Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality

Abstract

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10-15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.

Fig. 1.

Effects of skin location. A, Distribution of interval discrimination thresholds expressed as Weber fractions for a base interval of 125 msec. Stimuli were delivered to five skin locations, as shown in the abscissa. Thenumbers in parentheses represent the number of subjects tested at each skin location. B, Interval discrimination thresholds for stimuli delivered to digit 4 of the left hand are plotted against thresholds for stimuli delivered to digit 3 of left hand. The linear regression fit is shown in thedashed line (r = 65;p < 0.05). C, Interval discrimination thresholds for stimuli delivered to digit 4 of the left hand are plotted against thresholds for stimuli delivered to digit 4 of the right hand. The linear regression fit is shown in the dashed line (r = 41; ns, not statistically significant).

Fig. 2.

Effect of base interval and modality.A, Somatosensory interval discrimination thresholds for different base intervals expressed as Weber fractions. Thresholds at a base interval of 75 msec are significantly different from thresholds at other base intervals. B, Distribution of interval discrimination thresholds plotted at different base intervals with somatosensory stimuli (base intervals of 75,125, and 225 msec) and auditory stimuli (base intervals of 50, 100, and200 msec).

Fig. 3.

Learning curves for training at a base interval of 125 msec. A, Individual subject thresholds, expressed as Weber fractions, plotted as a function of training sessions. Stimuli were delivered to the distal tip of the fourth digit for subjectsS1–S8 and to the thenar eminence for subjectsS9–S12. Bars are SEM within subjects. The filled symbols indicate pre- and post-test thresholds for each subject. Note that the pre- and post-test thresholds were estimated from five to six blocks of 60 trials, whereas training thresholds were obtained for 15 blocks. B, Mean of all of the subjects trained at a base interval of 125 msec plotted as a function of training days. Here, error bars represent 1 SEM across subjects.

Fig. 4.

Learning curves for training at a base interval of 75 msec. A, Individual subject thresholds, expressed as Weber fractions, plotted as a function of training sessions. Stimuli were delivered to the distal tip of the fourth digit for subjectsS13–S16. Bars are SEM within subjects. Thefilled symbols indicate pre- and post-test thresholds for each subject. Note that the pre- and post-test thresholds were estimated from five to six blocks of 60 trials, whereas training thresholds were obtained for 15 blocks. B, Mean of all four subjects plotted as a function of training days. Here, error bars represent 1 SEM across subjects.

Fig. 5.

Summary of learning. A, Distribution of fractional changes between pre- and post-tests are shown for three groups: (1) subjects trained at a base interval of 75 msec, (2) subjects trained at a base interval of 125 msec, and (3) untrained subjects tested with no training during the 10–15 d between pre- and post-tests. B, Individual subjects’ pretest (left panel) and post-test thresholds (right panel) are plotted as a function of their fractional change in the trained condition. Dashed linesare linear regression fits, which account for 31% of the variance between pretest threshold and fractional changes (r= 0.56; p < 0.05) and for <4% of the variance between post-test thresholds and fractional changes (r = −0.2, ns). C, The variability of thresholds decreases with training both within (left panel) and across subjects (right panel). Within subjects, the SEMs of average pre- and post-test thresholds are shown. Across subjects, the SEMs of the pre-and post-test thresholds are shown.

Fig. 6.

Spatial generalization. Distribution of fractional changes at a base interval of 125 msec for stimuli delivered to the trained skin location and an untrained skin location on the same hand and to an untrained but corresponding location on the contralateral hand is shown.

Fig. 7.

Temporal generalization. A, Distribution of fractional change at base intervals of 75, 125, and 225 msec for subjects trained at a base interval of 125 msec (indicated by the gray box). B, Distribution of fractional change at base intervals of 75 and 125 msec for subjects trained at a base interval of 75 msec (indicated by the gray box).

Fig. 8.

Generalization across modality: somatosensory to auditory. A, Distributions of fractional changes in interval discrimination for somatosensory stimuli (at base intervals of75, 125, and 225 msec) and for untrained auditory stimuli (at base intervals of 50,100, and 200 msec). Note that the 125 msec somatosensory base interval is the trained condition (gray box). Fractional changes at 225 msec with somatosensory stimuli and at 200 msec with auditory stimuli are significantly different from the others (shown byasterisks). B, Fractional changes in the trained condition plotted against fractional changes in auditory interval discrimination at a base interval of 100 msec. The data show a moderate linear correlation of 0.89; a linear regression fit (dashed line) accounts for 78% of the variance.