Online repetitive transcranial magnetic stimulation (TMS) to the parietal operculum disrupts haptic memory for grasping

Abstract

The parietal operculum (OP) contains haptic memory on the geometry of objects that is readily transferrable to the motor cortex but a causal role of OP in memory-guided grasping is only speculative. We explored this issue by using online high-frequency repetitive transcranial magnetic stimulation (rTMS). The experimental task was performed by blindfolded participants acting on objects of variable size. Trials consisted in three phases: haptic exploration of an object, delay, and reach-grasp movement onto the explored object. Motor performance was evaluated by the kinematics of finger aperture. Online rTMS was applied to the left OP region separately in each of the three phases of the task. The results showed that rTMS altered grip aperture only when applied in the delay phase to the OP. In a second experiment a haptic discriminative (match-to-sample) task was carried out on objects similar to those used in the first experiment. Online rTMS was applied to the left OP. No psychophysical effects were induced by rTMS on the detection of explicit haptic object size. We conclude that neural activity in the OP region is necessary for proficient memory-guided haptic grasping. The function of OP seems to be critical while maintaining the haptic memory trace and less so while encoding it or retrieving it.

Keywords: affordance; dorsal stream; graspable; object; parietal; premotor; reaching; sensorimotor; somatosensory; tactile.

Figure 1

Schematization of trial structure of Experiment 1.In the inter‐trial phase (phase A) participants were required to hold a small plate fixed to the table, so that the same finger aperture was ensured at the start of the active phases of each trial. The subjects were then prompted by a sound to explore haptically the object in front of them (phase B). After 2 s a sound instructed the participants to go back to the resting position. They were then required to stop the haptic exploration for a delay of 2 s (phase C). Finally a sound prompted them to perform a thumb‐and‐index grasping (phase D) of the previously explored object.

Figure 2

Scalp positions in the 32 subjects of the projection of OP.

Figure 3

Example of the fleximeter's output in a representative trial. On the upper part of the figure the electrical signal obtained by the fleximeter during a whole trial is represented. On the lower part of the figure, the corresponding hand configuration producing the signal is reported. In the upper left corner the fleximeter is schematized. The fleximeter was sticked to the hand as showed in order to obtain an electrical signal variable according to the hand aperture indicating the hand preshaping during the thumb‐index grasping (phase D). The baseline‐peak amplitude of the finger aperture that was measured in phase D and then used for further analysis is indicated by a double arrow.

Figure 4

Schematization of trial structure of Experiment 2.

Figure 5

Results of Experiment 1: mean fleximeter outputs for the two diameters in the trials without TMS. Error bars indicate 95% confidence intervals of the mean.

Figure 6

Results of Experiment 1: mean fleximeter outputs in the different experimental conditions. P values refer to pairwise planned contrasts. Only significant P values are shown. Note that the condition in which TMS had been delivered to OP in the delay period was the only one in which no differentiation was observed between the two cylinders. Error bars indicate 95% confidence intervals of the mean.

Figure 7

Results of Experiment 2. Error bars indicate 95% confidence intervals of the mean.