What constitutes evidence for neural plasticity?

There is tremendous debate about what constitutes evidence for neural plasticity. Perhaps the most unambiguous evidence is when training increases the thickness or volume of neural structure. It has been demonstrated that sedentary older adults who engage in aerobic exercise can delay shrinkage in prefrontal cortex, an area maximally sensitive to age-related volumetric shrinkage. In terms of cognitive interventions, actual gains in neural volume relative to a control group were demonstrated by Boyke et al33 in the mid temporal regions, hippocampus and nucleus accumbens, when older adults were trained to juggle for 90 days. These regions are associated with complex motor behaviors, so this finding was an important demonstration of plasticity in older adults. Importantly, however, the gains were not maintained after a 90-day period of non-juggling, providing important evidence that there are many constraints on plasticity, and that the familiar “use or lose it” adage was disappointingly relevant in this particular study. Other evidence shows that older men who played a demanding spatial navigational game every other day for 4 months exhibited stability of hippocampal volume over a 4-month period, whereas control subjects declined.34 Additionally, these trained subjects showed an increase in structural integrity of the hippocampus which was maintained when training ceased. Overall, however, the evidence that one can improve volume of neural structures through training is relatively sparse. The limited data available suggest that gains that are realized from a sustained training program most likely need to be maintained with continued performance. An important question is whether continuous improvement and challenge on a task is required to maintain gains, or whether mere maintenance of a high level of improved but asymptotic performance would be sufficient to preserve gains. It seems likely that it will be important for individuals to enjoy the tasks they are performing over the very long term so that the behavior can be sustained and gains maintained. This may be the greatest challenge associated with training the aging human brain. From a clinical perspective, daily “brain training” could become a boring and effortful task, such that gains realized might be offset by the negative consequences of performing a task that over time could become a dreaded obligation rather than a pleasurable and stimulating activity

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