Mental health

Injection for curing 'fish fears'

“Scientists have created fearless goldfish,” the Daily Mirror reported. The Daily Telegraph and_ Daily Mail_ also cover the same study, saying that an injection may be able to “cure phobias”. The Mirror reports that experts aim to use the method (an injection of local anaesthetic into the brain) “to help cure people of common phobias, such as fear of flying, of heights or of spiders”.

In this study, goldfish were trained to fear a green light by combining it with a mild electric shock. After training, the fishes’ hearts would slow down when the light switched on; an automatic response that indicates that the fish are afraid. The researchers found that if they injected lidocaine (a local anaesthetic) into a region at the back of the fishes’ brains before training, the fish did not develop this “fear response” to light.

This study tells us more about the biology of fear in fish than in humans. It certainly cannot tell us whether an injection of local anaesthetic into the brain could reduce phobias in humans, and this crude method is very unlikely to ever be used in humans.

Where did the story come from?

Masayuki Yoshida and Ruriko Hirano from Hiroshima University in Japan carried out this research. The study was funded by the Japan Society for the Promotion of Science. The study is currently in press and awaiting publication in the open access peer-reviewed journal Behavioral and Brain Functions.

The Daily Telegraph, Daily Mirror _ and Daily Mail_ all cover this story and report that the study was in goldfish. All of the papers suggest that this research could lead to treatments for human phobias, and one of the study authors is quoted in the Telegraph as saying, “Imagine if your fear of spiders, heights or flying could be cured with a simple injection - our research suggests that one day this could be a reality.”

It is not possible to say, based on the current study, whether an injection of lidocaine could be used to treat phobias in humans. The Mail reports that an injection of lidocaine an hour before the experiment stopped fear from developing, but this was not the case. Only a lidocaine injection just before the fear conditioning had this effect.

What kind of research was this?

This animal research investigated whether the cerebellum (an area at the back of the brain) in goldfish is involved in learning to fear an event (fear conditioning). The cerebellum is involved in fear conditioning in mammals, and the researchers thought that if they could show that it played a similar role in fish, the fish could be used as a model to study fear conditioning.

Studies in animal models that show similarities to humans are important in that they give us insight into human biology. However, differences between species mean that results obtained in animals may not be directly applicable to humans. For example, the researchers reported that goldfishes’ hearts slow down in response to fear, whereas the human heart speeds up. Also, some conditions can be difficult to replicate in animals. For example, although the fish in this study did show fear and develop fear conditioning, it is unlikely that this could be considered the direct equivalent of a human phobia.

What did the research involve?

The researchers took 30 goldfish and split them into three groups: a lidocaine group, a group that would receive only the solution used to dissolve the lidocaine (called the “vehicle”) and a control group receiving no injection. Injecting lidocaine into a region of the brain reduces the activity of that area.

All the fish were given fear conditioning while their heartbeat was monitored. The researchers began by shining a light into the fishes’ eyes 10 times (this method is called “habituation”). They then repeated this process 20 times, giving the fish a mild electrical shock at the same time (this is called “acquisition”). Finally, they shone the light into the fishes’ eyes a further 15 times without the shocks (this is called “extinction”). The lidocaine and vehicle groups were injected into the fishes’ cerebellum after the “habituation” part of the trial.

The researchers compared what happened to the fishes’ heartbeats in response to the light in the three groups in these different periods. They also tested whether a lidocaine injection into the cerebellum an hour before the start of the training procedure had the same effect.

What were the basic results?

The researchers found that an injection of lidocaine into the cerebellum had no effect on the fishes’ normal heart rate (that is, heart rate when they were not being shown the light). Fish injected with lidocaine shortly before being trained to fear the light showed less of a learned fear response to light than the control or vehicle groups, meaning that their hearts slowed less in response to light.

The researchers found that if they injected the fishes’ cerebella with lidocaine an hour before the fear conditioning experiment, this did not affect their fear learning.

How did the researchers interpret the results?

The researchers conclude that their results “further confirm the idea that the cerebellum in [goldfish], as in mammals, is critically involved in classical fear conditioning”.


This study aimed to look at whether the cerebellum is involved in fear conditioning in goldfish, as it is in mammals. The findings suggest that this does appear to be the case. This suggests that these fish could be used to study how fear conditioning develops at the level of the individual cells of the brain; something possibly unattainable in humans.

The study did not aim to determine whether lidocaine injections can reduce fear or phobias in humans, and cannot tell us whether this would be the case. It is unlikely that such a crude method would ever be used in humans. Greater understanding of how fear conditioning works may eventually suggest ways in which it could be manipulated in humans, but such an advance is a long way off.

NHS Attribution