Low-dose Propofol-induced Amnesia Is Not due to a Failure of Encoding: Left Inferior Prefrontal Cortex Is Still Active

Background: Propofol may produce amnesia by affecting encoding. The hypothesis that propofol weakens encoding was tested by measuring regional cerebral blood flow during verbal encoding.

Methods: Seventeen volunteer participants (12 men; aged 30.4 +/- 6.5 yr) had regional cerebral blood flow measured using H2O15 positron emission tomography during complex and simple encoding tasks (deep vs. shallow level of processing) to identify a region of interest in the left inferior prefrontal cortex (LIPFC). The effect of either propofol (n = 6, 0.9 mu]g/ml target concentration), placebo with a divided attention task (n = 5), or thiopental at sedative doses (n = 6, 3 mu]g/ml) on regional cerebral blood flow activation in the LIPFC was tested. The divided attention task was expected to decrease activation in the LIPFC.

Results: Propofol did not impair encoding performance or reaction times, but impaired recognition memory of deeply encoded words 4 h later (median recognition of 35% interquartile range, 17-54%] of words presented during propofol vs. 65% 38-91%] before drug; P < 0.05). Statistical parametric mapping analysis identified a region of interest of 6.6 cm2 in the LIPFC (T = 7.44, P = 0.014). Regional cerebral blood flow response to deep encoding was present in this region of interest in each group before drug (T > 4.41, P < 0.04). During drug infusion, only the propofol group continued to have borderline significant activation in this region (T = 4.00, P = 0.063).