Tactile selective attention and temporal masking

The presentation of a nontarget stimulus to one fingerpad interferes with the identification of a target stimulus presented to a second fingerpad. This interference has been attributed to a failure of selective attention and, more specifically, to the nontarget’s eliciting a competing response. In the present study, the temporal interval between the target and nontarget was varied to determine the extent to which a nontarget primes a competing response. The results showed more interference when the nontarget was presented after the target than when it was presented before the target. Although still consistent with a response-competition explanation, this result offered no support for a priming explanation. The function relating the amount of interference to the temporal separation between the target and nontarget was similar to the functions obtained in studies of temporal masking, and this prompted a second experiment in which temporal masking was examined. These results, obtained with stimuli presented to the same fingerpad, indicate that response competition may be a major factor in temporal masking and that similar processes are involved in temporal masking and selective attention.     

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.     

Identification of scanned and static tactile patterns

Most of the studies in which the interactions between target and nontarget spatial patterns have been examined have tested patterns that are generated statically. Static patterns are those in which all the elements of the pattern are presented at the same time and at a fixed location on the skin; however, most tactile information comes to the skin by means of patterns' being scanned across the surface of the skin. In the present study, the interactions between target and nontarget patterns were measured for patterns generated in both the static and the scanned modes. Nontarget patterns often interfere with the perception of target patterns. Using patterns generated in the static mode, previous studies have identified two factors that produce interference in pattern identification: response competition and masking. Masking, in turn, appears to be the result of temporal integration of the target and nontarget patterns, as well as the displacement of target features. In the present study, these factors were examined for patterns generated in both static and scanned modes. Regardless of the mode in which the patterns were generated, similar functions were obtained relating identification performance to the temporal separation between the target and the nontarget patterns. Although statically generated patterns are more easily identified than scanned patterns, particularly at brief durations, mechanisms such as response competition, temporal integration, and the displacement of target features appear to be factors that affect scanned patterns to nearly the same degree as static patterns.     

Processing of sequential tactile patterns: effects of a neutral stimulus

The perception of a target pattern may be interfered with by the presentation of a nontarget pattern in close temporal and spatial proximity with it. The results from previous studies suggested that much of this interference is the result not of masking but of response competition, subjects responding with the nontarget instead of the target. Using a 4-to-2 paradigm in which four target patterns are mapped onto two responses, it was shown that neutral patterns (i.e., patterns with no responses associated with them) produce considerable interference. The amount of interference is less than that produced by patterns associated with incorrect responses but greater than that produced by patterns associated with correct responses. The amount of interference produced by neutral patterns did not vary as a function of the form of the neutral pattern (Experiments 1 and 2); however, the amount of interference did depend on the degree and nature of the similarity between the neutral and target patterns (Experiments 3 and 4). The results indicate that recent studies have underestimated the amount of interference due to masking and overestimated the amount due to response competition. Response competition may either hinder or help target categorization depending on the nontarget pattern.     

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.     

The effect of shape and location on temporal masking of spatial vibrotactile patterns

Target patterns presented to uncued locations on a single fingerpad were followed by either same-shape (SS) maskers or different-shape (DS) maskers presented to either the same location (SLoc) or a different location (DLoc). DS maskers interfered with identification more when they were at SLoc than when they were at Dloc, but the reverse was true for SS maskers; they interfered more at DLoc than at SLoc. When targets were presented to cued locations, performance in the absence of maskers improved, but the pattern of interference from maskers resembled that for uncued presentation. The proportion of masker responses on incorrect trials revealed that both temporal masking and response competition may be involved in the effects of location on pattern identification.     

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.     

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 spatial orientation on the perception of moving tactile stimuli

Previous studies have shown that the perception of spatial patterns, such as letters, presented to the hand is affected by the spatial orientation of the hand. The present study investigated how the perception of direction of motion across the fingerpads changes with the position of the hand in space. The moving stimuli were generated on two displays. In one condition, the displays were placed horizontally in front of the subject, with the subject’s thumb (target site) and index finger (nontarget site) placed flat on the displays. In a second condition, the displays were vertically oriented and gripped between the thumb and index finger. Using a selective-attention paradigm in which subjects are instructed to respond only to the direction of motion at the target site, performance was still affected by the direction of motion at the nontarget site. Changing the orientation of the displays changed the effectiveness of the nontarget in interfering with the identification of the target movement. Nontarget stimuli that produced no interference in the horizontal orientation did so in the vertical, and vice versa. It appears that subjects are not using the local direction of movement across the fingerpads to judge the relative direction of movement at the two sites; rather, they are using the external direction of movement.     

Interference in localizing tactile stimuli

A series of experiments investigated the ability of subjects to localize a tactile stimulus in the presence of an additional, extraneous tactile stimulus. The subject's task was to localize a tactile stimulus (target) presented at one of several locations on his or her left index fingerpad. The target stimulus, generated on a 6 x 24 array of stimulators, was presented either by itself or in the presence of an extraneous stimulus (masker) that either preceded or followed the target. The localizability of the target was affected by the temporal separation between the target and masker in much the same way as previous studies have shown identification of tactile patterns to be affected. Unlike previous identification results, presenting the masking stimulus to the same location as the target interfered with localizability, although not as much as did presenting the masker to a different location. The results are discussed in terms of their implications for identification and discrimination of tactile patterns.     

Temporal order and tactile patterns

Temporal order judgments (TOJs) were obtained for tactile stimuli presented to subjects' fingerpads. In one set of measurements, pairs of spatial patterns were presented successively to a single fingerpad (same-site condition), to two fingers on the same hand (ipsilateral condition), or to two fingers on opposite hands (bilateral condition). The subjects were instructed to report which one of the two patterns was presented first. TOJs were more accurate in the same-site condition than in either the ipsilateral or the bilateral conditions. In the ipsilateral and bilateral conditions, performance improved when judging which one of two locations received a stimulus first, although performance levels were still lower than in the same-site condition. Increasing the size of the pattern set from which the two patterns to be judged were drawn had only a slight effect on same-site performance and no effect on ipsilateral/bilateral performance; however, changing the nature of the patterns had a considerable effect on same-site performance and a smaller effect on ipsilateral/bilateral performance. Introducing an intensity imbalance between members of the pair of stimuli also had a large effect on same-site TOJs: a less intense stimulus tended to be judged as being presented first. In the bilateral condition, however, there was a small effect in the reverse direction: more intense stimuli tended to be judged as being presented first. The intensity imbalance had no effect in the ipsilateral condition. The results suggest that different mechanisms are responsible for TOJs for patterns presented to the same-site and to separate sites and, furthermore, that separate sites may constitute separate channels for spatial information.     

A comparison of tactile spatial sensitivity on the palm and fingerpad

Studies of tactile spatial pattern perception have, for the most part, been carried out using the fingerpad. On the basis of these studies, models have been developed linking spatial pattern identification and resolution with underlying neural structures. It has been suggested that with appropriate scaling, these models would apply to the processing of spatial patterns presented to other sites on the body. Spatial sensitivity was examined on another site on the body, the palm, using two measures, letter identification and grating orientation. The results from these measures were compared with results from similar studies conducted on the fingerpad and with estimates of the density of innervation of the fingerpad and palm. To produce levels of performance similar to those on the fingerpad required letters on the palm 50 mm in height, seven to nine times larger than those used on the fingerpad. Gratings had to be six to more than seven times larger on the palm to produce the same levels of performance achieved on the fingerpad. For the two types of receptor systems sensitive to spatial information, the ratio of density of innervation between the fingerpad and the palm is estimated to be 5.7:1 and 8.8:1. Performance of spatial tasks on the palm can be predicted quantitatively from fingerpad data with a moderate degree of accuracy. Qualitative comparisons between the palm and fingerpad data indicate that spatial patterns are processed similarly at the two sites.     

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.     

The effects of complexity on the perception of vibrotactile patterns.

Vibrotactile patterns were presented to subjects' left index fingerpads using the array from the Optacon. A set of simple (one-line) patterns and a set of complex (two-line) patterns were constructed so that they were equally identifiable when presented individually. In Experiment 1, discrimination performance was lower for two-line patterns than for one-line patterns. Communality, the number of lines that two patterns share in common, appeared to be the major factor in reducing discrimination performance for two-line patterns. Experiment 2 measured the time required to identify individual patterns. There was no significant difference in identification times for one- and two-line patterns, suggesting that features within a pattern were processed simultaneously. In the presence of a temporal masking stimulus (Experiment 3), two-line patterns were more difficult to identify than one-line patterns, but reaction times were similar for the two sets of patterns. The results suggest that varying complexity affects perception of patterns at later stages of processing.     

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.     

Vibrotactile pattern isolation/integration

Temporal integration has been cited as a major factor in temporal masking. Two experiments were designed to examine the conditions under which temporal integration may aid or hinder the perception of vibrotactile spatial patterns. In Experiment 1, the subject’s task was to discriminate between pairs of patterns. Each pattern was composed of two temporally separated pattern elements. When the task required the subjects to perceive the individual pattern elements, performance improved with temporal isolation—that is, performance improved as the temporal separation between the elements increased. In a second task, when the discrimination could be based on either the overall pattern shape or the pattern elements, temporal integration appeared to improve performance—that is, performance improved as the temporal separation decreased. In Experiment 2, an identification task was used. Several factors appeared to determine whether temporal integration aided or hindered pattern identification. When pattern elements similar to those in Experiment 1 were tested, performance improved with increasing temporal separation (isolation). A single function was fit to the discrimination (isolation) and identification (isolation) results. Whether temporal integration aids or hinders pattern perception appears to depend on pattern shape, the pattern elements, and the nature of the task.     

A comparison of discrimination and identification of vibrotactile patterns

Both discrimination and identification tasks have been used to assess subjects' abilities to perceive vibratory spatial patterns presented to the skin. The present study examined discrimination and identification performance under comparable conditions. In Experiment 1, subjects attempted to discriminate a pair of patterns on some blocks of trials and to identify both members of a pair on other blocks. For both tasks, the time between the members of the pair was varied. Discrimination performance could be predicted accurately from identification data. Analysis of performance on identification trials indicated that subjects used discriminability information to identify pairs. In Experiment 2, discrimination and identification were compared when the temporal separation between patterns was fixed and a masking stimulus followed each pattern after a variable delay. Results suggest that temporal masking, rather than the time available for processing pattern information, is the major limitation in both discrimination and identification of sequences of tactile patterns.     

The effect of location on the discrimination of spatial vibrotactile patterns

The present study examined whether the locations of patterns on the skin affected the ability to process information about their shapes. In Experiment 1, pairs of spatial vibrotactile patterns, using the array from the Optacon, were presented sequentially to subjects’ left index fingerpads. The location of each pattern in a pair was varied randomly among four locations on the skin. The subjects responded “same” or “different” on the basis of the shapes of patterns, regardless of their locations. Discrimination accuracy was highest and response time fastest when patterns occupied identical locations (ILs), and performance suffered with increasing distance between patterns. In Experiment 2, pairs were presented to corresponding points or to noncorresponding points on separate fingerpads. When patterns occupied corresponding points on separate fingers, accuracy was lower than when patterns occupied ILs on a single finger, but higher than when patterns occupied noncorresponding points on separate fingers. The results suggested that discriminability declined partly because patterns did not occupy ILs, and partly because separate locations had different densities of innervation.