This experiment attempted to further understand the neural systems that activate in response to auditory stimuli during the execution of an isometric ankle-dorsiflexion task. A brief musical excerpt was used to draw participants’ attentional focus toward task-irrelevant stimuli. In line with Rejeski’s (
1985) conceptualization, the research team expected a parallel processing mechanism to emerge given that participants were required to monitor a range of task-relevant factors, such as the force generated and work duration (10 s of contraction and rest). Accordingly, the alerting system (i.e. immediate reallocation of attentional focus to the auditory stimulus; Fan, McCandliss, Fossella, Flombaum, & Posner
2005) would influence selective attention. Nonetheless, parallel information processing was expected to have immediate bearing over attentional focus in order to prevent likely detriments in neural activation of the working muscles.
The present results appear to uphold the veracity of Rejeski’s (
1985) and Tenenbaum’s (
2001) theoretical propositions, as parallel channels partially suppressed the processing of task-irrelevant signals, allowing participants to focus more intently on the task at hand. Moreover, the findings indicate that exercise complexity and intensity could have similar effects on selective attention, given that a distractive auditory stimulus was not sufficiently potent to disrupt task performance. In such instances, the frontoparietal network appears to activate at ~360 ms following stimulus onset. This is a means by which task-unrelated signals can be blocked and the neural activation of working muscles can be maintained. Nonetheless, auditory stimuli could have a more potent effect on the execution of low-demanding cognitive-motor tasks such as self-paced walking. Everyday tasks performed at a light intensity only require partial awareness to be executed successfully, meaning that environmental sensory stimuli have a strong bearing on attentional focus, which subsequently forces individuals to rearrange the motor plan (e.g. Haga et al.
2015).
According to Broadbent’s (
1958) theoretical proposition, low-demanding cognitive tasks leave greater capacity for parallel processing, and thus, there is a reduced likelihood of task disruption to a primary task. Interestingly, recent evidence indicates that even walking tasks can be negatively affected by the presence of environmental distractions (e.g. smartphones), leading to detriments in task performance (Haga et al.
2015; Vredeveldt & Perfect
2014). This is predicated on the notion that low-demanding cognitive tasks only require partial awareness to be executed, leaving scope for environmental distractions to guide attentional focus toward task-irrelevant cues. Attentional shifts that are prompted by the presence of internal and external sensory cues appear to force the prefrontal cortex to inhibit inappropriate actions and maintain the motor plan. Nonetheless, this proposed mechanism does not appear to prevent auditory stimuli from entering focal awareness and disrupting task performance (e.g. during walking; Takeuchi et al.
2016).
Electrical activity in the muscle
The electrical activity produced by the anterior tibialis was assessed in order to identify the likely negative effects of the auditory stimulus on neural activation and voluntary control. Electrical activity in the muscle was used as the primary index of attentional distraction given that the experimental task only required participants to contract the anterior tibialis at 20% of MVC. Minimal differences caused by attentional distractions should have elicited immediate changes in the neural control of movements with subsequent influence on EMG activity. Accordingly, a hypothetical decrease in the recruitment of motor units (i.e. measured by RMS) caused by the auditory stimulus would indicate that participants were only partially capable of processing task-irrelevant information during the execution of an isometric ankle-dorsiflexion task performed at low intensity (Petersen & Posner
2012). However, EMG signals were not influenced by the auditory stimuli, indicating that the immediate electrical signals evoked by the stimuli were rapidly inhibited via the mechanism of attentional suppression (Geng
2014) or parallel processed by alternative brain networks (Caputo & Guerra
1998).
The number of task-related factors can also influence the attentional system (Lavie et al.
2004). For example, if the motor task involves fine motor control of movements and high levels of concentration, even insignificant sensory stimuli can compromise the neural activation of the working muscles (see Bernstein & Bernstein
2015). Fortunately, the attentional system is trainable, and humans have developed psychological techniques that normally involve the control of physiological indices as a means by which to avoid the detrimental effects of task-irrelevant factors on task performance (Bernier et al.
2011; Desbordes et al.
2012). Tenenbaum (
2001) suggested that exercise intensity can moderate the processing of environmental sensory stimuli. For example, whole-body exercises performed at high intensity force attentional focus toward interoceptive sensory cues and increase the prevalence of associative thoughts. In such instances, task-irrelevant factors remain outside of focal awareness because the brain has limited the capacity to process signals from multiple sources. It is noteworthy that the execution of repetitive movements appears to reallocate the organism’s attentional resources in accord with the relevance of both internal and external sensory stimuli; a potential confound that Broadbent (
1958) did not contemplate in his original theoretical contribution.
Cerebral responses
Electrical activity in the brain was compared primarily between AD and SO; thus, only task-related factors could be responsible for the differences in the evoked potential. Luck, Woodman, & Vogel (
2000) pointed out that attentional processes would only suppress perceptual pathways if the sensory system is overloaded. In the present study, a highly demanding cognitive-motor task was used as a means by which to guide attentional focus toward task-relevant information. Statistically significant differences were identified in the left frontal, frontal-central, central, central parietal, right parietal, parietal-occipital, and occipital regions of the cortex. The presence of task-related factors (e.g. executing the motor task and monitoring the level of force produced) modulated N2 at 0.368 s after the onset of the stimulus. Such differences can be attributed to a parallel processing mechanism that initially occurred in the superior and inferior parietal regions of the cortex (Corbetta & Shulman
2002; Katsuki & Constantinidis
2014; Lee et al.
2013). No statistical differences were identified
during the stimulus cessation; we contend that this cerebral response occurred owing to the fact that the auditory stimulus had already been partially suppressed approximately 0.360 s after the stimulus onset (Berti & Schröger
2003; Polich
2007). Therefore, the stimulus cessation would not have differed between AD and SO.
The high or low activity in the parietal lobe is primarily influenced by the number of task-related factors (Yin et al.
2012). The control of produced force and time duration serve to reallocate one’s attentional focus to task-related information. Irrelevant auditory stimuli are therefore supposed to force one’s attentional focus toward sensory pathways. This reallocation of attentional focus could possibly explain the differences in N2. Time domain analysis indicated that the presence of task-related factors prevented the sharp decrease of the EEG activity after approximately 0.360 s.
Auditory distractions have been commonly associated with changes in P300. Previous authors have suggested that up/down modulations in the time-series waveform that occur at ~350 ms following stimulus onset are induced primarily by stimulus-driven attentional processes originated in the frontal cortex (Berti & Schröger
2003; Polich
2007). There is evidence emerging to suggest that up modulations in P300 reflect a direct response to stimulus evaluation and decision-making processes (see Nieuwenhuis, Aston-Jones, & Cohen
2005; Twomey et al.
2015). P300 amplitude tends to increase during NoGo tasks (i.e. requiring self-control to elicit successful outcomes) as a form of response inhibition (Salisbury, Griggs, Shenton, & McCarley
2004), and similar responses have been successfully replicated in social contexts (see Nash, Schiller, Gianotti, Baumgartner, & Knoch
2013). Accordingly, the neural faculties and cognitive processes associated with up/down modulations in P300 appear to be far more complex than previously thought.
The results of the present study are in line with the extant literature (Linden
2005; Wang, Zheng, Zheng, & Sun
2015). We believe that changes in AD could have been caused by the presence of task-related factors, such as afferent feedback from working muscles and performance-related information, such as visual feedback (Vredeveldt & Perfect
2014). Up-regulation of the EEG waveform at ~ 360 ms following stimulus onset might be indicative of a swift decision strategy to reduce processing of potential distractors (for review, see Linden
2005). We hypothesize that up modulations at ~360 ms following stimulus onset could represent neurophysiological mechanisms that underlie Rejeski’s (
1985) and Tenenbaum’s (
2001) theoretical propositions. This electrophysiological response would partially block task-irrelevant information from entering focal awareness and thus causing disruption to exercise performance.
We hypothesize that the parietal lobe initially evaluated and successively reduced the processing of irrelevant stimuli such as a distracting musical excerpt (cf. Suzuki & Gottlieb
2013). The frontal lobe possibly received the signals from the parietal regions of the cortex and initiated appropriate action (i.e. stimulus interpretation; see Chadick & Gazzaley
2011; Prado, Carp, & Weissman
2011). Reduced activity in the left frontal regions induced by the presence of task-related factors (see Fig.
3) is believed to be caused by the previous suppression of irrelevant information in the parietal lobe. In summary, motor tasks performed in the presence of sensory stimuli appear to activate the parietal-frontal pathways. The parietal cortex not only functions as an informant in the parietal-frontal neural connection, but also performs initial evaluation of sensory signals (Bisley & Goldberg
2010) and thus may partially suppress or enable future processing in the frontal lobe, not only at rest, but also during exercise-related situations.