Effects of chewing on cognitive processing speed

Abstract

In recent years, chewing has been discussed as producing effects of maintaining and sustaining cognitive performance. We have reported that chewing may improve or recover the process of working memory; however, the mechanisms underlying these phenomena are still to be elucidated. We investigated the effect of chewing on aspects of attention and cognitive processing speed, testing the hypothesis that this effect induces higher cognitive performance. Seventeen healthy adults (20–34 years old) were studied during attention task with blood oxygenation level-dependent functional (fMRI) at 3.0 T MRI. The attentional network test (ANT) within a single task fMRI containing two cue conditions (no cue and center cue) and two target conditions (congruent and incongruent) was conducted to examine the efficiency of alerting and executive control. Participants were instructed to press a button with the right or left thumb according to the direction of a centrally presented arrow. Each participant underwent two back-to-back ANT sessions with or without chewing gum, odorless and tasteless to remove any effect other than chewing. Behavioral results showed that mean reaction time was significantly decreased during chewing condition, regardless of speed-accuracy trade-off, although there were no significant changes in behavioral effects (both alerting and conflict effects). On the other hand, fMRI analysis revealed higher activations in the anterior cingulate cortex and left frontal gyrus for the executive network and motor-related regions for both attentional networks during chewing condition. These results suggested that chewing induced an increase in the arousal level and alertness in addition to an effect on motor control and, as a consequence, these effects could lead to improvements in cognitive performance.

Introduction

Recently, behavioral studies were performed to examine the relationship between chewing and cognitive performance including memory, attention and executive function. With regard to memory, it has been reported that gum chewing improves episodic and working memory during chewing, suggesting at least in part that chewing promotes regional cerebral blood flow and glucose delivery (Stephens and Tunney, 2004, Wilkinson et al., 2002, Zoladz and Raudenbush, 2005). However, the existence of enhanced performance of episodic memory task by context-dependent effects induced by chewing has remained controversial (Baker et al., 2004, Johnson and Miles, 2007, Johnson and Miles, 2008, Stephens and Tunney, 2004). As for attention, it was reported that sustained attention (Smith, 2009a, Smith, 2010, Tucha et al., 2004) and language-based attention (Stephens & Tunney, 2004) were improved by chewing. On the other hand, Tucha et al. (2004) claimed not only that memory functions were not improved but also that tonic and phasic alertness were adversely affected by chewing. With respect to executive function, a study claimed that chewing gum does not appear to be of benefit to word association executive function (Stephens & Tunney, 2004), but another study reported a beneficial effect (Onyper, Carr, Farrar, & Floyd, 2011).

To elucidate these inconsistent results and their mechanisms, several functional neuroimaging studies have been conducted. These studies suggested that chewing facilitated the process of working memory and also that it was related to attention (Hirano et al., 2008, Wang et al., 2009). As well, several studies mentioned that chewing affects arousal (Onyper et al., 2011, Sakamoto et al., 2009, Smith, 2010, Stephens and Tunney, 2004). Sakamoto et al. (2009) studied the effect of chewing on the central nervous system by measuring reaction time (RT) and event-related potentials (ERPs). They suggested that chewing influences the state of arousal via the ascending reticular activating system, and that it accelerates cognitive processing.

Based on these studies, we assumed that chewing also affects aspects of attention and accelerates cognitive processing. Indeed, recent studies, pointing out that reaction times were shortened by chewing in the categoric search task (Allen and Smith, 2012a, Smith, 2010), vigilance task (Allen & Smith, 2012a), language-based attention task (Stephens & Tunney, 2004), and the encoding of new information in the focused attention task (Smith, 2010). Smith (2010) speculated that positive cognitive performance may come from the fact that subjects feel more alert as described, being energetic, quick-witted and attentive, all based on mood improvement. However, some studies reported not only that the performance of sustained attention was not accelerated (Kohler et al., 2006, Smith, 2010, Tucha et al., 2010) but also that vigilance task was decelerated (Tucha et al., 2010). Tucha et al. (2010) indicated that the psychodynamics of gum chewing might be an important factor, and these conflicts of cognitive performance may originate from the duration of the study (Tucha et al., 2010) and time of the task (Allen and Smith, 2012a, Allen and Smith, 2012b, Tucha and Simpson, 2011). Indeed, Tänzer, Von Fintel, and Eikermann (2009) reported that chewing benefit in concentration performance showed up after 14 min from the initiation of the test. To elucidate the mechanism of this issue, we considered that a functional magnetic resonance imaging (fMRI) assessment might be helpful. The attentional network test (ANT) provided a way of testing for the efficiency of the alerting, orienting and executive (conflict resolution) functions of attention (Fan, McCandliss, Sommer, Raz, & Posner, 2002), and it was adapted within a single session of event-related fMRI (Fan, McCandliss, Fossella, Flombaum, & Posner, 2005). In the current study, we examined the effects of chewing on alerting and executive attention and their processing speed by comparing the behavioral and fMRI results of ANT.