We report an aftereffect in perception of the extent (or degree or range) of joint movement, showing for the first time that a prolonged exposure to a passive back-and-forth movement of a certain extent results in a change in judgment of the extent of a subsequently presented movement. The adapting stimulus, movement about the wrist, had an extent of either 30 degrees or 75 degrees , while the test stimulus was a 50 degrees movement. Following a 4-min adaptation period, the estimated magnitudes of the test stimuli were 61 degrees and 36 degrees in the 30 degrees and 75 degrees condition, respectively (t test(6)= 9.6; p < 0.001). The observed effect is an instance of repulsion or contrast commonly described in perception literature, with perceived value of the test stimulus pushed away from the adapting stimulus.

While viewing an unambiguously rotating circular array of bars for an extended period, most perceive the array to occasionally move in the direction opposite to its true motion. We find that this alternation in perception has similar dynamics to rivalry, including little correlation among the durations of successive percepts. We also describe analogous reversals in touch and in proprioception. In the proprioceptive case, biceps vibration induces illusory forearm extension. Occasionally, although the same stimulation continues, reversals occur-flexion is perceived rather than extension. Temporal sampling is often invoked to explain the visual reversals but it cannot explain these proprioceptive reversals. Instead, after initial adaptation to the stimulus, rivalry between signals indicating the opposing directions could potentially explain reversals in all three modalities.

Vibration of the dorsolateral neck stimulates proprioceptors that are normally active during head movement; this induces a visual illusion of contralateral motion and displacement of a stationary target seen against a homogenous background. The spatial constancy explanation of the illusion argues that it occurs because information about head movement is necessary for accurate egocentric localization of visual objects. Accurate egocentric localization, in turn, is necessary for the success of object-directed motor action, but previous studies failed to find evidence that vibration affects pointing toward visual targets in a normally illuminated, structured field. Our goal was to provide this evidence. Vibration lasting 12 s was applied to either side of the neck while observers (N = 11) pointed at the visual target with an unseen hand. Vibration of the right side of dorsal neck in the illuminated visual field induced a 26-mm lateral bias in pointing responses in comparison to the vibration of the left side. We conclude that the mechanism that takes into account neck proprioceptive signals also operates in full cues. The pointing bias in full cues generally co-occurred with reported stationariness of the visual target, suggesting a conflict between cues used in perception of body-centric position used to guide action, which include neck proprioception, and those used in perception of motion, for which object-relative retinal information is sufficient.

BACKGROUND: Adaptation to constant stimulation has often been used to investigate the mechanisms of perceptual coding, but the adaptive processes within the proprioceptive channels that encode body movement have not been well described. We investigated them using vibration as a stimulus because vibration of muscle tendons results in a powerful illusion of movement. METHODOLOGY/PRINCIPAL FINDINGS: We applied sustained 90 Hz vibratory stimulation to biceps brachii, an elbow flexor and induced the expected illusion of elbow extension (in 12 participants). There was clear evidence of adaptation to the movement signal both during the 6-min long vibration and on its cessation. During vibration, the strong initial illusion of extension waxed and waned, with diminishing duration of periods of illusory movement and occasional reversals in the direction of the illusion. After vibration there was an aftereffect in which the stationary elbow seemed to move into flexion. Muscle activity shows no consistent relationship with the variations in perceived movement. CONCLUSION: We interpret the observed effects as adaptive changes in the central mechanisms that code movement in direction-selective opponent channels.

Vibratory stimulation of the neck muscles can elicit illusory drift of a visual target; after vibration stops, motion in the opposite direction is perceived. This motion aftereffect (MAE) could be due to adaptation of proprioceptive mechanisms that encode head orientation, or at a stage where visual and proprioceptive information are combined. To distinguish between these two possibilities, we applied vibratory stimulation to dorsolateral neck muscles for 15-s periods alternating with 15-s periods without vibration. Twenty-six observers used a hand-held tracker to indicate perceived motion of a stationary light-emitting diode (LED) in an otherwise dark room. In the critical condition, observers were in complete darkness during vibration, and the LED was only turned on in post-vibration periods. If adaptation was purely proprioceptive, a visual MAE should have occurred in this condition, but it did not. In a follow-up experiment (N = 9), the LED was presented intermittently to determine if there was a position aftereffect that might have been inhibited by processes signalling an absence of motion. No aftereffect occurred under these conditions either. In both experiments, a visual stimulus had to be present during the adaptation period in order to elicit an aftereffect. Results from our previous study ruled out an explanation based on suppression of eye movements. Thus, the most likely site responsible for the visual aftereffect lies with bimodal mechanisms combining proprioceptive and visual information. We conclude that the bimodal mechanisms adapted more quickly than the proprioceptive mechanisms from which they received input.

If two demarcated dots are embedded in separate clusters of similar dots in off centre positions, their perceived separation is biased towards the separation between the centres of the clusters (). We replicated these results and went on to determine whether a similar bias is present for orientation judgments, using a staircase method and a range of cluster orientations and separations. A complex pattern of biases was found including biases for targets at centroids. Orientation attraction towards tangents to the clusters seemed to be involved. We conclude that orientation is subject to different contextual constraints from separation, and that bias towards the edges of clusters needs to be included in models of position coding.

Eye movements are thought to account for a number of visual motion illusions involving stationary objects presented against a featureless background or apparent motion of the whole visual field. We tested two different versions of the eye movement account:(a) the retinal slip explanation and (b) the nystagmus-suppression explanation, in particular their ability to account for visual motion experienced during vibration of the neck muscles, and for the visual motion aftereffect following vibration. We vibrated the neck (ventral sternocleidomastoid muscles, bilaterally, or right dorsal muscles) and measured eye movements in conjunction with perceived illusory displacement of an LED presented in complete darkness (N=10). To test the retinal-slip explanation, we compared the direction of slow eye movements to the direction of illusory motion of the visual target. To test the suppression explanation, we estimated the direction of suppressed slow-phase eye movements and compared it to the direction of illusory motion. Two main findings show that neither actual nor suppressed eye movements cause the illusory motion and motion aftereffect. Firstly, eye movements do not reverse direction when the illusory motion reverses after vibration stops. Secondly, there are large individual differences with regards to the direction of eye movements in observers who all experience a similar visual illusion. We conclude that, rather than eye movements, a more global spatial constancy mechanism that takes into account head movement is responsible for the illusion. The results also argue against the notion of a single central signal that determines both perceptual experience and oculomotor behaviour.

This paper is concerned with the information used in open-loop pointing to visually perceived targets. Stereoscopic stimuli were used to produce illusory relative egocentric distances, which were inconsistent with the angles of vergence required to fuse the targets. One of the stimuli was a rectangle slanted around a vertical axis. Four participants in Experiment 1 reported its slant and pointed to its edges. The slant was hugely underestimated (condition A) unless the rectangle was flanked by other surfaces (condition B). The relative depth of a pair of dots placed in front of the rectangle was also misperceived due to depth-contrast effect. The critical finding is that pointing responses were not based on vergence but were consistent with depth estimates, both for the rectangle and for the dots. Experiment 2 revealed the conditions necessary for pointing to be consistent with perceived relative position. The different target distances were either randomised allowing inter-trial comparisons, or presented only one per session to prevent them. Pointing was similar to estimates only in the randomised condition showing the significance of inter-trial comparisons. It is proposed that participants used the remembered motor command and kinesthetic sensations of a previous movement as a reference, attempting to make the difference between successive movements the same as a visually perceived depth difference between successive targets.

When two sizes, one perceived by vision and the other by kinesthesia, are apparently equal, the physical relationship between them varies: The sizes may be equal, or the visual size may be larger than the kinesthetic size, or vice versa. In this study, the method of cross-modal matching and the method of magnitude production were used to explore the relationship between apparently equal sizes (5-40 cm) perceived by vision and by kinesthesia. The sizes were linear or circular, and the mode of standard presentation was visual, kinesthetic, or verbal. The size and the direction of the intermodal mismatch varied with the size of the standard. It was also found that an apparent length of movement varied with the direction of movement. In all conditions, the relationship between apparently equal visual and kinesthetic sizes was well approximated by a power function.