Children assigned to chew sugar-free gum purportedly score 3% higher on standardized tests of math skills (as widely reported in the press). But is this just one of the 5% of all possible untrue hypotheses statistically guaranteed to have some significant result in its favor (in fact, it's worse than that)? Is the effect due to some other aspect of gum chewing (as Michael Posner asks)? Or might there be a real effect here of chewing (i.e., "mastication"), and if so, how can you use it to your maximum advantage?
What we know - and what we don't - about gum and cognition
To cut to the chase: there's ample evidence that chewing gum enhances cognition - it's just unclear what precisely the effect is, how it occurs and why (that's all!). As reviewed in more detail below, some work suggests the effect is limited to memory, independent of sustained attention and the so-called "executive" functions that operate on memory. In contrast, a few neuroimaging studies suggest the effect is rather specific to the executive network (involving the prefrontal and parietal cortices) and that it does indeed involve sustained attention (as assessed through electrophysiology). The mechanism by which chewing has these effects is clearly related to cortical blood flow, but changes in blood flow are not merely a side effect of either the act of chewing, as indicated by numerous studies using "sham chewing" controls, nor merely to the difficulty of chewing, since "moderately hard" gum is superior to harder gum. While the caloric content of the gum appears to have some effect, even zero-calorie gum has been shown to improve cognition, and some sugar gum is sometimes associated with decrements in cognitive function. Stress reduction likely has a role here as well, but the direction of causality is unclear - is stress reduced as a result of the increased cognitive functioning induced by masticuation, or is stress reduction by chewing the precursor to that improved cognitive functioning? (Not to mention why chewing would reduce stress at all.)
So let's start with what we do know. The effects of mastication are typically examined by having subjects chew gum while undergoing cognitive testing, neuroimaging with functional magnetic resonance imaging (fMRI) or electroencephalography (EEG), or some combination of these techniques. In general, subjects are better at a number of tasks while or immediately after chewing gum (including tests of working and episodic memory) and show both increased activation of the cortex during cognitive tasks (including the frontal operculum and insula, the thalamus, and posterior parietal cortex) as well as shorter-latency peaks in electrophysiological signatures of high-level cognitive processing.
By my count there are more than15 studies in the literature showing effects of this kind, many employing fairly sophisticated control conditions. For example, the positive effects of gum persist even when controlling for the influence of motor movement in general, or of jaw movement in particular; when controlling for the possible influence of glucose, taste, or odor; and even when considering the helpful influence of distinctiveness that gum chewing may lend to any psychological testing (although this particular point is currently a topic of debate). Rather than covering in detail all of the studies that support the generalizations above, I'll just hit some highlights, list some speculative tips on maximizing the beneficial effects of masticulation, and list out the references so you can read more if you like.
Is it all about the sugar?
An immaculate behavioral study by Stephens & Tunney examined whether the gum chewing might improve performance by improving the delivery of glucose to the brain. The authors administered sugar-free mint-flavored gum and a glucose drink, both separately and together, in a double-blind manner, and followed this with testing on a battery of complex cognitive tasks. They also used as their baseline condition the effects of the mint flavor alone, and used the conservative Bonferroni correction in data analysis (increasing the required significance threshold by a factor of 9!). The results showed significant improvement of performance of gum chewing, independent of glucose, relative to the mint flavor control condition on tests of immediate recall, delayed recall, and working memory (the latter assessed with digit and spatial span), with no significant effects of gum chewing on sustained attention, "processing speed", or executive function (assessed via digit cancellation, digit symbol substitution, trail making A & B, and COWAT).
Maybe it's about stress reduction, like other forms of exercise
A reasonable hypothesis is that mastication might have its effects by reducing stress, because we know that stress is negatively correlated with cognitive functioning, and that other forms of exercise can reduce stress. A new 2009 article by Scholey et al is maybe most impressive for the fact that subjects were able to complete the
torture tasks at all - subjects had to simultaneously complete four tasks that are actually pretty difficult even in isolation (Stroop, mental arithmetic, a Sternberg task, and a visual monitoring task). The authors also measured salivary cortisol, a physiological marker of stress, as well as self-reported levels of calmness, alertness, and anxiety. As you'd expect, this "extreme multitasking" reduced calmness and alertness, and increased anxiety. More interesting, gum-chewing significantly mitigated these effects, even reducing stress as measured by salivary cortisol. Moreover, performance across all four tasks was improved by chewing gum (although these effects might be weak, since no task showed improvement when considered alone). Of course, causality remains unclear; mastication might have reduced stress because subjects found the tasks easier, or might have made the tasks easier because mastication directly reduces stress.
So what about arousal - maybe chewing just keeps people awake for the cognitive testing?
A 2009 study by Sakamoto et al showed that both reaction time and the "contingent negative variation" (often considered an electrophysiological index of the early stages of response preparation) gradually improved with gum chewing, showing significant differences from a rest control condition only after around 10 minutes of chewing. Critically, this study showed no effect of gum chewing on motor-related electrophysiological measures (although they stopped short of analyzing the data for a real interaction). This data is at least suggestive that if the effect of mastication is on arousal, it is more visible in the preparatory stages of task performance than in motoric processing. This interpretation is supported by a number of fMRI studies showing that the chewing of gum relative to chewing without gum is associated with a number of regions of the prefrontal cortex, as in this image borrowed from Takada and Miyamoto, as though the influence of masticuation occurs at a relatively high level in the putative "hierarchy" of cognitive processing.
Tips for Getting the Most Out of Mastication
OK, so what's the take-home? More research is needed, but I can make some speculative suggestions:
1) Chew sugar-free gum that includes sugar alcohols - for example, xylitol. Not only is it potentially better for your teeth, but the nonzero caloric content may "feed your head" in a more sustained fashion than a giant spike of quickly-metabolized simple sugars. Some studies even show a marginal reduction in performance after chewing gum with those simple sugars.
2) Give it time. Some work suggests that the beneficial effects of gum chewing emerge (at the group level) only after two or more five-minute sessions of gum chewing.
3) Go for flavored gum, and use the same flavor at study and test!. Some of the evidence suggests that there are nonadditive effects of the act of chewing and gum flavor - perhaps because the flavoring adds an additional dimension to the experience, differentiating all stimuli processed under that context from others, and perhaps thereby aiding in recall of that information. This general phenomenon is sometimes called the encoding specificity principle.
4) Chew with rhythm. I think most people do this naturally, but if there's anything to the story that gum chewing has its effects on arousal by enhancing the activity of central pattern generators in the brainstem (which some have proposed), it's probably ideal to do your chewing with a regular frequency (most studies use 1Hz).
5) The placebo effect is your friend. As described above, I think there's ample reason to believe that chewing gum actually does benefit cognition, but there have been a few failures in the literature to replicate some of the more detailed effects. Even if the entire phenomenon turns out to be false (through some alternative explanation I can't fathom at the moment) - one could still get some benefit through just believing it would work.
References:Allen, K. L.; Norman, R. G.; Katz, R. V. 2008. The effect of chewing gum on learning as measured by test performance. Nutrition Bulletin. 33(2):102-107, June 2008.J.R. Baker, J.B. Bezance, E. Zellaby and J.P. Aggleton, Chewing gum can produce context-dependent effects upon memory, Appetite 43 (2004), pp. 207-210.Y. Hirano, T. Obara, K. Kashikura, H. Nonaka, A. Tachibana and H. Ikehira et al., Effects of chewing in working memory processing, Neurosci Lett 436 (2008), pp. 189-192.A.J. Johnson and C. Miles, Evidence against memorial facilitation and context-dependent memory effects through the chewing of gum, Appetite 48 (2007), pp. 394-396.A.J. Johnson and C. Miles, Chewing gum and context-dependent memory: the independent roles of chewing gum and mint flavour, Br J Psychol 99 (2008), pp. 293-306.Y. Masumoto, T. Morinushi, H. Kawasaki and M. Takigawa, Spectral analysis of changes in electroencephalographic activity after the chewing of gum, Psychiatry Clin Neurosci. 52 (1998), pp. 587-592.Y. Masumoto, T. Morinushi, H. Kawasaki, T. Ogura and M. Takigawa, Effects of three principal constituents in chewing gum on electroencephalographic activity, Psychiatry Clin Neurosci 53 (1999), pp. 17-23C. Miles and A. Johnson, Chewing gum and context-dependent memory effects: a re-examination, Appetite 48 (2007), pp. 154-158. I. Momose, J. Nishikawa, T. Watanabe, Y. Sasaki, M. Senda and K. Kubota et al., Effect of mastication on regional cerebral blood flow in humans examined by positron-emission tomography with 15O-labelled water and magnetic resonance imaging, Arch Oral Biol 42 (1997), pp. 57-61.T. Morinushi, Y. Masumoto, H. Kawasaki and M. Takigawa, Effect on electroencephalogram of chewing flavored gum, Psychiatry Clin Neurosci 54 (2000), pp. 645-651.Y. Nakamura and N. Katakura, Generation of masticatory rhythm in the brainstem, Neurosci Res 23 (1995), pp. 1-19.M. Onozuka, M. Fujita, K. Watanabe, Y. Hirano, M. Niwa and K. Nishimura et al., Mapping brain region activity during chewing: a functional magnetic resonance imaging study, J Dent Res 81 (2002), pp. 743-746.Sakamoto K, Nakata H, Kakigi R. 2009. The effect of mastication on human cognitive processing: a study using event-related potentials. Clin Neurophysiol. 2009 Jan;120(1):41-50. Epub 2008 Nov 20.A. Scholey, Further issues regarding the possible modulation of cognitive function by the chewing of gum: Response to Stephens and Tunney (2004) and Tucha et al. (2004), Appetite 43 (2004), pp. 221-223.A. Scholey, Chewing gum and cognitive performance: a case of a functional food with function but no food?, Appetite 43 (2004), pp. 215-216.Scholey A, Haskell C, Robertson B, Kennedy D, Milne A, Wetherell M. 2009. Chewing gum alleviates negative mood and reduces cortisol during acute laboratory psychological stress.Physiol Behav. 2009 Jun 22;97(3-4):304-12. Epub 2009 Mar 5Smith A. Effects of caffeine in chewing gum on mood and attention. Human Psychopharmacology: Clin Exp 2009; 24:239-47.R. Stephens and R.J. Tunney, Role of glucose in chewing gum-related facilitation of cognitive function, Appetite 43 (2004), pp. 211-213T. Takada and T. Miyamoto, A fronto-parietal network for chewing of gum: a study on human subjects with functional magnetic resonance imaging, Neurosci Lett 360 (2004), pp. 137-140.O. Tucha, L. Mecklinger, K. Majer, M. Hammerl and K.W. Lange, Chewing gum differentially affects aspects of attention in healthy subjects, Appetite 42 (2004), pp. 327-329.L. Wilkinson, A. Scholey and K. Wesnes, Chewing gum selectively improves aspects of memory in healthy volunteers, Appetite 38 (2002), pp. 235-236.
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