What is external stimulus

Reaction to external stimuli

Anyone who hears a fire alarm has only one reflex: to run away as quickly as possible. When our brain perceives external stimuli, it is excellent at predicting threats and making us flee. The neuroscientists Massimo Trusel and Manuel Mameli from the University of Lausanne have published a study in the journal Neuron in which they identify the neural circuits and mechanisms associated with this “learning of dangers”.

The survival instinct, which is based on our five senses, proves that we have learned to associate an external stimulus (alarm) with a potentially aversive event (danger to life). To explain this mechanism, the researchers carried out a five-day experiment with mice. They made a tone every day, followed by a small surge of electricity. With each day of the experiment, the mice understood better that they had to flee when the noise was heard in order to avoid the electrical surge. "During this learning process, the synapses (contact areas between the neurons) between the hypothalamus and the lateral habenula gradually improved," the researchers explain. In a next step, they tried to block this connection in the brain. The result: the mice learned less well.

With this finding, the researchers at the University of Lausanne hope to gain a better understanding of certain mental illnesses. For example, the habenula is known to be implicated in depression. Manuel Mameli emphasizes: "It has been observed that depression patients find it difficult to assess the associated risk potential when exposed to external stimuli and therefore have difficulties in behaving appropriately." The research group now wants to investigate more closely how depressed mice react when threatened and whether they can escape.