Differential activation in somatosensory cortex for different discrimination tasks

Abstract

Maps of the body surface in somatosensory cortex have been shown to be highly plastic, altering their configuration in response to changes in use of body parts. The current study investigated alterations in the functional organization of the human somatosensory cortex resulting from massed practice. Over a period of 4 weeks, subjects were given synchronous tactile stimulation of thumb (D1) and little finger (D5) for 1 hr/d. They had to identify the orientation of the stimuli. Neuroelectric source localization based on high-resolution EEG revealed that, when subjects received passive tactile stimulation of D1 or D5, the representations of the fingers in primary somatosensory cortex were closer together after training than before. There was also an apparently correlative tendency to anomalously mislocalize near-threshold tactile stimuli equally to the distant finger costimulated during training rather than preferentially to the finger nearest to the finger stimulated in a post-training test. However, when the stimulus discrimination had to be made, neuroelectric source imaging revealed that the digital representations of D1 and D5 were further apart after training than before. Thus, the same series of prolonged repetitive stimulations produced two different opposite effects on the spatial relationship of the cortical representations of the digits, suggesting that differential activation in the same region of somatosensory cortex is specific to different tasks.

Fig. 1.

The timing of tactile discrimination training (top row), EEG-recording (middle row), and mislocalization testing (bottom row) across the experimental sessions.

Fig. 2.

Stimulation sites and stimulation patterns for tactile discrimination training (a), and pretraining and post-training (nondiscrimination) (b) sessions. The frequencies of the various stimulation patterns are indicated in the bottom of the figure. Conditions that were entered separately in the neuroelectric source localization are framed by gray rectangles. The rectangles in a contain two stimulation patterns each, indicating that the two trial types were collapsed for the source analysis.

Fig. 3.

A superposition of an evoked response to stimulation of D5 from all 128 channels is illustrated for a single subject (a). Stimulus onset is at 0 msec. Thearrow marks the SI peak used for source analysis; its topography is shown on the right(b). (The subsequent later peaks receive contributions from SII and also posterior parietal cortex, making source modeling more difficult).

Fig. 4.

Neuroelectric source imaging data for tactile discrimination training sessions. a, Coronal section through the postcentral gyrus (somatosensory cortex) of one experimental subject showing the increase in polar angle between the cortical representations of the first and fifth digit from the first period of training (left) to the last period of training (right). b, Mean polar angle for experimental subjects (group data) between the D1 and D5 dipoles during the first, second, and last period of discrimination training.

Fig. 5.

Neuroelectric source imaging data for passive tactile stimulation sessions. a, Coronal section through the postcentral gyrus of one experimental subject showing the decrease in polar angle between the cortical representations of the first and fifth digit from the pretraining to the post-training sessions.b, Mean polar angle for experimental subjects (group data) between the D1 and D5 dipoles for the pretraining and post-training passive stimulation sessions.

Fig. 6.

Number of mislocalizations attributed to the first and fourth neighbor fingers during the first week of training and the final week of training for the trained left hands (top row) and the untrained right hands of the experimental subjects and for the left and right untrained hands of 18 additional control subjects (bottom row).