Tactile coactivation-induced changes in spatial discrimination performance

Abstract

We studied coactivation-based cortical plasticity at a psychophysical level in humans. For induction of plasticity, we used a protocol of simultaneous pairing of tactile stimulation to follow as closely as possible the idea of Hebbian learning. We reported previously that a few hours of tactile coactivation resulted in selective and reversible reorganization of receptive fields and cortical maps of the hindpaw representation of the somatosensory cortex of adult rats (Godde et al., 1996). In the present study, simultaneous spatial two-point discrimination was tested on the tip of the right index finger in human subjects as a marker of plastic changes. After 2 hr of coactivation we found a significant improvement in discrimination performance that was reversible within 8 hr. Reduction of the duration of the coactivation protocol revealed that 30 min was not sufficient to drive plastic changes. Repeated application of coactivation over 3 consecutive days resulted in a delayed recovery indicating stabilization of the improvement over time. Perceptual changes were highly selective because no transfer of improved performance to fingers that were not stimulated was found. The results demonstrate the potential role of sensory input statistics (i.e., their probability of occurrence and spatiotemporal relationships) in the induction of cortical plasticity without involving cognitive factors such as attention or reinforcement.

Fig. 1.

Learning curves of naive (left) and non-naive (right) subjects. Thresholds for spatial two-point discrimination as a function of the day of training before induction of changes by a coactivation protocol [day −4 (d −4) to day 0 (d0)] are shown. In this and subsequent figures dots represent the mean thresholds; horizontallineswithinboxes represent the medians.Boxes show the top and bottom quartiles, and theoutliercaps are placed on the top and bottom deciles.

Fig. 2.

Effects of coactivation on discrimination thresholds (n = 21). Thresholds were measured at the end of the training period (day 0) before and after application of the coactivation protocol (day 0 pre, day 0 post, respectively) and on the 2 following days (day 1, day 2).

Fig. 3.

Effects of different durations of the coactivation protocol. Shown are relative changes of discrimination thresholds (comparing day 0 pre with day 0 post) after 6, 2, and 0.5 hr of coactivation.

Fig. 4.

Recovery of the coactivation effect on discrimination thresholds (n = 4). Thresholds are shown for the before and after conditions (day 0 pre, day 0 post, respectively) and for measurements 2, 4, 6, 8, and 24 hr after termination of the coactivation protocol.

Fig. 5.

Cumulative effects of repeated coactivation (n = 5). Discrimination thresholds before and after (pre, post, respectively) coactivation applied on 3 consecutive days (day 0, day 1, day 2) and on 3 d after the last coactivation application (day 3, day 4, day 5) are shown.

Fig. 6.

Controls. Relative changes of discrimination thresholds (comparing day 0 pre with day 0 post) were measured on the tip of the right-IF, the right-MF, and the left-IF after coactivation was applied to the right index finger. In addition, the result of a sham stimulation protocol (sham) applied to the right index finger is shown.

Fig. 7.

Transfer of improvement of discrimination performance during the initial training period. Discrimination thresholds were measured for the test finger (right-IF) on the first day of testing (day −4) and at the end of the initial training period (day 0) and for the control fingers on day 0 (corresponding to the first day of testing of the control fingers). Left,Subjects with the right-MF as the control finger. Right,Subjects with the left-IF as the control finger.