(Page 2 of 6)

Blackouts represent episodes of amnesia, during which subjects are capable of participating even in salient, emotionally charged events - as well as more mundane events - that they later cannot remember. Like milder alcohol-induced memory impairments, these periods of amnesia are primarily "anterograde," meaning that alcohol impairs the ability to form new memories while the person is intoxicated, but does not typically erase memories formed before intoxication. Formal research into the nature of alcohol-induced blackouts began in the 1940s with the work of E.M. Jellinek. Jellinek's initial characterization of blackouts was based on data collected from a survey of Alcoholics Anonymous members. Noting that recovering alcoholics frequently reported having experienced alcohol-induced amnesia while they were drinking, Jellinek concluded that the occurrence of blackouts is a powerful indicator of alcoholism.

In 1969, Goodwin and colleagues published two of the most influential studies in the literature on blackouts. Based on interviews with 100 hospitalized alcoholics, 64 of whom had a history of blackouts, the authors posited the existence of two qualitatively different types of blackouts: en bloc and fragmentary blackouts. People experiencing en bloc blackouts are unable to recall any details whatsoever from events that occurred while they were intoxicated, despite all efforts by the drinkers or others to cue recall. Referring back to our general model of memory formation, it is as if the process of transferring information from short-term to long-term storage has been completely blocked. En bloc memory impairments tend to have a distinct onset. It is usually less clear when these blackouts end because people typically fall asleep before they are over. Interestingly, people appear able to keep information active in short-term memory for at least a few seconds. As a result, they can often carry on conversations, drive automobiles, and engage in other complicated behaviors. Information pertaining to these events is simply not transferred into long-term storage. Ryback wrote that intoxicated subjects in one of his studies "could carry on conversations during the amnesic state, but could not remember what they said or did 5 minutes earlier. Their immediate and remote memory were intact". Similarly, in their study of memory impairments in intoxicated alcoholics, Goodwin and colleagues reported that subjects who experienced blackouts for testing sessions showed intact memory for up to 2 minutes while the sessions were taking place.

Unlike en bloc blackouts, fragmentary blackouts involve partial blocking of memory formation for events that occurred while the person was intoxicated. Goodwin and colleagues reported that subjects experiencing fragmentary blackouts often become aware that they are missing pieces of events only after being reminded that the events occurred. Interestingly, these reminders trigger at least some recall of the initially missing information. Research suggests that fragmentary blackouts are far more common than those of the en bloc variety.

Blackouts: State-Dependent Memory Formation?

Early anecdotal evidence suggested that blackouts might actually reflect state-dependent information storage - that is, people might be able to remember events that occurred while they were intoxicated if they returned to that state. State-dependent memory can be viewed as a special case of a broader category known as context-dependent memory, in which cues that are associated with an event when a memory is formed tend to help trigger recall for that event at a later time. For instance, in a classic study by Godden and Baddeley divers who learned word lists either on land or under water remembered more words when tested in the same context in which learning took place. Likewise, returning to the same emotional or physiological state that was present when a memory was formed often can facilitate recall of that memory. It is not uncommon to hear stories of drinkers who stash alcohol or money while intoxicated and can locate the hiding places only after becoming intoxicated again. Regardless of how compelling such stories can be, clear evidence of state-dependent learning under the influence of alcohol is lacking. In one recent study, Weissenborn and Duka examined whether subjects who learned word lists while intoxicated could recall more items if they were intoxicated again during the testing session. No such state-dependency was observed. Similarly, Lisman tried unsuccessfully to help subjects resurrect lost information for events occurring during periods of intoxication by getting them intoxicated once again.

Blood Alcohol Concentrations and Blackouts

Drinking large quantities of alcohol often precedes blackouts, but several other factors also appear to play important roles in causing such episodes of memory loss. As Goodwin and colleagues stated with regard to subjects in one of their studies, "Although blackouts almost always were associated with heavy drinking, this alone seemed insufficient to produce one. On many other occasions, subjects said they had drunk as much or more without memory loss". Among the factors that preceded blackouts were gulping drinks and drinking on an empty stomach, each of which leads to a rapid rise in BAC.

Subsequent research provided additional evidence suggesting a link between blackouts and rapidly rising BACs. Goodwin and colleagues examined the impact of acute alcohol exposure on memory formation in a laboratory setting. The author recruited 10 male subjects for the project, all but one through the unemployment office in St. Louis, Missouri. Most subjects met diagnostic criteria for alcoholism and half had a history of frequent blackouts. The men were asked to consume roughly 16 to 18 ounces of 86-proof bourbon in approximately 4 hours. Beginning 1 hour after subjects began drinking, memory was tested by presenting subjects with several different stimuli, including a series of children's toys and scenes from erotic films. Subjects were asked to recall details regarding these stimuli 2 minutes, 30 minutes, and 24 hours after the stimuli were shown. Half of the subjects reported no recall for the stimuli or their presentation 30 minutes and 24 hours after the events, though most seemed to recall the stimuli 2 minutes after presentation. Lack of recall for the events 24 hours later, while sober, represents clear experimental evidence for the occurrence of blackouts. The fact that subjects could remember aspects of the events 2 minutes after they occurred but not 30 minutes or 24 hours afterward provides compelling evidence that the blackouts stemmed from an inability to transfer information from short-term to long-term storage. For all but one subject in the blackout group, memory impairments began during the first few hours of drinking, when BAC levels were still rising. The average peak BAC in this group, which was roughly 0.28 percent, occurred approximately 2.5 hours after the onset of drinking.

In a similar study, Ryback examined the impact of alcohol on memory in seven hospitalized alcoholics given access to alcohol over the course of several days. All subjects were White males between the ages of 31 and 44. Blackouts occurred in five of the seven subjects, as evidenced by an inability to recall salient events that occurred while drinking the day before (one subject could not recall preparing to hit another over the head with a chair). Estimates of BAC levels during blackout periods suggested that they often began at levels around 0.20 percent and as low as 0.14 percent. The duration of blackouts ranged from 9 hours to 3 days. Based on his observations, Ryback concluded that a key predictor of blackouts was the rate at which subjects consumed their drinks. He stated, "It is important to note that all the blackout periods occurred after a rapid rise in blood alcohol level". The two subjects who did not black out, despite becoming extremely intoxicated, experienced slow increases in blood alcohol levels.

Tags: Brain

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.