Abstract
To efficiently select important features in the environment, processes that categorize input are necessary. Using a visual search paradigm (Vuong & Thornton, 2006; Mayer & Vuong, 2008), we found that observers performed better when searching for biological targets compared to mechanical targets in search arrays of natural videos. This category effect was indicated by faster search times. Furthermore, observers fixated on biological targets earlier in the search process and the durations of fixation on biological targets were shorter than on mechanical targets. As search times and eye movements are indirect indicators of neural processes, we used an fMRI study to investigate whether dynamic natural scenes with biological motion activate different cortical areas compared to dynamic natural scenes with mechanical motion. Fourteen participants were scanned in a 3T scanner while they passively viewed natural scenes belonging to three categories: humans, animals, or machines. They were encouraged to freely move their eyes while viewing the scenes. Videos of each category were presented in separate blocks. We found greater BOLD responses to humans compared to machines in the superior temporal sulcus (STS) and the inferior frontal gyrus (IFG). Similarly, animals compared to machines lead to greater responses in STS and middle frontal gyrus (MFG). No regions were found that were more strongly activated by animals compared to humans, or by machines compared to either biological category. This study shows that the category effect found for search times and eye movements is also reflected in the BOLD response. Importantly, the results point to possible top-down influences from frontal regions, which have been found to represent object categories, on STS, which is involved in processing biological motion. Together with our behavioural studies, these results suggest that both visual and higher cortical areas appear to be tuned to process biological motion.