Localization of tactile stimuli and body parts in space: two dissociated perceptual experiences revealed by a lack of constancy in the presence of position sense and motor activity

Two motor acts were analyzed at the level of tongue and fingers. These motor acts generated illusions. When subjects voluntarily rotated the tongue by 90 degrees, the perceived orientation of a tactile stimulus applied to the tongue did not covary with the perceived orientation of the tongue itself. Analogously, when subjects voluntarily crossed two adjacent fingers, the perceived position of two tactile stimuli applied to the fingers did not covary with the perceived position of the fingers themselves. Although tongue and fingers were positioned accurately in space, a lack of perceptual constancy occurred for tactile stimuli applied to these body parts. Therefore, whereas position sense was preserved, correct localization of objects was lost. The occurrence of this perceptual dissociation suggests that spatial localization of tactile stimuli may be independent both of knowledge of body part location and motor activity.     

The effect of visual threat on spatial attention to touch

Viewing a threatening stimulus can bias visual attention toward that location. Such effects have typically been investigated only in the visual modality, despite the fact that many threatening stimuli are most dangerous when close to or in contact with the body. Recent multisensory research indicates that a neutral visual stimulus, such as a light flash, can lead to a tactile attention shift towards a nearby body part. Here, we investigated whether the threat value of a visual stimulus modulates its effect on attention to touch. Participants made speeded discrimination responses about tactile stimuli presented to one or other hand, preceded by a picture cue (snake, spider, flower or mushroom) presented close to the same or the opposite hand. Pictures of snakes led to a significantly greater tactile attentional facilitation effect than did non-threatening pictures of flowers and mushrooms. Furthermore, there was a correlation between self-reported fear of snakes and spiders and the magnitude of early facilitation following cues of that type. These findings demonstrate that the attentional bias towards threat extends to the tactile modality and indicate that perceived threat value can modulate the cross-modal effect that a visual cue has on attention to touch.     

Mental body representations retain homuncular shape distortions: Evidence from Weber’s illusion

Mental body representations underlying tactile perception do not accurately reflect the body’s true morphology. For example, perceived tactile distance is dependent on both the body part being touched and the stimulus orientation, a phenomenon called Weber’s illusion. These findings suggest the presence of size and shape distortions, respectively. However, whereas each morphological feature is typically measured in isolation, a complete morphological characterization requires the concurrent measurement of both size and shape. We did so in three experiments, manipulating both the stimulated body parts (hand; forearm) and stimulus orientation while requiring participants to make tactile distance judgments. We found that the forearm was significantly more distorted than the hand lengthwise but not widthwise. Effects of stimulus orientation are thought to reflect receptive field anisotropies in primary somatosensory cortex. The results of the present study therefore suggest that mental body representations retain homuncular shape distortions that characterize early stages of somatosensory processing.     

Spatial organisation in passive tactile perception: is there a tactile field?

The perceptual field is a cardinal concept of sensory psychology. 'Field' refers to a representation in which perceptual contents have spatial properties and relations which derive from the spatial properties and relations of corresponding stimuli. It is a matter of debate whether a perceptual field exists in touch analogous to the visual field. To study this issue, we investigated whether tactile stimuli on the palm can be perceived as complex stimulus patterns, according to basic spatial principles. Subjects judged the intensity of a target stimulus to the palm, ignoring two brief preceding touches at nearby flanker locations. We found that the judgements of the target intensity were boosted by flankers when the target lay on the line joining the flankers in comparison to when the target lay away from this line. Therefore, we suggest that a tactile spatial organisation, i.e. a tactile field, exists; the field supports the relation of collinearity; it is automatically and implicitly activated by touch, and it groups spatially coherent perceptual contents.     

Absolute localization of vibrotactile stimuli on the torso

Vibrotactile mobility systems present spatial information such as the direction of a waypoint through a localized vibration on the torso. Using these systems requires the ability to determine the absolute location of the stimulus. Because data are available only on the ability to determine the relative location of stimuli on the torso, we developed a novel method for measuring absolute localization on the basis of triangulation. For 15 observers, we calculated the subjective location of visual and tactile stimuli on the frontal half of the torso. The size of the 95% confidence intervals around the subjective tactile locations is about the size of the stimuli (1.66 cm) and is slightly larger than that around the subjective visual locations (mean difference, 0.17 cm). The error in tactile judgments over and above that in the visual judgments is present only for locations near the body midline. When the subjective visual and tactile locations are not co-located, the difference can best be described by a shift along the radius from the body midaxis. The same holds for the differences between the veridical and the subjective locations. Therefore, the difference between the veridical and the subjective directions of a stimulus is small. The results make us believe that stimulus locations on the torso are coded in polar coordinates of which the angle is perceptual invariant and the distance is less important, probably because it varies with changes in, among other things, posture and breathing.     

Are the spatial features of bodily threat limited to the exact location where pain is expected?

Previous research has revealed that anticipating pain at a particular location of the body prioritizes somatosensory input presented there. The present study tested whether the spatial features of bodily threat are limited to the exact location of nociception. Participants judged which one of two tactile stimuli, presented to either hand, had been presented first, while occasionally experiencing a painful stimulus. The distance between the pain and tactile locations was manipulated. In Experiment 1, participants expected pain either proximal to one of the tactile stimuli (on the hand; near condition) or more distant on the same body part (arm; far condition). In Experiment 2, the painful stimulus was expected either proximal to one of the tactile stimuli (hand; near) or on a different body-part at the same body side (leg; far). The results revealed that in the near condition of both experiments, participants became aware of tactile stimuli presented to the “threatened” hand more quickly as compared to the “neutral” hand. Of particular interest, the data in the far conditions showed a similar prioritization effect when pain was expected at a different location of the same body part as well as when pain was expected at a different body part at the same body side. In this study, the encoding of spatial features of bodily threat was not limited to the exact location where pain was anticipated but rather generalized to the entire body part and even to different body parts at the same side of the body.     

Tactile "capture" of audition

Previous research has demonstrated that the localization of auditory or tactile stimuli can be biased by the simultaneous presentation of a visual stimulus from a different spatial position. We investigated whether auditory localization judgments could also be affected by the presentation of spatially displaced tactile stimuli, using a procedure designed to reveal perceptual interactions across modalities. Participants made left-right discrimination responses regarding the perceived location of sounds, which were presented either in isolation or together with tactile stimulation to the fingertips. The results demonstrate that the apparent location of a sound can be biased toward tactile stimulation when it is synchronous, but not when it is asynchronous, with the auditory event. Directing attention to the tactile modality did not increase the bias of sound localization toward synchronous tactile stimulation. These results provide the first demonstration of the tactile capture of audition.     

Tactile “capture” of audition

Previous research has demonstrated that the localization of auditory or tactile stimuli can be biased by the simultaneous presentation of a visual stimulus from a different spatial position. We investigated whether auditory localization judgments could also be affected by the presentation of spatially displaced tactile stimuli, using a procedure designed to reveal perceptual interactions across modalities. Participants made left—right discrimination responses regarding the perceived location of sounds, which were presented either in isolation or together with tactile stimulation to the fingertips. The results demonstrate that the apparent location of a sound can be biased toward tactile stimulation when it is synchronous, but not when it is asynchronous, with the auditory event. Directing attention to the tactile modality did not increase the bias of sound localization toward synchronous tactile stimulation. These results provide the first demonstration of the tactilecapture of audition.     

Part‐based representations of the body in early childhood: evidence from perceived distortions of tactile space across limb boundaries

Studies show that touch in adults is referenced to a representation of the body that is structured topologically according to body parts; the perceived distance between two stimuli crossing over a body part boundary is elongated relative to the perceived distance between two stimuli presented within one body part category. Here we investigate this influence of body parts on tactile space perception in children of 5, 6 and 7 years of age. We presented children with pairs of tactile stimuli on the left hand/arm, either within the hand, within the forearm, or over the wrist. With their eyes closed children were asked to adjust the distance between the thumb and forefinger of their right hand to represent the felt distance between the two tactile stimuli. Like adults, the children perceived the distance between two stimuli that cross the body part boundary to be further apart than those that were presented within the hand or arm. They also perceive tactile distance to be greater on the hand than the arm which is the first observation of Weber's illusion in young children. We propose that a topological mode of body representation is particularly advantageous during early life given that body part categories remain constant while the metric proportions of the body change substantially as the child grows.     

Spatial specificity of tactile enhancement during reaching

Tactile signals on a hand that serves as movement goal are enhanced during movement planning and execution. Here, we examined how spatially specific tactile enhancement is when humans reach to their own static hand. Participants discriminated two brief and simultaneously presented tactile stimuli: a comparison stimulus on the left thumb or little finger from a reference stimulus on the sternum. Tactile stimuli were presented either during right-hand reaching towards the left thumb or little finger or while holding both hands still (baseline). Consistent with our previous findings, stimuli on the left hand were perceived stronger during movement than baseline. However, tactile enhancement was not stronger when the stimuli were presented on the digit that served as reach target, thus the perception across the whole hand was uniformly enhanced. In experiment 2, we also presented stimuli on the upper arm in half of the trials to reduce the expectation of the stimulus location. Tactile stimuli on the target hand, but not on the upper arm, were generally enhanced, supporting the idea of a spatial gradient of tactile enhancement. Overall, our findings argue for low spatial specificity of tactile enhancement at movement-relevant body parts, here the target hand.     

Internal versus external references with respect to perception of stimuli on the body surface

Previous studies on the determinants of locus of perception of stimuli on the body have suggested that the position of the stimulus has a significant effect whether a subject perceives a tactile pattern as seen from inside or outside the body. However, it is possible that previous investigators confounded stimulus location--dorsal or frontal--and experimenter's position--behind or in front of the subject. Using 42 male subjects in a 2 X 2 design, the effects of experimenter's position and stimulus location were studied by a new technique for inferring locus of perception. Experimenter's position, rather than stimulus location, affects subjects' locus of perception. Perception of stimuli on the body involves three independent factors, the demand characteristics of the experiment, the manner in which an individual perceives the boundaries of his own body, and an individual's ability to adopt the experimenter's perceptual standpoint.     

Principles of feature integration in visual perception

Several recent theories of visual information processing have postulated that errors in recognition may result not only from a failure in feature extraction, but also from a failure to correctly join features after they have been correctly extracted. Errors that result from incorrectly integrating features are called conjunction errors. The present study uses conjunction errors to investigate the principles used by the visual system to integrate features. The research tests whether the visual system is more likely to integrate features located close together in visual space (the location principle) or whether the visual system is more likely to integrate features from stimulus items that come from the same perceptual group or object (the perceptual group principle). In four target-detection experiments, stimuli were created so that feature integration by the location principle and feature integration by the perceptual group principle made different predictions for performance. In all of the experiments, the perceptual group principle predicted feature integration even though the distance between stimulus items and retinal eccentricity were strictly controlled.     

Depth separation and lateral interference

In four experiments, the perceptual interaction between an annulus and a Landolt C enclosed within it was investigated as a function of their perceived relative depth positions and of the perceived lateral distance between the inner edge of the annulus and the outer edge of the C. To permit facile and unconfounded manipulation of perceived depth, the stimuli were stereoscopic contours formed from dynamic random-lement stereograms. Either one or both stimuli were visible continuously. The effect of the annulus on the Landolt C was assessed by forced-choice recognition thresholds of the C and by judgments of its apparent clarity. The main results were: (1) For both threshold recognition and apparent clarity, perceived depth separation has a strong effect on the strength of perceptual interaction; (2) the effect is asymmetrical in that the stimulus perceived as in front of its partner and closer to the observer has greater perceptual potency; and (3) as spacing between the elements increases, perceptual interaction declines independently of depth position. The implications of these data for general theories of stimulus interaction in three-dimensional space are discussed.     

Task-irrelevant sounds influence both temporal order and apparent-motion judgments about tactile stimuli applied to crossed and uncrossed hands

It has been suggested that judgments about the temporal–spatial order of successive tactile stimuli depend on the perceived direction of apparent motion between them. Here we manipulated tactile apparent-motion percepts by presenting a brief, task-irrelevant auditory stimulus temporally in-between pairs of tactile stimuli. The tactile stimuli were applied one to each hand, with varying stimulus onset asynchronies (SOAs). Participants reported the location of the first stimulus (temporal order judgments: TOJs) while adopting both crossed and uncrossed hand postures, so we could scrutinize skin-based, anatomical, and external reference frames. With crossed hands, the sound improved TOJ performance at short (≤300 ms) and at long (>300 ms) SOAs. When the hands were uncrossed, the sound induced a decrease in TOJ performance, but only at short SOAs. A second experiment confirmed that the auditory stimulus indeed modulated tactile apparent motion perception under these conditions. Perceived apparent motion directions were more ambiguous with crossed than with uncrossed hands, probably indicating competing spatial codes in the crossed posture. However, irrespective of posture, the additional sound tended to impair potentially anatomically coded motion direction discrimination at a short SOA of 80 ms, but it significantly enhanced externally coded apparent motion perception at a long SOA of 500 ms. Anatomically coded motion signals imply incorrect TOJ responses with crossed hands, but correct responses when the hands are uncrossed; externally coded motion signals always point toward the correct TOJ response. Thus, taken together, these results suggest that apparent-motion signals are likely taken into account when tactile temporal–spatial information is reconstructed.

     

Tactile perceptual processes and their relationship to somatoform disorders

The Somatic Signal Detection Task (SSDT) is a recent paradigm serving to examine perceptual processes likely relevant for somatoform disorders. We tested whether touch illusions are more easily induced in individuals suffering from somatoform disorders (SFD) and whether their perceptual threshold for tactile stimuli is lower compared to healthy controls. Thirty-three participants with SFD and 32 healthy controls reported whether they recognized near-threshold tactile stimuli at their fingertip, which were presented in half of the test trials. With a probability of 0.5, an auxiliary visual stimulus was additionally presented. Tactile detection thresholds, tactile sensitivity, response bias, and the rate of false-positive perceptions of the tactile stimulus were assessed. In both groups, the light stimulus led to an amelioration of tactile sensitivity as well as to a more liberal response style. The SFD group was characterized by a more liberal response bias in the first half of the light-absent condition compared to the healthy controls. Within the SFD group, the report of somatoform (especially pseudoneurological) symptoms correlated positively with illusory tactile perceptions in the SSDT. Tactile thresholds in the SSDT were measured reliably (rtt = .86) and were significantly lower in the SFD group. The notion that general perceptual dispositions influence the formation of symptom perception may thus complement cognitive models of SFD.     

Distinctive features,dichotic competition,and the encoding of stop consonants

“Same”-“different” response latencies were measured for dichotic CV syllables at stimulus onset asynchronies (SOAs) of 20–480 msec. The results showed a consistent effect of distinctive feature separation on latencies and error rates, which persisted at longer SOAs. This supports the idea that phonemes are retained in memory in a distinctive feature code at least for half a second, and that such a code is also involved in the comparison of successive phonemes. The pattern of errors and latencies suggested two components in dichotic competition: “perceptual integration” at short SOAs and “perceptual noise” at longer SOAs. A “perceptual space” analogy is suggested: two stimuli must be separated by a minimum distance (including time separation as a dimension) to be perceived as distinct; if they are perceived as distinct, they are first categorized and then compared by assessing their distance in a phonetic space.     

Duration discrimination by musicians and nonmusicians

This study investigated the effects of stimulus modality, standard duration, sex, and laterality in duration discrimination by musicians and nonmusicians. Seventeen musicians (M age = 24.1 yr.) and 22 nonmusicians (M age = 26.8 yr.) participated. Auditory (1,000 Hz) and tactile (250 Hz) sinusoidal suprathreshold stimuli with varying durations were used. The standard durations tested were 0.5 and 3.0 sec. Participants discriminated comparison stimuli which had durations slightly longer and shorter than the standard durations. Difference limens were found by the method of limits and converted to Weber fractions based on the standard durations. Musicians had lower, i.e., better, Weber fractions than nonmusicians in the auditory modality, but there was no significant difference between musicians and nonmusicians in the tactile modality. Auditory discrimination was better than tactile discrimination. Discrimination improved when the standard duration was increased both for musicians and nonmusicians. These results support previous findings of superior auditory processing by musicians. Significant differences between discrimination in the millisecond and second ranges may be due to a deviation from Weber's law and the discontinuity of timing in different duration ranges reported in the literature.     

Asymmetrical Perception of Changing Intensity in Short Tonal Stimuli: Duration of Stimulus

A number of reports have suggested that changing intensity in short tonal stimuli is asymmetrically perceived. In particular, steady stimuli may be heard as growing louder; stimuli must decrease in intensity to be heard as steady in loudness. The influence of stimulus duration on this perceptual asymmetry was examined. Three participants heard diotic tonal stimuli of eight durations between 0.8 s and 2.5 s. Each stimulus increased, decreased, or remained steady in intensity; initial intensity was 40 dB SPL (sound pressure level relative to 0.0002 dynes/cm²), and carrier frequency was 1 kHz. Participants made forced binary responses of “growing louder” or “growing softer” to each stimulus. For each duration, that value of intensity change eliciting equal numbers of both responses was determined. The results indicated a pronounced perceptual asymmetry for 0.8-s stimuli, which diminished for longer stimuli; changing intensity in 2.5-s stimuli was perceived symmetrically. Additionally, sensitivity to changing intensity improved as stimulus duration increased, suggesting that responses may be based in part on the difference in intensity between the beginning and end of the stimulus. Possible ramifications of the asymmetry reside in (a) the percussive nature of many natural sounds and (b) selective responding to approaching sound sources.     

Viewing a face (especially one's own face) being touched enhances tactile perception on the face

Observing touch on another person's body activates brain regions involved in tactile perception, even when the observer's body is not directly stimulated. Previous work has shown that in some synaesthetes, this effect induces a sensation of being touched. The present study shows that if perceptual thresholds are experimentally manipulated, viewing touch can modulate tactile experience in nonsynaesthetes as well. When observers saw a face being touched by hands, rather than a face being merely approached by hands, they demonstrated enhanced detection of subthreshold tactile stimuli on their own faces. This effect was specific to observing touch on a body part, and was not found for touch on a nonbodily stimulus, namely, a picture of a house. In addition, the effect was stronger when subjects viewed their own faces rather than another person's face. Thus, observing touch can activate the tactile system, and if perceptual thresholds are manipulated, such activation can result in a behavioral effect in nonsynaesthetes. The effect is maximum if the observed body matches the observer's body.