Functional specialization of the left ventral parietal cortex in working memory

Abstract

The function of the ventral parietal cortex (VPC) is subject to much debate. Many studies suggest a lateralization of function in the VPC, with the left hemisphere facilitating verbal working memory and the right subserving stimulus-driven attention. However, many attentional tasks elicit activity in the VPC bilaterally. To elucidate the potential divides across the VPC in function, we assessed the pattern of activity in the VPC bilaterally across two tasks that require different demands, an oddball attentional task with low working memory demands and a working memory task. An anterior region of the VPC was bilaterally active during novel targets in the oddball task and during retrieval in WM, while more posterior regions of the VPC displayed dissociable functions in the left and right hemisphere, with the left being active during the encoding and retrieval of WM, but not during the oddball task and the right showing the reverse pattern. These results suggest that bilateral regions of the anterior VPC subserve non-mnemonic processes, such as stimulus-driven attention during WM retrieval and oddball detection. The left posterior VPC may be important for speech-related processing important for both working memory and perception, while the right hemisphere is more lateralized for attention.

Keywords: language; short-term memory; stimulus-driven attention; verbal working memory; voluntary attention; working memory.

FIGURE 1

Experimental design of the oddball (A) and working memory [WM; (B)] task. The oddball task consisted of novel targets (Korean and English letters), non-targets (percentage signs) and control targets (same as non-targets, but used as the baseline condition for comparison purposes; one-third of the time the percentage signs were denoted as the control targets); stimulus duration was 600 ms, while the interstimulus interval was 600 ms. (A). Presented is an example sequence of a novel target (Korean letter) followed by a series of standard stimuli (%) and another novel target (English letter). Participants indicated whenever they saw a stimulus that was not a percentage sign. The WM task consisted of verbal (English letters only) and object (Korean letters only) trials. Presented is a verbal trial followed by a correctly ordered probe (B). A fixation cross marked the onset of each trial (2 s), followed by the presentation of five randomly selected stimuli (all verbal or all object; 8 s). Participants were to rehearse the stimuli in serial order (16 s) and make a response when the probe (r→c) was presented (4 s). A fixation cross marked the end of the trial (12 s).

FIGURE 2

Average accuracy and reaction time (RT) for the oddball [(A,B) ± SEM] and working memory [WM; (C,D) ± SEM] task. For the oddball task, there were no differences in detecting verbal or object targets in accuracy (A) or RT (B). For the WM task, participants had both higher performance (C) and faster RT (D) for verbal trials. *Asterisks indicate a significant difference between the verbal and object trials of the WM task at p < 0.05. For better visualization of the differences in accuracy and RT, the y-axis starts a higher value in (A,C,D), this is represented by the jagged line in these graphs.

FIGURE 3

Regions of the VPC active in the oddball and WM tasks. Blue indicates significant activity in the oddball task, yellow indicates significant activity at encoding in the WM task, and green denotes significant activity at retrieval in the WM task (D–F). Left and right aVPC regions [left: (D); right: (E)] in the supramarginal gyrus were active in the oddball task [left: (A); right: (B)] and at retrieval during the WM task [left: (G); right: (I)]. A separate region in the left STG/VPC (D) was active at encoding and retrieval (H), but not for oddball detection whereas the homologous region in the right hemisphere (E) displayed the opposite pattern; namely, the right STG/VPC was reliably active in the oddball task (C) but not the WM task. The BOLD percent signal change is defined with respect to the image signal of the first time point of a trial, which is at the baseline. Note that only activity in regions of the STG and VPC is shown in the figure.

FIGURE 4

Time course of average (±SEM) activity for the left VPC during verbal (A) and object (B) trials of the WM task. Error (solid line) and correct (dashed line) trials are shown for each condition. A binary logistic regression indicated that higher activity averaged during the maintenance period (timepoints 14–26; shaded area) of the verbal condition was associated with a greater likelihood of making an error.