It was often said that alcohol killing off brain cells is behind the blackouts, but according to a new study what’s actually happening is that booze is preventing new memories being recorded in the first place.

Yukitoshi Izumi, research professor of psychiatry at Washington University School of Medicine in St. Louis, said: “We’ve found that exposure to alcohol inhibits some receptors and later activates others, causing neurons to manufacture steroids that inhibit memory formation.”

The brain cells affected by alcohol are found in the hippocampus and other brain structures involved in advanced cognitive functions.

Yukitoshi Izumi studied slices of the hippocampus from the rat brain.

When he treated hippocampal cells with moderate amounts of alcohol, the key areas for memory formation were unaffected, but exposing the cells to large amounts of alcohol inhibited the memory formation mechanism.

“It takes a lot of alcohol to block memory,” said senior investigator Charles F. Zorumski.

“But the mechanism isn’t straightforward. The alcohol triggers receptors to behave in seemingly contradictory ways, and that’s what actually blocks the neural signals that create memories. It also may explain why individuals who get highly intoxicated don’t remember what they did the night before.”

The study is echoed by the work of researchers at the University of California, San Diego, who found that some people are actually more prone to blackouts than others.

“Our study’s findings suggest that some people are more likely to experience alcohol-induced blackouts than others due to the way alcohol affects brain activity in areas involved in self-monitoring, attention, and working memory,” said lead author Reagan R. Wetherill.

“Some people may be more vulnerable to alcohol’s effects than others. In other words, just because your friend may be able to drink a certain number of drinks and appear to be functioning fine, it does not mean that you or everyone else can.”

“Blackouts”, where very heavy drinkers wake up and are unable to remember anything that happened, are quite rare, says Reagan R. Weatherill – but their cousin, the “brown out”, where details vanish, is much more common.

The study – using 24 students, 12 of whom suffered “brown outs” while drinking, and 12 who did not, challenged the students to complete memory exercises both while drunk and while sober.

The study found that the students had similar abilities to remember and to multi-task while sober – but alcohol acted as a “magic switch” for some which stopped them remembering.

It suggests there is a biochemical trigger for some people to forget what happens while drinking.

“Through use of imaging technology, this study has made the really intriguing finding that the unique patterns of blood flow and neural activity seen in persons prone to experience those amnestic phenomena emerged only after they became intoxicated,” she said.

“That finding, taken together with results from prior research on fragmentary blackouts, suggests there are individual differences in how alcohol impacts memory.”

“Alcohol intoxication reduced activity in a brain region involved in <<multitasking>>,” said Reagan R. Wetherill.

“Thus, alcohol appears to affect a person’s ability to multitask, and also affects some people’s ability to engage brain areas required for encoding and remembering previous experiences.”

“Irrespective of the specific type of alcohol-related memory loss involved, if one is experiencing blackouts it is an important signal that negative personal and health consequences are more likely to occur.

“Not fully recalling one’s life experiences, particularly those that occur while one is intoxicated, creates a state of vulnerability where the chances increase for the individual to incur all kinds of problems.”