James C. Makous

VIBROTACTILE FORWARD MASKING : EFFECTS OF THE AMPLITUDE AND DURATION OF THE MASKING STIMULUS

Tactile thresholds for detecting a 50-ms signal presented 25 ms after the termination of a masking stimulus increased as a function of the amplitude level and duration of the masking stimulus. The effects were similar in both the P and NP I channels measured at 250 and 20 Hz, respectively. It was concluded that the increased masking caused by increasing the duration of the masking stimulus resulted from processes other than or in addition to temporal integration--the latter being a characteristic of the P, but not the NP I, channel. The slopes of the masking functions, in which threshold shifts were plotted as a function of masking-stimulus sensation level, were consistently greater for 20-Hz than for the 250-Hz stimuli, suggesting that masking efficiency is greater in the NP I than in the P channel.

The effects of static indentation on vibrotactile threshold

Vibrotactile thresholds were measured on the thenar eminence and the volar forearm at different static depths of skin indentation. Three stimulus frequencies (1, 20, and 200 Hz) were delivered through either a 0.008- or a 2.9-cm2 contractor. The indentation depths ranged from 0 to 1 mm (0.25-mm steps) relative to the point of skin contact with the stimulator. There was a significant effect of indentation in all stimulus combinations of contactor size, location, and frequency. These results resolve an apparent discrepancy in the literature regarding threshold reduction with increasing contactor size observed on the forearm at low frequencies.

Decay in the effect of vibrotactile masking

Vibrotactile forward masking experiments were conducted on the thenar eminence of the hand to determine the time course of masking in the Pacinian (P) or a non-Pacinian (NP I) channel. Brief masking and test stimuli that contained energy centered either below 27 or at 500 Hz were used to preferentially activate the NP I or P channels, respectively. Test thresholds were measured at different fixed delays ( Delta t) between the masking and test stimuli that ranged from 5 to 995 microseconds. Masking stimulus level was also varied from 5 to 25 dB above threshold. The masking effect followed an exponential decay with different time constants for each of the channels, but a similar asymptote (residual masking) for both channels. At the higher masking stimulus levels, the time constant for the P channel was about 40 microseconds, while the time constant for the NP I channel was about 100 microseconds. Residual masking in both channels increased about 1 dB for every 5-dB increase of masking stimulus level.

New discoveries in taction and their implications for tactile communication

The implementation of tactile aids for the deaf and blind has progressed largely through the development of new technologies and strategies adapted to adequately and efficiently activate previously understood tactile mechanisms and processes. In the past decade, however, important new discoveries have been made relating to the basic mechanisms of taction such as the effects of aging and disease, submodality interactions, skin mechanics, and learning and cortical plasticity. These advances in our knowledge will impact heavily on future developments and applications of tactile aids. Several of these new discoveries will be described in the context of basic tactile mechanisms and in their implications for significant improvements in the effectiveness of aids for tactile communication. [Work supported by NIH.]

The effects of the intensity and duration of a masking stimulus, Δt, and signal duration on vibrotactile forward masking

Vibrotactile thresholds were found to increase as a function of the duration and intensity of a forward masking stimulus and to decrease as a function of the time interval between the termination of the masking stimulus and the onset of the signal (Δt) and signal duration. The effects of the intensity and duration of the masking stimulus were similar for both the P and NPI channels as indicated by the nearly identical results measured at 250 and 20 Hz. The observed declines in threshold as a function of Δt and signal duration were both attributed to an increase in the time interval between the offset of the masking stimulus and the offset of the signal.

Influences of pain and learning on vibrotaction

The parallel processing of tactile information by independent channels as defined psychophysically, physiologically and anatomically has been amply described in the literature. For a unified percept to occur, however, information carried by these separate channels must be combined centrally. Furthermore, the unified percepts are affected by submodality interactions, such as the gate theory of pain whereby tactile stimulation suppresses pain, and cognitive factors (e.g., attention). Several series of experiments are presented showing that painfully hot and cold stimuli can substantially diminish vibrotactile sensation, but only when co‐localized with the tactile stimuli. The effect occurs regardless of the tactile channel activated. This tough‐gate acts oppositely to the pain gate and indicates centrally located touch–pain interactions. The results of additional experiments show that training of observers in an intensity‐discrimination task can significantly affect the ability to distinguish differences in...

Tactile forward masking in Pacinian and non‐Pacinian channels

Four subjects were tested with a forward masking paradigm to determine the time course of masking in the Pacinian (P) and a non‐Pacinian (NPI) channel. Stimuli presented to the thenar eminence were either centered below 27 Hz or at 500 Hz to preferentially activate the NPI or P channels, respectively. Two‐interval forced‐choice tracking was used to measure thresholds. A masking stimulus was presented 100 ms after each interval onset and a test occurred in one of the two intervals at some specified delay (delay t) relative to the masking stimulus. Delta t was varied from 10 to 1000 ms and the masking stimulus level varied from 5 to 25 dB above thresholds in 5‐dB steps. The results showed that the masking functions varied significantly with delta t. At longer delta t’s there was less masking that followed an exponential decay. The results from the individual channels differed from each other with respect to the slope of the masking functions and the exponential decay of masking with delta t. The time constants for the PC channel ranged from 20 to 40 ms, whereas the time constants for NPI ranged from 50 to 120 ms. [Work supported by NIH, NIDCD.]

Temporal factors may affect the binaural directionality of hearing in the mustache bat

The directional sensitivity of two Pteronotus parnellii bats to the first, second, and third harmonics of their echolocating signal was measured binaurally, under free field conditions, by recording the summated neural response at the lateral lemniscus. A speaker was mounted on a perimeter that could be rotated to form a hemisphere in front of the bat. Speaker angles were selected randomly, contralateral to the recording site, 50 cm from the bats head. The area of greatest sensitivity to the first harmonic was 40‐ to 60‐deg azimuth and −20 to −40 deg elevation. The response to the second harmonic was more medial ranging from 10‐ to 30‐deg azimuth and 0 to −10 deg elevation. Most medial was the response to the third harmonic frequency that ranged from 0‐ to 15‐deg azimuth and 0 to −10 deg elevation. The areas of greatest sensitivity (2 dB above threshold) to the first and third harmonics were more medial than areas where intensity differences at opposite ears afford the most precise localization [Z. M. Fu...