To the Editor:—
I recently read the interesting study of Stonell et al.1 about the use of the word stem completion test to measure implicit and explicit memory formation during general anesthesia. I think that the word stem completion test is one of the most useful and clinically feasible models to study the implicit form of awareness in general anesthesia, and it is an important tool to validate systems for awareness prevention.
Stonell et al. used a word presentation counterbalancing scheme based on a previous study by Lubke et al.2,3 In this scheme, two of four words lists are given to patients during surgery, after induction of anesthesia; these lists are defined as inclusion target and exclusion target, whereas the two lists not given are defined as inclusion distracter and exclusion distracter.
Postoperatively, patients are asked to complete a column of word stems (inclusion target and inclusion distracter) with the words that they remember hearing during anesthesia, and then they are asked to complete another column of word stems (exclusion target and exclusion distracter) with words they have surely not heard during surgical anesthesia.
The word list presentation scheme is composed of four groups, but in this way, other possible cross-associations between the four lists are excluded.
Statistically, by probability calculation, with four lists to choose two by two, the number of audio tracks to administer to patients is six (track A: list 1 and list 2; track B: list 1 and list 3; track C: list 1 and list 4; track D: list 2 and list 3; track E: list 2 and list 4; track F: list 3 and list 4). Then, any list can be associated with four different types of function (inclusion target, inclusion distracter, exclusion target, or exclusion distracter). In this way, the possibility number is 24 [(number of lists − 1)! * number of list functions]; the total cross-association is shown in table 1.
