Measures of tactual detection and temporal order resolution in congenitally deaf and normal-hearing adults

Abstract

To guide the development of tactile speech aids, tactual detection and temporal order discrimination by congenitally deaf and normal-hearing adults have been examined. Tactual detection thresholds for sinusoidal vibrations between 2 and 300 Hz were measured at the left thumb and index finger using an adaptive paradigm. Temporal onset- and offset-order discrimination were tested using stimuli of 50 Hz at the thumb and 250 Hz at the index finger, delivered asynchronously and varied independently in amplitude and duration. Mean detection thresholds for the deaf and normal-hearing groups did not differ significantly at any frequency tested. Temporal onset-order discrimination thresholds varied widely, particularly among congenitally deaf individuals, but no statistically significant difference was found between group means. Both experimental groups exhibited a broad range of discrimination thresholds for temporal offset-order, and mean thresholds did not differ significantly. On the whole, tactual offset-order thresholds were substantially higher than onset-order thresholds. Differences in the relative levels of paired stimuli systematically affected sensitivity to both onset- and offset-orders in most subjects. Differences in the relative durations of paired stimuli had little effect on onset-order discrimination, but had a robust effect on offset-order discrimination thresholds, which was consistent across all subjects.

Figure 1

Illustration of Tactuator finger-interface configuration.

Figure 2

Two trials of the temporal onset-order experimental paradigm. Trial 1 has a positive-valued SOA (index onset precedes thumb) and trial 2 has a negative-valued SOA (thumb onset precedes index). The upper traces represent 50-Hz vibration to the thumb, and the lower traces represent 250-Hz vibration to the index finger.

Figure 3

Two trials of the temporal offset-order experimental paradigm. Trial 1 has a positive-valued SOFA (thumb offset follows index) and trial 2 has a negative-valued SOFA (index offset follows thumb). The upper traces represent 50-Hz vibration to the thumb, and the lower traces represent 250-Hz vibration to the index finger.

Figure 4

Tactual detection thresholds (in dB re 1.0 μm peak displacement) are plotted as a function of stimulation frequency. Measurements for congenitally deaf subjects are presented in the two left-hand panels, and those for normal-hearing subjects are presented in the two right-hand panels. Data in the upper panels correspond to the left thumb of each subject, while those in the lower panels correspond to the left index finger.

Figure 5

Mean values of d^′ (top) and bias (bottom) are plotted as a function of |SOA| for congenitally deaf (left) and normal-hearing (right) subjects in the temporal onset-order discrimination experiment. Each subject was tested at three |SOA| values. In the top panels, a straight line that crosses the origin has been fit to the data of each subject by linear regression; onset-order discrimination threshold is defined as the value of |SOA| at which this line crosses d^′=1. Error bars indicate standard deviation.

Figure 6

Mean values of d^′ (top) and bias (bottom) are plotted as a function of |SOFA| for congenitally deaf (left) and normal-hearing (right) subjects in the temporal offset-order discrimination experiment. Each subject was tested at three |SOFA| values, except NH2, who was tested at four. In the top panels, a straight line that crosses the origin has been fit to the data of each subject by linear regression; offset-order discrimination threshold is defined as the value of |SOFA| at which this line crosses d^′=1. Error bars indicate standard deviation.

Figure 7

Percent correct performance plotted as a function of amplitude category (left panels) and duration category (right panels) in the tactual temporal onset-order and offset-order experiments (top and bottom panels, respectively). Data are averaged across the 12 deaf and normal-hearing subjects who were tested with SOA values of 50, 100, and 160 ms, and SOFA values of 100 and 160 ms. In each panel, illustrative icons depict either the relative level or duration relationships characteristic of paired stimuli in the lowest and highest numbered categories, in conjunction with indicators of their relative temporal onset or offset ordering. The precise relationships characteristic of each amplitude and duration category are described in Tables 3, 4, respectively.

Figure 8

For each subject who participated in both temporal order discrimination experiments, tactual offset-order thresholds are plotted as a function of corresponding tactual onset-order thresholds (Correlation coefficient r=0.852, p<0.0005).