Abstract:

The visual system can be divided into two major pathways: the dorsal stream, which is responsible for the volumetric and spatial analysis; and the ventral stream, which allows the form-based object identification. In this experiment, we focused on manipulable objects and hands because of their common features (they are both involved in action, execution and planning). Here we wanted to understand how the two visual streams interact in their involvement in the processing of these two neurally and functionally interrelated object categories – hands and tools. We conducted a categorization experiment using pictures of hands and tools, within a priming paradigm, in healthy and apraxic subjects (i.e., patients with lesions within the inferior parietal lobule, so dorsal stream is impaired). We measured how unconscious processing of images from one these categories (e.g., tools) affect the recognition of images from the other category (i.e., hands). Our findings with healthy subjects demonstrated a functional relation between hands and tools – processing an image of a hand hinders the subsequent processing of an image of a tool, and vice versa. These results were not present when we tested apraxic patients (N=3) in the same task. Instead, the apraxic individuals demonstrated a pattern in which tool primes facilitated categorization of hand targets. This provides a novel perspective on how the dorsal and ventral streams interact and process visual information and suggest local and global inhibitory processes working together to co-register information across the two streams.