Differential representation of “toolness” and the elongated shape of tools revealed by continuous flash suppression and backward masking

Abstract

A crucial characteristic that distinguishes human beings from animals is using tools to perform complicated actions. A tool usually corresponds to a particular action. Therefore, although viewing other objects such as animals and faces mainly activate the ventral visual stream, viewing tools activate both the ventral visual stream mediating object recognition and the dorsal visual stream mediating object manipulation. Previous studies show that tools can be represented in the dorsal visual pathway unconsciously, but others argued that this is just due to its elongated shape. It is essential to solve this debate because it relates to which level of perceptual functions (shape at the middle level vs. category at the high level) can take place unconsciously along the visual ventral or dorsal stream. Here, we address this question by investigating the temporal evolution of the global shape (elongated vs. stubby) and “toolness” representation of tools using electroencephalography (EEG) when visual stimuli were visible or made invisible by backward masking (BM) or continuous flash suppression (CFS). With a combination of univariate, multivariate, and computational modeling (deep convolutional neural network, DCNN) analyses, we provide compelling evidence that even when the global shape (i.e., elongated vs. stubby) of the objects were controlled, the “toolness” representation was still available in unconsciousness caused by CFS but not in unconsciousness caused by BM. These findings suggest that there is category information beyond the global elongated shape that enabled the unconscious processing of tools.