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

Human Electroretinography Shows Little Polarity Specificity Following Full-Field Ramp Adaptation

Purpose

The ramp aftereffect, a visual phenomenon in which perception of light changes dynamically after exposure to sawtooth-modulated light, was first described in 1967. Despite decades of psychophysical research, location and mechanisms of its generation remain unknown. In this study, we investigated a potential retinal contribution to effect formation with specific emphasis on on-/off-pathway involvement.

Methods

A 100 ms flash electroretinogram (ERG) was employed to probe the adaptive state of retinal neurons after presentation of stimuli that were homogenous in space but modulated in time following a sawtooth pattern (upward or downward ramps at 2 Hz). Additionally, a psychophysical nulling experiment was performed.

Results

Psychophysics data confirmed previous findings that the ramp aftereffect opposes the adapting stimuli in ramp direction and is stronger after upward ramps. The ERG study revealed significant changes of activity in every response component in the low-frequency range (a-wave, b-wave, on-PhNR, d-wave and off-PhNR) and high-frequency range (oscillatory potentials) in amplitudes, peak times, or both. The changes are neither specific to the on- or off-response nor antagonistic between ramp directions. With downward ramp adaptation, effects were stronger. Neither amplitudes nor peak times were correlated with perception strength. Amplitudes and peak times were uncorrelated, and the effect diminished over time, ceasing almost completely with three seconds.

Conclusions

Despite abundant effects on retinal responses, the pattern of adaptational effects was not specific to the sawtooth nature of adaptation. Although not ruling out retinal contributions the present findings favor post-retinal mechanisms as the primary locus of the ramp aftereffect.

Evaluating integration of letter fragments through contrast and spatially targeted masking

Four experiments were conducted to gain a better understanding of the visual mechanisms related to how integration of partial shape cues provides for recognition of the full shape. In each experiment, letters formed as outline contours were displayed as a sequence of adjacent segments (fragments), each visible during a 17-ms time frame. The first experiment varied the contrast of the fragments. There were substantial individual differences in contrast sensitivity, so stimulus displays in the masking experiments that followed were calibrated to the sensitivity of each participant. Masks were displayed either as patterns that filled the entire screen (full field) or as successive strips that were sliced from the pattern, each strip lying across the location of the letter fragment that had been shown a moment before. Contrast of masks were varied to be lighter or darker than the letter fragments. Full-field masks, whether light or dark, provided relatively little impairment of recognition, as was the case for mask strips that were lighter than the letter fragments. However, dark strip masks proved to be very effective, with the degree of recognition impairment becoming larger as mask contrast was increased. A final experiment found the strip masks to be most effective when they overlapped the location where the letter fragments had been shown a moment before. They became progressively less effective with increased spatial separation from that location. Results are discussed with extensive reference to potential brain mechanisms for integrating shape cues.

Delay and Speed of Visual Feedback of a Keystroke Cause Illusory Heaviness and Stiffness

Imposing a delay between an action (e.g., a limb movement) and its related visual feedback (e.g., a cursor movement on the display) induces a peculiar sensation of heaviness or stiffness. Earlier studies have examined this delay-induced heaviness or stiffness sensation in relation to the non-arbitrary causal relationship between an action and its effect. Here, “non-arbitrary causal relationship” means that an action produces a specific and deterministic pattern of visual feedback; for example, a leftward limb movement consistently and deterministically causes a leftward visual motion. In modern graphical user interfaces, on the other hand, users often control visual information by pressing keys, wherein the relationship between the keystroke and the change in visual information is arbitrary. The present study examined whether the sensation of heaviness, stiffness and bumpiness could be caused when participants' keystroke produced a delayed arbitrary visual feedback. Participants were asked to press and hold down an assigned key to cause temporal luminance changes in a square centered on the display, an arbitrary visual feedback of their keystroke. Not only the onset delay of the temporal luminance change from the participant's keystroke but also the speed of the temporal luminance change were examined as a visual cue to heaviness, stiffness, or bumpiness. In Experiment 1, the participants' task was to give a rating for the strength of the heaviness, stiffness, or bumpiness perceived when they pressed the key. Our results showed that the heaviness and stiffness ratings increased as the delay increased and decreased as the speed increased. To check whether the manipulation of the delay and speed of the visual feedback caused changes in the subjective evaluation of sensorimotor incongruence, in Experiment 2, we asked the participants to give a rating for the sense of agency. The rating scores decreased as the delay increased and increased as the speed increased. The delay and speed influenced the rating scores for the sense of agency in the opposite direction to those for heaviness/stiffness. We discuss that the brain determines the heaviness and stiffness during a keystroke based on internalized statistics relating to the delay and speed of the action feedback.

Diamond Patterns: Cumulative Cornsweet Effects and Motion-Induced Brightening

A Cornsweet edge creates the perception of a step in surface lightness between two adjacent regions of identical mean luminance due to a gradient on both sides. We might imagine that in a concatenated set of these gradients, the lightness steps would accumulate, but they do not. However, a diamond pattern, with each diamond filled with an identical luminance gradient does give a cumulative Cornsweet effect. Here, we offer an illumination explanation for why the cumulative effect is visible in the diamonds but not in the basic ramp grating and we demonstrate that when the diamonds drift, they produce a strong brightening effect (depending on the direction of the motion) and a dimming aftereffect. These effects are consistent with the local luminance gradients and not with the global lightness shift of the cumulative Cornsweet effect.

Brightening and Dimming Aftereffects at Low and High Luminance

Adaptation to a spatially uniform field that increases or decreases in luminance over time yields a “ramp aftereffect”, whereby a steady, uniform luminance appears to dim or brighten, and an appropriate non-uniform test field appears to move. We measured the duration of this aftereffect of adaptation to ascending and descending luminance for a wide range of temporal frequencies and luminance amplitudes. Three types of luminance ramp profiles were used: linear, logarithmic, and exponential. The duration of the motion aftereffect increased as amplitude increased, regardless of the frequency, slope, or ramp profile of the adapting pattern. At low luminance, this result held for ascending luminance adaptation, but the duration of the aftereffect was significantly reduced for descending luminance adaptation. This reduction in the duration of the aftereffect at low luminance is consistent with differential recruitment of temporally tuned cells of the ON and OFF pathways, but the relative independence of the effect from temporal frequency is not.

Dynamic coding of temporal luminance variation

The range of variation in environmental stimuli is much larger than the visual system can represent. It is therefore sensible for the system to adjust its responses to the momentary input statistics of the environment, such as when our pupils contract to limit the light entering the eye. Previous evidence indicates that the visual system increasingly centers responses on the mean of the visual input and scales responses to its variation during adaptation. To what degree does adaptation to a stimulus varying in luminance over time result in such adjustment of responses? The first two experiments were designed to test whether sensitivity to changes in the amplitude and the mean of a 9.6° central patch varying sinusoidally in luminance at 0.6 Hz would increase or decrease with adaptation. This was also tested for a dynamic peripheral stimulus (random patches rotating on the screen) to test to what extent the effects uncovered in the first two experiments reflect retinotopic mechanisms. Sensitivity to changes in mean and amplitude of the temporal luminance variation increased sharply the longer the adaptation to the variation, both for the large patch and the peripheral patches. Adaptation to luminance variation leads to increased sensitivity to temporal luminance variation for both central and peripheral presentation, the latter result ruling retinotopic mechanisms out as sole explanations for the adaptation effects.

Attention regulates the plasticity of multisensory timing

Evidence suggests than human time perception is likely to reflect an ensemble of recent temporal experience. For example, prolonged exposure to consistent temporal patterns can adaptively realign the perception of event order, both within and between sensory modalities (e.g. Fujisaki et al., 2004 Nat. Neurosci., 7, 773-778). In addition, the observation that 'a watched pot never boils' serves to illustrate the fact that dynamic shifts in our attentional state can also produce marked distortions in our temporal estimates. In the current study we provide evidence for a hitherto unknown link between adaptation, temporal perception and our attentional state. We show that our ability to use recent sensory history as a perceptual baseline for ongoing temporal judgments is subject to striking top-down modulation via shifts in the observer's selective attention. Specifically, attending to the temporal structure of asynchronous auditory and visual adapting stimuli generates a substantial increase in the temporal recalibration induced by these stimuli. We propose a conceptual framework accounting for our findings whereby attention modulates the perceived salience of temporal patterns. This heightened salience allows the formation of audiovisual perceptual 'objects', defined solely by their temporal structure. Repeated exposure to these objects induces high-level pattern adaptation effects, akin to those found in visual and auditory domains (e.g. Leopold & Bondar (2005) Fitting the Mind to the World: Adaptation and Aftereffects in High-Level Vision. Oxford University Press, Oxford, 189-211; Schweinberger et al. (2008) Curr. Biol., 18, 684-688).

Afterimages and the breathing light illusion

Gori and Stubbs (2006 Perception 35 1573-1577) have published some visual illusions elicited in observers who fixate very blurred disks while they move their head towards and away from them. We interpret these illusions as afterimages. We support this with examples of eccentric, colored, and striped afterimages.

Metacontrast masking is specific to luminance polarity

A 1 degrees -spot was flashed up on a screen, followed by a snugly fitting annular mask. We measured the amount of masking as a function of stimulus luminance. The surround was always mid-gray, the masking ring was either black or white, and the luminance of the spot target ranged from 0% to 100% of white in 4% steps. Observers reported the apparent lightness of the masked spot by adjusting a matching spot.

RESULTS

A black annular mask made all spots that were darker than the gray surround appear to be transparent, that is, of the same luminance as the surround (complete masking). The black ring had virtually no masking effect on spots that were lighter than the surround. Conversely, a white ring made all spots that were lighter than the gray surround look apparently the same luminance as the surround (complete masking), but had virtually no masking effect on spots that were darker than the surround. In summary, a black ring masked spatial decrements but not increments, whilst a white ring masked spatial increments but not decrements. Thus masking occurred only when the spot and the ring had the same luminance polarity. This same-polarity masking still occurred when the target spot was larger than the 'donut hole' of the masking ring, so that the target and ring partly overlapped. This ruled out simple edge-cancellation theories. Instead, masking disrupts the filling-in process that normally propagates inward from the edges of a spot [Vision Res. 31 (7-8) (1991) 1221]. We conclude that metacontrast masking occurs within, but not between, separate visual ON and OFF pathways.

Adaptation of spatiotemporal mechanisms by increment and decrement stimuli

Sawtooth modulation has been used in the past to examine visual sensitivity to luminance increments and decrements. The threshold elevation caused by adaptation depends on the spatial profile of the stimulus field and the polarities of the adaptation and test stimuli. We hypothesized that the adaptation effects reflect a change in the sensitivity of the spatiotemporal channels that detect the stimuli. We used a 2-deg disk centered in a larger surround field. Five levels of contrast between the test field and surround were investigated: equiluminant, three intermediate levels, and dark. At each contrast, observers adapted for 5 s to 2-Hz sawtooth modulation (rapid-on or rapid-off). Immediately after adaptation, thresholds were measured for detection of a single cycle of either a rapid-on or a rapid-off waveform. Varying the contrast of the surround affected observers' sensitivity to the polarity of the sawtooth stimulus to the extent that the pattern of sensitivity with the equiluminant surround was the opposite of that with the dark surround. To examine temporal factors, we measured thresholds for slow (500-ms ramps) and fast (8.3-ms pulses) test stimuli. The adaptation effect was preserved with the ramp stimuli but not with the pulse stimuli. Blurring the edge between the test and surround fields in the equiluminant surround condition raised thresholds for all sawtooth test stimuli, suggesting that spatiotemporal channels sensitive to high spatial frequencies and low temporal frequencies facilitate detection in that condition. These findings suggest that adaptation to sawtooth modulation can differentially effect the sensitivity of ON and OFF pathways, but the relative desensitization of each pathway depends on an interaction with the adaptation state of spatiotemporal channels that are involved in detection.

S-cone signals to temporal OFF-channels: asymmetrical connections to postreceptoral chromatic mechanisms

Psychophysical tests of S-cone contributions to temporal ON- and OFF-channels were conducted. Detection thresholds for S-cone modulation were measured with two kinds of test stimuli presented on a CRT: a rapid-on sawtooth test and a rapid-off sawtooth test, assumed to be detected differentially by temporal ON- and OFF-channels, respectively. S-cone related ON- and OFF-temporal responses were separated by adapting for 5 min to 1 Hz monochromatic (420, 440, 450, 540, or 650 nm in separate sessions) sawtooth flicker presented in Maxwellian view. Circular test stimuli, with a sawtooth temporal profile and a Gaussian spatial taper, were presented for 1 s in one of four quadrants 1.0 degree from a central fixation point. A four-alternative forced-choice method combined with a double-staircase procedure was used to determine ON- and OFF-thresholds in the same session. Following adaptation, the threshold elevation was greater if the polarity of the test stimulus was the same as the polarity of the sawtooth adaptation flicker, consistent with separate ON- and OFF-responses from S-cones. This asymmetrical pattern was obtained, however, only when the adaptation stimuli appeared blue with a little redness. When the adaptation flicker had a clear reddish hue component, the threshold elevation did not depend on the polarity of the sawtooth test stimuli. These results are consistent with a model in which OFF-signals originating from S cones are maintained by a postreceptoral mechanism signaling redness, but not by a postreceptoral chromatic mechanism signaling blueness.

Sensitivity in changing-loudness aftereffects as indicated by an adjustment procedure: implications regarding mechanisms

The reduced sensitivity that accompanies some auditory aftereffects has been linked to sensory fatigue. However, changing-loudness aftereffects are unaffected by reduced sensitivity, according to previous evidence from a single-interval forced-choice procedure. That result was confirmed in the present study, in which an adjustment procedure was used to measure changing-loudness aftereffects. In each condition, the listener set the rate of intensity change in test stimuli until they were heard as steady in loudness. The mean of 10 such settings was taken as a measure of the aftereffect's magnitude. The standard deviation of the 10 settings indicated the listener's sensitivity in perceiving changing intensity: The greater the standard deviation, the less the sensitivity. Consistent with previous data, the magnitude of increasing-loudness aftereffects (Experiment 1) and decreasing-loudness aftereffects (Experiment 2) varied according to the adaptation condition, but sensitivity did not. Although sensory fatigue may contribute to aftereffects, the author concluded that reduced sensitivity is not a reflection of sensory fatigue. Instead, it may be explained as a methodological artifact dependent on whether the adapted property is processed by direct or indirect mechanisms. Aftereffects only concern direct mechanisms, but the test stimuli used in their measurement may entail both types of mechanisms: If the measurement entails both types of mechanisms, sensitivity is reduced; if not, sensitivity is unaltered.

Changing-loudness aftereffects: slope of response functions and spectral dependence

Aftereffects of azimuthal auditory motion may have two components. A sensory component is inferred from strong aftereffects, because they are spectrally dependent and have shallower response functions than those for non-adaptation. Neither property applies to weak aftereffects, suggesting a cognitive component. Two experiments determined whether changing-loudness aftereffects (CLA) might be understood similarly. In a single-interval forced-choice procedure, listeners responded "growing softer" or "growing louder" to test stimuli changing in intensity. In Exp. 1, adapting and test stimuli were diotic and had the same 1-kHz sinusoidal carrier. Although response functions following adaptation were displaced from response functions for non-adaptation-indicating CLA-their slopes were broadly similar. In Exp. 2, stimuli were monotic; adapting frequency was 1 kHz and test frequencies were between 0.5 and 2.0 kHz. CLA was present in most adaptation conditions, but was strongest when the test frequency was 1.0 kHz; functions' slopes again evinced no systematic variation. The two-component hypothesis for CLA is supported by spectral dependence alone. It is argued that the slope of response functions is due to the nulling procedures for measuring auditory aftereffects. The slope depends on whether the adapted property is processed by "direct" and "indirect" mechanisms; aftereffects tap "direct" mechanisms alone, which may affect sensitivity during measurement.

Flicker brightness enhancement and visual nonlinearity

The purpose of this study was to investigate the nonlinear mechanism underlying brightness enhancement, in which a flickering stimulus appears brighter than a steady stimulus of equal mean luminance. The flickering and matching stimuli were temporally alternated. Both were cosine windowed to minimize the potential effects of temporal transients. Subjects adjusted the amplitude of the matching stimulus to match it in brightness to the flickering stimulus. The temporal frequency, modulation, and waveform of the flickering stimulus were varied. With sinusoidal flicker, brightness enhancement increased with increasing modulation at all frequencies, peaking at about 16 Hz at full modulation. The results were modeled by a broad temporal filter followed by a single accelerating nonlinearity. The derived temporal sensitivity of the early filter inferred from brightness enhancement decreased more slowly at high frequencies than the filter(s) inferred from flicker modulation thresholds. With low frequency sawtooth flicker, brightness enhancement was phase-dependent at low, but not at high modulations, suggesting that multiple neural mechanisms may also be involved in addition to an early nonlinearity.

Temporal thresholds and reaction time to changes in velocity of visual motion

A random dot pattern moved at a velocity V1. The velocity then increased or decreased abruptly to another value V2 for some time and again returned to V1. The temporal threshold, i.e. the duration of V2 that was necessary to detect the change was measured. Thresholds for the detection of the same velocity increment, V2 = 2 x V1, were shorter when the baseline velocity V1 increased from 1 to 8 deg/sec (Expt 1). The temporal threshold decreased as the velocity contrast (V2 - V1)/(V1 + V2) increased from 0.33 to 0.77. The thresholds for the detection of velocity decrements were in general longer than those for the detection of increments (Expt 3). In Expts 2 and 4 the random-dot pattern moved with velocity V1, which abruptly increased or decreased to V2, without returning to V1. The reaction time to the change was measured for the same velocity pairs as those used in the temporal threshold measurements. There was a good correspondence between changes in the reaction times and changes in the thresholds under the various conditions. The data are interpreted on the basis of two hypotheses: higher velocities are detected by mechanisms that respond more rapidly; and integration of velocities occurs when temporally-adjacent motions are presented.