Design of the main imaging experiment.

Example trials of the multiple object tracking (MOT) task with low and high attentional load conditions corresponding to tracking two and four targets among distractors, respectively. γ-aminobutyric acid (GABA) was measured with MEGAPRESS scans. Glx (glutamate and its precursor glutamine) was measured with PRESS scans.

fMRS checklist proposed by Choi et al. (2021).

Tracking performance across fMRS runs in the main imaging experiment.

The thick line shows mean results across participants. Thin lines show results for each participant. The lower the inverse efficiency score, the faster and more accurate participants responded. The result of a paired-sample t-test between tracking with high load and low load is denoted by asterisks. *** p < 0.001.

Metabolite concentrations in the main imaging experiment.

Thick lines show mean results across participants for the parietal and visual volumes-of-interest (VOIs). Thin lines show results for each participant. GABA and Glx concentrations are normalized to the concentration of N-acetyl-aspartate (NAA). Results of paired-sample t-tests between tracking with high load and low load are denoted by asterisks. * p < 0.05, ** p < 0.01.

Correlational results in the main imaging experiment.

Load-dependent changes in GABA and Glx concentrations during tracking were calculated by subtracting low load from high load concentrations for each participant. More positive values on the x-axis correspond to greater concentration increase from low to high load. Load-dependent changes in tracking performance were calculated by subtracting the inverse efficiency score in low load from high load for each participant. More positive values on the y-axis correspond to greater increase in inverse efficiency score from low to high load (i.e., greater decrease of response accuracy and greater increase of response time from low to high load). Each correlational analysis was carried out between metabolite concentrations and tracking performance from the same fMRS run. Each dot shows the result from a different participant.

Design of control behavioral and imaging experiments for the measurement of target enhancement and distractor suppression.

In the control imaging experiments, GABA and Glx concentrations during MOT were measured in the parietal VOI exactly as in the main imaging experiment with MEGAPRESS and PRESS scans, respectively (see Figure 1). Cueing and response periods were 3 s-long each for MEGAPRESS scans and 2 s-long each for PRESS scans. A jittered intertrial interval was included after each MOT trial exactly as in the main imaging experiment.

Tracking performance across runs in each control behavioral experiment.

(A) Target enhancement experiment. (B) Distractor suppression experiment. Otherwise same as Figure 2. *** p < 0.001.

Tracking performance across fMRS runs in each control imaging experiment.

(A) Target enhancement experiment. (B) Distractor suppression experiment. Otherwise same as Figure 6. ** p < 0.01, *** p < 0.001.

GABA and Glx measurements in the parietal VOI in the control imaging experiments.

Otherwise same as Figure 3.

Correlational results in the control imaging experiments.

More positive values on the x-axis correspond to greater concentration increase from low to high target enhancement and distractor suppression. More positive values on the y-axis correspond to greater increase in inverse efficiency score from low to high target enhancement and distractor suppression (i.e., greater decrease of response accuracy and greater increase of response time). Each correlational analysis was carried out between metabolite concentrations and tracking performance from the same fMRS run. Otherwise same as Figure 4.

Mean MEGAPRESS difference spectra across participants in the main imaging experiment.

The gray shaded area shows standard-error-of-the mean. GABA at 3 ppm and NAA at 2 ppm.

Mean PRESS spectra across participants in the main imaging experiment.

Glx between 2.1 and 2.5 ppm and NAA at 2 ppm. Otherwise same as Supplementary Figure 1.

Mean MEGAPRESS difference spectra across participants in the control imaging experiments.

Otherwise same as Supplementary Figure 1.

Mean PRESS spectra across participants in the control imaging experiments.

Otherwise same as Supplementary Figure 2.

Tracking performance across fMRS runs in the main imaging experiment.

(A) Response accuracy. Participants responded less accurately in tracking with high load than with low load [paired-sample t-test on arcsin-square-root transformed response accuracy; t(34) = −4.40, p < 0.001, d = −0.74]. Otherwise same as Figure 2. (B) Median response time. Participants responded slower in tracking with high load than with low load [paired-sample t-test on log-transformed response time; t(34) = 5.42, p < 0.001, d = 0.92]. Otherwise same as (A). *** p < 0.001.

Tracking performance across runs in each control behavioral experiment.

(A) Response accuracy in the target enhancement experiment. Participants responded less accurately in tracking with high than with low target enhancement [paired-sample t-test on arcsin-square-root transformed response accuracy; t(19) = −4.84, p < 0.001, d = −1.08]. Otherwise same as Supplementary Figure 5A. (B) Median response time in the target enhancement experiment. Participants responded slower in tracking with high than with low target enhancement [paired-sample t-test on log-transformed response time; t(19) = 8.24, p < 0.001, d = 1.84]. Otherwise same as Supplementary Figure 5B. (C) Response accuracy in the distractor suppression experiment. Participants responded less accurately in tracking with high than with low distractor suppression [paired-sample t-test on arcsin-square-root transformed response accuracy; t(19) = −6.05, p < 0.001, d = −1.35]. Otherwise same as (A). (D) Median response time in the distractor suppression experiment. Participants responded slower in tracking with high than with low distractor suppression [paired-sample t-test on log-transformed response time; t(19) = 8.12, p < 0.001, d = 1.82]. Otherwise same as (B). *** p < 0.001.

Tracking performance across fMRS runs in each control imaging experiment.

(A) Response accuracy in the target enhancement experiment. Participants responded less accurately in tracking with high than with low target enhancement [paired-sample t-test on arcsin-square-root transformed response accuracy; t(18) = −3.89, p = 0.001, d = −0.89]. Otherwise same as Supplementary Figure 6A. (B) Median response time in the target enhancement experiment. Participants responded slower in tracking with high than with low target enhancement [paired-sample t-test on log-transformed response time; t(18) = 6.41, p < 0.001, d = 1.47]. Otherwise same as Supplementary Figure 6B. (C) Response accuracy in the distractor suppression experiment. Participants responded less accurately in tracking with high than with low distractor suppression [paired-sample t-test on arcsin-square-root transformed response accuracy; t(18) = −3.50, p = 0.003, d = −0.80]. Otherwise same as (A). (D) Median response time in the distractor suppression experiment. Participants responded slower in tracking with high than with low distractor suppression [paired-sample t-test on log-transformed response time; t(18) = 2.76, p = 0.01, d = 0.63]. Otherwise same as (B). * p < 0.05, ** p < 0.01, *** p < 0.001.