March 9, 2010 – Mind Hacks

How cannabis makes thoughts tumble

Cannabis smokers often report that when stoned, their thoughts have a free-wheeling quality and concepts seem connected in unusual and playful ways. A study just published online in Psychiatry Research suggests that this effect may be due to the drug causing ‘fast and loose’ patterns of spreading activity in memory, something known as ‘hyper-priming’.

Priming is a well studied effect in psychology where encountering one concept makes related concepts more easily accessible. For example, classic experiments show that if you see the word ‘bird’ you will react more quickly to words like ‘wing’ and ‘fly’ than words like ‘apple’ and ‘can’ because the former words are more closely related in meaning than the latter.

In fact, it has been shown that the more closely related the word, the quicker we react to it, demonstrating a kind of ‘mental distance’ between concepts. Think of it like dropping a stone into a pool of mental concepts. The ripples cause activity that reduces in strength as it moves away from the central idea.

‘Hyper-priming’ is an effect where priming happens for concepts at a much greater distance than normal. For example, the word ‘bird’ might speed up reaction times to the the word ‘aeroplane’. To return to our analogy, the ripples are much stronger and spread further than normal.

The effect has been reported, albeit inconsistently, in people with schizophrenia and some have suggested it might explain why affected people can sometimes make false or unlikely connections or have disjointed thoughts.

As cannabis has been linked to a slight increased risk for psychosis, and certainly causes smokers to have freewheeling thoughts, the researchers decided to test whether stoned participants would show the ‘hyper-priming’ effect.

The experiment used a classic ‘lexical decision task‘ where the volunteers are shown an initial word (‘time’) and then after a short gap are shown a nonsense word (‘yipt’) and a true word (‘date’) at the same time and have to indicate as quickly as possible which is the real world.

The experimenters altered how related the initial word and true word were to test for the semantic distance effect, and also varied the gap between the initial word and the test to see how long the priming effect might last.

Volunteers who were under the influence of cannabis showed a definite ‘hyper-priming’ tendency where distant concepts were reacted to more quickly. Interestingly, they also showed some of this tendency when straight and sober .

Cannabis also had the effect of causing temporary psychosis-like distortions as would be expected from a psychedelic drug, but the smokers did not make more errors and were not more likely to report psychosis-like symptoms when sober, suggesting the effect was not due to general mental impairment and couldn’t be explained by underlying tendency to mental distortion.

Although the debate is not completely settled, there is now good evidence that cannabis causes a small increased risk for developing schizophrenia particularly when smokers start young. In fact, additional evidence on this front was published only this week.

The researchers discuss the possibility that long-term smokers who spend a lot of time in a chronic ‘hyper-primed’ state might make psychosis more likely by loosening the boundaries of well-grounded thought, although exactly how cannabis raises the risk of psychosis, and indeed, how exactly it affects the brain, is still not understood well-enough to make a firm judgement.

Link to PubMed entry for cannabis ‘hyper-priming’ study.

Tracking the unborn brain into childhood

A brain scanning technology called MEG is being used to track the function of unborn babies’ brains as they grow inside the womb until after they’ve been born.

The full name for MEG is magnetoencephalography and it works by reading the magnetic fields created by the electrical signalling in the brain.

One of the advantages is that it can be used at various angles, doesn’t require the person to be in a cramped space, and is less sensitive to movement, so is ideally suited to scanning babies.

This includes unborn babies and with a bit of modification, as illustrated in the picture, researchers can pick up signals from the fetal brain in response to flashes or light or sounds.

We discussed the use of fMRI to scan the fetal brain previously, but this is a remarkable study that scanned the brains of babies inside the womb, every two weeks from week 27 until delivery, and then once after they were born.

Clearly, unborn babies are not the best at doing tasks set by experimenters, but there are various tests that just require the individual to experience changes in what’s presented to them.

One is called the auditory oddball task, where a series of tones are played that can either be similar (‘beep beep beep’) or can have include an ‘oddball’ (‘beep beep boop’). The brain is very good at picking out differences and the oddball is known to reliably trigger brain signals related to detecting changes.

This was the exact task used with the babies and the researchers looked to see if they could pick out a brain reaction to the ‘oddball’.

They found that they could detect this response 83% of the time in unborn babies, and that the reaction to the ‘oddball’ increased in speed throughout pregnancy. The newly born babies showed the response every time without fail.

This is an impressive finding as it shows how the brain development of the unborn child can be tracked over time with a brain scanner.

In a recent review article that discusses the development of this technology, the same group of researchers suggest that these and similar techniques could help track how different conditions in the mother affect the developing brain and even how the brain begins to develop its understanding of speech sounds before birth.

Link to PubMed entry for MEG study of developing fetus.
Link to PubMed entry for review article on fetal MEG.

A man with virtually no serotonin or dopamine

Neuroskeptic covers a fascinating case of a man born with a genetic mutation meaning he had a severe lifelong deficiency of both serotonin and dopamine.

The case report concerns a gentleman with sepiapterin reductase deficiency, a genetic condition which prevents the production of the enzyme sepiapterin reductase which is essential in the synthesis of both dopamine and serotonin.

The most widely recognised symptoms of the condition, linked to the deficiency in dopamine which has an important role in controlling movement, are problems coordinating both conscious movements and the unconscious control of muscles that allows simple actions. Unconscious control requires that the brain signals one muscle to contract while releasing the complementary muscle, and problems with this process cause spasticity.

The effects the condition on serotonin, often stereotyped as the ‘happy chemical’, are less well known, but in this case it was clear that the patient wasn’t depressed but did some other difficulties:

These included increased appetite – he ate constantly, and was moderately obese – mild cognitive impairment, and disrupted sleep:

“The patient reported sleep problems since childhood. He would sleep 1 or 2 times every day since childhood and was awake during more than 2 hours most nights since adolescence. At the time of the first interview, the night sleep was irregular with a sleep onset at 22:00 and offset between 02:00 and 03:00. He often needed 1 or 2 spontaneous, long (2- to 5-h) naps during the daytime.”

After doctors did a genetic test and diagnosed SRD, they treated him with 5HTP, a precursor to serotonin. The patient’s sleep cycle immediately normalized, his appetite was reduced and his concentration and cognitive function improved (although that may have been because he was less tired)…

Overall, though, the biggest finding here was a non-finding: this patient wasn’t depressed, despite having much reduced serotonin levels. This is further evidence that serotonin isn’t the “happy chemical” in any simple sense.

This is another piece of evidence against the common myth that depression is “caused by low serotonin” although Neuroskeptic speculates whether the link between disrupted sleep and depression may indicate an effect of serotonin dysfnction.

Link to Neuroskeptic on ‘Life Without Serotonin’.
Link to summary of scientific paper.