Some neurons are more active than others, even when they are positioned right next to each other and are one and the same neuron type. Researchers from the Max Delbrück Center (MDC) and the NeuroCure Cluster of Excellence at Charité have discovered the cause for this phenomenon. They found that the more active neurons in the somatosensory area of the brain respond to a broader receptive field and theorise that they play a particularly important role in human sensory perception. The opensource study is published in the journal Neuron.
Billions of neurons process signals in the human brain. In the sensory part of the cerebral cortex, which is responsible for perceptions of the outside world, not all neurons are equally active; even neurons positioned directly next to each other can be differentially active. If there is input of a stimulus, some neurons respond more than their neighbours. Until now, the reason for this remained elusive. Are the more active neurons perhaps more strongly connected within the cortex? Or do they get more information from upstream areas of the brain?
To clarify this, the researchers stimulated the whiskers of mice and investigated how different neurons in the brain react. For this purpose, they measured the activity of two neurons simultaneously. The active cells are characterized by a high concentration of the protein cFos. Since this was coupled to the green fluorescent protein (GFP), the researchers were able to distinguish more active cells from less active ones.
First the team stimulated only one central whisker. Surprisingly, no differences showed up between the two neurons. However, if the researchers stimulated many whiskers at the same time with a short airpuff, the response of the GFP-labeled neuron was significantly earlier and larger. Apparently, the more active neurons are distinguished by the fact that they respond to a wider receptive field. But where does this information come from?
Before a person perceives a stimulus from their environment, it must pass through the thalamus in the brain. This area is therefore also called ‘the gateway to consciousness’. In mice, the signals from the whiskers are processed in two areas of the thalamus, the so-called ventral posteromedial nucleus (VPM) and the area of the posteromedial nucleus (POm). Using optogenetic stimulation, the team determined which of these two nuclei is responsible for the enhanced response of specific neurons. By means of light impulses in the brain, they could specifically activate the thalamic nuclei and thus selectively simulate a flow of information through one of the two nuclei.
If the scientists activated the VPM, both types of neurons showed an equally strong response. They behaved exactly as if only a single whisker was touched. This specific reaction is thus apparently mediated by the ventral posteromedial nucleus. The posteromedial nucleus, elicited, just like the stimulation of several whiskers, a contrasting stronger and faster response of the GFP-labeled neurons.
The posteromedial nucleus is known for covering a broad receptive field and for transmitting the signals to widely distributed areas in the cerebrum. According to the current study, the most active neurons in the somatosensory (touch-sensitive) cortex are characterized by the fact that they not only get specific information from the ventral posteromedial nucleus, but can also draw on the wide receptive field of the posteromedial nucleus.
This parallel processing of specific and large-scale stimulus information by separate groups of neurons could be a fundamental mechanism of sensory perception. The more active neurons may have a particularly important role in sensory perception, with the researchers stating that this needs more investigation.