Inattentional aftereffects: The role of attention on the strength of the motion aftereffect.

The way that attention affects the processing of visual information is one of the most intriguing fields in the study of visual perception. One way to examine this interaction is by studying the way perceptual aftereffects are modulated by attention. In the present study, we have manipulated attention during adaptation to translational motion generated by coherently moving random dots, in order to investigate the effect of the distraction of attention on the strength of the peripheral dynamic motion aftereffect (MAE). A foveal rapid serial visual presentation task (RSVP) of varying difficulty was introduced during the adaptation period while the adaptation and test stimuli were presented peripherally. Furthermore, to examine the interaction between the physical characteristics of the stimulus and attention, we have manipulated the motion coherence level of the adaptation stimuli. Our results suggested that the removal of attention through an irrelevant task modulated the MAE's magnitude moderately and that such an effect depends on the stimulus strength. We also showed that the MAE still persists with subthreshold and unattended stimuli, suggesting that perhaps attention is not required for the complete development of the MAE.

Relative numerosity is constructed from size and density information: Evidence from adaptation.

To dissociate aftereffects of size and density in the perception of relative numerosity, large or small adapter sizes were crossed with high or low adapter densities. A total of 48 participants were included in this preregistered design. To adapt the same retinotopic region as the large adapters, the small adapters were flashed in a sequence so as to "paint" the adapting density across the large region. Perceived numerosities and sizes in the adapted region were then compared to those in an unadapted region in separate blocks of trials, so that changes in density could be inferred. These density changes were found to be bidirectional and roughly symmetric, whereas the aftereffects of size and number were not symmetric. A simple account of these findings is that local adaptations to retinotopic density as well as global adaptations to size combine in producing numerosity aftereffects measured by assessing perceived relative number. Accounts based on number adaptation are contraindicated, in particular, by the result of adapting to a large, sparse adapter and testing with a stimulus with a double the density but half number of dots.

Invisible Adaptation: The Effect of Awareness on the Strength of the Motion Aftereffect.

The ability to process information despite the lack of perceptual awareness is one of the most fascinating aspects of the visual system. Such unconscious processing is often investigated using adaptation, where any presence of the former can be traced by its footprint on aftereffects following the latter. We have investigated the mechanisms of the motion aftereffect (MAE) using random dot displays of varying motion coherence as well as crowding to modulate both the physical as well as the perceptual strength of the adaptation stimulus. Perceptual strength was quantitatively measured as the performance in a forced-choice direction-discrimination task. A motion-nulling technique was used to quantitatively measure the strength of the MAE. We show that the strength of the dynamic MAE is independently influenced by both the physical stimulus strength as well as the subjective perceptual strength, with the effect of the former being more prominent than that of the latter. We further show that the MAE still persists under conditions of subthreshold perception. Our results suggest that perceptual awareness can influence the strength of visual processing, but the latter is not fully dependent on the former and can still take place at its partial or even total absence.

Individual movement features during prism adaptation correlate with after-effects and interlimb transfer.

The human nervous system displays such plasticity that we can adapt our motor behavior to various changes in environmental or body properties. However, how sensorimotor adaptation generalizes to new situations and new effectors, and which factors influence the underlying mechanisms, remains unclear. Here we tested the general hypothesis that differences across participants can be exploited to uncover what drives interlimb transfer. Twenty healthy adults adapted to prismatic glasses while reaching to visual targets with their dominant arm. Classic adaptation and generalization across movement directions were observed but transfer to the non-dominant arm was not significant and inter-individual differences were substantial. Interlimb transfer resulted for some participants in a directional shift of non-dominant arm movements that was consistent with an encoding of visuomotor adaptation in extrinsic coordinates. For some other participants, transfer was consistent with an intrinsic coordinate system. Simple and multiple regression analyses showed that a few kinematic parameters such as peak acceleration (or peak velocity) and variability of movement direction were correlated with interlimb transfer. Low peak acceleration and low variability were related to extrinsic transfer, while high peak acceleration and high variability were related to intrinsic transfer. Motor variability was also positively correlated with the magnitude of the after-effect systematically observed on the dominant arm. Overall, these findings on unconstrained movements support the idea that individual movement features could be linked to the sensorimotor adaptation and its generalization. The study also suggests that distinct movement characteristics may be related to different coordinate frames of action representations in the nervous system.

Motion-form interaction: Motion and form aftereffects induced by distorted static natural scenes.

Spatially varying distortions (SVDs) are common artifacts of spectacles like progressive additional lenses (PALs). To habituate to distortions of PALs, the visual system has to adapt to distortion-induced image alterations, termed skew adaptation. But how this visual adjustment is achieved is largely unknown. This study examines the properties of visual adaptation to distortions of PALs in natural scenes. The visual adaptation in response to altered form and motion features of the natural stimuli were probed in two different psychophysical experiments. Observers were exposed to distortions in natural images, and form and motion aftereffects were tested subsequently in a constant stimuli procedure where subjects were asked to judge the skew, or the motion direction of an according test stimulus. Exposure to skewed natural stimuli induced a shift in perceived undistorted form as well as motion direction, when viewing distorted dynamic natural scenes, and also after exposure to static distorted natural images. Therefore, skew adaptation occurred in form and motion for dynamic visual scenes as well as static images. Thus, specifically in the condition of static skewed images and the test feature of motion direction, cortical interactions between motion-form processing presumably contributed to the adaptation process. In a nutshell, interfeature cortical interactions constituted the adaptation process to distortion of PALs. Thus, comprehensive investigation of adaptation to distortions of PALs would benefit from taking into account content richness of the stimuli to be used, like natural images.

Limited Attention Diminishes Spatial Suppression From Large Field Glass Patterns.

Glass patterns (GPs) consist of randomly distributed dot pairs (dipoles) whose orientations are determined by specific geometric transforms. We investigated the role of visuospatial attention in the processing of global form from GPs by measuring the effect of distraction on adaptation to GPs. In the nondistracted condition, observers were adapted to coherent GPs. After the adaptation period, they were presented with a test GP divided in two halves along the vertical and were required to judge which side of the test GP was more coherent. In the attention-distracted condition, a high-load rapid serial visual presentation task was performed during the adapting period. The magnitude of the form after-effect was measured using a technique that measures the coherence level at which the test GP appears random. The rationale was that if attention has a modulatory effect on the spatial summation of dipoles, in the attention-distracted condition, we should expect a weaker form after-effect. However, the results showed stronger form after-effect in the attention-distracted condition than in the nondistracted condition, suggesting that distraction during adaptation increases the strength of form adaptation. Additional experiments suggested that distraction may reduce the spatial suppression from large-scale textures, strengthening the spatial summation of local-oriented signals.

Mental Imagery Induces Cross-Modal Sensory Plasticity and Changes Future Auditory Perception.

Can what we imagine in our minds change how we perceive the world in the future? A continuous process of multisensory integration and recalibration is responsible for maintaining a correspondence between the senses (e.g., vision, touch, audition) and, ultimately, a stable and coherent perception of our environment. This process depends on the plasticity of our sensory systems. The so-called ventriloquism aftereffect-a shift in the perceived localization of sounds presented alone after repeated exposure to spatially mismatched auditory and visual stimuli-is a clear example of this type of plasticity in the audiovisual domain. In a series of six studies with 24 participants each, we investigated an imagery-induced ventriloquism aftereffect in which imagining a visual stimulus elicits the same frequency-specific auditory aftereffect as actually seeing one. These results demonstrate that mental imagery can recalibrate the senses and induce the same cross-modal sensory plasticity as real sensory stimuli.

Bidirectional aftereffects in perceived contrast.

It is well known that prolonged observation of a high-contrast stimulus alters the perception of a subsequent test stimulus. Previous studies of perceived contrast shifts only reported perceived contrast reductions. Here, we used successive presentations of test and reference stimuli and found that perceived contrast was reduced if tests had a lower contrast than adaptors but was significantly enhanced when tests had a higher contrast than adaptors. Such bidirectional contrast aftereffects were not observed for single adaptor flashes but became increasingly pronounced for repeated adaptor presentations, thereby suggesting that the aftereffect is a consequence of adaptation rather than of attentional cuing or temporal repulsion. In addition, perceived contrast reduction weakened as we increasingly jittered the spatial position of the adaptor, but perceived contrast enhancement was observed for large spatial range of jittered adaptor positions. We conclude that aftereffects involve adaptation in distinct mechanisms with narrow and broad spatial tunings. Results suggest that the visual system not only possesses low-level contrast encoding units, which monotonically increase their responses as physical contrast increases, but is also equipped with high-level channels selectively tuned for particular contrast ranges.

The contribution of local and global motion adaptation in the repulsive direction aftereffect.

After adapting to a certain motion direction, our perception of a similar direction will be repelled away from the adapting direction, a phenomenon known as the direction aftereffect (DAE). As the motion system consists of local and global processing stages, it remains unclear how the adaptation of the two stages contributes in producing the DAE. The present study addresses this question by independently inducing adaptation at local and global motion-processing levels. Local adaptation was manipulated by presenting test stimuli at either adapted or nonadapted locations. Global adaptation was manipulated by embedding one or five global motion directions in the adapting motion. Repulsive DAE, when measured using a multiple-element test pattern, was stronger when it was produced by global adaptation than when produced by local adaptation. Specifically, the DAE resulting from local adaptation (a) decreased when test orientations differed from adapting orientation, (b) decreased when local directions were disambiguated using plaid stimuli, (c) remained the same even when attention was focused at specific test locations during adaptation, and (d) increased when tested with a single element. Overall, these findings suggest that the strength of repulsive DAE depends on both the motion-processing level at which adaptation occurs and the level at which the DAE was tested. Furthermore, the repulsive DAE arising from local adaptation alone can be explained by the propagation of local speed repulsion instead of local direction repulsion. Findings are discussed in the context of how motion aftereffects arise from the adaptation of a hierarchical motion system.

Watching the brain recalibrate: Neural correlates of renormalization during face adaptation.

The face perception system flexibly adjusts its neural responses to current face exposure, inducing aftereffects in the perception of subsequent faces. For instance, adaptation to expanded faces makes undistorted faces appear compressed, and adaptation to compressed faces makes undistorted faces appear expanded. Such distortion aftereffects have been proposed to result from renormalization, in which the visual system constantly updates a prototype according to the adaptors' characteristics and evaluates subsequent faces relative to that. However, although consequences of adaptation are easily observed in behavioral aftereffects, it has proven difficult to observe renormalization during adaptation itself. Here we directly measured brain responses during adaptation to establish a neural correlate of renormalization. Given that the face-evoked occipito-temporal P2 event-related brain potential has been found to increase with face prototypicality, we reasoned that the adaptor-elicited P2 could serve as an electrophysiological indicator for renormalization. Participants adapted to sequences of four distorted (compressed or expanded) or undistorted faces, followed by a slightly distorted test face, which they had to classify as undistorted or distorted. We analysed ERPs evoked by each of the adaptors and found that P2 (but not N170) amplitudes evoked by consecutive adaptor faces exhibited an electrophysiological pattern of renormalization during adaptation to distorted faces: P2 amplitudes evoked by both compressed and expanded adaptors significantly increased towards asymptotic levels as adaptation proceeded. P2 amplitudes were smallest for the first adaptor, significantly larger for the second, and yet larger for the third adaptor. We conclude that the sensitivity of the occipito-temporal P2 to the perceived deviation of a face from the norm makes this component an excellent tool to study adaptation-induced renormalization.

Prism adaptation speeds reach initiation in the direction of the prism after-effect.

Damage to the temporal-parietal cortex in the right hemisphere often leads to spatial neglect-a disorder in which patients are unable to attend to sensory input from their contralesional (left) side. Neglect has been associated with both attentional and premotor deficits. That is, in addition to having difficulty with attending to the left side, patients are often slower to initiate leftward vs. rightward movements (i.e., directional hypokinesia). Previous research has indicated that a brief period of adaptation to rightward shifting prisms can reduce symptoms of neglect by adjusting the patient's movements leftward, toward the neglected field. Although prism adaptation has been shown to reduce spatial attention deficits in patients with neglect, very little work has examined the effects of prisms on premotor symptoms. In the current study, we examined this in healthy individuals using leftward shifting prisms to induce a rightward shift in the egocentric reference frame, similar to neglect patients prior to prism adaptation. Specifically, we examined the speed with which healthy participants initiated leftward and rightward reaches (without visual feedback) prior to and following adaptation to either 17° leftward (n = 16) or 17° rightward (n = 15) shifting prisms. Our results indicated that, following adaptation, participants were significantly faster to initiate reaches towards targets located in the direction opposite the prism shift. That is, participants were faster to initiate reaches to right targets following leftward prism adaptation and were faster to initiate reaches to left targets following rightward prism adaptation. Overall, these results are consistent with the idea that prism adaptation can influence the speed with which a reach can be initiated toward a target in the direction opposite the prism shift, possibly through altering activity in neural circuits involved in reach planning.

Bidirectional Gender Face Aftereffects: Evidence Against Normative Facial Coding.

Facial appearance can be altered, not just by restyling but also by sensory processes. Exposure to a female face can, for instance, make subsequent faces look more masculine than they would otherwise. Two explanations exist. According to one, exposure to a female face renormalizes face perception, making that female and all other faces look more masculine as a consequence-a unidirectional effect. According to that explanation, exposure to a male face would have the opposite unidirectional effect. Another suggestion is that face gender is subject to contrastive aftereffects. These should make some faces look more masculine than the adaptor and other faces more feminine-a bidirectional effect. Here, we show that face gender aftereffects are bidirectional, as predicted by the latter hypothesis. Images of real faces rated as more and less masculine than adaptors at baseline tended to look even more and less masculine than adaptors post adaptation. This suggests that, rather than mental representations of all faces being recalibrated to better reflect the prevailing statistics of the environment, mental operations exaggerate differences between successive faces, and this can impact facial gender perception.

The contributions of temporal delay and face exposure to the decay of gaze direction aftereffects.

Gaze direction is a dynamic social signal that provides real-time insight into another person's focus of attention. Gaze adaptation induces aftereffects in the perception of gaze in subsequent faces, typically biasing it away from the adapted direction. Previous studies found that such gaze direction aftereffects persist for about 7 min when repeatedly tested immediately after adaptation, but can survive at least 24 hr when there is no testing immediately after adaptation. These findings suggest that exposure to test faces after adaptation might affect the persistence of gaze direction aftereffects more than the passing of time. The present study systematically established the contributions of time and intervening testing on the longevity of gaze direction aftereffects. Aftereffects were induced and then traced over six postadaptation tests. Participants were assigned to four groups with a delay of either 30 s, 3 min, 5.5 min, or 8 min between adaptation and the first postadaptation test. Aftereffects were strongly affected by the number of preceding postadaptation tests, but unaffected by the delay between adaptation and test, revealing that face exposure affects the longevity of aftereffects more strongly than the passing of time, at least over the time frame studied here. Our findings suggest that exposure to a substantial number of faces with an unbiased distribution of gaze directions may be necessary to overcome gaze direction aftereffects.

The Role of Familiarity on Viewpoint Adaptation for Self-Face and Other-Face Images.

An adaptation method was used to investigate whether self-face processing is dissociable from general face processing. We explored the viewpoint aftereffect with face images having different degrees of familiarity (never-before-seen faces, recently familiarized faces, personally familiar faces, and the participant's own face). A face viewpoint aftereffect occurs after prolonged viewing of a face viewed from one side, with the result that the perceived viewing direction of a subsequently presented face image shown near the frontal view is biased in a direction which is the opposite of the adapting orientation. We found that (1) the magnitude of the viewpoint aftereffect depends on the level of familiarity of the adapting and test faces, (2) a cross-identity transfer of the viewpoint aftereffect is found between all categories of faces, but not between an unfamiliar adaptor face and the self-face test, and (3) learning affects the processing of the self-face in greater measure than any other category of faces. These results highlight the importance of familiarity on the face aftereffects, but they also suggest the possibility of separate representations for the self-face, on the one side, and for highly familiar faces, on the other.

Biological Form is Sufficient to Create a Biological Motion Sex Aftereffect.

In a series of five experiments we sought to determine what causes the biological motion sex aftereffect-adaptation of a general representation of the stimulus sex, adaptation to the motion in the stimulus, or adaptation to the form in the stimulus. The experiments showed that (a) adaptation to gendered faces and gendered full body images did not create a biological motion sex aftereffect; (b) adaptation to moving partial biological motion displays containing the most important motion cues for sex discrimination (shoulders and hips or shoulders, hips, and feet) did not create a biological motion sex aftereffect; and (c) adaptation to a static frame or shapes derived from a static frame did create a biological motion sex aftereffect. These results suggest that form information is sufficient to create a biological motion sex aftereffect and suggests that biological motion sex aftereffects may be a result of lower level rather than higher level adaptation in the visual system.

Global shape aftereffects in composite radial frequency patterns.

Individual radial frequency (RF) patterns are generated by modulating a circle's radius as a sinusoidal function of polar angle and have been shown to tap into global shape processing mechanisms. Composite RF patterns can reproduce the complex outlines of natural shapes and examining these stimuli may allow us to interrogate global shape mechanisms that are recruited in biologically relevant tasks. We present evidence for a global shape aftereffect in a composite RF pattern stimulus comprising two RF components. Manipulations of the shape, location, size and spatial frequency of the stimuli revealed that this aftereffect could only be explained by the attenuation of intermediate-level global shape mechanisms. The tuning of the aftereffect to test stimulus size also revealed two mechanisms underlying the aftereffect; one that was tuned to size and one that was invariant. Finally, we show that these shape mechanisms may encode some RF information. However, the RF encoding we found was not capable of explaining the full extent of the aftereffect, indicating that encoding of other shape features such as curvature are also important in global shape processing.

Expectations and visual aftereffects.

The tilt aftereffect (TAE) is traditionally regarded as a consequence of orientation-selective sensory adaptation, a low-level stimulus-driven process. Adaptation has been recently suggested to be the outcome of predictive coding. Here, we tested whether the TAE is modulated by predictability, and specifically, whether TAE depends on the congruency of adapted and expected orientations. Observers were presented with successive pairs of oriented Gabor patches. Pairs were arranged in blocks, forming two conditions with the orientation of the second pair member either predictable or not. For all pairs, the orientation of the first Gabor was tilted clockwise (CW) or counterclockwise (CCW) (±20° relative to vertical, randomized). In the "Expected" conditions, the orientation of the second Gabor was fixed relative to the first Gabor (the same or a mirror orientation, blocked). In the "no-expectation" condition, the orientation of the second Gabor was independent of the first Gabor (randomized ±20°). Intermixed test pairs were used to measure observers' perceived vertical, with the second pair member serving as a target, oriented around the vertical, permitting an estimate of the TAE produced by the presentation of the first Gabor. Results show an increase in TAE with the expected orientation matching the inducing orientation, but a decrease with the expected mirror orientation, consistent with additivity of the adaptation and the expectation effects. A second experiment, with the first oriented Gabor replaced by a colored circular blob, showed that expectation alone does not modulate the perceived orientation. These findings indicate a role for expectation in generating the perceptual TAE and are in line with predictive coding models of perception. We suggest that orientation dependent adaptation is affected by both the mean orientation (first order statistics) and by temporal contingencies (second order statistics).

The timecourse of expression aftereffects.

Adaptation to facial expressions produces aftereffects that bias perception of subsequent expressions away from the adaptor. Studying the temporal dynamics of an aftereffect can help us to understand the neural processes that underlie perception, and how they change with experience. Little is known about the temporal dynamics of the expression aftereffect. We conducted two experiments to measure the timecourse of this aftereffect. In Experiment 1 we examined how the size of the aftereffect varies with changes in the duration of the adaptor and test stimuli. We found that the expression aftereffect follows the classic timecourse pattern of logarithmic build-up and exponential decay that has been demonstrated for many lower level aftereffects, as well as for facial identity and figural face aftereffects. This classic timecourse pattern suggests that the adaptive calibration mechanisms of facial expression are similar to those of lower level visual stimuli, and is consistent with a perceptual locus for the adaptation aftereffect. We also found that aftereffects could be generated by as little as 1 s of adaptation, and in some conditions lasted for as long as 3200 ms. We extended this last finding in Experiment 2, exploring the longevity of the expression aftereffect by adding a stimulus-free gap of varying duration between adaptation and test. We found that significant expression aftereffects were still present 32 s after adaptation. The persistence of the expression aftereffect suggests that they may have a considerable impact on day-to-day expression perception.

Tilt representation beyond the retinotopic level.

We perceive a stable visual world, which enables successful interaction with our environment, despite movements of the eyes, head, and body. How are such perceptions formed? One possibility is that retino-centric visual input is transformed into representations at higher levels, such as head-, body-, or world-centered representations. We investigated this hypothesis using the tilt aftereffect in a balanced adaptation paradigm designed to isolate head-, body-, and world-centered aftereffects. Observers adapted to two oppositely tilted stimuli, each contingent on one of two different gaze, head, or body directions. We found aftereffects contingent on gaze direction, but not head or body direction. This demonstrates that adaptable tilt representations exist in a head-centric frame but not in higher reference frames. These aftereffects may be attributed to adaptation of retinotopic tilt-sensitive neurons whose responses are modulated by gaze direction (gain fields). Such neurons could support functionally head-centric tilt representations and are found as early as V1. On the basis of our results we would not expect activity in tilt-sensitive neurons to be modulated by head or body direction. The balanced adaptation paradigm is a useful tool for examining properties of the process responsible for gaze modulation of activity in visual neurons.

Time dilation in a perceptually jittering dot pattern.

Although it is known that a moving stimulus appears to dilate in duration compared to a stationary stimulus, whether subjective motion devoid of stimulus motion is sufficient remains unknown. To elucidate this, we used a motion illusion in which an actually static stimulus clearly appears to move, a useful dissociation between actual and subjective motions. We used the jitter aftereffect resulting from adaptation to dynamic noise as such a tool and measured subjective durations of a static random-dot pattern in which illusory jitter was seen, an actually oscillating pattern mimicking the illusory jitter, and a static pattern without illusory jitter. Pattern oscillation as tiny as fixational eye movements robustly evoked time dilation, and time dilation to a similar extent was also induced by an actually static but subjectively jittering pattern. Taken together with the previous knowledge that this subjective jitter is related to a visually based compensation of spurious retinal image motions due to fixational eye movements, these findings demonstrate that visual duration computation is influenced by a representation at a high-level motion processing stage at which a stable visual world despite jittery retinal inputs has been established.