The effect of hand position and pattern motion on temporal order judgments

Subjects made temporal order judgments (TOJs) of tactile stimuli presented to the fingerpads. The subjects judged which one of two locations had been stimulated first. The tactile stimuli were patterns that simulated movement across the fingerpads. Although irrelevant to the task, the direction of movement of the patterns biased the TOJs. If the pattern at one location moved in the direction of the second location, the subjects tended to judge the first location as leading the second location. If the pattern moved in the opposite direction, that location was judged as trailing. In a series of experiments, the effect of the spatial position of the hands and fingers on TOJs and the perception of the direction of pattern movement were examined. Changing the position of the hands so that the patterns no longer moved directly toward each other reduced or eliminated the effect of motion on TOJs. In a variation of Aristotle's illusion, the moving patterns were presented to crossed and uncrossed fingers. The results indicated that, contrary to Aristotle's illusion, the subjects processed the moving patterns relative to an environmental framework, rather than to the local direction of motion on the fingerpads. Presenting the patterns to crossed hands produced results similar to those obtained with crossed fingers: The subjects processed the patterns according to an environmental framework.     

The effect of motion on tactile and visual temporal order judgments

Four experiments were conducted, three with tactile stimuli and one with visual stimuli, in which subjects made temporal order judgments (TOJs). The tactile stimuli were patterns that moved laterally across the fingerpads. The subject's task was to judge which finger received the pattern first. Even though the movement was irrelevant to the task, the subjects' TOJs were greatly affected by the direction of movement of the patterns. Accuracy in judging temporal order was enhanced when the patterns moved in a direction that was consistent with the temporal order of presentation--for example, when the movement on each fingerpad was from right to left and the temporally leading site of stimulation was to the right of the temporally trailing site of stimulation. When movement was inconsistent with the temporal order of presentation, accuracy was considerably reduced, often well below chance.The bias in TOJs was unaffected by training or by presenting the stimuli to fingers on opposite hands. In a fourth experiment, subjects judged the temporal order of visual stimuli that, like the tactile stimuli, moved in a direction that was either consistent or inconsistent with the TOJ. The results were similar to those obtained with tactile stimuli. It is suggested that the bias may be affected by attentional mechanisms and by apparent motion generated between the two sites on the skin.     

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.     

Tactile attention and the perception of moving tactile stimuli

Three experiments investigated the ability of subjects to identify the direction of movement of a pattern across the skin. In Experiments 1 and 2, subjects were required to identify the direction of movement of a pattern presented to one fingerpad while another moving pattern was being presented to an adjacent fingerpad. Subjects were instructed to attend only to the target location. The results showed that accuracy was consistently higher and reaction times were consistently faster when the two patterns moved in the same direction than when they moved in opposite directions. Both effects were largest when the two patterns were presented simultaneously. In Experiment 3, the nontarget location was the contralateral hand. In this case, performance was not affected by the presentation of the nontarget. Combined, the results suggest that movement information is processed across adjacent fingers even when subjects are explicitly instructed to attend only to one finger. Subjects do appear to be able to restrict attention to a single hand.     

Perceptual processing at adjacent location son a single finger: Masking and response competition

When target patterns and nontarget patterns are presented either to the same or to adjacent locations on the distal pad of the index finger, the amount of interference in identifying targets depends on both the shape and the location of the nontarget (Horner, 1997). In the present study, the question of whether such interference is caused by masking (the masker in some way distorts the initial representation of the target) or by response competition (the observer mistakenly responds with the masker, rather than with the target) was investigated. A 4-to-2 paradigm was used (Craig, 1995), in which four stimuli were mapped to only two responses. Targets and nontargets were randomly selected from the set of four stimuli and presented to the same or adjacent locations on the same fingerpad. Both the distal pad and the medial pad of the index finger were tested, because innervation density varies proximodistally on the distal pad, but not on the medial pad. The results indicated that response competition was an important factor limiting perception. Furthermore, perception was affected by varying location on the distal pad, but not on the medial pad. Finally, varying location on the distal pad affected perception only when responses were based on pattern shape, not when responses were based on direction of motion. The results are discussed in terms of differences in innervation density between adjacent locations and possible resultant differences in the spatial filtering properties of the skin.     

Perceptual processing at adjacent locations on a single finger: masking and response competition

When target patterns and nontarget patterns are presented either to the same or to adjacent locations on the distal pad of the index finger, the amount of interference in identifying targets depends on both the shape and the location of the nontarget (Horner, 1997). In the present study, the question of whether such interference is caused by masking (the masker in some way distorts the initial representation of the target) or by response competition (the observer mistakenly responds with the masker, rather than with the target) was investigated. A 4-to-2 paradigm was used (Craig, 1995), in which four stimuli were mapped to only two responses. Targets and nontargets were randomly selected from the set of four stimuli and presented to the same or adjacent locations on the same fingerpad. Both the distal pad and the medial pad of the index finger were tested, because innervation density varies proximodistally on the distal pad, but not on the medial pad. The results indicated that response competition was an important factor limiting perception. Furthermore, perception was affected by varying location on the distal pad, but not on the medial pad. Finally, varying location on the distal pad affected perception only when responses were based on pattern shape, not when responses were based on direction of motion. The results are discussed in terms of differences in innervation density between adjacent locations and possible resultant differences in the spatial filtering properties of the skin.     

Response competition: A major source of interference in a tactile identification task

Two experiments investigated the ability of subjects to identify a moving, tactile stimulus. In both experiments, the subjects were presented with a target to their left index fingerpad and a nontarget (also moving) to their left middle fingerpad. Subjects were instructed to attend only to the target location and to respond “1” if the stimulus moved either to the left or up the finger, and to respond “2” if the stimulus moved either right or down the finger. The results showed that accuracy was better and reaction times were faster when the target and nontarget moved in the same direction than when they moved in different directions. When the target and nontarget moved in different directions, accuracy was significantly better and reaction times were significantly faster when the two stimuli had the same assigned response than when they had different responses. The results provide support for the conclusion that movement information is processed across adjacent fingers to the level of incipient response activation, even when subjects attempt to focus their attention on one location on the skin.     

Response competition: a major source of interference in a tactile identification task.

Two experiments investigated the ability of subjects to identify a moving, tactile stimulus. In both experiments, the subjects were presented with a target to their left index fingerpad and a nontarget (also moving) to their left middle fingerpad. Subjects were instructed to attend only to the target location and to respond "1" if the stimulus moved either to the left or up the finger, and to respond "2" if the stimulus moved either right or down the finger. The results showed that accuracy was better and reaction times were faster when the target and nontarget moved in the same direction than when they moved in different directions. When the target and nontarget moved in different directions, accuracy was significantly better and reaction times were significantly faster when the two stimuli had the same assigned response than when they had different responses. The results provide support for the conclusion that movement information is processed across adjacent fingers to the level of incipient response activation, even when subjects attempt to focus their attention on one location on the skin.     

Tactile pattern perception by two fingers: Temporal interference and response competition

The identification of a spatial pattern (target) presented to one fingerpad may be interfered with by the presentation of a second pattern (nontarget) to either the same fingerpad or a second fingerpad. A portion of the interference appears to be due to masking and a portion to response competition. In the present study, vibrotactile spatial patterns were designed to extend over two fingerpads. Target and nontarget patterns were presented to the same two fingerpads with a temporal separation between the two patterns. The function relating target identification to the temporal separation between the target and nontarget was very similar to the functions obtained with one-finger patterns in temporal masking studies. Subsequent measurements showed that a substantial portion of the interference resulted from response competition. Pattern categorization was better when patterns were presented to two fingers on opposite hands than to two fingers on the same hand; however, there was more interference for patterns presented bilaterally than for patterns presented ipsilaterally. The results supported the conclusion that similar processes are involved in the perception of sequences of spatial patterns whether the patterns are presented to one or to two fingers.     

Psychological analyses of haptic and haptic-visual judgements of extent

Magnitude estimates of haptic extent resulted in positively accelerated psychophysical power function with an exponent of 1.18. However, in two further experiments right-handed male subjects made rating-scale judgements of the combined width of two stimulus blocks. Six widths were used and five replications of the 36 factorial combinations were presented to each subject. In Experiment II both stimuli were out of view and one was held between the thumb and index finger of each hand. In Experiment III one stimulus was held out of view between thumb and finger of the right hand and the second was shown to the subject. Mean ratings in both experiments were fit by a model which assumes that responses are a weighted average of the scale values of the two stimuli (Anderson, 1974a).     

Perceptual processing of adjacent and nonadjacent tactile nontargets

Previous research has shown that subjects appear unable to restrict processing to a single finger and ignore a stimulus presented to an adjacent finger. Furthermore, the evidence suggests that, at least for moving stimuli, an adjacent nontarget is fully processed to the level of incipient response activation. The present study replicated and expanded upon these original findings. The results of Experiment 1 showed that an equally large response-competition effect occurred when the nontarget was presented to adjacent and nonadjacent fingers4on the same hand). The results of Experiment 2 showed that the effects observed in Experiment 1 (and in previous studies) were also obtained with stationary stimuli. Although small, there was some indication in the results of Experiment 2 that interference may dissipate more rapidly with distance with stationary stimuli. An additional finding was that interference effects were observed in both experiments with temporal separations between the target and nontarget of up to 100 msec. In Experiment 3, target and nontarget stimuli were presented to opposite hands. Although reduced, interference was still evident with target and nontarget stimuli presented to opposite hands. Varying the physical distance between hands did not produce any change in the amount of interference. The results suggest that the focus of attention on the skin extends nearly undiminished across the fingers of one hand and is not dependent upon the physical distance between sites of stimulation.     

Perceptual processing of adjacent and nonadjacent tactile nontargets.

Previous research has shown that subjects appear unable to restrict processing to a single finger and ignore a stimulus presented to an adjacent finger. Furthermore, the evidence suggests that, at least for moving stimuli, an adjacent nontarget is fully processed to the level of incipient response activation. The present study replicated and expanded upon these original findings. The results of Experiment 1 showed that an equally large response-competition effect occurred when the nontarget was presented to adjacent and nonadjacent fingers (on the same hand). The results of Experiment 2 showed that the effects observed in Experiment 1 (and in previous studies) were also obtained with stationary stimuli. Although small, there was some indication in the results of Experiment 2 that interference may dissipate more rapidly with distance with stationary stimuli. An additional finding was that interference effects were observed in both experiments with temporal separations between the target and nontarget of up to 100 msec. In Experiment 3, target and nontarget stimuli were presented to opposite hands. Although reduced, interference was still evident with target and nontarget stimuli presented to opposite hands. Varying the physical distance between hands did not produce any change in the amount of interference. The results suggest that the focus of attention on the skin extends nearly undiminished across the fingers of one hand and is not dependent upon the physical distance between sites of stimulation.     

Vibrotactile masking: The role of response competition

When two tactile patterns, a target and a nontarget pattern, are presented in close temporal proximity to the same location, the nontarget pattern may interfere with the identification of the target. A series of experiments examined the extent to which the interference in target identification results from masking (interference in the representation of the target at an early stage of processing) or from response competition. A response competition view of pattern perception holds that both the target and nontarget are fully processed to the level of evoking responses. Interference is produced when subjects select the nontarget rather than the target. This view was tested with a paradigm developed in studies of selective attention. Pairs of tactile patterns were presented to subjects’ left index fingerpads. The amount of interference produced by a nontarget that is physically different from a target depends on whether the nontarget is associated with the same response as the target or a different response. The amount of masking also depends on the set of target and nontarget patterns that are used. The results support the conclusion that subjects have available a representation of both the target and the nontarget and that a substantial portion of the interference previously attributed to masking may be due to response competition.     

Haptic perception of linear extent.

The perception of linear extent in haptic touch appears to be anisotropic, in that haptically perceived extents can depend on the spatial orientation and location of the object and, thus, on the direction of exploratory motion. Experiments 1 and 2 quantified how the haptic perception of linear extent depended on the type of motion (radial or tangential to the body) when subjects explored different stimulus objects (raised lines or solid blocks) varying in length and in relative spatial location. Relatively narrow, shallow, raised lines were judged to be longer, by magnitude estimation, than solid blocks. Consistent with earlier reports, stimuli explored with radial arm motions were judged to be longer than identical stimuli explored with tangential motions; this difference did not depend consistently on the lateral position of the stimulus object, the direction of movement (toward or away from the body), or the distance of the hand from the body but did depend slightly on the angular position of the shoulder. Experiment 3 showed that the radial-tangential effect could be explained by temporal differences in exploratory movements, implying that the apparent anisotropy is not intrinsic to the structure of haptic space.     

Haptic perception of linear extent

The perception of linear extent in haptic touch appears to be anisotropic, in that haptically perceived extents can depend on the spatial orientation and location of the object and, thus, on the direction of exploratory motion. Experiments 1 and 2 quantified how the haptic perception of linear extent depended on the type of motion (radial or tangential to the body) when subjects explored different stimulus objects (raised lines or solid blocks) varying in length and in relative spatial location. Relatively narrow, shallow, raised lines were judged to be longer, by magnitude estimation, than solid blocks. Consistent with earlier reports, stimuli explored with radial arm motions were judged to be longer than identical stimuli explored with tangential motions; this difference did not depend consistently on the lateral position of the stimulus object, the direction of movement (toward or away from the body), or the distance of the hand from the body but did depend slightly on the angular position of the shoulder. Experiment 3 showed that the radial-tangential effect could be explained by temporal differences in exploratory movements, implying that the apparent anisotropy is not intrinsic to the structure of haptic space.     

Perceived motion in structure from motion: Pointing responses to the axis of rotation

We investigated the ability to match finger orientation to the direction of the axis of rotation in structure-from-motion displays. Preliminary experiments verified that subjects could accurately use the index finger to report direction. The remainder of the experiments studied the perception of the axis of rotation from full rotations of a group of discrete points, the profiles of a rotating ellipsoid, and two views of a group of discrete points. Subjects’ responses were analyzed by decomposing the pointing responses into their slant and tilt components. Overall, the results indicated that subjects were sensitive to both slant and tilt. However, when the axis of rotation was near the viewing direction, subjects had difficulty reporting tilt with profiles and two views and showed a large bias in their slant judgments with two views and full rotations. These results are not entirely consistent with theoretical predictions. The results, particularly for two views, suggest that additional constraints are used by humans in the recovery of structure from motion.     

On the hand transport component of prehensile movements

Jeannerod (1981) proposed that prehensile movements involve two independent visuomotor channels that are responsible for hand transport and hand aperture. In many studies, the movement of a marker placed on the wrist has been used as an index of hand transport because wrist movement is unaffected by the movements of the digits responsible for hand aperture. In the present study, the spatial paths of the wrist, index finger, and thumb of 5 adults, each performing 50 reaching movements, were measured with a WATSMART movement tracking system, and their variability was analyzed. The measures of movement variability suggest that the motor system is more concerned with thumb position than with wrist position during hand transport. Although the wrist is a technically convenient index of hand transport, the thumb may be a more appropriate index from the point of view of motor control     

On the Hand Transport Component of Prehensile Movements

Jeannerod (1981) proposed that prehensile movements involve two independent visuomotor channels that are responsible for hand transport and hand aperture. In many studies, the movement of a marker placed on the wrist has been used as an index of hand transport because wrist movement is unaffected by the movements of the digits responsible for hand aperture. In the present study, the spatial paths of the wrist, index finger, and thumb of 5 adults, each performing 50 reaching movements, were measured with a WATSMART movement tracking system, and their variability was analyzed. The measures of movement variability suggest that the motor system is more concerned with thumb position than with wrist position during hand transport. Although the wrist is a technically convenient index of hand transport, the thumb may be a more appropriate index from the point of view of motor control.     

The influence of exploration mode, orientation, and configuration on the haptic Müller-Lyer illusion

We studied the impact of manner of exploration, orientation, spatial position, and configuration on the haptic Müller-Lyer illusion. Blindfolded sighted subjects felt raised-line Müller-Lyer and control stimuli. The stimuli were felt by tracing with the index finger, free exploration, grasping with the index finger and thumb, or by measuring with the use of any two or more fingers. For haptic judgments of extent a sliding tangible ruler was used. The illusion was present in all exploration conditions, with overestimation of the wings-out compared to wings-in stimuli. Tracing with the index finger reduced the magnitude of the illusion. However, tracing and grasping induced an overall underestimation of size. The illusion was greatly attenuated when stimuli were felt with the index fingers of both hands. Illusory misperception was not altered by the position in space of the Müller-Lyer stimuli. No effects of changes in the thickness of the line shaft were found, but there were effects of the length of the wing endings for the smaller, 5.1 cm stimuli. The theoretical and practical implications of the results are discussed.     

Influence of shape on haptic curvature perception

Curvature discrimination of hand-sized doubly curved surfaces by means of static touch was investigated. Stimuli consisted of hyperbolical, cylindrical, elliptical and spherical surfaces of various curvatures. In the first experiment subjects had to discriminate the curvature along a specified orientation (the discrimination orientation) of a doubly curved surface from a flat surface. The curvature to be discriminated was oriented either along the middle finger or across the middle finger of the right hand. Independent of the shape of the surface, thresholds were found to be about 1.6 times smaller along the middle finger than across the middle finger. Discrimination biases were found to be strongly influenced by the shape of the surface; subjects judged a curvature to be more convex when the perpendicular curvature was convex than when this curvature was concave. With the results of the second experiment it could be ruled out that the influence of shape on curvature perception was simply due to a systematic error made by the subject regarding the discrimination orientation.