What cognitive abilities are involved in trail-making performance?

Salthouse, T. A. (2011). What cognitive abilities are involved in trail-making performance?  Intelligence, 39(4), 222-232.‏

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3141679/

In this paper, the renowned scholar Salthouse attempted to find out what cognitive abilities are involved in the TMT test.  The research was done with adults.  Will children's results be the same?  That's an interesting question.

Salthouse is a professor of psychology in Virginia University, where he studies the process of cognitive aging.  He published 10 books and over 250 chapters and papers.

 

 

Salthouse used a slightly different version of the TMT test than the version most of us are familiar with.  This version, the Connections test, has 49 circles containing numbers or letters in each stage of the task.  Like in the TMT test, one has to connect numbers only, numbers alternating with letters (1-A-2-B-3-C etc.) and letters only (A-B-C…this part does not exist in all familiar versions).  One of the differences between the Connections test and the TMT test is that in the Connections test the influences of visual search and hand movements have been minimized since the "targets" (the circles one has to reach in the sequence performed) are close together and not widely scattered on the page.  Another difference is that one gets 20 seconds to work on each page, instead of measuring the time needed to complete the page.  Research has shown that the Connections test is loaded with the same factors as the TMT test.  This means that the tests are highly equivalent. 

Despite their simplicity, researchers think that TMT tests reflect a wide variety of cognitive processes including attention, visual search and scanning, sequencing and shifting, psychomotor speed, abstraction, flexibility, the ability to perform and to alter a plan for action, and an ability to keep two lines of thought in mind simultaneously.

Some scholars argue that performance in the TMT reflects executive functions.  Salthouse writes that since there is a very close connection between fluid ability and executive functions, it can be predicted that fluid ability will affect performance on the TMT.  Other scholars argue that working memory affects performance in the TMT because of the need to keep track of the number and letter sequences.  Since there is a very strong empirical relation between fluid ability and working memory, Salthouse asked himself if working memory has a unique contribution to performance in the TMT, beyond the presumed contributions of processing speed and fluid ability. 

The study was done with a large sample of adults aged 18 to 98 (I admire 98 year olds who are willing to participate in such studies!).  The participants performed the Connections test and tests measuring fluid ability, long term memory, processing speed and vocabulary.

  Like other studies, this study reveals unpleasant facts about the influence of aging on cognitive abilities, and one ray of hope.  As we can see in the attached figure, fluid ability, long term memory and processing speed deteriorate from the age of 20 (participants in this study were recruited through advertisement.  In each age group – in each decade, there were between 220 and 830 subjects.  Generally, the participants in this study performed better on the tests than normative samples).  Vocabulary is the ray of hope.  Vocabulary keeps rising between ages 20 to 65, and then it deteriorates slightly, but remains in the average level.

The difference in processing speed between part A and part B of the Connections test was smaller at the older ages (performance speed in part A deteriorated faster than performance speed in part B, and thus the two parts came closer together).

In part A, processing speed affected performance more than fluid ability, which also affected performance significantly.  In part B, the results were reversed:  fluid ability affected performance more than processing speed, which also affected performance significantly.  The difference in performance speed between part A and part B was affected only by processing speed.  People with a high processing speed had a larger difference between speed of performance in part A and part B than people with low processing speed. 

Here we can see something odd – inferences about relations of cognitive abilities on Connections performance  vary depending on the particular measure of performance examined. Performance on the Connections test was affected in a moderate to a large degree by processing speed in parts A and B.  Processing speed probably affects performance because of the need for fast response on both parts.  Performance in the Connections test was affected in a moderate to a large degree by fluid ability, both in part A and in part B.  Fluid ability probably affects performance because of the need to keep one's place in the current sequence while one looks for the next element in the sequence.  This is required on both parts of the test.  There was a special relation between fluid ability and performance in part B.  This means that fluid ability affects the ability to switch between sequences.  There was no relation of fluid ability with the difference score (B–A) measure. The difference score reflected mainly processing speed.

There was no significant relation between working memory and performance on the Connections test.  I think that working memory was assessed here with very complex tasks, resembling fluid tasks.  This may be the reason for the lack of influence of working memory on the performance on the TMT beyond the influence of fluid ability.

Other studies, like this of Hoelzle, also found that the TMT test measures processing speed (and not working memory)

 

Hoelzle, J. B. (2008). Neuropsychological assessment and the Cattell-Horn-Carroll (CHC) cognitive abilities model. ProQuest.‏    PAGE 114   http://utdr.utoledo.edu/cgi/viewcontent.cgi?article=2213&context=theses-dissertations