Dartmouth researchers have found the first direct evidence showing how the vestibular system’s horizontal canals play a key role in sensing our direction in the environment.
The findings, which appear in the Journal of Neuroscience, shed light on brain activity that helps us to get around and that is impaired by trauma, stroke and neurodegenerative diseases.
“Knowing what direction you’re facing, your location, how to navigate our environment and your spatial orientation at any given moment is fundamental to survival,” says Professor Jeffrey Taube, the study’s senior author.
The vestibular system consists of otolith organs and semicircular canals in each ear that provide sensory information about motion, balance and spatial orientation, allowing us to establish our location and direction and to navigate our environment. A number of cell types in the brain respond in relation to where you are (place cells) and your perceived directional heading (head direction cells, which fire when an animal faces a particular direction). A third cell type (grid cells) is activated at multiple places in the environment. Previous studies have shown that vestibular information is critical for generating the head direction signal but have not confirmed whether information from all three semicircular canals or just the horizontal canals, which are primarily sensitive to horizontal head rotation, are critical for the head direction signal.
To understand how head direction cells generate their activity, the Dartmouth researchers monitored brain cells in mice that have a mutant gene that prevents their vestibular system from developing properly. Specifically, it was a part of the vestibular system responsible for sensing how you are rotating or turning in the horizontal plane. They found that these mutant mice did not have normal head direction cells, which no longer fired in a directional manner.
“Our results suggest that the neural structure for the head direction network remains intact in mutant mice, but the absence of normal horizontal canals results in an inability to control the network properly and brings about an unstable head direction signal,” Taube says. “These findings are important for two reasons — they confirm previous theoretical views that this portion of the vestibular system is important for generating the head direction cell signal, and they show how the neural network, as a whole, functions in this brain area.”
Funding: The research was supported by the National Institute of Health.
Source: John Cramer – Dartmouth College
Image Source: The image is credited to Dartmouth College
Original Research: Abstract for “Head Direction Cell Activity Is Absent in Mice without the Horizontal Semicircular Canals” by Stephane Valerio and Jeffrey S. Taube in Journal of Neuroscience. Published online January 20 2016 doi:10.1523/JNEUROSCI.3790-14.2016
Head Direction Cell Activity Is Absent in Mice without the Horizontal Semicircular Canals
Head direction (HD) cells fire when an animal faces a particular direction in its environment, and they are thought to represent the neural correlate of the animal’s perceived spatial orientation. Previous studies have shown that vestibular information is critical for generating the HD signal but have not delineated whether information from all three semicircular canals or just the horizontal canals, which are primarily sensitive to angular head rotation in the horizontal (yaw) plane, are critical for the signal. Here, we monitored cell activity in the anterodorsal thalamus (ADN), an area known to contain HD cells, in epstatic circler (Ecl) mice, which have a bilateral malformation of the horizontal (lateral) semicircular canals. Ecl mice and their littermates that did not express the mutation (controls) were implanted with recording electrodes in the ADN. Results confirm the important role the horizontal canals play in forming the HD signal. Although normal HD cell activity (Raleigh’s r > 0.4) was recorded in control mice, no such activity was found in Ecl mice, although some cells had activity that was mildly modulated by HD (0.4 > r > 0.2). Importantly, we also observed activity in Ecl mice that was best characterized as bursty—a pattern of activity similar to an HD signal but without any preferred firing direction. These results suggest that the neural structure for the HD network remains intact in Ecl mice, but the absence of normal horizontal canals results in an inability to control the network properly and brings about an unstable HD signal.
SIGNIFICANCE STATEMENT Cells in the anterior dorsal thalamic nucleus normally fire in relation to the animal’s directional heading with respect to the environment—so-called head direction cells. To understand how these head direction cells generate their activity, we recorded single-unit activity from the anterior dorsal thalamus in transgenic mice that lack functional horizontal semicircular canals. We show that the neural network for the head direction signal remains intact in these mice, but that the absence of normal horizontal canals results in an inability to control the network properly and brings about an unstable head direction signal.
“Head Direction Cell Activity Is Absent in Mice without the Horizontal Semicircular Canals” by Stephane Valerio and Jeffrey S. Taube in Journal of Neuroscience. Published online January 20 2016 doi:10.1523/JNEUROSCI.3790-14.2016