Two waves of a long-lasting aftereffect of prism adaptation measured over 7 days

Prism adaptation is a useful paradigm to study the integration and reorganization of various sensory modalities involved in sensory-motor tasks. By prolonging the prismatic aftereffect and well-timed observation, we aimed to dissociate the components and mechanisms involved in human prism adaptation by their differential decay and development time courses. Here, we show that a single session of prism adaptation training, combining small increments of prism strength below the subjects' awareness threshold, during a pointing task with a free walk session with total prism exposure duration of 75 min, generated a surprisingly long-lasting aftereffect. The aftereffect was measured by the magnitude of the proprioceptive shift (assessed by straight-ahead pointing in the dark) for 7 days. An aftereffect was observed, which lasted for more than 6 days, by a single prism adaptation session. The aftereffect did not decay gradually. Unlike previous descriptions, the aftereffect showed two separate time-courses of decay and increase. After a significant initial decay within 6 h, the aftereffect increased again from 1 day up to 3 days. The novel decay and delayed development profile of this adaptation aftereffect suggests two separate underlying neural mechanisms with different time scales. Our experimental paradigms promise to reveal directly the temporal characteristics of early versus late long-term neural plasticity in complex human adaptive behavior.

Motion induced spatial conflict following binocular integration

When a moving border defined by small changes in luminance (or by differences in colour) is placed in close proximity to moving borders defined by large changes in luminance, the low contrast border can appear to jitter. Previously, the existence and characteristics of this phenomenon were established using subjective reports. Here, we show that spatial judgments become more difficult in the presence of illusory jitter, presumably because of the positional uncertainty that is induced. We also explore the influence of the distance between the different types of moving border. We find that this manipulation influences the salience and amplitude, but not the perceived rate, of illusory jitter. Finally, we show that illusory jitter remains when the different types of moving border are presented to different eyes. These observations suggest that this phenomenon arises at the cortical level and are consistent with our earlier proposal--that illusory jitter can occur because the visual system periodically resolves a spatial conflict that arises when a rigid moving object contains different apparent speeds.

Stimulus selectivity of figural aftereffects for faces

Viewing a distorted face induces large aftereffects in the appearance of an undistorted face. The authors examined the processes underlying this adaptation by comparing how selective the aftereffects are for different dimensions of the images including size, spatial frequency content, contrast, and color. Face aftereffects had weaker selectivity for changes in the size, contrast, or color of the images and stronger selectivity for changes in contrast polarity or spatial frequency. This pattern could arise if the adaptation is contingent on the perceived similarity of the stimuli as faces. Consistent with this, changing contrast polarity or spatial frequency had larger effects on the perceived identity of a face, and aftereffects were also selective for different individual faces. These results suggest that part of the sensitivity changes underlying the adaptation may arise at visual levels closely associated with the representation of faces.

Selective attention and cyclopean motion processing

The effect of diverted selective attention on the induction of the cyclopean motion aftereffect (aftereffect induced from dynamic disparity information) was investigated. The luminance motion aftereffect was examined for comparison. During diverted-attention trials, observers ignored background adapting motion and performed a low-load or high-load rapid serial visual presentation (RSVP) task presented in the center of the motion display. Baseline motion aftereffects were obtained with no diverted attention. The results showed that the cyclopean motion aftereffect, similar to the luminance motion aftereffect, declined only modestly under diverted-attention conditions. Selective attention appears to play a modest role in the visual processing of cyclopean motion.

Aftereffect of adaptation to Glass patterns

Our visual systems constantly adapt their representation of the environment to match the prevailing input. Adaptation phenomena provide striking examples of perceptual plasticity and offer valuable insight into the mechanisms of sensory coding. Here, we describe an aftereffect of adaptation to a spatially structured image whereby an unstructured test stimulus takes on illusory structure locally perpendicular to that of the adaptor. Objective measurement of the strength of the aftereffect for different patterns suggests a neural locus of adaptation prior to the extraction of complex form in the visual processing hierarchy, probably at the level of primary visual cortex. This view is supported by further experiments showing that the aftereffect exhibits partial interocular transfer but complete transfer across opposite contrast polarities. However, the aftereffect does show weak position invariance, suggesting that adaptation at higher levels of the visual system may also contribute to the effect.

Global orientation aftereffect in multi-attribute displays: implications for the binding problem

We investigated the binding problem (e.g. the combination of edge information across attributes), using an orientation aftereffect paradigm (OAE). Horizontal layers of vertical edges were phase-shifted to create a global near-vertical orientation. Multi-attribute displays were created by alternating the attribute defining edges (e.g. luminance, colour, texture or motion) across layers. OAE magnitude was dependent only on the attributes used in the adaptation phase, and the similarity of attributes from adaptation to testing phase had no significant effect. Moreover, compared to single-attribute conditions, the cooperation between attributes is moderate. These results favour segregation models of the binding mechanism.

The motion aftereffect of transparent motion: Two temporal channels account for perceived direction

Adaptation to orthogonal transparent patterns drifting at the same speed produces a unidirectional motion aftereffect (MAE) whose direction is opposite the average adaptation direction. If the patterns move at different speeds, MAE direction can be predicted by an inverse vector average, using the observer's motion sensitivity to each individual pattern as vector magnitudes. These weights are well approximated by the duration of each pattern's MAE, as measured with static test patterns. However, previous efforts to use the inverse-vector-average rule with dynamic test patterns have failed. Generally, these studies have used spatially and temporally broadband test stimuli. Here, in order to gain insight into the possible contribution of temporal channels, we filtered our test pattern in the temporal domain to produce five ideal, octave-width pass-bands. MAE durations were measured for single-component stimuli drifting at various adaptation speeds and tested at a range of temporal frequencies. Then, two components with orthogonal directions and different speeds were combined and the direction of the resulting MAE was measured. The key findings are that: (i) for a given adaptation speed, the duration of a single component's MAE is dependent on test temporal frequency; (ii) the direction of MAEs produced by transparent motion (i.e., bivectorial adaptation) also varies strongly as a function test temporal frequency (by up to 90 degrees for some speed pairings); and (iii) the inverse-vector-average rule predicts the direction of the transparent MAE provided the MAE durations used to weight the vector combination were obtained from stimuli matched in adaptation speed and test temporal frequency. These results are discussed in terms of the number and shape of temporal channels in our visual system.

Components of motion perception revealed: two different after-effects from a single moving object

If motion that one has been looking at for some time suddenly stops, or if one shifts one's gaze to a static object, one will see motion in the opposite direction: the motion after-effect. If two transparent surfaces move with different speeds in different directions, then the direction of the motion after-effect will depend on the test pattern. For such transparent surfaces both the local motion and the global percept have two components. When looking at a normal moving object, there is only one perceived global motion. However, we know that locally there can be considerable ambiguity (the aperture problem). Does one adapt to all the local components, including those that one does not perceive, or only to the perceived global motion? We designed a stimulus that is perceived to be a fast rotating object, but also has a slow local radial component of motion. By selecting an appropriate test pattern we could either get a radial or a rotating motion after-effect. Thus we show that adaptation to motion must (also) occur at a stage at which local motions have not yet been integrated to give a unified percept.

Eye direction aftereffect

Three experiments using computer-generated human figures showed that after a prolonged observation of eyes looking to the left (or right), eyes looking directly toward the viewer appeared directed to the right (or left). Observation of an arrow pointing left or right did not induce this aftereffect on the perceived eye direction. Happy faces produced the aftereffect more effectively than surprised faces, even though the image features of the eyes were identical for both the happy and the surprised faces. These results suggest that the eye direction aftereffect may reflect the adaptation of relatively higher-level mechanisms analyzing the other's eye direction.

Effect of aging on visual shape distortion

BACKGROUND

Individuals experience a visual illusion created by shape interaction: when two shapes are presented successively and briefly, the form of the second (test) shape appears distorted due to the form of the first (prime) shape; this shape interaction is called the shape distortion effect. While age-related deterioration in performance is found in various aspects of visual perception, the effect of aging on the shape distortion effect has not been evaluated.

OBJECTIVE

The purpose of this study is to determine the effect of aging on the shape distortion effect.

METHODS

We measured the perception of briefly presented elementary shapes and of the shape distortion effect in 29 healthy volunteers, with ages ranging from 18 to 83 years. For each shape interaction trial, a prime rectangle was presented (vertical or horizontal) for 45 ms, followed by an interstimulus interval for 135 ms, a test circle for 60 ms and finally a random dot mask for 300 ms. The test circle was presented in each quadrant of the visual field. The prime rectangle was flashed either at the same position as the test circle (0 degrees offset, intrahemifield) or 11 degrees apart, displaced horizontally in the opposite hemifield (11 degrees offset, interhemifield). In the elementary-shape trials, there was no prime, and the test shape was a circle or an ellipse. Using the method of adjustment, the percent elongation [(longer diameter - shorter diameter)/(shorter diameter) x100] of the reproduced ellipse was computed.

RESULTS

The mean percent elongation in response to the elementary shapes did not vary with increasing age. The shape distortion effect decreased significantly with increasing age during both intra- and interhemifield conditions. The mean shape distortion effect was larger for the intrahemifield condition than for the interhemifield one.

CONCLUSION

The shape distortion effect decreases with advancing age while the perception of elementary shapes does not. These results indicate a severer age-related dysfunction of the cerebral processing of shape interaction than that of elementary-shape perception.

Motion adaptation shifts apparent position without the motion aftereffect

Adaptation to motion can produce effects on both the perceived motion (the motion aftereffect) and the position (McGraw, Whitaker, Skillen, & Chung, 2002; Nishida & Johnston, 1999; Snowden, 1998; Whitaker, McGraw, & Pearson, 1999) of a subsequently viewed test stimulus. The position shift can be interpreted as a consequence of the motion aftereffect. For example, as the motion within a stationary aperture creates the impression that the aperture is shifted in position (De Valois & De Valois, 1991; Hayes, 2000; Ramachandran & Anstis, 1990), the motion aftereffect may generate a shift in perceived position of the test pattern simply because of the illusory motion it generates on the pattern. However, here we show a different aftereffect of motion adaptation that causes a shift in the apparent position of an object even when the object appears stationary and is located several degrees from the adapted region. This position aftereffect of motion reveals a new form of motion adaptation--one that does not result in a motion aftereffect--and suggests that motion and position signals are processed independently but then interact at a higher stage of processing.

Influence of viewing distance on aftereffects of moving random pixel arrays

Viewing-distance invariance of visual perception has evolutionary advantages, but it is of necessity limited by spatial and temporal resolution. Even within these resolution limits viewing-distance invariance might not be perfect or even good, but there are remarkably few studies of its precise limits. Here we ask to what extent viewing-distance invariance holds for motion aftereffects (MAEs). There are (at least) two different MAEs: one can be seen on a static test pattern (sMAE) and is tuned to low speeds, the other only becomes manifest on a dynamic noise test stimulus (dMAE) and is sensitive to higher adaptation speeds. We show that each of these MAEs has a limited viewing-distance invariance, the dMAE only for higher screen-speeds and the sMAE only for lower screen-speeds. In both cases upper angular-speed limits shift to higher values for smaller viewing-distances (lower spatial frequencies, larger fields). This upper limit is constant, independent of viewing distance, if expressed in terms of screen-speed. On the other hand the lower speed limit is fixed in angular-speed and variable in screen-speed terms. Explanations for these findings are provided. We show that there is no fixed optimum viewing-distance or optimum angular stimulus-size for either of the two MAEs.

First-trial adaptation to prism exposure

Terminal target-pointing error on the 1st trial of exposure to optical displacement is usually less than that expected from the optical displacement magnitude. Such 1st trial adaptation was confirmed in 2 experiments (N = 48 students in each) comparing pointing toward optically displaced targets and toward equivalent physically displaced targets (no optical displacement), with visual feedback delayed until movement completion. First-trial performance could not be explained by ordinary target undershoot, online correction, or reverse optic flow information about true target position and was unrelated to realignment aftereffects. Such adaptation might be an artifact of the asymmetry of the structured visual field produced by optical displacement, which induces a felt head rotation opposite to the direction of the displacement, thereby reducing the effective optical displacement.

The effect of trajectory on the auditory motion aftereffect

The auditory motion aftereffect (aMAE) can be induced in listeners after repeated presentation of a horizontally moving sound source. Aftereffects have also been found for the individual acoustic consequences of source motion such as amplitude or frequency modulations (AM, FM). No study, however, has investigated whether combining these changes would enhance the magnitude of the aMAE, which has appeared otherwise weak relative to its visual counterpart. AM, FM and binaural changes can occur simultaneously when sources move along common translational trajectories rather than the restricted rotational paths used in previous adaptation studies. This raises the question whether the observed weakness of the aMAE is due to the improper stimulation of units responsive to the entire macrostructure induced by translational motion. The hypothesis is tested here that if integrated motion detectors exist, then including lawful amplitude and frequency changes in adapting stimuli may enhance aftereffects. Though results indicate that interaurally moving stimuli in general induce an aMAE, the acoustic macrostructure of translational motion does not appear to increase the aftereffect. A simple cross-correlation model is used to illustrate that such acoustic modulations may allow brainstem auditory centers time to recover from adaptation to translational motion.

The origin of the oblique effect examined with pattern adaptation and masking

The decreased visibility of obliquely oriented patterns as compared to horizontal or vertical ones is termed the oblique effect. The origin of the oblique effect in the chain of visual processing was examined by comparing the potency of oblique adapting gratings to the potency of horizontal ones. Oblique gratings (which were less visible but of equal physical contrast) were as powerful or more powerful than horizontal gratings as adapting stimuli. Obliquely oriented stimuli also produced a slightly stronger tilt aftereffect than stimuli near the cardinal axes. These results suggest that the diminished neural representation of oblique stimuli arises in the human cortex, rather than from impairments of sensitivity or resolution in the initial geniculo-cortical projection.

Illusory jitter in a static stimulus surrounded by a synchronously flickering pattern

The eyes are always moving even during fixation, making the retinal image move concomitantly. While these motions activate early visual stages, they are excluded from one's perception. A striking illusion reported here renders them visible: a static pattern surrounded by a synchronously flickering pattern appears to move coherently in random directions. There was a positive correlation between the illusion and fixational eye movements. A simulation revealed that motion computation artificially creates a motion difference between center and surround, which is usually a cue to object motion but now a wrong cue to seeing eye movements of oneself on-line. Therefore, this novel illusion indicates that the visual system normally counteracts shaky visual inputs due to small eye movements by using retinal, as opposed to extraretinal, motion signals. As long as they comprise common image motions over space, they are interpreted as coming from a static outer world viewed through moving eyes. Such visual stability fails in the condition of artificial flicker, because common image motions due to eye movements are registered differently between flickering and non-flickering regions.

Attentional selection of overlapped shapes: a study using brief shape aftereffects

Prior studies using brief stimulus sequences revealed "opponent shape aftereffects", indicative of direct opponent coding of global shape attributes such as aspect ratio, skew, taper, curvature, and convexity (perhaps in IT). Further, aftereffects from overlapped opponent pairs of adaptor shapes (e.g., concave and convex shapes) were substantially modulated by attention [Vision Res. 41 (2001) 3883]. Hypothetically, (1) attention might weight the attended and ignored contours at early stages of processing, or (2) it might sway opposing neural activity (e.g., of convex- vs. concave-tuned units) at the stage of opponent shape coding. Attentional modulation was equivalent for opponent pairs (producing opposite aftereffects) and non-opponent pairs (producing orthogonal aftereffects) of overlapped adaptor shapes, whether convexity or aspect-ratio aftereffects were measured. Further, the degree of attentional modulation obtained for these aftereffects (approximately 60%) was comparable to that obtained for V4 cells [J. Neurosci. 19 (1999) 1736]. Taken together, differential contour weighting appears to be the primary mechanism of attentional modulation of brief shape aftereffects.

Buildup and decay of a three-dimensional rotational aftereffect obtained with a three-dimensional figure

Gaps in past literature have raised questions regarding the kinds of stimuli that can lead to three-dimensional (3-D) rotation aftereffects. Further, the characteristics of the buildup and decay of such aftereffects are not clear. In the present experiments, rotation aftereffects were generated by projections of cube-like stimuli whose dynamic perspective motions gave rise to the perception of rotation in unambiguous directions; test stimuli consisted of similar cubes whose rotation directions were ambiguous. In experiment 1, the duration of the adaptation stimulus was varied and it was found that the 3-D rotation aftereffect develops with a time constant of approximately 26 s. In experiment 2, the duration between adaptation and testing was varied. It was found that the 3-D rotation aftereffect has a decay constant of about 9 s, similar to that observed with 2-D motion aftereffects. Experiment 3 showed that the rotation aftereffects were not simple depth aftereffects. To account for these aftereffects and related data, a modification of an existing neural-network model is suggested.

Compelling classroom demonstrations that link visual system anatomy, physiology, and behaviour

One of our approaches to teaching a course in anatomy and physiology is to stress the fundamental, systems-level concepts. One successful strategy we use is to continually highlight the relationships among anatomy, physiology, and behavior. In this article, we describe a set of classroom demonstrations that stress these links while fostering critical thinking. These demonstrations, on the topic of sensory system structure and function, rely on two perceptual consequences of neural adaptation in the visual system: afterimages and aftereffects. Viewing specific visual stimuli under binocular or monocular conditions with interocular transfer permits several concepts to be observed and discussed, including neural adaptation, anatomical and functional segregation of visual system pathways, and the relationship among visual system structure, function, and perception. This article discusses how to produce and present the required visual stimuli, suggests a set of questions to stimulate critical thinking, and presents student evaluation of this activity.

Color-selective analysis of luminance-varying stimuli

Alternating adaptation to red and green luminance-varying gratings of different spatial frequencies simultaneously induces opposing color-selective size aftereffects (Blakemore & Sutton, 1969) in the same retinal locus. With single-color adaptation, the aftereffect is larger and affects test patterns of both colors, though not equally. The color-insensitive portion of the effect shows very substantial interocular and cross-orientation transfer. The color-selective aftereffect, which accounts for about 1/3 of the total effect, is highly selective for both orientation and eye of origin. Thus, both color-selective and color-insensitive mechanisms participate in determining the perceptual characteristics of luminance-varying patterns.

Effects of attentional modulation of a stationary surround in adaptation to motion

The effect of varying the spatial relationships between an adapt/test grating and a stationary surrounding reference grating, and their interaction with diversion of attention during adaptation, were investigated in two experiments on the movement aftereffect (MAE). In experiment 1, MAEs were found to increase as the separation between the surrounding grating and the adapt/test grating decreased, but not with the area of the adapt/test grating. Although diversion during adaptation (repeating changing digits at the fixation point) reduced MAE durations, its effects did not interact with any of the stimulus variables. In experiment 2, MAE durations increased as the outer dimensions of the reference grating were increased, and this effect did interact with diversion, so that the effects of diversion were smaller when the surround grating was larger. This suggests that diversion may be affecting the inputs to an opponent process in motion adaptation, with a smaller effect on the surrounds than on the centres of antagonistic motion-contrast detectors with large receptive fields. A third experiment showed that, although repeating the word 'zero' during adaptation reduced MAEs, this reduction was smaller than that from naming a changing sequence of digits (and not significantly different from that from simply observing the changing digits), suggesting that MAE reductions are not produced only, if at all, by putative movements of the head and eyes caused by speaking.

Context and the motion aftereffect: occlusion cues in the test pattern alter perceived direction

A horizontally moving vertical grating viewed through a diamond-shaped aperture can be made to appear to move either upwards or downwards by introduction of appropriate depth-ordering cues at the boundaries of the aperture (Duncan et al. 2000 Journal of Neuroscience 20 5885-5897). The grating is perceived to move towards (and sliding under) occluding 'near' surfaces, and parallel to 'far' surfaces. Here we show that these depth-ordering cues affect the perceptual interpretation of the motion aftereffect (MAE) as well. After adaptation to unambiguous horizontal motion, the MAE direction deviates from horizontal towards near surfaces. However, the influence of depth-ordering cues on the illusory motion of the MAE is generally less than that seen for 'real' motion. Implications for theories of depth-motion and depth-MAE interactions are discussed.

Thalamocortical dynamics of the McCollough effect: boundary-surface alignment through perceptual learning

This article further develops the FACADE neural model of 3-D vision and figure-ground perception to quantitatively explain properties of the McCollough effect (ME). The model proposes that many ME data result from visual system mechanisms whose primary function is to adaptively align, through learning, boundary and surface representations that are positionally shifted due to the process of binocular fusion. For example, binocular boundary representations are shifted by binocular fusion relative to monocular surface representations, yet the boundaries must become positionally aligned with the surfaces to control binocular surface capture and filling-in. The model also includes perceptual reset mechanisms that use habituative transmitters in opponent processing circuits. Thus the model shows how ME data may arise from a combination of mechanisms that have a clear functional role in biological vision. Simulation results with a single set of parameters quantitatively fit data from 13 experiments that probe the nature of achromatic/chromatic and monocular/binocular interactions during induction of the ME. The model proposes how perceptual learning, opponent processing, and habituation at both monocular and binocular surface representations are involved, including early thalamocortical sites. In particular, it explains the anomalous ME utilizing these multiple processing sites. Alternative models of the ME are also summarized and compared with the present model.

The conjunction of feature and depth information

By inducing feature-contingent depth aftereffects, we show that the human visual system combines feature information with depth information. These contingent aftereffects were revealed through the use of a novel selective adaptation paradigm whose stimuli required the combination of feature and depth information in order to segment two interleaved, transparent surfaces. We argue that this combined processing exemplifies the remarkable resourcefulness of a visual system that has adapted to exploit conjunctions of cues that can aid in the segmentation of visual surfaces.

Calibration and alignment are separable: evidence from prism adaptation

In 2 prism adaptation experiments, the authors investigated the effects of limb starting position visibility (visible or not visible) and visual feedback availability (early or late in target pointing movements). Thirty-two students participated in Experiment 1 and 24 students participated in Experiment 2. Independent of visual feedback availability, constant error was larger and variable error was smaller for target pointing when limb starting position was visible during prism exposure. Independent of limb starting position visibility, aftereffects of prism exposure were determined by visual feedback availability. Those results support the hypothesis that calibration is determined by limb starting position visibility, whereas alignment is determined separately by visual feedback availability.