Indeed, the study found that 27 of 5,713 patients -- all thought to be at high risk for intraoperative awareness -- had either definite or possible awareness during their surgery, based on interviews afterward.
Only nine had what experts considered to be definite awareness.
The patients had been randomly assigned to be monitored either by the bispectral index -- the BIS group -- or the standard method, known as end tidal anesthetic-agent concentration, or ETAC.
When the researchers looked at what happened in those groups, they found seven of the 2,861 patients in the BIS group had definite intraoperative awareness, compared with two of 2,852 in the ETAC group.
Also, there were 11 cases of possible intraoperative awareness in the BIS group and six in the ETAC group.
But the numbers were not considered to reflect a real difference in the risk of awareness during surgery, the researchers reported.
Patients who are aware during their surgery rarely feel pain, according to Dr. Mark Schlesinger of Hackensack University Medical Center in Hackensack, NJ -- although some patients have described unpleasant sensations.
"Usually, they are just aware without discomfort," he said in an email to ABC News/MedPage Today.
In the study, Avidan and colleagues reported, none of the patients thought have been definitely or possibly aware reported feeling pain.
The researchers cautioned that the results may not apply to all surgical patients; those in the study were at high risk for intraoperative awareness and were sedated with gases. Results might differ for patients at lower risk or sedated intravenously, they noted.