We read with interest the article by Iselin-Chaves et al.  1in which implicit learning was studied during general anesthesia. Forty words were played 25 times during anesthesia, and each played word was associated with a Bispectral Index (BIS) value recorded at the moment the word was played. The authors showed that implicit learning persists for words played during light (BIS 61–80) and adequate anesthesia (BIS 41–60) but not during deep anesthesia (BIS 21–40). Because the words were repeated 25 times throughout anesthesia, each word was associated with 25 BIS values for each patients, and it is not clear wether the authors, to classify each word in a BIS category (21–40, 41–60, or 61–80), used the mean of the 25 BIS values or used the BIS range of these 25 values to be associated with each word. If the mean BIS value was used, it is possible that some of the word presentations have been played at a higher BIS value than stated in the study. So implicit learning presented as occurring at a BIS below 60 may actually have occurred at a higher BIS.

Moreover, the time for BIS processing was not taken into account; this processing is responsible for a 15- to 30-s delay between raw electroencephalogram recordings and BIS value display (depending on the “BIS Smoothing Rate” setup).2So each BIS value associated with a word should have actually been associated with the word played 15–30 s earlier. This could have changed the assumption that implicit learning occurs with a BIS below 60. Indeed, during word presentation, BIS was above 60 during 18.5% of the time, which is far from what can be considered as an adequate anesthesia as stated in the article.

*Centre Hospitalier Universitaire-Tivoli, La Louviere, Belgium. pilequeu@ulb.ac.be

Iselin-Chaves IA, Willems SJ, Jermann FC, Forster A, Adam SR, Van der Linden M: Investigation of implicit memory during isoflurane anesthesia for elective surgery using the process dissociation procedure. Anesthesiology 2005; 103:925–33
A-2000 Bispectral Index (BIS) Monitoring System: Operating Manual. Newton, MA, Aspect Medical Systems, 2004, chapter 3.4.2, pp 3–10