Increased Alpha-Band Power during the Retention of Shapes and Shape-Location Associations in Visual Short-Term Memory

Abstract

Studies exploring the role of neural oscillations in cognition have revealed sustained increases in alpha-band (~8-14 Hz) power during the delay period of delayed-recognition short-term memory tasks. These increases have been proposed to reflect the inhibition, for example, of cortical areas representing task-irrelevant information, or of potentially interfering representations from previous trials. Another possibility, however, is that elevated delay-period alpha-band power (DPABP) reflects the selection and maintenance of information, rather than, or in addition to, the inhibition of task-irrelevant information. In the present study, we explored these possibilities using a delayed-recognition paradigm in which the presence and task relevance of shape information was systematically manipulated across trial blocks and electroencephalographic was used to measure alpha-band power. In the first trial block, participants remembered locations marked by identical black circles. The second block featured the same instructions, but locations were marked by unique shapes. The third block featured the same stimulus presentation as the second, but with pretrial instructions indicating, on a trial-by-trial basis, whether memory for shape or location was required, the other dimension being irrelevant. In the final block, participants remembered the unique pairing of shape and location for each stimulus. Results revealed minimal DPABP in each of the location-memory conditions, whether locations were marked with identical circles or with unique task-irrelevant shapes. In contrast, alpha-band power increases were observed in both the shape-memory condition, in which location was task irrelevant, and in the critical final condition, in which both shape and location were task relevant. These results provide support for the proposal that alpha-band oscillations reflect the retention of shape information and/or shape-location associations in short-term memory.

Keywords: alpha band; electroencephalography; inhibition; oscillations; short-term memory.

Figure 1

Behavioral task. (A) General timing of fixation, stimuli, delay period, probe, and ITI used during all task blocks of VSTM task. Note that Block 3 also included an additional 1-s presentation of task instructions at the beginning of each trial. (B) Example memory target and probe item for each task block and condition (in each case, a match trial is depicted). Stimuli are not drawn to scale.

Figure 2

Behavioral data. (A) Repeated-measures ANOVA revealed no significant main effect of memory condition on accuracy (F < 1). (B) A repeated-measures ANOVA revealed a significant difference in RT across conditions [F(4,40) = 5.11, p = 0.002]. Follow up paired comparisons revealed faster RTs in each of the location-with-shapes conditions versus the shape and shape-in-location conditions (all p's < 0.01). All other comparisons were non-significant. Error bars reflect within-subject confidence intervals (Cousineau, 2007).

Figure 3

Delay-period alpha-band power. (A) Topographical plots of mean power in the 8- to 15-Hz frequency band averaged over the 500- to 2500-ms portion of the delay across each block/condition. (B) Time–frequency plots for each trial block across the full delay period (delay marked by black lines at 0 and 3000 ms) in the 5- to 30-Hz frequency range for the cluster of posterior electrodes highlighted in black in the inset topographical plot. (C) Comparison of uncorrected mean alpha-band power during the baseline interval (black solid line) and the delay period (red solid line).

Figure 4

Results of cluster analysis plotted over the difference in delay-period alpha-band power between, from left to right, Conditions 3 and 1, 4 and 3, and 5 and 3, respectively. Asterisks overlayed on each topoplot indicate sensors showing significant differences in delay-period alpha power between conditions.