Amy A. Collins

Vibrotactile localization on the arm: Effects of place, space, and age

Although tactile acuity has been explored for touch stimuli, vibrotactile resolution on the skin has not. In the present experiments, we explored the ability to localize vibrotactile stimuli on a linear array of tactors on the forearm. We examined the influence of a number of stimulus parameters, including the frequency of the vibratory stimulus, the locations of the stimulus sites on the body relative to specific body references or landmarks, the proximity among driven loci, and the age of the observer. Stimulus frequency and age group showed much less of an effect on localization than was expected. The position of stimulus sites relative to body landmarks and the separation among sites exerted the strongest influence on localization accuracy, and these effects could be mimicked by introducing an “artificial” referent into the tactile array.

The generation of vibrotactile patterns on a linear array: influences of body site, time, and presentation mode.

In order to provide information regarding orientation or direction, a convenient code employs vectors (lines) because they have both length and direction. Potential users of such information, encoded tactually, could include persons who are blind, as well as pilots, astronauts, and scuba divers, all of whom need to maintain spatial awareness in their respective unusual environments. In these situations, a tactile display can enhance environmental awareness. In this study, optimal parameters were explored for lines presented dynamically to the skin with vibrotactile arrays on three body sites, with veridical and saltatory presentation modes. Perceived length, straightness, spatial distribution, and smoothness were judged while the durations of the discrete taps making up the “drawn” dotted lines and the times between them were varied. The results indicate that the two modes produce equivalent sensations and that similar sets of timing parameters, within the ranges tested, result in “good” lines at each site.

Vibrotactile threshold in young and old observers: The effects of spatial summation and the presence of a rigid surround

Human psychophysical detection thresholds for ten frequencies of sinusoidal vibration ranging from 10 to 400 Hz were obtained on the left index fingertip and thenar eminence of young and older observers using a three-alternative forced-choice tracking procedure. The first experiment utilized a 7-mm (0.38 cm2) contactor and rigid surround with 1-mm gap. In the second experiment, three contactor sizes (1.6-, 7.0-, and 25.4-mm diameter) and two surround configurations (1-mm gap between contactor and surround, and no surround) were used. The results indicate that, although the shapes of the threshold versus frequency functions were similar in the two age groups, there was a reduction in sensitivity for the older group at all frequencies. Furthermore, taking into account the difference in sensitivity between the two age groups, spatial summation and the effects of a surround did not seem to differ between the two groups. These results are discussed in the context of physiological models of cutaneous sensitivity and changes in receptor function with age.

VIBROTACTILE PATTERN DISCRIMINATION AND COMMUNALITY AT SEVERAL BODY SITES

In a series of experiments, the effects of spatial layout on vibrotactile pattern perception were explored by testing the ability to discriminate between two sequentially presented patterns that share active elements in the same spatial locations. Two-dimensional displays were used in order to examine the functional relationship between discrimination performance and patterncommunality, defined as the sharing elements, on different body sites. Accuracy of discrimination judgments was inversely proportional to communality, regardless of the number of pattern elements. For compact arrays fitted to the finger, palm, and thigh, the effects of communality appeared equivalent. The similarity between finger and thigh functions is remarkable, considering the dramatic differences between these sites in receptor components and structure. When these data were compared with those from arrays with distributed contactors, performance was substantially better with well-separated pattern elements. Such findings help to explicate how information from apposed patterns can best be delivered to the skin through tactile communication systems.

Individual differences in the vibrotactile perception of a “simple” pattern set

Discriminative capacities for vibrotactile spatiotemporal patterns were examined in 62 college students with three tasks: identification, masking, and discrimination of the letters “X” and “O” presented tactually on the Optacon, a reading machine for blind persons. Individual differences in performance and interrelations among scores within and across paradigms were explored. In identification, most persons quickly achieved consistently better than 90% performance, but others failed to identify the patterns above 80%, even after prolonged training. The same performance variance was found when the task was repeated by 23 Naval student pilots. Masking and discrimination measured susceptibility to interference when patterns followed one another closely in time. The resulting functions were typical, with poorest performance at short stimulus onset asynchronies. Again, a wide range of performance was seen. Individual performance, however, appeared to be consistent across tasks, suggesting that abilities in a variety of pattern-perception tasks might be predictable.

The effects of a plastic-film covering on vibrotactile pattern perception with the Optacon

Three experiments were conducted to determine whether the perception of vibrotactile patterns presented to the fingertip on the Optacon (a reading machine for blind persons) would be affected by covering the array with a plastic membrane. Occasionally in psychophysical and physiological studies that involve the Optacon, the array is covered with a plastic film to minimize electrical transients during electrophysiological recording and to maintain cleanliness. In the three studies described here, observers performed a spatial acuity task, identified patterns of a previously learned set, and judged the loudness of vibrotactile noise patterns by means of absolute magnitude estimation, on both the covered and the uncovered-array. Performance in the spatial acuity and pattern identification tasks was not affected by the presence of the protective film. The rate of growth of perceived intensity was also the same with and without the plastic film, although there was a slight increase in vibrotactile detection threshold with the film.

A comparison of complex vibrotactile pattern perception on the OPTACON by young and old observers

College‐aged and, more recently, senior observers are being surveyed for the ability to identify and recognize complex vibrotactile patterns. These patterns are presented to the left index fingertip on the 144‐pin vibratory array from the OPTACON, a reading machine for the blind, in a ‘‘static’’ mode in which all elements that spatially define the pattern are turned on for its 26‐ms duration. Pattern complexity ranges from simple line segments, identified only by number, to the 26 letters of the alphabet. The ability of these observers to discriminate between two simple patterns is first measured. They are then trained to identify these patterns, and are tested in a masking situation. Observers are then trained to identify a larger set of abstract patterns, and finally the alphabet. In addition, two measures of basic sensitivity are taken on both groups: threshold at ten different frequencies (10–400 Hz) of sinusoidal vibration, and the growth of magnitude for suprathreshold levels of four of these freque...

The shape of the vibrotactile loudness function: the effect of stimulus repetition and skin-contactor coupling.

In the psychophysical literature describing the relationships between physical and psychological magnitudes, as physical intensity increases, perceived intensity often grows much faster near threshold than at higher levels. In this laboratory, however, the loudness curve for sinusoidal vibrotactile stimuli was best fit by a single-limbed function rather than by the expected two-limbed function. In the present study, we measured the growth of vibrotactile loudness of 250-Hz sinusoidal stimuli by the method of absolute magnitude estimation to explore the source of the one- versus two-limbed discrepancy. The number of times that the stimulus was presented was varied, as well as whether the stimulator contacted the skin with constant force or constant penetration. Neither of these manipulations affected the shape of the loudness function consistently. Number of repetitions influenced the shapes of the magnitude estimation functions, but only for a few individuals. Skin-contactor coupling did not affect the shapes of the functions, although the absolute level (vibrotactile loudness) was consistently greater for constant indentation.

A comparison between the growths of loudness of vibrotactile pulse and sinusoidal stimuli

The growths of loudness of pulsatile and sinusoidal stimuli were examined by the method of absolute magnitude estimation. Fourteen observers judged 32‐pps pulse and 250‐Hz sine stimuli on two sites on the palm: the thenar and hypothenar eminences. Estimates revealed that pulse stimuli were perceived as being louder than the sine stimuli on both sites. Power functions fit to the sine data had exponents of 0.84, while those fit to the upper limbs of the pulse data had slopes of 0.82. Cross‐quality matches between the two types of stimuli were made by nine of these observers. The group matches were not found to be internally consistent. That is, those made when the sine was the standard were not equivalent to matches made when the pulse was the standard. Such consistency was apparent, however, in some individual observers data. In addition, slopes of matching functions were often well predicted by magnitude estimates using a procedure based upon the absolute scaling hypothesis. Intercepts were also predicte...

Regression effects in magnitude judgments of line length, and vibrotactile sine and pulse loudness

Five observers performed absolute magnitude estimation (AME) and absolute magnitude production (AMP) of line lengths and vibrotactile pulse and sine loudnesses as well as matching of each possible pair. Line lengths, presented visually, could extend over a 7‐ to 950‐mm range. Vibrotactile stimuli were 32‐pps haversine pulses and 250‐Hz sinusoids presented to the palm with 7‐mm diameter contactors and static surrounds. When the AME and AMP data were plotted together, perfect correlation was not seen indicating the influence of regression effects on the data. Regression effects occur when observers making judgments on two perceptual continua (i.e., line magnitude and number magnitude) restrict the range of their responses gravitating toward the mean response. Magnitude balance functions (MB) were calculated from AME and AMP data for the line, sine, and pulse. The cross‐modality matches, made with each stimulus type serving as the standard, showed similar regression effects. These data are discussed with reference to transitivity and the relationships among the data set predicted from the absolute scaling hypothesis. [Work supported by NIH.]Five observers performed absolute magnitude estimation (AME) and absolute magnitude production (AMP) of line lengths and vibrotactile pulse and sine loudnesses as well as matching of each possible pair. Line lengths, presented visually, could extend over a 7‐ to 950‐mm range. Vibrotactile stimuli were 32‐pps haversine pulses and 250‐Hz sinusoids presented to the palm with 7‐mm diameter contactors and static surrounds. When the AME and AMP data were plotted together, perfect correlation was not seen indicating the influence of regression effects on the data. Regression effects occur when observers making judgments on two perceptual continua (i.e., line magnitude and number magnitude) restrict the range of their responses gravitating toward the mean response. Magnitude balance functions (MB) were calculated from AME and AMP data for the line, sine, and pulse. The cross‐modality matches, made with each stimulus type serving as the standard, showed similar regression effects. These data are discussed with ref...