Authors:

CUSACK, R., MITCHELL, D.J. & DUNCAN, J.

Reference:

Year of publication:

2007

CBU number:

6596

Abstract:

A vast array of information arrives from the senses, which must be structured, to allow for integration of parts that belong together, and segregation of parts that do not. This formation of discrete yet coherent objects strongly affects performance in many tasks, such as those involving selective attention, short term memory or multiple object tracking. A number of studies have implicated posterior parietal cortex (PPC) in the representation of discrete objects. However, there is disagreement on whether it these object representations are recruited only for visual short-term memory (VSTM), or also for perception. In an FMRI experiment, we found that activity in the PPC tracked the number of objects (coloured circles) up to a capacity limit of around four items, during perceptual as well as VSTM tasks. In a second experiment, we investigated whether multiple objects must be distributed in space to recruit parietal cortex. We contrasted random dot kinematograms that are perceived as one surface or two overlapping surfaces, and found that the PPC is recruited even when objects are not distributed in space. In a third FMRI experiment, we investigated whether attention switching between objects is the critical factor. We presented overlapping surfaces, and in different conditions, asked subjects to attend to one throughout, or to switch attention between them. Attention switching (and the performance of a more difficult discrimination) activated a broad characteristic fronto-parietal (“multiple demands”, MD) network. However, the parietal activity was anterior, and did not extend into the PPC. In contrast, the perception of discrete objects did recruit the PPC (transitorily at onset), despite recruiting anterior parietal regions less strongly than attention switching, and without the broader MD network. Finally, we present some neuropsychological and behavioural evidence that not all VSTM tasks recruit the PPC, and discuss possible critical factors. Taken together, our experiments suggest that the PPC is important in dividing sensory input into multiple, discrete objects, either in perception or in short-term memory. We propose that these objects are the output of perceptual organization processes, and are influenced by both bottom-up cues to segmentation, and task demands.