Attention-modulated alpha-band oscillations protect against intrusion of irrelevant information

Abstract

Combining high-density scalp EEG recordings with a sensitive analog measure of STM's fidelity, we characterized the temporal dynamics of intentional ignoring and related those dynamics to the intrusion of task-irrelevant information. On each trial of the task, two study Gabors were briefly presented in succession. A green or red disc preceding each Gabor signified whether that Gabor should be remembered or ignored, respectively. With cue-stimulus intervals of 300, 600, or 900 msec presented in separate sessions, we found that the onset of posterior, prestimulus alpha oscillations varied with the length of the interval. Although stimulus onset time was entirely predictable, the longer the cue-stimulus interval, the earlier the increase in prestimulus alpha power. However, the alpha-band modulation was not simply locked to the cue offset. The temporal envelopes of posterior alpha-band modulation were strikingly similar for both cued attending and cued ignoring and differed only in magnitude. This similarity suggests that cued attending includes suppression of task-irrelevant, spatial processing. Supporting the view that alpha-band oscillations represent inhibition, our graded measure of recall revealed that, when the stimulus to be ignored appears second in the sequence, peristimulus alpha power predicted the degree to which that irrelevant stimulus distorted subsequent recall of the stimulus that was to be remembered. These results demonstrate that timely deployment of attention-related alpha-band oscillations can aid STM by filtering out task-irrelevant information.

Figure 1

Schematic diagram illustrating a trial’s event structure. Each trial began with a fixation cross that oriented the subject to the region of the computer display within which the trial’s stimuli would be presented. The fixation point was replaced either by a green disc or a red disc. The green disc cued the subject that the spatial frequency of the ensuing Gabor stimulus should be remembered; a red disc cued the subject that the next Gabor’s spatial frequency should be ignored. A cue-stimulus interval of either 300, 600 or 900 ms followed (blocked design), and then the first of two Gabor stimuli was presented. Immediately thereafter, a second cue was presented. This cue was a disc that was green if the first cue had been red, or red if the first cue had been green. Then a second cue-stimulus interval followed; this interval was always the same as the trial’s initial cue-stimulus interval, that is, either 300, 600 or 900 ms. Next a second Gabor stimulus was presented, which was followed by a one-second long retention interval. Finally, a comparison Gabor appeared whose spatial frequency could be adjusted to match the remembered spatial frequency of the Gabor that the subject had been cued to remember. Top row: on half the trials, the target Gabor to be attended appeared first and the nonTarget Gabor to be ignored appeared second (hereafter we refer to this sequence as T₁N₂). Bottom row: on half of the trials the nonTarget Gabor to be ignored appeared first and the Target Gabor to be attended appeared second (hereafter, N₁T₂).

Figure 2

Bar charts showing absolute values of nRE (left set of bars), the effect of the nonTarget stimulus (middle set of bars), and the effect of the prototypical stimulus (right set of bars) for the 300ms, 600ms and 900ms cue-stimulus interval conditions. In each pair of bars, results for T₁N₂ and N₁T₂ trials are shown separately. Data are means over subjects. Error bars are ± 1 within-subject standard errors of the mean. A significant difference between two test conditions at p < 0.05 is indicated by the asterisk (*).

Figure 3

Grand averaged, topographic maps of alpha power for the first Attend and Ignore stimuli (left panels) and the second Attend and Ignore stimuli (right panels). (Top) Results with 300 ms between cue’s end and onset of stimulus; (Middle) results with 600 ms between cue’s end and onset of stimulus; (Bottom) results with 900 ms between cue’s end and onset of stimulus. Results are averaged over the 0 to 100 ms time window from the stimulus onset. Rounded ears are to the right and left sides and a triangle nose is at the top of each map.

Figure 4

Topographic display of ignore-related alpha power (nonTarget > Target cluster; p<0.01). Electrodes within the cluster showing a significant difference between the two test conditions at p < 0.025 are indicated by the asterisk (*).

Figure 5

Grand averaged, time-frequency wavelets averaged across the cluster of 26 posterior electrodes for the first Attend and Ignore stimuli (left panels) and the second Attend and Ignore stimuli (right panels). The time scale is the same for each wavelet (500 ms is the same length on all wavelets). The Attend and Ignore columns are lined up on the stimulus onset for ease of comparing the timing of the increases in alpha power. (Top) Results with 300 ms between cue’s end and onset of stimulus; (Middle) results with 600 ms between cue’s end and onset of stimulus; (Bottom) results with 900 ms between cue’s end and onset of stimulus.

Figure 6

Grand Averaged alpha power over time for attend (green) and ignore (red) for the first stimulus (left panel) and the second stimulus (right panel). (Top) 300 ms condition, (Middle) 600 ms condition and (Bottom) 900 ms condition. Ribbons indicate ± 1 within-subject standard errors of the mean. The black rectangles along the time axis mark the duration between the onset and offset of 50% peak power.

Figure 7

Alpha power predicts the nonTarget effect (intrusion of the nonTarget information) when the nonTarget stimulus follows the Target stimulus. The relationship shows that as alpha power increases, the nonTarget effect decreases. The header depicts the trial type with a box around the nonTarget stimulus of interest. A) Ignore-first trials (N₁T₂) and B) Ignore-second trials (T₁N₂) were sorted into eight equally populous bins according to the ongoing alpha power across the cluster of posterior electrodes. Bin 1 comprises trials with the lowest alpha amplitude, and bin 8 includes trials on which alpha amplitude was highest. (Top) Pre-stimulus 100 ms time window; (Bottom) 100 ms time window following nonTarget onset.