Functional MRI of working memory and selective attention in vibrotactile frequency discrimination

Abstract

Background: Focal lesions of the frontal, parietal and temporal lobe may interfere with tactile working memory and attention. To characterise the neural correlates of intact vibrotactile working memory and attention, functional MRI was conducted in 12 healthy young adults. Participants performed a forced-choice vibrotactile frequency discrimination task, comparing a cue stimulus of fixed frequency to their right thumb with a probe stimulus of identical or higher frequency. To investigate working memory, the time interval between the 2 stimuli was pseudo-randomized (either 2 or 8 s). To investigate selective attention, a distractor stimulus was occasionally presented contralaterally, simultaneous to the probe.

Results: Delayed vibrotactile frequency discrimination, following a probe presented 8 s after the cue in contrast to a probe presented 2 s after the cue, was associated with activation in the bilateral anterior insula and the right inferior parietal cortex. Frequency discrimination under distraction was correlated with activation in the right anterior insula, in the bilateral posterior parietal cortex, and in the right middle temporal gyrus.

Conclusion: These results support the notion that working memory and attention are organised in partly overlapping neural circuits. In contrast to previous reports in the visual or auditory domain, this study emphasises the involvement of the anterior insula in vibrotactile working memory and selective attention.

Figure 1

Brain activation associated with processing of the probe. The figure shows brain activation and deactivation associated with the processing of the probe (either 25 Hz or higher) across all conditions (clustered activation images with an overall corrected p < 0.05). Activated areas are colour-coded in yellow and red, deactivated areas are displayed in blue. Activation is seen in the left cerebellar hemisphere (1), the bilateral anterior insula (2, 3), the bilateral head of the caudate nucleus and the globus pallidus (4, 5), the bilateral thalamus (6, 7), the right inferior frontal cortex (8), the anterior cingulate cortex (9), the left (contralateral) sensorimotor cortex (10), the right posterior parietal cortex (11) and the supplementary motor area (12). Deactivation was found in the right parahippocampal gyrus (13), the bilateral medial frontal gyrus (14), the right cuneus (15), the bilateral posterior cingulate gyrus (16), the bilateral precuneus (16) and the left superior frontal gyrus (17). Brain images are shown in radiological convention (the right hemisphere is seen on the left side of the image).

Figure 2

The effect of a delayed probe. For this figure, brain activation associated with the processing of the probe was compared between conditions with long and short interstimulus interval (8 s vs. 2 s interval between cue and probe). Clustered activation images with an overall corrected p < 0.05 are shown. The right (1) and left insula (2) as well as the right head of the caudate nucleus and (3) and the right inferior parietal cortex (4) were significantly stronger activated following the probe in trials with an ISI of 8 s compared to trials with an ISI of 2 s. Brain images are shown in radiological convention (the right hemisphere is seen on the left side of the image).

Figure 3

The effect of a simultaneous distractor. For this figure, brain activation associated with the processing of the probe was compared between conditions with and without distractor. Areas with significantly stronger activation following the probe with simultaneous distractor compared to frequency discrimination without distractor are colour-coded in yellow and red, areas with less activation are coded in blue (clustered activation images with an overall corrected p < 0.05). Processing of the probe with distractor was associated with increased activity in the right middle temporal gyrus (1), the right anterior insula (2), the left precuneus (3) and the bilateral posterior parietal cortex (4, 5). Deactivation was seen in the right posterior cingulate gyrus (6), the left medial frontal gyrus (7) and the left precentral gyrus (8). Brain images are shown in radiological convention (the right hemisphere is seen on the left side of the image).

Figure 4

Experimental setup. A magnetomechanical vibrotactile device is taped to the right thumb. The index and the middle finger rested on a two-button response pad. The arms were extended during the measurement. Pressure points were avoided using foam pads.

Figure 5

Illustration of the experiment. Upper graph: A vibrotactile stimulus (frequency: 25 Hz; duration: 2 s) was delivered to the right thumb (cue) followed by an analogous probe of either identical frequency or higher frequency (25 Hz + individual discrimination threshold f). The interstimulus interval (ISI) was either 2 s (as illustrated here) or 8 s. Lower graph: In 25% of trials the probe was paired with a distractor to the left thumb. The stimulation parameters of the distractor were identical to those of the cue. Functional MRI data were obtained continously. Every 2 s, a brain volume consisting of 26 axial was acquired, starting with the beginning of each trial.