Flexible, Capacity-Limited Activity of Posterior Parietal Cortex in Perceptual as well as Visual Short-Term Memory Tasks

Abstract

It has recently been shown, using functional magnetic resonance imaging with a change detection paradigm, that activity in posterior parietal cortex (PPC) correlates with the limited number of objects held in visual short-term memory (VSTM). We replicate this finding and extend it to tasks that use similar stimuli, but without explicit memory requirements. As well as a perceptual task used previously (detecting an item at fixation), 2 additional tasks were designed to increase attentional demands across space (searching for a red item anywhere in the array) and across both space and time (detecting a staggered offset after prolonged viewing of the array). During the VSTM task, a capacity-limited set-size effect was seen in PPC as well as occipital and frontal regions. However, the PPC showed similar activity during 2 of the tasks not requiring VSTM. These findings cannot easily be explained by behavioral performance measures or memory demands alone, suggesting a role of the PPC in processing a limited number of discrete object representations, whether in the current perceptual scene or working memory. The differential influence of item load across perceptual tasks is consistent with task requirements affecting the form of these representations.

Transient Perceptual Neglect: Visual Working Memory Load Affects Conscious Object Processing

“…Furthermore, object recognition performance was also worse under bilateral conditions, relative to when only a single object was presented. These results are consistent with the findings that regions related to VWM and object processing share overlapping neural and cognitive resources (Xu & Chun, 2006Mitchell & Cusack, 2008). That is, when VWM capacity has been exceeded, the amount of information that can be subsequently processed by these regions is limited, resulting in impairments in conscious object processing.…”

“…That is, we predicted that, because regions that mediate VWM are sensitive to perceptual load (Mitchell & Cusack, 2008) and overlap with those involved in establishing "object files" (Xu & Chun, 2009), object processing would be impaired under a high VWM load but that these deficits would be most pronounced at the level of conscious recognition, with sensory-driven processes remaining relatively intact. In other words, the detection of sudden onsets in the periphery can proceed regardless of VWM manipulations; the higher-order process of object recognition, however, is crucially affected by VWM load.…”

“…This process seems to be subserved by the superior IPS (sIPS) and the lateral occipital complex (LOC), an area associated with conscious object recognition (Grill-Spector, Kushnir, 1 University of Toronto, Hendler, & Malach, 2000). Furthermore, a recent study has demonstrated that capacity-limited areas subserving VWM are also sensitive to perceptual load (Mitchell & Cusack, 2008). Thus, VWM and object recognition are not only mediated by partially overlapping neural resources but the mechanisms defining VWM capacity limits may be related to limits in perceptual processes (i.e., individuation and representation).…”

Cholinergic Augmentation Modulates Visual Task Performance in Sleep-Deprived Young Adults

“…Activation in the intraparietal sulcus (IPS) in the VSTM task has been shown to track storage, whereas extrastriate activation associated with both tasks reflects increased perceptual demands (Todd and Marois, 2004;Chee and Chuah, 2007). However, part of the IPS activation in VSTM-type tasks can also be attributed to focusing attention (Majerus et al, 2007;Mitchell and Cusack, 2007). These previous studies compared activation in the IPS across several VSTM tasks performed by the same individuals that tapped on common as well as distinct cognitive processes.…”

“…Impairment of attention in sleep-deprived persons (Lim and Dinges, 2008) is an important cognitive deficit that affects other "higher" cognitive operations, for example, visual short-term memory (VSTM) (Chee and Chuah, 2007). VSTM is used in service of many higher cognitive operations and has a capacity limit (Luck and Vogel, 1997;Cowan, 2001) that has neuroimaging correlates (Linden et al, 2003;Majerus et al, 2007;Mitchell and Cusack, 2007). SD-induced decline in VSTM capacity has been shown to originate from depressed visual attention or visual processing (Chee and Chuah, 2007).…”