The effect of a concurrent cognitive task on cortical potentials evoked by unpredictable balance perturbations

Abstract

Background: Although previous studies suggest that postural control requires attention and other cognitive resources, the central mechanisms responsible for this relationship remain unclear. To address this issue, we examined the effects of altered attention on cortical activity and postural responses following mechanical perturbations to upright stance. We hypothesized that cortical activity would be attenuated but not delayed when mechanical perturbations were applied during a concurrent performance of a cognitive task (i.e. when attention was directed away from the perturbation). We also hypothesized that these cortical changes would be accompanied by alterations in the postural response, as evidenced by increases in the magnitude of anteroposterior (AP) centre of pressure (COP) peak displacements and tibialis anterior (TA) muscle activity. Healthy young adults (n = 7) were instructed to continuously track (cognitive task) or not track (control task) a randomly moving visual target using a hand-held joystick. During each of these conditions, unpredictable translations of a moving floor evoked cortical and postural responses. Scalp-recorded cortical activity, COP, and TA electromyographic (EMG) measures were collected.

Results: Results revealed a significant decrease in the magnitude of early cortical activity (the N1 response, the first negative peak after perturbation onset) during the tracking task compared to the control condition. More pronounced AP COP peak displacements and EMG magnitudes were also observed for the tracking task and were possibly related to changes in the N1 response.

Conclusion: Based on previous notions that the N1 response represents sensory processing of the balance disturbance, we suggest that the attenuation of the N1 response is an important central mechanism that may provide insight into the relationship between attention and postural control.

Figure 1

A) Average cortical activity, electromyographic activity, and platform acceleration of one subject during the no-tracking condition (n = 32 trials). Early components of perturbation-evoked potentials (PEPs) from CZ include the N1 response and late PEPs, as denoted by the hatched area within a 300-ms time interval following the N1 response. The gray dashed line indicates the onset of right tibialis anterior activity (RTA) and the black dashed line indicates the onset of perturbation. B) Example trace of a tracking trial from a single subject. Black line represents the subject's tracking performance and the gray line represents the target waveform.

Figure 2

A) Grand average cortical activity (CZ) during tracking (black line) and no-tracking (gray line) conditions (n = 7 subjects). The black dashed line indicates the onset of perturbation. B) Average N1 latencies and standard errors for tracking (black bar) and no-tracking (gray bar) conditions. C) N1 magnitudes shown for individual subjects and average N1 magnitudes (AVG) for all subjects. Data from tracking (black bar) and no-tracking (gray bar) conditions are shown. Average N1 magnitudes were statistically different between tasks (* p < 0.05).

Figure 3

Anteroposterior (AP) centre of pressure (COP) displacements of all trials in one subject during tracking (black lines) and no-tracking (gray lines) conditions. The platform acceleration of a typical trial is also shown. AP COP peak displacements involving the first (backward) peak and second (forward) peak were increased during tracking when compared to the control condition; average magnitudes of the second peak displacement were statistically different between tasks (p < 0.05).

Figure 4

A) Average tibialis anterior (TA) electromyographic (EMG) activity from left (LTA) and right (RTA) legs during tracking (black line) and no-tracking (gray line) conditions (n = 7 subjects). The vertical dashed line indicates the onset of perturbation. B) Average onset latencies and standard errors of left and right TA EMG activity. C) Average magnitudes and standard errors for left and right TA EMG activity. Integrated EMG activity is computed for three time windows (initial 50 ms, 50–100 ms, and 100–150 ms). Magnitude is expressed as a percentage difference between tracking and no-tracking conditions (positive values indicate greater relative activity in the tracking condition). * denotes statistically significant differences between tasks (p < 0.05).