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.     

Vibrotactile masking: A comparison of energy and pattern maskers

Subjects were required to identify vibrotactile patterns presented to their fingertips. The patterns, letters of the alphabet, were presented singly or in the presence of other vibrotactile masking stimuli. Two types of masking stimuli were used: an energy masker and a pattern masker. The effectiveness of these two types of maskers in interfering with letter recognition was tested, using them as forward and as backward maskers and presenting them at several different levels of intensity. The results showed more masking by the pattern masker, more backward than forward masking, and more masking as intensity increased. In addition, compared with the energy masker, the pattern masker showed both a greater difference between forward and backward masking and a greater increase in masking as masker intensity increased. The results are discussed in terms of a two-factor model of vibrotactfle masking.     

Vibrotactile signal detection

The theory of signal detectability was applied to vibrotactile sensitivity in two experiments. The first experiment showed The psychometric function to be satisfactorily linear when a sensitivity index of the d family was plotted against the signal intensity expressed in decibels. The second experiment yielded receiver-operating-characteristic (ROC) curves of a familiar form for the yes-no and rating response methods. Reasonably consistent estimates of sensitivity were obtained in the second experiment from the yes-no. rating, and forced-choice methods. The sensitivity indices examined were d’ and d_e’, based on Gaussian density functions; A, based on Rayleigh density functions; and the distribution-free indices, P(A) and F(C). For each type of index a tendency was observed for the forced-choice value to be lower than the yes-no and rating values.     

Vibrotactile loudness addition

Os adjusted the intensity of vibration at a single locus on the right hand to a value equal in vibratory loudness to various patterns of vibration on the left hand. The patterns were created by 1 to 5 equated vibration generators, varied with respect to sensation level and distances among the vibrators. The results were: (a) increasing from 1 to 5 vibrators produced a doubling in vibratory loudness, (b) neither loudness level of the components nor distance among vibrators had any effect on the slope of the overall loudness growth function. as also adjusted the intensity of a white noise to equal in magnitude the patterns of vibration presented (a) to the left hand as before and (b) to loci distributed over the surface of the body. The results were the same as those obtained using a single vibrator as standard. The specific loci stimulated did not appear to have any effect on vibrotactile loudness addition.     

Vibrotactile spatial summation

By means of a two-interval, forced-choice technique, absolute thresholds were obtained at several sites on the skin for short bursts of mechanical vibration. The difference between the energy required for a single vibrator, in dB, to reach threshold and the energy required for two vibrators presented successively to reach threshold was taken as the measure of probability summation. Spatial summation was computed as the difference between the energies required for threshold for the two vibrators presented successively and simultaneously. Both probability summation, 0.52 dB lowering in threshold, and spatial summation, 1.94 dB lowering in threshold, remained constant as a function of the distance between the two vibrators on the thigh. Similar amounts of summation were obtained unilaterally and bilaterally on the fingers. When the frequency of vibration was lowered to 9 Hz, no spatial summation was found. When the frequencies of vibration at the two test sites were different, 160 Hz at one locus, 360 Hz at the other locus, there was no spatial summation.     

Vibrotactile loudness addition

Os adjusted the intensity of vibration at a single locus on the right hand to a value equal in vibratory loudness to various patterns of vibration on the left hand. The patterns were created by 1 to 5 equated vibration generators, varied with respect to sensation level and distances among the vibrators. The results were: (a) increasing from 1 to 5 vibrators produced a doubling in vibratory loudness, (b) neither loudness level of the components nor distance among vibrators had any effect on the slope of the overall loudness growth function. Os also adjusted the intensity of a white noise to equal in magnitude the patterns of vibration presented (a) to the left hand as before and(b) to loci distributed over the surface of the body. The results were the same as those obtained using a single vibrator as standard. The specific loci stimulated did not appear to have any effect on vibrotactile loudness addition.     

Vibrotactile frequency discrimination

Thresholds for vibrotactile discrimination of pulse interval were determined for pulse frequencies between 1 and 384 Hz. The results point to a temporal resolution significantly more accurate than that demonstrated in earlier studies. Although touch as a vibratory sensor is in general much inferior to audition, the present results show a striking resemblance to those obtained on auditory pitch. The neurophysiological implications for the tactile as well as for the auditory system are discussed.     

Vibrotactile adaptation on the face

Threshold amplitude for vibration is elevated if testing is preceded by extended exposure to a vibratory adapting stimulus of appropriate amplitude and frequency. This phenomenon, previously studied almost exclusively on the hand, is here shown for the first time to occur on the face as well. Adaptation is then used analytically to determine that the two-branched threshold-versus-frequency function obtained on the face by Verrillo and Ecker (1977) represents the activity of two distinct mechanisms. Action spectra of vibrotactile adaptation reveal the presence of both mechanisms even in subjects whose unadapted threshold function (like that reported by Barlow, 1987) shows no sign of duplexity. Finally, the data suggest that on the face (unlike the hand), cross-channel adaptation may occur at high adapting amplitudes.     

Vibrotactile discrimination of musical timbre

Five experiments investigated the ability to discriminate between musical timbres based on vibrotactile stimulation alone. Participants made same/different judgments on pairs of complex waveforms presented sequentially to the back through voice coils embedded in a conforming chair. Discrimination between cello, piano, and trombone tones matched for F0, duration, and magnitude was above chance with white noise masking the sound output of the voice coils (Experiment 1), with additional masking to control for bone-conducted sound (Experiment 2), and among a group of deaf individuals (Experiment 4a). Hearing (Experiment 3) and deaf individuals (Experiment 4b) also successfully discriminated between dull and bright timbres varying only with regard to spectral centroid. We propose that, as with auditory discrimination of musical timbre, vibrotactile discrimination may involve the cortical integration of filtered output from frequency-tuned mechanoreceptors functioning as critical bands. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Vibrotactile learning and the locus dimension

Forty subjects, each of whom was randomly allocated to one of four experimental groups, learned a 26-element vibrotactile code. Each group used either 3, 4, 5 or 6 vibrators. As predicted, the performance of each group improved in the order 3-, 4-, 5- and 6-vibrators. Although there were no significant differences between the groups there was a highly significant learning effect over ten trials. Error analyses showed that duration errors were the most frequent type of error and that intensity errors were the least frequent. It is suggested that both the types of errors and the optimum number of loci which can be separately stimulated in this type of experiment will depend upon the spacing and arrangement of signal elements.     

Vibrotactile frequency discrimination at short durations

This experiment investigated how frequency discrimination of a sinusoidal, mechanical vibration applied to the tip of the right index finger is affected by shortening the duration of the stimuli from 200 ms to 30 ms. Using a standard stimulus of 100 Hz at 30 dB above threshold, seven comparison frequencies (at intervals of 10 Hz) were judged as higher or lower in frequency according to the method of constant differences. Vibrotactile stimuli were matched for subjective intensity across both frequency and duration. Difference limens for vibrotactile frequency were found to decline slightly from 200 ms to 50 ms (attributable to practice) and to increase noticeably at 30 ms. This result is discussed in relation to the seemingly contradictory results for auditory pitch discrimination.     

Vibrotactile thresholds measured at the finger

Absolute vibrotactile thresholds were determined over the distal pad of the middle finger and thenar eminence of the right hands of five Ss. Measurements were made using eight frequencies between 25 and 700 Hz and seven contactor sizes between .0008 and 1.3 cm². When plotted as a function of frequency, the threshold curve measured at the fingerpad with a .005-cm² contactor is U shaped, with a maximum sensitivity in the region of 250 Hz. When plotted as a function of contactor size, the threshold decreases at a rate of 3 dB per doubling of the area.     

Vibrotactile masking: Effects of multiple maskers

Threshold elevations (TE) were determined at a testsite on the left thigh with the simultaneous presentation of combinations of maskers positioned on the trunk. It was found that the resultant TE from groups of up to 10 multiple maskers is nearly equal to the addition of the TE produced by each masker separately.