To communicate a message powerfully, well-timed pauses can be as important as the words, if not more so. In this, humans are not much different from other animals, it seems – even fish.
Researchers studying African electric fish called mormyrids (Brienomyrus brachyistius) have revealed an underlying neurological mechanism for the phenomenon, as reported in the journal Current Biology.
The upshot is that when you bombard someone with continuous stimuli – such as words – the response of sensory neurons starts getting weaker as they become accustomed to the stimuli, and they tune out. The effect of pausing helps to wake them up again.
How did electric fish show us this? These little marvels communicate with each other through electric signals with the same all-or-nothing spikes that are used by the nervous system. They also frequently pause when communicating with other fish.
Lead author Tsunehiko Kohashi, from Japan’s Nagoya University, had been running experiments on behaviourally relevant activity in the fishes’ electrical signals when he realised the effect of pauses hadn’t been studied.
He decided to investigate how pauses affect the responses of sensory neurons and followed up with an elegant series of behavioural experiments.
“Overall, the basic approach was to stimulate with two trains of pulses separated by a pause of variable duration, and measure the responses of neurons using electrophysiology, or measure behavioural responses,” explains senior author Bruce Carlson from Washington University, US.
When fish were housed with other fish, they tended to produce more frequent and longer pauses and high-frequency bursts of pulses that carried a large amount of information just after each pause, presumably to communicate better with fish around them.
“We found that sensory neurons in the midbrain became progressively less responsive to repeated stimuli, and that pauses of sufficient duration reset their sensitivity,” says Carlson.
“This was due to a form of synaptic plasticity called short-term synaptic depression, in which repeated activation of a synapse causes a progressive weakening of synaptic strength, leading to smaller responses in the postsynaptic neuron.”
They also found that pauses of sufficient duration manifested in fish behaviour, measured by their electrical discharge rate. This became weaker as the sensory neuron response died out, and recovered at a similar rate, suggesting a link between them.
“There is a remarkable parallel here between humans and our fish,” says Carlson.
Research with humans has shown that after pauses, our auditory system responds to words more strongly and we recognise words more effectively. We also tend to pause just before delivering information-rich content or want to have a big impact.
“The underlying mechanism for this effect that we discovered in our fish, recovery from short-term synaptic depression, is extremely common and is found throughout the animal kingdom,” Carlson explains.
“So we think it is highly likely that this mechanism is also responsible for this same phenomenon that occurs during vocal communication in humans.”
The findings are testament to the fascinating gems that can be found throughout nature, he adds. “This study is a great example of how novel insights of broad general significance can come from studies of species with unique, or distinctive, behaviours.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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