More than 10 years after the discovery of the role of pyramidal neurons in the mental representation of space and location, a group of Brazilian and American researchers have made an important contribution to the understanding of the brain's navigation system by identifying a subtype of these neurons that can detect speed with high accuracy. An article about the study was published in the journal Scientific Reports. Excitatory neurons and inhibitory interneurons are located in the hippocampus, a bilateral brain structure in the temporal lobe that is associated with the formation of new memories, learning, emotions and, as recently discovered, spatial representation. Excitatory neurons activate other neurons, while inhibitory interneurons inhibit or block them. Together they can modulate signals in areas of the brain to avoid excessive activity associated with, for example, seizures. By analyzing video images of neural activity in mice, the group also noticed that inhibitory neurons responded differently to changes in the environment, such as rotation or size. They were more stable than excitatory neurons, which sometimes change position encoding in response to environmental changes, the study authors noted. This is the first study to confirm the function of inhibitory cells, which make up less than 10% of pyramidal neurons.  First, mice were injected with a virus containing instructions to infect the cells and cause them to produce a protein that becomes fluorescent in the presence of elevated calcium levels, which occurs when the cells are activated. A cylindrical lens was then placed over the hippocampus, and a miniature microscope was attached to the surface of the skull to document neuronal activity. During the experiments, the animals were placed in aviaries that rotated and changed size and shape. The scientists said the findings contribute to a better understanding of how humans track and retain information about trails and learn new places. The researchers noted that these latest findings point the way toward exploring difficulties in learning new routes and locations in people with Alzheimer's disease and other neurodegenerative diseases.