Enhanced accuracy in novel mirror drawing after repetitive transcranial magnetic stimulation-induced proprioceptive deafferentation

Abstract

When performing visually guided actions under conditions of perturbed visual feedback, e.g., in a mirror or a video camera, there is a spatial conflict between visual and proprioceptive information. Recent studies have shown that subjects without proprioception avoid this conflict and show a performance benefit. In this study, we tested whether deafferentation induced by repetitive transcranial magnetic stimulation (rTMS) can improve mirror tracing skills in normal subjects. Hand trajectory error during novel mirror drawing was compared across two groups of subjects that received either 1 Hz rTMS over the somatosensory cortex contralateral to the hand or sham stimulation. Mirror tracing was more accurate after rTMS than after sham stimulation. Using a position-matching task, we confirmed that rTMS reduced proprioceptive acuity and that this reduction was largest when the coil was placed at an anterior parietal site. It is thus possible, with rTMS, to enhance motor performance in tasks involving a visuoproprioceptive conflict, presumably by reducing the excitability of somatosensory cortical areas that contribute to the sense of hand position.

Figure 1.

The mirror tracing experiment. A, Study design. Normal tracing skill was tested before and immediately after 15 min of 1 Hz rTMS. Mirror tracing was tested during six trials given after rTMS. Each trial lasted 10 sec with an intertrial interval of 10 sec. B, Results. Box plot of trajectory error during novel mirror tracing in the groups that received either real or sham rTMS. Post-rTMS error is expressed as percentage of the baseline error sampled before rTMS. Mann-Whitney U test, p < 0.05; n = 6 subjects in each group.

Figure 2.

The apparatus used to test proprioceptive accuracy. The apparatus consisted of two LEGO bricks fixated on a horizontal table so as to be parallel with the subject's midsagittal plane. Before each trial, the examiner positioned both fingers on the studs situated at the proximal end of the apparatus. Then, the subject's right index finger was passively moved to one of the studs on the right brick, at variable distance from the start position. Finally, the subject moved the left index finger to place it on a stud of the left brick so as to match the position of the right index finger. The subjects had their eyes closed throughout the experiment and received no feedback on performance.

Figure 3.

Box plot of proprioceptive error after rTMS applied at various scalp sites. The coil was placed over three different brain sites: M1, at the motor hotspot; APC, at 3 cm posterior to the motor hotspot; and PPC, at 6 cm posterior to the motor hotspot. The effect of rTMS site on proprioceptive error was significant (Kruskal-Wallis test, p = 0.023). Post-rTMS error is expressed as percentage of the baseline error, sampled before rTMS. n = 15 subjects in each group.