The Perils of Deep Air Diving

As you know, scuba diving comes with associated risks, and nitrogen narcosis, i.e., getting "narked, "is one of them if you're diving on air. Increasing ambient pressure causes a form of intoxication in the brain, and the symptoms, a range of cognitive and motor deficits, start becoming apparent at depths below 100 feet.

One symptom of narcosis is memory loss, which research has shown results from impairment of longterm memory, rather than short-term, working memory. Studies have consistently shown that narcosis impairs free recall (such as words) but not recognition memory in depth ranges of 100 to 165 feet. One possible explanation is that narcosis disrupts the encoding process of breaking down the information into a form the person understands. Material is learned but the quality of encoding it is reduced, because the nervous system has a harder time memorizing -- and thus retaining -- the information, resulting in a weaker memory trace.

Testing a Hypothesis

Wendy Kneller and Malcolm Hobbs, psychology professors at the University of Winchester in England, tested the hypothesis that narcosis disrupts processing at the encoding stage. In a typical level of processing (LoP) task, research participants are presented with words to process either in a "shallow" manner (deciding whether the word is written in capital or lower-case letters, or whether it contains an "e"), or a "deep" manner (deciding whether the word fits into a particular sentence, or whether it's pleasant or unpleasant). If narcosis does affect the encoding of information, it's predicted that deeper processing learned under narcosis would continue to lead to better recall than shallow processing. In contrast, another hypothesis -- that narcosis impairs self-guided search, and the impairment rests in the ability to retrieve information, not on how well it was encoded -- then deep processing under narcosis shouldn't improve free recall.

Levels of Processing under Narcosis

Kneller and Hobbs tested the LoP effect in shallow water (a swimming pool with a depth of six feet) and in deep water (125 feet off Egypt's Red Sea coast), using a total of 67 divers. They predicted that in shallow water, deeper processing should lead to improved recall, and that at deeper depths, free recall would be reduced but deeper processing would lessen or extinguish narcosis's effect.

The free-recall test used 30 target words, presented in the same order, displayed one at a time on a laminated card for five seconds. To test shallow processing, each word was preceded by a question about whether the word was in capital letters or in lower-case letters. After all words were shown, there was a three-minute break, then the divers were shown an underwater slate with the directive, "Write down as many target words as you can remember from those I just showed you," and were given 90 seconds to complete the task. Then they were led back to the surface to take a break before the second test. To test deep processing, each target word was preceded by a sentence with a word missing, such as "The ____ fell off the table." The target word below either made sense as part of the sentence or didn't, with an equal number of both options. The divers indicated their response to each card by writing their answers on a slate.

Recall in the shallow-processing experiment was slightly lower in the ocean compared to the pool, and the same went for the deep-processing test. As predicted by LoP, recall was significantly higher in both deep-processing conditions compared to shallow-processing conditions at each depth. However, the mean age of divers in the swimming pool (age 39) was older than in the Red Sea (age 29). Prior research shows that in adults age 60-plus the ability on free-recall tests is significantly lower than that of 20- to -30 year olds, and may begin to deteriorate at age 45.

They ran the analysis again with age taken into account, and results found a significant effect of depth and level of processing, but no significant link between depth and level of processing interaction. The results demonstrated a detrimental effect of narcosis at depth on free recall performance. More importantly, once age was considered, regardless of depth, divers in the deep-processing conditions recalled more words compared to those in the shallow-processing conditions. This supports earlier studies showing that free recall performance is impaired by narcosis at depths of 110 feet and more. The results also support that deeper processing leads to much greater recall, supporting the hypothesis that narcosis disrupts the encoding stage of memory.

The Good News and the Bad News

The researchers believe the outcomes support the hypothesis that narcosis affects the brain's process of storing memory and later retrieving it. While recall is significantly poorer at deep depths as a result of narcosis, this deficit can be improved by utilizing deeper processing of the materials to be learned in the encoding stage. Therefore, as altering encoding can lead to changes in recall, it's likely that narcosis acts to reduce encoding effectiveness.

On one hand, that's good news for divers, as it suggests that the knowledge most impaired by narcosis is information presented at depth. Thus, prior knowledge gained in training on the surface or in shallow water (and which is important for safety or an underwater task) may be more resistant to the effects of narcosis. That' s supported by previous findings that recall of information learned in shallow water isn't impaired at depth. The bad news is that important information that comes to divers in deep water may not be encoded properly, and may be lost when it's important later in the dive or back on the surface. For example, divers entering a deep wreck may not recall important markers that lead to the exit or recall information they recently viewed on their computers and air gauges. While they state that further studies need to be done to ascertain the reliability of their findings, Kneller and Hobbes suggest that divers would benefit from using memory strategies that encourage deeper processing when learning the information that needs to be remembered later at deep depths.

"The Levels of Processing Effect under Nitrogen Narcosis," by Wendy Kneller and Malcolm Hobbs, Undersea and Hyperbaric Medicine, vol. 40, no. 3, pgs -239-245. We have taken the liberty to edit, revise and summarize this study, with apologies in advance for any inadvertent errors we might have made.