We tested the idea that real-world situations, such as the highly strenuous exercise involved in marathon running, that impose extreme physical demands on an individual may result in neurohormonal changes that alter the functioning of memory. Marathon runners were given implicit and explicit memory tasks before or immediately after they completed a marathon. Runners tested immediately upon completing the marathon showed impairment in the explicit memory task but enhancement in the implicit memory task. This postmarathon impairment in explicit memory is similar to that seen with amnesic patients with organic brain damage. However, no previous studies have shown a simultaneous enhancement in the implicit memory task, as shown by the marathon runners in the present study. This study indicates that human memory functioning can be dynamically altered by such activities as marathon running, in which hundreds of thousands of healthy normal individuals routinely partake.
With the increasing understanding of the selective effects of neuromodulators, including stress hormones, that vary with the vicissitudes of daily living, comes the need to investigate human memory under the kinds of naturalistic conditions that may give rise to changes in the balance of these neuromodulators. Patterns of memory responses exhibited in the laboratory may or may not generalize to real-world situations in which neuromodulator levels are altered. Memory dissociations have often been found with patient populations with focal brain damage. But such lesions are not the only, or even the most common, situation in which different memory systems or processes may be selectively engaged. The possibility exists that the fluctuations in neuromodulators obtained under real-world conditions could produce in normal individuals memory dissociations similar to those seen in patient populations; an understanding of such dissociations may be crucial for understanding the dynamics of human memory in the wild.
Exercise is an interesting (and pervasive) example of a behavior that has an influence on neuromodulators affecting memory. The emotional, cognitive, and physiological effects of moderate levels of aerobic exercise, which we would not consider stressful, have been well documented. These include beneficial effects on mood (Byrne & Byrne, 1993), cognitive speed, auditory and visual attention (Angevaren, Aufdemkampe, Verhaar, Aleman, & Vanhees, 2008), and neurogenesis in the dentate gyrus of the hippocampus, a brain structure implicated in memory (Pereira et al., 2007).
But what about the effects of heavy, strenuous exercise? Research into the physiological effects of marathon running, the event on which we focus in the present study, suggests the possibility of maladaptive consequences. Dehydration, gastrointestinal bleeding, muscle damage, immune suppression, and even sudden cardiac death have all been reported (see Uchakin, Gotovtseva, & Stray- Gundersen, 2003). Marathon running also greatly increases production of cortisol and norepinephrine, hormones that are elevated by physiological and emotional stress. Marathon runners' cortisol levels have been documented (Cook, Ng, Read, Harris, & Riad-Fahmy, 1987) to rise fourfold above the highest levels induced by the most common laboratory-based stress task, the Trier Social Stress Task (Kirschbaum, Pirke, & Hellhammer, 1993). Indeed, cortisol levels recorded 30 min after completion of a marathon rival those reported in military training and interrogation (Taylor et al., 2007), rape victims being treated acutely (Resnick, Yehuda, Pitman, & Foy, 1995), severe burn injury patients (Norbury, Herndon, Branski, Chinkes, & Jeschke, 2008), and first-time parachute jumpers (Aloe et al., 1994). Other neurotransmitters, such as norepinephrine, are similarly elevated in marathon running (Demers, Harrison, Halbert, & Santen, 1981). Leading experts (Sapolsky, 2004, p. 104) have concluded that marathon running is one of the most stressful activities in which normal, neurologically intact humans engage. …