Temporal dynamics of neural activity underlying unconscious processing of manipulable objects

The primate visual system is assumed to comprise two main pathways: a ventral pathway for shape and color perception and a dorsal pathway for spatial processing and visuomotor control. Previous studies consistently reported strong activation in the dorsal pathway (especially in the inferior parietal region) induced by manipulable object images such as tools. However, it is controversial whether the dorsal pathway retains this preferential activity to tool images under unconscious perception. In the present study, we used magnetoencephalography (MEG) and investigated spatio-temporal dynamics of neural responses to visible and invisible tool images. A presentation of visible tool images elicited a strong neural response over the parietal regions in the left hemisphere peaking at 400 msec. This response unique to the processing of tool information in the left parietal regions was still observed when conscious perception of tool images was inhibited by interocular suppression. Furthermore, analyses of neural oscillation signals revealed a suppression of μ rhythm (8-13 Hz), a neural index of movement execution or imagery, induced by both visible and invisible tools. Those results indicated that the neural circuit to process the tool information was preserved under unconscious perception, highlighting an implicit aspect of the dorsal pathway.