The results showed that 1) P300 amplitudes decreased under optimal, fast and slow conditions relative to the control condition at Cz and Pz, 2) P300 amplitudes decreased under fast and slow conditions compared to that under the optimal condition at Pz, and 3) P300 amplitudes were not significantly different between fast and slow conditions at Fz, Cz, and Pz. Our results showed that the optimal cadence is slow compared to previous studies[20]. It could be because the participants performed pedaling on the stationary bike like a bicycle in previous studies, while performed on the reclining chair in present study.
Studies have reported that P300 amplitude is proportional to the amount of attentional resources devoted to a given task [10, 12] and decreases in more difficult tasks, such as dual tasks compared to a single task [21]. MA Just, PA Carpenter, TA Keller, L Emery, H Zajac and KR Thulborn [22] reported that brain activation distributes between two tasks in dual tasks. Our results suggested that attentional resources, which were shared between pedaling and counting, contributed to the reduction in P300 amplitude.
By contrast, P300 amplitude at Pz under fast and slow conditions decreased compared to that under the optimal condition. A study suggested that pedaling at optimal cadence may be a good reflection of movement frequency output generated by the central pattern generator (CPG) [23]. CPG, which is composed of spinal interneuronal networks, is a major component of the rhythm generating system [24, 25]. Both descending supraspinal drive and sensory feedback assist in fine tuning the output from central pattern generators [24, 25]. Moreover, studies have reported that the activation of frontal and parietal lobes increased when the count task was performed [26, 27]. Sensory input during the count task may be integrated at frontal and parietal lobes through the peripheral nerve, spinal cord, thalamus, and primary somatosensory cortex. Pedaling at optimal cadence may demand a greater contribution of CPG, thus motor control at the cerebral cortex declines. Our results showed that attentional resources for the count task decreased under fast and slow conditions possibly because motor control increased at the cerebral cortex. Therefore, pedaling at non-optimal cadence demanded greater attentional resources than at optimal cadence, and attentional resources allocated to external stimuli decreased. However, it is our limitation that the slow or fast condition could be not only a dual task, but a multi task situation, i.e. motor activity, controlling the pace, counting the targets.
In this study, P300 amplitudes at all electrodes were not significantly different between fast and slow conditions. We hypothesized that P300 amplitude increased under the fast condition and decreased under the slow condition [6]. In that study, the participants performed reaction time tasks with auditory stimuli. Their results showed that compared to slow and self-selected speeds, accelerated walking generated faster RTs. The slowing of gait has been shown to result in greater lateral instability, increased attentional cost, reduced trunk smoothness, increased stride time variability, altered muscular activity, and higher energy costs, suggesting that slow walking increases equilibrium demands and decreases energy efficiency [28]. Thus, attentional resources during walking is decided by levels of posture control because gait speeds and posture control have a close relationship. However, in this study, participants performed alternating movement of the lower limbs on the reclining chair; therefore, the factor of posture control may be removed. For the reasons, a difference between the hypothesis and results may occur, irrespective of whether the task demands posture control.
Our results revealed that EMG activities in the VM and BF increased under the fast condition compared to slow and optimal conditions. A study reported that the value of EMG is decided by the activation of the primary motor cortex [29], and P300 amplitudes decrease with increasing activation of the primary motor cortex [30]. We suggested that a decrease in P300 amplitudes under the fast condition was caused by the increased activation of the motor cortex during pedaling at high cadence.
The error rate of the slow condition was the highest, thus slow conditions may be the most difficult tasks. A study revealed that executing smooth rhythmic motions very slowly is challenging for humans [31]. Similarly, it may be difficult to perform pedaling at a low cadence. This could suggest that the slow condition was a more complex task than optimal and fast conditions, thus pedaling at low cadence results in less attention being paid to external stimuli compared with pedaling at optimal cadence.
In this study, no significant changes in P300 latency were observed under any condition. Studies have reported that P300 latency is related to stimulus evaluation [11]. T Kida, Y Nishihira, A Hatta, T Wasaka, H Nakata, M Sakamoto and T Nakajima [19] showed that P300 latency measured during ignoring stimulus task, counting task, or reaction task is not different. Therefore, it is perhaps P300 latency was not affected by characteristics of tasks, such as pedaling at different cadence. Our results showed that no significant correlation between P300 amplitudes and EMG activities existed under any condition. Thus, in this study, P300 amplitude and muscle activation may be independent.