Decoding haptic and imagined stimulus size in the human cortex

Human neuroimaging studies indicate that the early visual cortex (EVC), including the primary visual cortex (V1), is involved in haptic exploration of objects, even when visual information is not available. However, it remains unknown whether the features of haptically explored objects, like size, are represented in the EVC. Here, we investigated whether we can use the activity patterns in the EVC and other task-relevant brain regions to decode stimulus size during haptic exploration, and whether this effect is due to visual imagery. Twenty-five right-handed participants haptically explored or imagined the size of three rings (small, medium, large) in a slow-event-related fMRI study. Participants were blindfolded during the training and fMRI sessions. Using multivariate pattern analysis, we found that V1 and the occipital pole (OP) showed accurate decoding of stimulus size during haptic exploration, but not imagery trials. This suggests that the activity patterns observed in the haptic condition cannot be explained by visual imagery. Frontal and parietal regions, as well as the multisensory lateral occipital tactile-visual area (LOtv), showed accurate size decoding during both haptic and imagery conditions, suggesting a flexible representation of stimulus size that adapts to task demands. In addition, stimulus size could be decoded across tasks in the anterior and posterior intraparietal sulcus (aIPS, pIPS), and dorsal premotor cortex (dPM). Psychophysiological interaction analysis indicated that V1 and OP showed stronger functional connectivity with ventral and dorsal visual stream areas during the haptic as compared to the imagery task. Overall, stimulus size information is similarly represented in frontal and parietal cortices across haptic exploration and imagery, but not in early visual areas, demonstrating that only regions specialized for haptic exploration and imagery support generalized size representations.