Please sir, my brain is full...We're not stupid

- english cognitive biases decision-making gareth lock incident analysis Jun 03, 2023

An experienced cave diver (cave CCR, cave DPV, full cave, sidemount cave) undertakes what was planned to be a relatively simple OC sidemount (SM) dive for them. Go into the cenote, drop the O2 deco cylinder at 6m/20ft, scooter several thousand feet back into the cave, drop the bottom stage, scooter some more, photo shoot, and then return along the same route. They had executed similar dives without an issue in the past. This time it ended differently. They ended the dive with their left SM cylinder almost empty, their right SM cylinder half full, and their bottom stage one-third full. They thought that they had run out of gas in the bottom stage and thought they had been balancing their SM rig, so thought the right SM cylinder was also close to empty when they saw their left cylinder only had <10 bar/100 psi in it. They had no idea that the situation had developed in the manner it did until they were completing their O2 deco and started to think back over the dive and take stock of their equipment.

Why is it that the ‘obvious’ isn’t obvious at the time when something is going wrong, but it is afterwards? Why is it we forget basic tasks or skills when they should be executed without error? Even more importantly for divers, why is it that our concentration and performance are reduced when we are underwater?

There are some very good reasons for this, and that is what this blog is about. This article looks at some of the physiological and cognitive reasons why we don’t necessarily perform as we should as divers, especially when underwater.


How much mental capacity do we have?

There is no easy way to describe how much capacity we have. Internally, we should consider how the skills, strategies, and expertise that the diver has, allow them to notice what is going on, what is going to happen, and what to do to make sure the outcome is what we expect and/or want it to be (this might be considered the ability to have ‘good’ situation awareness, but it is slightly different). How these skills, strategies and expertise are applied is sometimes considered workload.

We also have to consider both physical and cognitive task loads. Task load is a measurement of human performance that broadly refers to the levels of difficulty that a diver encounters when executing a task. For diving, we are interested more in cognitive task load than physical task load. However, high levels of physical task load can impact cognitive functions as carbon dioxide can build up and this has an impact on cognitive function. Cognitive task load is based on three factors: time taken to complete the task, level of information processing, and the number of task switches needed. Time refers to the amount of time that is allocated for task completion. The level of information processing is made up of automatic processes (skills), routine problem-solving (rules), and the more complex analysis of information to generate strategies (knowledge). Task switching occurs when components of a task have competing goals. There is also a loss in performance when we switch between tasks.

Finally, we also have to consider our background cognitive loading – this is the emotional or cognitive ‘stuff’ you bring with you to the pre-dive and dive. It might be a row with a family member or close friend, it might be financial worries, it might be a broken piece of dive gear that is expensive and needs replacing, it might be about delayed flights, it might be the instructor check that is going on now…etc. Anything that takes up mental space in your head. You might be able to drop all of this when you get in the water, but when you’re on the surface, you might not. The contributory factors leading to the death of Carl Spencer appeared to be heavily influenced by cognitive loading.

There are two analogies that might help to get this point across. 



One analogy considers our working memory. This is used to hold temporary information that is going to be used in the next few seconds to a few minutes. Think about remembering a telephone number, street directions, the next stop on decompression, and the navigation route through the wreck… our working memory is like your mental scratch pad or RAM in a computer. It has limited capacity though and is limited to an approximate value of 7+/-2 elements or chunks of information (not just individual ‘numbers’). Note, an element or chunk might be made up of a number of different things or chunks that we intuitively know what to do because they are heavily practised and we have patterns to match.

You could liken this to a juggler holding 7 balls, one for each of the elements. As a new task (or ball) comes in, the juggler might have to drop a ball because he might not be able to handle more than seven (depending on their skills, knowledge and competence). In some cases, the juggler can choose which ball to drop – delegation. In others, one of the balls gets dropped without realising it because in their mind the seven slots are still full, and everything is ok.


Buckets of Water

Another analogy is that of a bucket which is being filled up with water. The water represents things that must be dealt with. Once the bucket fills to the top, water overflows and this represents tasks or information being missed. If the bucket is full, we can’t add more to it, or we can, but other stuff flows out. Different people have different-sized buckets based on their knowledge, skills, and competence. They also have different experiences to make sure they can empty the bucket under control.

In both cases, experts can deal with more balls or water. How they’ve done it, through perfect practice which involves feedback and reflection, means they are able to notice the important things in the scene and ignore the apparently irrelevant factors, moving the practised activities to unconscious competence. They ‘just do it’ and it is hard to get them to explain ‘how’ they do it.



Just being submerged impacts our performance

So, we have 7 balls to (sub-)consciously play with. What happens when we go diving?

We are not designed to live underwater. Being submerged can cause many psychological and physiological stressors which reduce our cognitive performance or thinking power. You don’t have to be doing ANYTHING to have a reduced amount of mental performance. Research by Daleki and colleagues in 2012 showed that individuals who were submerged in a 5m pool and were subject to simple cognitive tasks showed a reduction in cognitive performance of approximately 10% and a reduction in fine motor coordination between 24% and 50% (Dalecki et al., 2012). There is a saying that people leave part of their brains on the boat or the shore when they get in the water is metaphorically true. You have reduced cognitive capacity and physical coordination by just being submerged.

If we think back to the juggling analogy, one of those balls is now filled up with mental stimulation from being in the water. Despite having seven balls, this reduction in performance means we only have six balls available for task load and workload. In the analogy of the bucket, it could be the bottom 10% of the bucket already has water in it, or we’ve reduced the size of the bucket by 10%.

Given this situation, we need to recognise this reduced performance, and either increase our competence through practice and feedback or reduce the complexity of the activity we are doing. Note, just because you are able to do skills and activities in a controlled environment like a pool, or confined water, it doesn’t mean they will be at the same level of competence when the real world is encountered when there are more stimuli, and you will face environments where you have no patterns to match. The same problem applies with instructor development – using fellow instructor candidates as students has little resemblance to your class environment when you are dealing with ‘live’ students and their variabilities. It is quite easy to be task focused on a single student without realising the big picture.



As well as task loading, we also have to consider the impact of physiological changes caused by immersion and breathing compressed gases. We know that nitrogen is narcotic, having a noticeable effect at around 30m/100ft. Oxygen was thought to be narcotic, but recent research has shown that may not to be the case. Carbon dioxide is known to be narcotic too. The latter is important because air is relatively dense below 30m/100ft which means more CO2 is being produced, as well as being subject to the ‘normal’ nitrogen narcosis. While the research shows that motor skills are not impacted too much by narcosis, higher cognitive functions like problem-solving are negatively impacted. Consequently, while you might be able to execute a shutdown drill on command, are you able to problem solve and recognise the situation developing in the manner it does in a timely manner? The same issue applies when problem-solving on a CCR when you are breathing a dense gas.


What happened on the opening dive?

There were multiple minor issues on the dive that led to the diver having the inability to focus on the bigger issues developing. There was an equipment configuration problem that was easily fixed at the start of the dive but left residual ‘how stupid was that?’ thoughts running through their head. There were social pressures which made it harder to thumb the dive or at least slow it down. A fumbled gas switch while on the scooter on the way back led to more cognitive loading and them switching to their left SM cylinder because they thought their bottom stage was empty. They made an assumption that the SM cylinders were being switched and so when the left SM cylinder was checked at the end, the assumption was that because that cylinder was nearly empty, the right one must have been too. These ‘failures’ are down to normal human behaviours. What was different was the specific context in which the dive took place. ‘More training’ or being told ‘you shouldn’t have done that’ is never going to solve the problem of a competent person falling foul of the context driving their behaviour, because the training very rarely includes these sorts of cognitive failure modes.



We have a finite capacity when it comes to dealing with information coming in and processing it. Errors are normal. It is the ability to be resilient when failures are encountered that allows us to move from a catastrophic outcome to an ‘almost happened’ near miss. We were good, not lucky. More on the different types of near-misses and luck next week!

As always, when looking at adverse events, look to understand the local rationality of those involved. How did it make sense? Was their ‘brain full’ so that even if the information was in front of them, they wouldn’t recognise it for what it was? Did they even know the patterns to match to know what was in front of them and it's significance?

Gareth Lock is the owner of The Human Diver, a niche company focused on educating and developing divers, instructors and related teams to be high-performing. If you'd like to deepen your diving experience, consider taking the online introduction course which will change your attitude towards diving because safety is your perception, visit the website.