Most cameras are equipped with an auto-contrast feature that enables them to take high quality pictures in a wide range of lighting conditions. Auto-contrast works by increasing the sensitivity of the camera to light in dimly lit surroundings, but reducing it in bright conditions to ensure that images do not become saturated.

Our visual system is equipped with a similar feature. Neurons in the visual system increase or decrease their sensitivity to light as appropriate to enable us to see in both dimly lit rooms and dazzling sunshine.

This process, which is known as dynamic range adaptation, also occurs in neurons that are sensitive to sound or touch. Robert Rasmussen of the University of Pittsburgh, and his colleagues, wanted to know whether the same might hold true for neurons that encode non-sensory stimuli such as the direction of movement.

Would these neurons change their sensitivity to direction if presented with a wide range of possible directions instead of a narrow range? If so, this would suggest that dynamic range adaptation occurs throughout the nervous system.

Steven Chase, assistant professor in the Department of Biomedical Engineering and the Center for Neural Basis of Cognition, said:

“Our brain has an amazing ability to optimize its own information processing by changing how individual neurons represent the world. If we can understand this process as it applies to movements, we can design more precise neural prostheses. We can one day, for example, design robotic arms that more accurately implement a patient’s intended movement because we now better understand how our brain adjusts on a moment-by-moment basis when we are in motion."

Motor Neuron Sensitivity

To find out, Rasmussen et al. trained two rhesus macaque monkeys to use their brain activity to move a cursor on a virtual reality screen in either 2D or 3D. Studying this brain activity showed that neurons became less sensitive to the cursor’s direction of movement when the task switched from 2D to 3D.

This makes sense because in a 3D task, which also features depth, the neurons have a greater range of possible movement directions to encode. Conversely, the neurons became more sensitive to the direction of movement when the task switched from 3D to 2D.

Under these circumstances the neurons can use activity that was previously dedicated to encoding depth to instead represent the 2D space in finer detail. The results presented raise several additional questions.

Are the mechanisms that support dynamic range adaptation the same in sensory and motor neurons?

If these neurons also encode other aspects of movement, such as speed, would these also be included in the same range as direction or is the adaptation process segregated by specific parameter categories?

And how do these changes in sensitivity affect the movements that animals produce?

Robert G Rasmussen Andrew Schwartz Steven M Chase Dynamic range adaptation in primary motor cortical populations eLife 2017;6:e21409

Image: Carnegie Mellon University Materials Science and Engineering

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