Roger W. Cholewiak

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.

Vibrotactile localization on the abdomen: Effects of place and space

In this study, we explore the conditions for accurate localization of vibrotactile stimuli presented to the abdomen. Tactile orientation systems intended to provide mobility information for people who are blind depend on accurate identification of location of stimuli on the skin, as do systems designed to indicate target positions in space or the status of remotely operated devices to pilots or engineers. The spatial acuity of the skin has been examined for simple touch, but not for the types of vibrating signals used in such devices. The ability to localize vibratory stimuli was examined at sites around the abdomen and found to be a function of separation among loci and, most significantly, of place on the trunk. Neither the structures underlying the skin nor the types of tactor tested appeared to affect localization. Evidence was found for anatomically defined anchor points that provide localization referents that enhance performance even with wide target spacing.

Vibrotactile transduction and transducers

The bodys sense of touch is potentially a versatile channel for the conveyance of directional, spatial, command, and timing information. Most practical implementations of vibrotactile systems require compact, light-weight actuators that can be mounted against the body. Eccentric mass motors are widely used for this application, yet their output is limited and the effects of loading on the transducers due to the skin and mounting arrangement have been largely ignored. Conventional linear actuators are well suited as vibrotactile transducers and can provide high output, but are typically limited to laboratory research due to their large size and cost. The effect of loading on various practical vibrotactile transducers is investigated using a skin impedance phantom and measuring the transducer displacement with respect to additional mass loading. Depending on the transducer design, loading can dramatically reduce the vibratory displacement and, in the case of eccentric mass motors, also increase the operating frequency. In contrast, a new linear actuator design can be designed to be almost independent of skin loading, by considering the mechanical impedance of the load and optimizing the transducer contact area.

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.

The perception of tactile distance: influences of body site, space, and time.

Vibrotactile prostheses for deaf or blind persons have been applied to any number of different locations on the body, including the finger, wrist, forearm, abdomen, back, and nape of the neck. The discriminability of patterns presented by such devices can be affected by the acuity of the site of application and the resolution of the display. In addition, the mutual influences among stimuli close together in both space and time can affect percepts within a broad range of parameters. For example, consideration must be given to a variety of tactile illusions often associated with the spatial separations and the range of temporal intervals typically used in cutaneous communication displays. Experiments are reported in which magnitude estimates and cross-modality matches of perceived extent produced by pairs of vibrotactile taps presented to separate loci were obtained on three different body sites. Perceived distance was directly related both to the timing between the taps and to their physical separation. The findings show a consistent relationship to cortical magnification across body sites.

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 recognition and discrimination at several body sites

Although many body sites have been tested for the processing of vibrotactile patterns, very few studies have employed similar patterns and techniques to compare processing across loci. In the studies reported here, subjects judged comparable sets of spatial patterns presented in the same way to the finger, palm, and thigh in tests of recognition, discrimination, and masking. Discrimination varied as a function of stimulus onset asynchrony in a similar way across loci, but level of performance across sites depended, to different degrees, on duration. These differences were mirrored in the relative levels of recognition learning across sites. Levels of forward and backward masking on the fingers and thigh were less dependent on duration and site.

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.

A computer-controlled matrix system for presentation to the skin of complex spatiotemporal patterns

A system for presenting complex spatiotemporal patterns to the skin is described. The computer-controlled device consists of a matrix of 64 piezoceramic benders mounted in a support system that can be fitted to steeply contoured skin surfaces such as the thigh. Vibrotactile patterns that include movement across the skin’s surface and/or intensitive variation within or across display epochs are programmed and presented in a frame-by-frame manner, similar to the production of motion pictures. Patterns can be predefined, or modified by observers’ responses to allow for an interactive haptographic display. Design considerations for the construction of the tactile matrix, computer interface, and software are discussed.

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.