Attentional demands predict short-term memory load response in posterior parietal cortex

doi:10.1016/j.neuropsychologia.2009.02.015

Neuropsychologia

Volume 47, Issues 8–9, July 2009, Pages 1790-1798

https://doi.org/10.1016/j.neuropsychologia.2009.02.015 Get rights and content

Abstract

Limits to the capacity of visual short-term memory (VSTM) indicate a maximum storage of only 3 or 4 items. Recently, it has been suggested that activity in a specific part of the brain, the posterior parietal cortex (PPC), is correlated with behavioral estimates of VSTM capacity and might reflect a capacity-limited store. In three experiments that varied the delay period and the stimuli to be stored, we found dissociations between functional magnetic resonance imaging (fMRI) activity in PPC and behavioral measures of capacity. When the delay length increased, fMRI activity in this area increased with memory load beyond the behaviorally determined limits of capacity. The results suggest that activity in PPC may reflect the attentional demands of short-term memory rehearsal processes rather than capacity limitations, and imply that a larger number of items than that determined by behavioral measures of capacity may be rehearsed during STM tasks. This account is consistent with the role of PPC in attentional processes and with the close correlation between brain areas that are involved in attention and those that mediate STM.

Section snippets

Experiment 1

The goal of experiment 1 was to replicate previous findings that established the relation between fMRI activity in PPC and behavioral measures of capacity (Todd & Marois, 2004). A color VSTM task with a delay of 1200 ms was employed in the experiment.

Participants

Fourteen neurologically healthy participants were tested in experiment 1. All had normal or corrected-to-normal vision and gave written informed consent for participation, which was approved by the Institutional Review Panel of Princeton University.

Stimuli and design

Each trial began with the presentation of three digits for 500 ms appearing side by side and centered at fixation (the string of digits measuring approximately 1° in width and 0.3° in height). Participants were asked to remember the digits for later

Behavioral results

The behavioral estimates of K were calculated for each participant, for each load level and then averaged across participants. K increased significantly from set size 1 to 3 (t(13) = 13.0, P < 0.01), and from set size 3 to 5 (t(13) = 2.8, P < 0.01), and then leveled off (t(13) = 1.3, P > 0.2) between set sizes 5 and 7 (Fig. 2a). Reaction time (RT) increased significantly from each set size to the next (all Ps < 0.01, t(13) = 6.9, 4.6, 3.0 respectively). Accuracy in the digit task was high overall (95–98%) and

Experiment 2

Experiment 2 was critical in examining our hypothesis. It was aimed at testing whether the potentially greater demands of a longer delay period would change the pattern of fMRI activity, independently of any effects on the behavioral K function. Experiment 2 was identical to experiment 1 except for the delay length, which was increased to 6000 ms.

Participants

Ten neurologically healthy participants participated in experiment 2 (eight participated in experiment 1 as well). As in experiment 1, all had normal or corrected-to-normal vision and gave written informed consent for participation, which was approved by the Institutional Review Panel of Princeton University.

Stimulus and design

The delay length in experiment 2 was 6000 ms. All other aspects of the behavioral task were identical to experiment 1.

Data acquisition and analysis

Data acquisition procedures were identical to experiment 1.

Behavioral results

The behavioral results of experiment 2 showed a significant increase in K from set size 1 to 3 (t(9) = 14.4, P < 0.01), that leveled off thereafter (t(9) = 0.02, P > 0.9 for the difference between set sizes 3 and 5) (Fig. 2b). RT increased significantly from each set size to the next for all load levels (Ps < 0.01, t(9) = 6.2, 4.1, 2.6 respectively). Accuracy in the digit task was high (92–97%) and did not differ as a function of VSTM load (F(3,27) = 2.0, P > 0.1 in a repeated measures ANOVA).

Experiment 3

Experiment 3 was aimed at testing whether our results could be generalized to memory for spatial locations. As mentioned before, ample evidence, both behavioral and neural suggest that separate systems may be involved in storage of spatial and visual information (Baddeley, 1992, Courtney et al., 1996, Smith et al., 1995, Tresch et al., 1993). Nevertheless, if PPC is involved in attentional processes that mediate the increased demands of load, we would expect to find similar results with spatial

Participants

Twelve neurologically healthy participants were tested in experiment 3 (four participated in experiments 1 and 2). All had normal or corrected-to-normal vision and gave written informed consent for participation, which was approved by the Institutional Review Panel of Princeton University.

Stimuli and design

On each trial, 1, 3, 5 or 7 black squares (0.5°) were presented in a 4 × 6 matrix (10° × 15°) either on the left or the right sides of fixation with no separation between the two sides. The density again was

Behavioral results

The behavioral results in experiment 3 showed significant increases in K from set size 1 to 3 (t(11) = 8.3, P < 0.01) and 3 to 5 (t(11) = 3.7, P < 0.01) and then leveled off (t(11) = 0.5, P > 0.6) (Fig. 2c). Memory capacity in experiment 3 was marginally lower than in experiment 2 (K_max = 2.67 and 2.91 in experiments 3 and 2 respectively), but the difference was not significant. RT increased from set size 1 to 3 (t(11) = 4.1, P < 0.01), the difference between set sizes 3 and 5 was marginally significant (t(11) =

Discussion

The results of the three experiments reported here yield some important findings. First, the fMRI activations involved the same regions in PPC in all three experiments, suggesting that similar processes are involved in both visual and spatial STM. Secondly, the relation between behavioral measures of recognition and the profile of fMRI load activity in superior IPS differed for short and for long delays and for different sets of stimuli, dissociating fMRI load effects from the behavioral

Acknowledgements

This work was supported by grants to A.T. (NIH grant 2RO1 MH 058383, 1RO1 MH 062331) and to S.K. (1RO1 MH64043, 2P50 MH-62196).

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View full text

Attentional demands predict short-term memory load response in posterior parietal cortex

Abstract

Section snippets

Experiment 1

Participants

Stimuli and design

Behavioral results

Experiment 2

Participants

Stimulus and design

Data acquisition and analysis

Behavioral results

Experiment 3

Participants

Stimuli and design

Behavioral results

Discussion

Acknowledgements

Trends in Cognitive Sciences

Neuroimage

Trends in Cognitive Sciences

Cognitive Psychology

Neuroimage

Vision Research

Neuroimage

Neuroimage

Neuron

Neuropsychologia

Neuroimage

Neuroimage

The capacity of visual short-term memory is set both by visual information load and by number of objects

Psychological Science

Rehearsal in spatial working memory

Journal of Experimental Psychology: Human Perception & Performance

Rehearsal in spatial working memory: Evidence from neuroimaging

Psychological Science

Working memory

Is working memory working? The fifteenth Bartlett lecture

Quarterly Journal of Experimental Psychology

The Psychophysics Toolbox

Spatial Vision

Temporal dynamics of brain activation during a working memory task

Nature

Control of goal-directed and stimulus-driven attention in the brain

Nature Neuroscience

Object and spatial visual working memory activate separate neural systems in human cortex

Cerebral Cortex

Transient and sustained activity in a distributed neural system for human working memory

Nature

The magical number 4 in short-term memory: A reconsideration of mental storage capacity

Behavioral Brain Sciences

Working memory capacity