Passive tactile recognition of geometrical shape in humans: An fMRI study

Tactile shape discrimination involves frontal other than somatosensory cortex (Palva et al., 2005 [48]), but it is unclear if this frontal activity is related to exploratory concomitants. In this study, we investigated topographical details of prefrontal, premotor, and parietal areas during passive tactile recognition of 2D geometrical shapes in conditions avoiding exploratory movements. Functional magnetic resonance imaging (fMRI) was performed while the same wooden 2D geometrical shapes were blindly pressed on subjects' passive right palm in three conditions. In the RAW condition, shapes were pressed while subjects were asked to attend to the stimuli but were not trained to recognize them. After a brief training, in the SHAPE condition subjects were asked to covertly recognize shapes. In the RECOGNITION condition, they were asked to overtly recognize shapes, using response buttons with their opposite hand. Results showed that somatosensory cortex including contralateral SII, contralateral SI, and left insula was active in all conditions, confirming its importance in processing tactile shapes. In the RAW vs. SHAPE contrast, bilateral posterior parietal, insular, premotor, prefrontal, and (left) Broca's areas were more active in the latter. In the RECOGNITION, activation of (left) Broca's area correlated with correct responses. These results suggest that, even without exploratory movements, passive recognition of tactile geometrical shapes involves prefrontal and premotor as well as somatosensory regions. In this framework, Broca's area might be involved in a successful selection and/or execution of the correct responses.