Abstract
Object identification and object localization are processes that are thought to be mediated by two relatively segregated brain regions that are independent from each other (Mishkin, Ungerleider & Macko, 1983; Goodale & Milner, 1992). Much literature, however, argues that the two processes might not be as independent as previously assumed, given the evidence that, when both processes are engaged in a single task, the performance of one process interferes with the performance of the other (Creem & Proffitt, 2001). Most of the experiments that report this interference, however, rely on complex motor movements in reach-to-grasp tasks, and it remains possible that the interference arises from the fact that the two visual mechanisms, albeit independent in nature, both influence the motor component of the complex task (Milner & Goodale, 2008). In this study, in a series of perceptual tasks with minimal motor demands, we explore the extent to which object identification and object localization are truly independent. Participants are required to compare two pairs of objects, presented simultaneously on a computer screen, and to determine how many differences the pairs have between them using a numeric keypad. The pairs can differ by zero, one or two changes either in object shape or location. The results indicate that the two visual processes are not independent but, rather, that the shape processing mechanism recruits the location mechanism in, at least, some integral part of its function. This finding was confirmed in a second experiment in which, in separate blocks, participants made only a location change detection in which the objects either did or did not have a concurrent shape change, or made a shape change detection with location changes present or not. Importantly, any change in the orthogonal dimension was irrelevant to the participant's task. Whereas shape changes interfered with location judgments, the converse was not true.
NIH/NIDA R90 DA023420 Multimodal Neuroimaging Training Program.