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LETTERS TO THE EDITOR |
Department of Psychology
Bishop’s University
Lennoxville, QC, J1M 1Z7
Canada
E-mail: sblack{at}ubishops.ca
Dear Sir:
The article by Zhang et al (1) contains an apparent contradiction in its findings. In the abstract, the authors first state that "...we found inverse linear associations of serum total cholesterol and non-HDL cholesterol with visuomotor speed...". However, the conclusion of the abstract then states, "Low serum total cholesterol and non-HDL cholesterol are associated with slow visuomotor speed...," which implies a direct, not an inverse, association. Similarly, a portion of the title of the article, "Serum cholesterol concentrations are associated with visuomotor speed," and the statement in the discussion that "...we documented that low serum TC [total cholesterol] and low serum NHDLC [non-HDL cholesterol] concentrations are significantly associated with slow visuomotor speed..." imply a direct association.
The source of the confusion is the incorrect way in which Zhang et al define "visuomotor speed." They state that "Visuomotor speed was measured by the SRT [simple reaction time] test." and that "The measured response was the latency (in ms)...". Consistent with this definition, their Table 2 reports latency values in ms under the heading "Visuomotor speed." However, the term "speed" refers to rate of response, conventionally defined as distance traveled divided by time (2); thus, visuomotor speed would be the reciprocal of latency (ms–1), not latency itself. In the first statement quoted above in which Zhang et al declare their finding an inverse relation, they use "visuomotor speed" as they have incorrectly defined it, namely as latency. However, in the remaining quoted statements, they seem to be using it in its conventional and correct sense, as the reciprocal of latency. Hence, their statements appear contradictory.
The solution is to consistently use the term "visuomotor speed" to refer to the reciprocal of latency. Then their finding, when clearly stated, is that the higher the cholesterol concentration, the higher the visuomotor speed—a direct association. This is an interesting and provocative finding. It would be regrettable if their inconsistent use of an incorrect definition prevented this important finding from being readily appreciated.
ACKNOWLEDGMENTS
The author had no conflict of interest to declare.
REFERENCES
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