm in subjects whose ODT peaked at day 1, 0.08 ± 0.09 mm in subjects whose ODT peaked on 1-week, and 0.12 ± 0.15 mm in subjects whose ODT peaked on 2-week or later. There was no difference in axial growth among the subjects whose ODT peaked at different days (P = 0.734).

Conclusion: While half ODT time course resembled the changing speed of HOA with a transient elevation on day 1, about a quarter of the ODT time course resemble the steadily rising of HOA-RMS, and the rest was located in the middle. The ALGs in children with different types of ODT time courses were similar.

Spontaneous recovery of adaptation aftereffects of natural facial categories

Adaptation to a natural face attribute such as a happy face can bias the perception of a subsequent face in this dimension such as a neutral face. Such face adaptation aftereffects have been widely found in many natural facial categories. However, how temporally tuned mechanisms could control the temporal dynamics of natural face adaptation aftereffects remains unknown. To address the question, we used a deadaptation paradigm to examine whether the spontaneous recovery of natural facial aftereffects would emerge in four natural facial categories including variable categories (emotional expressions in Experiment 1 and eye gaze in Experiment 2) and invariable categories (facial gender in Experiment 3 and facial identity in Experiment 4). In the deadaptation paradigm, participants adapted to a face with an extreme attribute (such as a 100% angry face in Experiment 1) for a relatively long duration, and then deadapted to a face with an opposite extreme attribute (such as a 100% happy face in Experiment 1) for a relatively short duration. The time courses of face adaptation aftereffects were measured using a top-up manner. Deadaptation only masked the effects of initial longer-lasting adaptation, and the spontaneous recovery of adaptation aftereffects was observed at the post-test stage for all four natural facial categories. These results likely indicate that the temporal dynamics of adaptation aftereffects of natural facial categories may be controlled by multiple temporally tuned mechanisms.

Functional reallocation of sensory processing resources caused by long-term neural adaptation to altered optics

The eye’s optics are a major determinant of visual perception. Elucidating how long-term exposure to optical defects affects visual processing is key to understanding the capacity for, and limits of, sensory plasticity. Here, we show evidence of functional reallocation of sensory processing resources following long-term exposure to poor optical quality. Using adaptive optics to bypass all optical defects, we assessed visual processing in neurotypically-developed adults with healthy eyes and with keratoconus – a corneal disease causing severe optical aberrations. Under fully-corrected optical conditions, keratoconus patients showed altered contrast sensitivity, with impaired sensitivity for fine spatial details and better-than-typical sensitivity for coarse spatial details. Both gains and losses in sensitivity were more pronounced in patients experiencing poorer optical quality in their daily life and mediated by changes in signal enhancement mechanisms. These findings show that adult neural processing adapts to better match the changes in sensory inputs caused by long-term exposure to altered optics.

Evaluating the performance of the staircase and quick Change Detection methods in measuring perceptual learning

The staircase method has been widely used in measuring perceptual learning. Recently, Zhao, Lesmes, and Lu (2017, 2019) developed the quick Change Detection (qCD) method and applied it to measure the trial-by-trial time course of dark adaptation. In the current study, we conducted two simulations to evaluate the performance of the 3-down/1-up staircase and qCD methods in measuring perceptual learning in a two-alternative forced-choice task. In Study 1, three observers with different time constants (40, 80, and 160 trials) of an exponential learning curve were simulated. Each simulated observer completed staircases with six step sizes (1%, 5%, 10%, 20%, 30%, and 60%) and a qCD procedure, each starting at five levels (+50%, +25%, 0, −25%, and −50% different from the true threshold in the first trial). We found the following results: Staircases with 1% and 5% step sizes failed to generate more than five reversals half of the time; and the bias and standard deviations of thresholds estimated from the post hoc segment-by-segment qCD analysis were much smaller than those from the staircase method with the other four step sizes. In Study 2, we simulated thresholds in the transfer phases with the same time constants and 50% transfer for each observer in Study 1. We found that the estimated transfer indexes from qCD showed smaller biases and standard deviations than those from the staircase method. In addition, rescoring the simulated data from the staircase method using the Bayesian estimation component of the qCD method resulted in much-improved estimates. We conclude that the qCD method characterizes the time course of perceptual learning and transfer more accurately, precisely, and efficiently than the staircase method, even with the optimal 10% step size.

Body Size Adaptation Alters Perception of Test Stimuli, Not Internal Body Image

Recent studies have reported that adaptation to extreme body types produces aftereffects on judgments of body normality and attractiveness, and also judgments of the size and shape of the viewer's own body. This latter effect suggests that adaptation could constitute an experimental model of media influences on body image. Alternatively, adaptation could affect perception of test stimuli, which should produce the same aftereffects for judgments about participant's own body or someone else's body. Here, we investigated whether adaptation similarly affects judgments about one's body and other bodies. We were interested in participants' own body image judgments, i.e., we wanted to measure the mental representations to which the test stimuli were compared and not the perception of test stimuli per se. Participants were adapted to pictures of thin or fat bodies and then rated whether bodies were fatter or thinner than either: their own body, an average body (Experiment 1), or the body of another person (Experiments 2 and 3). By keeping the visual stimuli constant but changing the task/type of judgment, i.e., the internal criterion participants are asked to judge the bodies against, we investigated how adaptation affects different stored representations of bodies, specifically own body image vs. representations of others. After adaptation, a classic aftereffect was found, with judgments biased away from the adapting stimulus. Critically, aftereffects were nearly identical for judgments of one's own body and for other people's bodies. These results suggest that adaptation affects body representations in a generic way and may not be specific to the own body image.

Good Visual Performance Despite Reduced Optical Quality during the First Month of Orthokeratology Lens Wear

Purpose: To measure changes in visual performances and optical quality in myopic children during the first month of wearing orthokeratology lens, and to reveal the association between those two.Methods: Thirty-five myopic children participated in this study. Visual performances were evaluated with visual acuity and shape discrimination threshold (SDT) for radial frequency patterns. Placido disc-based corneal topography for central 4 mm and 6 mm zones was collected and decomposed by Fourier analysis into the spherical, asymmetric, and regular astigmatic components. Root-mean-square of third-order, fourth-order, and total higher-order aberrations (HOA) were extracted for the 4 mm and 6 mm zones. All examinations were conducted at baseline, 1-week, and 1-month after lens dispensing. The changing trends over time and association between SDT and optical quality were analysed with linear-mixed model.Results: All subjects' uncorrected visual acuity improved to 0.1 logMAR or better at 1-week and 1-month lens wear (P < .01). SDT did not change significantly from the baseline at 1-week and 1-month after lens wear (P > .05). For the two zones with diameters of 4 mm and 6 mm, the spherical component decreased significantly at 1-week (P < .01) and remained stable thereafter (P < .01); the asymmetric component increased significantly at 1-week (P < .01) and remained high at 1-month (P < .01); and the regular astigmatism did not show any significant change throughout (P > .05). At the two zones with diameters of 4 mm and 6 mm, the third-order, fourth-order, and total HOA increased significantly over time (P < .05). Change of SDT did not correlate with impairments in optical quality (P > .05 for all parameters).Conclusions: While corneal optical quality decreased steadily during the first month following lens wearing, the visual acuity and shape discrimination sensitivity assessed by SDT remained very satisfactory.

Augmented Reality as a Tool for Studying Visual Plasticity: 2009 to 2018

Augmented reality (AR) has developed rapidly since its conception less than 30 years ago and is now a hot topic for both consumers and scientists. Although much attention has been paid to its application in industry, medicine, education, and entertainment, the use of AR in psychological research has been less noted. In this article, we survey recent progress in basic research that uses AR to explore the plasticity of the adult visual system. We focus on a particular application of AR called altered reality, which has been used to shed new light on mechanisms of long-term contrast adaptation and ocular-dominance plasticity. The results suggest that AR could also be a useful tool for the treatment of visual disorders.

Dynamics of contrast adaptation in central and peripheral vision

Adaptation aftereffects are generally stronger for peripheral than for foveal viewing. We examined whether there are also differences in the dynamics of visual adaptation in central and peripheral vision. We tracked the time course of contrast adaptation to binocularly presented Gabor patterns in both the central visual field (within 5°) and in the periphery (beyond 10° eccentricity) using a yes/no detection task to monitor contrast thresholds. Consistent with previous studies, sensitivity losses were stronger in the periphery than in the center when adapting to equivalent high contrast (90% contrast) patterns. The time course of the threshold changes was fitted with separate exponential functions to estimate the time constants during the adapt and post-adapt phases. When adapting to equivalent high contrast, adaptation effects built up and decayed more slowly in the periphery compared with central adaptation. Surprisingly, the aftereffect in the periphery did not decay completely to the baseline within the monitored post-adapt period (400 s), and instead asymptoted to a higher level than for central adaptation. Even when contrast was reduced to one-third (30% contrast) of the central contrast, peripheral adaptation remained stronger and decayed more slowly. This slower dynamic was also confirmed at suprathreshold test contrasts by tracking tilt-aftereffects with a 2AFC orientation discrimination task. Our results indicate that the dynamics of contrast adaptation differ between central and peripheral vision, with the periphery adapting not only more strongly but also more slowly, and provide another example of potential qualitative processing differences between central and peripheral vision.

The Role of Bottom-Up and Top-Down Cortical Interactions in Adaptation to Natural Scene Statistics

Adaptation is a mechanism by which cortical neurons adjust their responses according to recently viewed stimuli. Visual information is processed in a circuit formed by feedforward (FF) and feedback (FB) synaptic connections of neurons in different cortical layers. Here, the functional role of FF-FB streams and their synaptic dynamics in adaptation to natural stimuli is assessed in psychophysics and neural model. We propose a cortical model which predicts psychophysically observed motion adaptation aftereffects (MAE) after exposure to geometrically distorted natural image sequences. The model comprises direction selective neurons in V1 and MT connected by recurrent FF and FB dynamic synapses. Psychophysically plausible model MAEs were obtained from synaptic changes within neurons tuned to salient direction signals of the broadband natural input. It is conceived that, motion disambiguation by FF-FB interactions is critical to encode this salient information. Moreover, only FF-FB dynamic synapses operating at distinct rates predicted psychophysical MAEs at different adaptation time-scales which could not be accounted for by single rate dynamic synapses in either of the streams. Recurrent FF-FB pathways thereby play a role during adaptation in a natural environment, specifically in inducing multilevel cortical plasticity to salient information and in mediating adaptation at different time-scales.

Spontaneous recovery and time course of biological motion adaptation

Adaptation to changes of the environment is an essential function of the visual system. Recent studies have revealed that prolonged viewing of a point-light display of a human walker can produce the perception of a point-light walker facing in the opposite direction in a subsequent ambiguous test. Similar effects of biological motion adaptation have been documented for various properties of the point-light walkers. However, the time course and controlling mechanisms for biological motion adaptation have not yet been examined. The present study investigated whether a single mechanism or multiple mechanisms controlled biological motion adaptation. In Experiment 1, a relatively long duration of initial adaptation to one facing direction of a point-light walker was followed by a relatively short duration of deadaptation in which the adapter was a point-light walker of the opposite facing direction. Chimeric ambiguous walkers were used to test the aftereffect in a top-up manner. We observed spontaneous recovery of the adaptation effects in the post-test period. The Experiment 2 further delineated the build-up and decay of biological motion adaptation that accorded well with the duration scaling law (i.e., effects of adaptation become stronger and longer-lasting as adaptation duration increases). Further analysis indicated that the slower but not the faster component of the adaptation effects complied with the law. These findings suggest that biological motion adaptation is controlled by the multiple mechanisms tuned to differing timescales.

The Best of Both Worlds: Adaptation During Natural Tasks Produces Long-Lasting Plasticity in Perceptual Ocular Dominance

In human vision, one eye is usually stronger than the other. This is called ocular dominance. Extremely imbalanced ocular dominance can be found among certain patient groups, for example, in patients with amblyopia. Here, we introduce a novel method to rebalance ocular dominance. We developed an altered-reality system that subjects used to interact with the natural world, the appearance of which was changed through a real-time image process. Several daily adaptation sessions lasting 3 hr each reduced sensory ocular dominance in adults who were not diagnosed with amblyopia and improved vision in patients with amblyopia. Surprising additional strengthening was found over the subsequent 2 months, when subjects experienced natural vision only. Our method effectively trains subjects to use both eyes in the wide variety of everyday tasks. The transfer of this training to everyday vision likely produced the continuing growth in effects during the months after the training. These findings are promising for the application of this method in future clinical research on amblyopia.

Adaptation in the visual cortex: a case for probing neuronal populations with natural stimuli

The perception of, and neural responses to, sensory stimuli in the present are influenced by what has been observed in the past—a phenomenon known as adaptation. We focus on adaptation in visual cortical neurons as a paradigmatic example. We review recent work that represents two shifts in the way we study adaptation, namely (i) going beyond single neurons to study adaptation in populations of neurons and (ii) going beyond simple stimuli to study adaptation to natural stimuli. We suggest that efforts in these two directions, through a closer integration of experimental and modeling approaches, will enable a more complete understanding of cortical processing in natural environments.

Spontaneous recovery of effects of contrast adaptation without awareness

Prolonged exposure to a high contrast stimulus reduces the neural sensitivity to subsequent similar patterns. Recent work has disclosed that contrast adaptation is controlled by multiple mechanisms operating over differing timescales. Adaptation to high contrast for a relatively longer period can be rapidly eliminated by adaptation to a lower contrast (or meanfield in the present study). Such rapid deadaptation presumably causes a short-term mechanism to signal for a sensitivity increase, canceling ongoing signals from long-term mechanisms. Once deadaptation ends, the short-term mechanism rapidly returns to baseline, and the slowly decaying effects in the long-term mechanisms reemerge, allowing the perceptual aftereffects to recover during continued testing. Although this spontaneous recovery effect is considered strong evidence supporting the multiple mechanisms theory, it remains controversial whether the effect is mainly driven by visual memory established during the initial longer-term adaptation period. To resolve this debate, we used a modified Continuous Flash Suppression (CFS) and visual crowding paradigms to render the adapting stimuli invisible, but still observed the spontaneous recovery phenomenon. These results exclude the possibility that spontaneous recovery found in the previous work was merely the consequence of explicit visual memory. Our findings also demonstrate that contrast adaptation, even at the unconscious processing levels, is controlled by multiple mechanisms.

Orientation and Spatial Frequency Selectivity following Adaptation: A Reaction Time Study

The aim of the study was to determine orientation and spatial frequency sensitivity using reaction times (RTs) in an adaptation paradigm. Simple RTs were measured to the onset of a Gabor patch (SD = 1.2 deg, spatial frequency = 4 cycles deg−1). Observers adapted for 10 s to a 4 cycles deg−1 grating presented at a series of orientations (0, 2, 5, 10, 22.5, 45, 90°) or spatial frequencies (±0.5, 1, and 2 octaves). The contrast of the test grating was 4x each participant's unadapted threshold. The effect of adaptation was evaluated by transforming RTs to effective contrast reduction using RT-based contrast response functions. RTs increased by between ∼ 100 ms to 150 ms when the test and adapting gratings were of the same orientation or spatial frequency. The effect became less pronounced as the difference in orientation or spatial frequency increased. The average bandwidths for orientation and spatial frequency were 17.4° and 1.24 octaves, respectively. The method has some advantages over traditional approaches. It reveals a rapid time course of adaptation recovery with a half-life of about 13 s to 23 s. RTs form a rapid and easily implemented technique for assessing the underlying physiological mechanisms that control adaptation at suprathreshold levels of contrast.

The time course of contrast adaptation measured with a new method: detection of ramped contrast

The present study introduces a new method to measure contrast detection thresholds before and after adaptation to contrast. A sinusoidal testing grating increased contrast at a constant rate from an initial subthreshold level. Subjects indicated when the grating was just visible, which terminated its presentation. The stimulus contrast at the time of response was taken as a measure of relative threshold. We used the method to measure the time courses of changes in threshold following adaptation, for several adapting contrasts (9, 19, 29, or 39 dB, ref. 1%) and adapting durations (10, 100, or 1000 s). The buildup and decay of adaptation to contrast was described well by a power function of time. These results agree with previous findings, but are more efficient in terms of subject time. This method promises to be a useful tool for studying the dynamics of adaptation to spatial contrast.