Although it is generally held that speed of processing declines with age, there have been few studies in which tactile temporal processing has been examined with older subjects. In the present study, temporal order judgments were obtained from a group of younger subjects (n 28, mean age 23.5 years) and a group of older subjects (n 93, mean age 69.8 years). The subjects judged the temporal order of two patterns presented to the same finger, four patterns presented to the same finger, and two patterns presented to different hands. Depending on the task, the average thresholds for the older subjects ranged from two to five times longer than the thresholds from the younger subjects. In absolute terms, the largest difference between the young and older subjects was seen in correctly identifying the order of four patterns, a difference of more than 500 msec. There was some support for the decline in temporal processing being due in part to a slowing in cognitive processing, and, depending on the task, in part to stimulus persistence and difficulty in pattern identification.
The ability to discriminate a smooth surface from a grooved one depends on several variables, including the width of the grooves and the force with which the skin is contacted. It has been hypothesized that this smooth-grooved discrimination with statically presented stimuli is based on intensity cues, namely, the overall difference in perceived intensity between the smooth and grooved surfaces. To test this hypothesis, the perceived intensities of test stimuli were varied on a trial-by-trial basis by varying the depth of penetration the contactor was allowed to travel into the skin. As compared to a control condition in which stimuli were presented with the same average penetration and contrary to the hypothesis, random variations in penetration produced no decline in smooth-grooved performance. The total amount of conformance was an accurate predictor of sensitivity across various penetrations and across two test sites (distal finger pad and finger base). It appears that subjects are making absolute rather than comparative judgments in the smooth-grooved task. A recently developed continuum mechanical model of the responses of first-order mechanoreceptive afferents to static stimuli provided both a good fit to the data and indicated what aspect of the peripheral neural image was relevant for discriminating smooth surfaces from grooved surfaces.
A tactile vocoder was evaluated in a word recognition task. The vocoder consisted of a pair of two-dimensional vibrotactile arrays on which subjects placed their fingers. For one group of subjects, the two arrays contacted two fingers on the same hand (ipsilateral condition); for the second group, the arrays contacted two fingers on opposite hands (bilateral condition). Performance was better in the bilateral condition; however, the rate at which words were acquired, even in the bilateral condition, was less than that reported by Brooks and Frost [J. Acoust. Soc. Am. 74, 34-39 (1983)] with a tactile vocoder placed on the arm.
We describe a procedure to measure the error oflocalization on the skin. The procedure, which provides for rapid measurement of the error of localization and rapid analysis of the data, uses a digitizing tablet interfaced with a computer. A photocopy of the part of the body to be tested is placed on the digitizing tablet. The subject localizes the stimulus by touching the pen of the digitizing tablet to the photocopy. The location of the pen contact is stored, and the error oflocalization is determined by the computer. A graphic representation of each subject's test area can be stored. Both stimulus and response locations can be displayed on this graphic representation. The procedure also allows the same sites on the skin to be tested over a period of weeks or months.511
The ability of subjects to identify vowels in vibrotactile transformations of consonant-vowel syllables was measured for two types of displays: a spectral display (frequency by intensity), and a vocal tract area function display (vocal tract location by cross-sectional area). Both displays were presented to the fingertip via the tactile display of the Optacon transducer. In the first experiments the spectral display was effective for identifying vowels in /b/V/ context when as many as 24 or as few as eight spectral channels were presented to the skin. However, performance fell when the 12- and 8-channel displays were reduced in size to occupy 1/2 or 1/3 of the 24-row tactile matrix. The effect of reducing the size of the display was greater when the spectrum was represented as a solid histogram ("filled" patterns) than when it was represented as a simple spectral contour ("unfilled" patterns). Spatial masking within the filled pattern was postulated as the cause for this decline in performance. Another experiment measured the utility of the spectral display when the syllables were produced by multiple speakers. The resulting increase in response confusions was primarily attributable to variations in the tactile patterns caused by differences in vocal tract resonances among the speakers. The final experiment found an area function display to be inferior to the spectral display for identification of vowels. The results demonstrate that a two-dimensional spectral display is worthy of further development as a basic vibrotactile display for speech.
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