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Department of Health Studies and Gerontology
University of Waterloo
BMH 2318
Ontario N2L 3G1
Canada
E-mail: wainwrig{at}healthy.waterloo.ca
Dear Sir:
I am writing about the recently published review by McCann and Ames (1). This review addresses the behavioral effects of n–3 fatty acids in humans and animals. The authors are to be commended on their thoroughness in researching the extensive literature on this topic and in invoking the necessary methodologic considerations when evaluating the outcomes of studies conducted in both animals and humans. In their discussion of the various behavioral studies included in this review, they raise an important issue—that different neural systems might be affected differently by nutritional manipulations, and, thus, some outcomes might be more sensitive indicators than others with respect to the role of a particular nutrient. From my perspective as a behavioral scientist who works with rodents, I would like to expand on this observation and explore the implications for future research.
Although the terms cognitive and behavioral are often used freely in relation to nutritional interventions, what is meant by these terms is not always defined precisely. Thus, whereas both the human and animal literature encompass a diverse array of behavioral outcomes, there is often little discussion of exactly which specific psychological construct is being measured by each test and how this relates to overall cognitive function. This is despite an accumulation of evidence in both humans and animals during the past 15–20 y that supports the hypothesis that the brain comprises multiple memory systems, each anatomically and biochemically distinct and each processing information in a specialized way (2). On the basis of behavioral dissociations in studies using lesions in rats, 3 independent systems have been identified: 1) flexible use of knowledge involving the hippocampus, 2) habitual responses (reinforced stimulus-response associations) involving the dorsal striatum, and 3) emotional responses (Pavlovian stimulus—affect conditioning) involving the amygdala. In most situations, these systems operate in parallel, and, depending on the circumstances, interact either competitively or cooperatively. For example, depending on the protocol, different types of learning, either cognitive or stimulus-response, can occur in the place version of the Morris water maze. If the start position is varied over trials, the animal is required to use spatial information flexibly, whereas if the start position remains the same, the animal can use the response system, eg, swim 45 degrees left. What is important to note, however, is that when one system is damaged, performance will be impaired on tasks associated with the function of that particular system, but very often enhanced on the tasks associated with the other. For example, animals with hippocampal damage often perform better than do control animals on stimulus-response type learning tasks. Thus, it is entirely possible for an intervention that improves one type of memory to have deleterious effects on another type of memory or to have effects on other measures, such as arousal, that affect task performance.
In summary, the point being made herein is that broad terms such as cognition need to be carefully operationalized and then tested by using a comprehensive approach based on a broad battery of valid and reliable neuropsychological tests before conclusions can be drawn as to which of the myriad aspects of brain function are affected by a particular nutritional manipulation. Only then will we be in a position to make fully informed population-based recommendations with respect to dietary interventions.
ACKNOWLEDGMENTS
The author had no conflict of interest in relation to this letter.
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