Working memory of somatosensory stimuli: an fMRI study

In a previous study, we have shown that passive recognition of tactile geometrical shapes (i.e. no exploratory movement) engages prefrontal and premotor areas in addition to somatosensory regions (Savini et al., 2010). In the present study we tested the hypothesis that these regions are involved not only in the perception but also during working memory of such somatic information. We performed functional magnetic resonance imaging (fMRI) during the execution of N-BACK tasks, with 2D geometrical shapes blindly pressed on the subjects' right hand palm. Three conditions with increasing memory load (0-BACK, 1-BACK, 2-BACK) were used. Results showed that primary somatosensory area (SI), secondary somatosensory area (SII) and bilateral Insula were active in all conditions, confirming their importance in coding somatosensory stimuli. Activation of fronto-parietal circuit in supplementary motor area (SMA), right superior parietal lobe (rSPL), bilateral middle frontal gyrus, left inferior frontal gyrus, and right superior frontal sulcus was significantly larger during 1-BACK and 2-BACK than 0-BACK. Left superior parietal lobe and right frontal eye field showed a higher activation during the 2-BACK than 0-BACK. Finally, SMA and rSPL were characterized by a statistically significant higher activation during 2-BACK than 1-BACK, revealing their sensitivity to the memory load. These results suggest that working memory of tactile geometrical shapes (no exploratory movement) involves a complex circuit of modal and supramodal fronto-parietal areas.