Effects of acute aerobic exercise on mnemonic discrimination performance in older adults

Lack of effects of acute exercise intensity on mnemonic discrimination

The hippocampus is thought to support episodic memory by pattern separation, thereby supporting the ability to discriminate high similarity items. Past research evaluating whether acute exercise can improve mnemonic discrimination of high similarity items is mixed. The present experiment attempts to extend these prior mixed findings by evaluating the effects of multiple exercise intensities on hippocampal-dependent, mnemonic discrimination and memory performance. Fifty-seven young adults completed a three-condition (control, moderate-intensity, and vigorous-intensity), within-subjects crossover pretest-posttest comparison. We observed no effects of acute exercise on recognition memory or mnemonic discrimination. We discuss the implications of these null findings with the broader literature by discussing the complexity of this potential exercise-mnemonic discrimination relationship, including the unique role of exercise intensity, differences in the level of processing (e.g., conceptual vs. perceptual), and unique brain regions involved in mnemonic discrimination.

Improvement of mnemonic discrimination with acute light exercise is mediated by pupil-linked arousal in healthy older adults

Physical exercise has positive impacts on hippocampal memory decline with aging. One of the postulated neurobiological mechanisms of the decline is reduced catecholaminergic projections from the locus coeruleus to the hippocampus. Recent human studies revealed that very light exercise rapidly enhances memory and pupil diameter, which suggests that light exercise may improve memory via neural circuits involved in the ascending arousal system, including the locus coeruleus, even in older adults. Thus, we aimed to clarify the effects of a single bout of light-intensity exercise (60% ventilatory threshold) on mnemonic discrimination performance, an index of hippocampal memory function, in healthy older adults using a randomized crossover design. Pupil diameter was measured during exercise as a physiological marker of the ascending arousal system. Discrimination of highly similar stimuli to the targets improved after exercise when compared to the resting control performance. Importantly, causal mediation analysis showed that pupil dilation during exercise mediated the memory improvement. These results suggest that brief light exercise rapidly enhances memory, possibly by upregulating the ascending arousal system.

Increased neural differentiation after a single session of aerobic exercise in older adults

Aging is associated with decreased cognitive function. One theory posits that this decline is in part due to multiple neural systems becoming dedifferentiated in older adults. Exercise is known to improve cognition in older adults, even after only a single session. We hypothesized that one mechanism of improvement is a redifferentiation of neural systems. We used a within-participant, cross-over design involving 2 sessions: either 30 minutes of aerobic exercise or 30 minutes of seated rest (n = 32; ages 55-81 years). Both functional Magnetic Resonance Imaging (fMRI) and Stroop performance were acquired soon after exercise and rest. We quantified neural differentiation via general heterogeneity regression. There were 3 prominent findings following the exercise. First, participants were better at reducing Stroop interference. Second, while there was greater neural differentiation within the hippocampal formation and cerebellum, there was lower neural differentiation within frontal cortices. Third, this greater neural differentiation in the cerebellum and temporal lobe was more pronounced in the older ages. These data suggest that exercise can induce greater neural differentiation in healthy aging.

Acute cycling exercise and hippocampal subfield function and microstructure in healthy older adults

Aging is associated with deterioration in dentate gyrus (DG) and CA3, both crucial hippocampal subfields for age susceptible memory processes such as mnemonic discrimination (MD). Meanwhile, a single aerobic exercise session alters DG/CA3 function and neural activity in both rats and younger adults and can elicit short-term microstructural alterations in the hippocampus of older adults. However, our understanding of the effects of acute exercise on hippocampal subfield integrity via function and microstructure in older adults is limited. Thus, a within subject-design was employed to determine if 20-min of moderate to vigorous aerobic exercise alters bilateral hippocampal subfield function and microstructure using high-resolution functional magnetic resonance imaging (fMRI) during an MD task (n = 35) and high angular resolution multi-shell diffusion imaging (n = 31), in healthy older adults, compared to seated rest. Following the exercise condition, participants exhibited poorer MD performance, particularly when their perception of effort was higher. Exercise was also related to lower MD-related activity within the DG/CA3 but not CA1 subfield. Finally, after controlling for whole brain gray matter diffusion, exercise was associated with lower neurite density index (NDI) within the DG/CA3. However, exercise-related differences in DG/CA3 activity and NDI were not associated with differences in MD performance. Our results suggest moderate to vigorous aerobic exercise may temporarily inhibit MD performance, and suppress DG/CA3 MD-related activity and NDI, potentially through neuroinflammatory/glial processes. However, additional studies are needed to confirm whether these short-term changes in behavior and hippocampal subfield neurophysiology are beneficial and how they might relate to long-term exercise habits.

Associations between cardiorespiratory fitness, monocyte polarization, and exercise-related changes in mnemonic discrimination performance in older adults

Biological aging is accompanied by a chronic pro-inflammatory state that may facilitate losses in hippocampal-dependent mnemonic discrimination. Aerobic exercise training promotes adaptations that include improved immune competency, higher cardiorespiratory fitness, and maintenance of hippocampal function. However, it is poorly understood whether, in active older adults, baseline immune cell profiles and cardiorespiratory fitness are possible mechanisms that facilitate the long-term benefits to hippocampal dependent mnemonic discrimination performance. This within-subjects study with counterbalanced conditions aimed to investigate whether baseline monocyte polarization and cardiorespiratory fitness influenced performance in the mnemonic similarity task (MST) and related Lure Discrimination Index (LDI) score after an acute bout of exercise. Twenty-one active older adults (M = 68 ± 5 yrs) underwent baseline testing in which blood samples were collected and cardiorespiratory fitness measured. Participants then returned and completed a seated rest or moderate intensity aerobic exercise condition in which the MST was proctored prior to and 5 min after each condition. A linear mixed effects model was used in which Participant ID was a random effect and Condition (rest v. exercise), Time (pre- v post-), and order were fixed main effects. Simple linear regression models were used to determine the variance accounted for by monocyte phenotypes and cardiorespiratory fitness for LDI scores post-condition. Post-rest LDI scores were significantly lower than post-exercise LDI scores (t(20) = -2.65, p < 0.02, d = -0.57). Intermediate monocytes were significant predictors of the change in pre- to post-exercise LDI scores (F(1, 19) = 6.03, p = 0.024, R² = 0.24) and cardiorespiratory fitness was a significant predictor of the difference between post-condition LDI scores (F(1, 19) = 6.71, p = 0.018, R² = 0.26). Our results suggest baseline cardiorespiratory fitness and intermediate monocytes may relate to the integrity of hippocampal-dependent mnemonic discrimination performance, and possibly the degree of responsiveness to aerobic exercise interventions.