Motion aftereffects and retinal motion

Two experiments are described in which it was investigated whether the adaptation on which motion aftereffects (MAEs) are based is a response to retinal image motion alone or to the motion signal derived from the process which combines the image motion signal with information about eye movement (corollary discharge). In both experiments observers either fixated a stationary point or tracked a vertically moving point while a pattern (in experiment 1, a grating; in experiment 2, a random-dot pattern) drifted horizontally across the field. In the tracking condition the adapting retinal motion was oblique. In the fixation condition it was horizontal. In every case in both conditions the MAE was horizontal, in the direction opposite to that of pattern motion. These results are consistent with the hypothesis that the adaptation is a response to the motion signal derived from the comparison of eye and image motion rather than to retinal motion per se. An alternative explanation is discussed.     

Motion integration during motion aftereffects

The perceived global motion of a stimulus depends on how its different local motion-direction vectors are distributed in space and time. When they are explicitly co-localized, as in the case of locally paired motion, competitive motion integration mechanisms produce a unitary global motion direction determined by their vector average. During motion aftereffects induced by simultaneous adaptation to multiple motion directions, just as in the case of locally paired motion, different directional signals originate simultaneously from exactly the same position in space. Therefore, the perceived global motion direction during motion aftereffects results from local vector averaging of the co-localized motion-direction signals induced by adaptation.     

Motion transparency: making models of motion perception transparent

In daily life our visual system is bombarded with motion information. We see cars driving by, flocks of birds flying in the sky, clouds passing behind trees that are dancing in the wind. Vision science has a good understanding of the first stage of visual motion processing, that is, the mechanism underlying the detection of local motions. Currently, research is focused on the processes that occur beyond the first stage. At this level, local motions have to be integrated to form objects, define the boundaries between them, construct surfaces and so on. An interesting, if complicated case is known as motion transparency: the situation in which two overlapping surfaces move transparently over each other. In that case two motions have to be assigned to the same retinal location. Several researchers have tried to solve this problem from a computational point of view, using physiological and psychophysical results as a guideline. We will discuss two models: one uses the traditional idea known as ‘filter selection’ and the other a relatively new approach based on Bayesian inference. Predictions from these models are compared with our own visual behaviour and that of the neural substrates that are presumed to underlie these perceptions.     

Motion detection and motion verbs: language affects low-level visual perception

Recent theories propose that semantic representation and sensorimotor processing have a common substrate via simulation. We tested the prediction that comprehension interacts with perception, using a standard psychophysics methodology. While passively listening to verbs that referred to upward or downward motion, and to control verbs that did not refer to motion, 20 subjects performed a motion-detection task, indicating whether or not they saw motion in visual stimuli containing threshold levels of coherent vertical motion. A signal detection analysis revealed that when verbs were directionally incongruent with the motion signal, perceptual sensitivity was impaired. Word comprehension also affected decision criteria and reaction times, but in different ways. The results are discussed with reference to existing explanations of embodied processing and the potential of psychophysical methods for assessing interactions between language and perception.     

Figural aftereffects in the perception of faces

We examined figural aftereffects in images of human faces, for which changes in configuration are highly discriminable. Observers either matched or rated faces before or after viewing distorted images of faces. Prior adaptation strongly biases face perception by causing the original face to appear distorted in a direction opposite to the adapting distortion. Aftereffects transferred across different faces and were similar for upright or inverted faces, but were weaker when the adapting and test faces had different orientations (e.g., adapt inverted and test upright). Thus the aftereffects depend on which images are distorted, and not simply on the type of distortion introduced. We further show that the aftereffects are asymmetric, for adapting to the original face has little effect on the perception of a distorted face. This asymmetry suggests that adaptation may play an important normalizing role in face perception. Our results suggest that in normal viewing, figural aftereffects may strongly influence form perception and could provide a novel method for probing properties of human face perception.     

Face encoding is not categorical: Consistent evidence across multiple types of contingent aftereffects

ABSTRACT

Race-contingent aftereffects indicate that faces of different races are encoded via dissociable visual channels. Race-contingent aftereffects reflect perceptions of typicality, indicating a gradual transition of activity across channels as faces change from looking more typical of one race to the other We examine whether faces forming more discrete categories (sex: male/female; species: human/monkey) are encoded in a similar fashion, or whether they are instead encoded via more dichotomous categorical judgements. Curves representing the rate of change of aftereffect magnitude as stimuli changed from male to female produced shallow slopes, closely resembling ratings of typicality, but differing significantly from slopes for categorical judgements. For species, aftereffect slopes were significantly shallower than both ratings of typicality and categorical judgements. Overall, these results suggest that the encoding of facial properties such as these is not categorical, but instead involves a graded response as stimulus typicality varies. Aftereffect slopes were similar across the two experiments, raising the possibility of a common system that is recruited during contingent adaptation, regardless of the specific dimension involved or the categories used by the experimenter.     

Language is not necessary for color categories

The origin of color categories is under debate. Some researchers argue that color categories are linguistically constructed, while others claim they have a pre‐linguistic, and possibly even innate, basis. Although there is some evidence that 4–6‐month‐old infants respond categorically to color, these empirical results have been challenged in recent years. First, it has been claimed that previous demonstrations of color categories in infants may reflect color preferences instead. Second, and more seriously, other labs have reported failing to replicate the basic findings at all. In the current study we used eye‐tracking to test 8‐month‐old infants’ categorical perception of a previously attested color boundary (green–blue) and an additional color boundary (blue–purple). Our results show that infants are faster and more accurate at fixating targets when they come from a different color category than when from the same category (even though the chromatic separation sizes were equated). This is the case for both blue–green and blue–purple. Our findings provide independent evidence for the existence of color categories in pre‐linguistic infants, and suggest that categorical perception of color can occur without color language.     

Contingent aftereffects: lateral interactions between color and motion

A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 sec. Its change in direction of rotation was paired with a change in surround color, which was either red or green. After 15 min of exposure, observers reported vivid motion aftereffects contingent on the color of both the stationary disk and the surround, even though during adaptation only motion or color was associated with either alone. In further experiments, it was established that a change in color (or direction of motion) of the disk could be associated with a change in direction of motion (or color) of the surround. Such lateral effects were found even when a wide (5 degree) annulus was introduced between the disk and the surround during adaptation and testing. Furthermore, the aftereffects generalized to the annulus, which was not associated with either color or motion during adaptation. However, when the disk alone was adapted to color and motion, no generalization to the surround was found (and vice versa), suggesting that the effects are not produced by adaptation of large receptive fields or by scatter of light within the eye. The results appear to conflict with the ideas that contingent aftereffects are confined to the adapted area of the retina and that they are built up by links between single-duty neurones, and with an extreme view of the segregation of color and motion early in human vision.     

Motion perception in the ageing visual system: minimum motion, motion coherence, and speed discrimination thresholds

We aimed to address two issues: first, to describe how the perception of motion differs in elderly observers as compared to younger ones; and, second, to see if these changes in motion perception could be accounted for by the known changes in the ability of elderly observers to detect patterns (as indexed via contrast sensitivity). The lower threshold of motion, motion coherence, and speed discrimination were measured, alongside contrast sensitivity, in a group of thirty-two older (mean age 61.5 years) and thirty-two younger (mean age 23.2 years) subjects. The older observers showed losses in their ability to detect slow motions as indexed via the lower threshold of motion for random-dot patterns and for gratings of a range of spatial frequencies. They also were impaired on a test of motion coherence, but only for stimuli of a slow to medium speed, whereas faster speeds showed no decline with age. Finally, at all speeds tested the older observers required greater differences in speed in order to discriminate between patterns moving at different speeds. The pattern of losses on motion perception tasks was not predicted by the deficits of the older groups, such as loss of detection thresholds for high spatial and/or temporal frequencies. It is concluded that these hypotheses do not provide an adequate account of the data, and therefore that the losses occurring with age are complex and probably are a result of the loss of several types of cell.     

Persistence of simple and contingent motion aftereffects

Jones and Holding (1975) showed that orientation-contingent color aftereffects can persist for at least 3 months, but are depleted by repeated testing. We applied the same paradigm to a simple motion aftereffect (MAE) and found that it can persist for up to 1week and is only slightly diminished by testing. It was further found that simple MAEs appear to persist longer than color-contingent MAEs, although when procedures for inducing and measuring both kinds of aftereffect are equalized, contingent MAEs last longer. Finally, no tendency was found for color-contingent MAEs to diminish with repeated testing. Although both simple and color-contingent MAEs can be relatively persistent, there are certain differences between them. Furthermore, contingent aftereffects should not be considered interchangeable, as there appear to be large differences in the persistence of orientation-contingent color aftereffect and color-contingent MAEs.     

Recall is not necessary for verbal sequence learning

The question of whether overt recall of to-be-remembered material accelerates learning is important in a wide range of real-world learning settings. In the case of verbal sequence learning, previous research has proposed that recall either is necessary for verbal sequence learning (Cohen & Johansson Journal of Verbal Learning and Verbal Behavior, 6, 139–143, 1967; Cunningham, Healy, & Williams Journal of Experimental Psychology: Learning, Memory, and Cognition, 10, 575–597, 1984), or at least contributes significantly to it (Glass, Krejci, & Goldman Journal of Memory and Language, 28, 189–199, 1989; Oberauer & Meyer Memory, 17, 774–781, 2009). In contrast, here we show that the amount of previous spoken recall does not predict learning and is not necessary for it. We suggest that previous research may have underestimated participants’ learning by using suboptimal performance measures, or by using manual or written recall. However, we show that the amount of spoken recall predicted how much interference from other to-be-remembered sequences would be observed. In fact, spoken recall mediated most of the error learning observed in the task. Our data support the view that the learning of overlapping auditory–verbal sequences is driven by learning the phonological representations and not the articulatory motor responses. However, spoken recall seems to reinforce already learned representations, whether they are correct or incorrect, thus contributing to a participant identifying a specific stimulus as either “learned” or “new” during the presentation phase.     

Visual marking in moving displays: feature-based inhibition is not necessary

Visual marking is a mechanism by which new visual stimuli can gain a selection advantage by the top-down attentional inhibition of stimuli already in the field. Previous work (Olivers, Watson, & Humphreys, 1999) has shown that, for moving stimuli, there must be a unique feature difference between the old items and the new items for marking to occur. The present study shows that this constraint is not necessary if the local spatial relationships between the old moving items remain constant. It is proposed that, with a fixed configuration, the old moving items can be grouped to form a single object. An inhibitory template set up to represent the object then coordinates the application of inhibition to the individual stimuli. Implications for the theory and ecological flexibility of visual marking are discussed.     

Orientation-specific aftereffects and illusions in the perception of brightness

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.     

Adaptation in motion perception: Alteration of induced motion

Prolonged exposure to a condition that causes induced motion was found to diminish this effect. The extent of a horizontal induced motion was measured by obtaining estimates of the direction of the apparent oblique path that resulted when a spot was visible on a horizontally moving pattern and was therefore in horizontal induced motion and, at the same time, moved vertically. Because the horizontal component of the perceived motion path represented the induced motion, the slope of the path measured the extent of the induced motion. After a 10-min exposure to induced motion, the apparent motion path was steeper; the mean change corresponded to a 15% smaller extent of the induced motion. Results were obtained that argue that this effect is not due to a diminished horizontal motion of the pattern but amounts to a smaller motion-inducing effect. The experiments were meant to support the view that the perceptual process that underlies induced motion is learned.     

Inhibiting doping in sports: deterrence is necessary,but not sufficient

The use of performance-enhancing drugs is a significant problem in sport. It cheats clean athletes of their hard-earned rewards from perfecting their skills though dedication and hard work. It defrauds fans by substituting a distorted playing field for a true competition. Anti-doping agencies have been charged with enforcing drug policies, primarily through the use of drug testing programs. We propose that drug testing, while important, is not sufficient to achieve deterrence. Engaging the principles of perceptual deterrence and development of a moral community of athletes can greatly enhance compliance with rules.     

Conflict between aftereffects of retinal sweep and looming motion

Summary Observers looked monocularly into a tunnel, with gratings on the left and right sides drifting toward the head. An exposure period was followed by a test with fixed gratings. With fixation points, left and right retinal fields could be stimulated selectively. When exposure and test were on the same retinal fields, but fixation was on opposite sides of the tunnel during exposure and test periods, aftereffects of retinal sweep and of perceived looming were in opposite directions. The two effects tended to cancel, yielding no perceived aftereffect. When they did occur, aftereffects in the retinal and the looming directions were equally likely. Cancellation was significantly more likely in the experimental conditions than in the control, when fixation always remained on the same side. When areas of retinal stimulation in the exposure and test periods did not overlap, cancellation was less frequent and aftereffects of looming were more frequent. Results were not significantly different for left and right visual fields, indicating that cortical vs. subcortical OKN pathways do not influence the illusion. Vection resulted for 16 of 20 observers under one or another of our conditions.     

Systematicity is necessary but not sufficient: on the problem of facsimile science

Paul Hoyningen-Huene argues that what makes scientific knowledge special is its systematic character, and that this can be used to solve the demarcation problem. He labels this STDC: “Systematicity Theory’s Demarcation Criterion.” This paper argues that STDC fails, because there are areas of intellectual activity that are highly systematic, but that the great majority of scientists and historians and philosophers of science do not accept as scientific. These include homepathy, creationism, and climate change denial. I designate these activities “facsimile sciences” because they mimic the appearance of science but are not, by the standards of philosophers and scientists, scientific. This suggests that we need additional criteria to demarcate science from non-science and/ or nonsense.

     

I thought you were looking at me: direction-specific aftereffects in gaze perception

Gaze direction is an important social signal in humans and other primates. In this study, we used an adaptation paradigm to investigate the functional organization of gaze perception in humans. Adaptation to consistent leftward or rightward gaze produced a powerful illusion that virtually eliminated observers' perception of gaze in the adapted direction; gaze to that side was seen as pointing straight ahead, though perception of gaze to the opposite side was unimpaired. This striking dissociation held even when retinotopic mapping between adaptation and test stimuli was disrupted by changes in size or head orientation, suggesting that our findings do not reflect adaptation to low-level visual properties. Moreover, adaptation to averted gaze did not affect judgments of line bisection, illustrating that our findings do not reflect a general spatial bias. Our findings provide evidence that humans have distinct populations of neurons that are selectively responsive to particular directions of seen gaze.