How Does the Human Brain Regulate Sensory Information?

Originally Published MDDI July 2002

R&D DIGEST

Physicists at the University of California, San Diego (UCSD) and psychologists at Vanderbilt University (Nashville, TN) have recently collaborated on studies to determine how the brains of higher animals—including humans—integrate sensory information and motor-control signals. It is through this process, for example, that humans can smell and identify a faint odor, or recognize one face in a crowd.

The researchers believe that their work could provide the foundation for future efforts to develop "neuroprosthetic" devices that might aid spinal-injury or stroke patients. Such systems might be used to move artificial limbs, for example. The studies could also facilitate development of procedures that might be used to reestablish motor control in stroke victims.

The researchers explain that the methods humans use to focus on a specific smell, taste, or sound by filtering out other sensory inputs are made possible by the manner in which the sensory and motor cortices of the brain handle sensory inputs. This subconscious mechanism enables motor signals to be directed immediately to the eyes, ears, nose, and hands, which are then used to enhance the individual's perception of the sensory information.

According to David Kleinfeld, UCSD professor of physics and one of the leaders of the research collaboration, "The act of sensation is inexplicably tied to that of motor control. If we spot a friend in the distance, our eyes move to track him or her. If we caress an object with our fingers, our hand moves to optimize the sense of texture. If a new odor permeates a room, we sniff to sample and identify the smell. In all of these processes, we separate the sensory input from the motor component that directs and defines the sensation."

The research encompassed novel experiments on rats to examine the specific mechanisms that are used to convert sensory signals into motor-control signals. The group used both physical and psychological tools to investigate the transformation of transient sensory inputs into a smooth motor-control signal for the position of the tactile whiskers on the rats' snouts. Use of these whiskers, known as the "vibrissa sensorimotor system," allowed the scientists to apply a variety of experimental techniques originally developed to study speech and other sound waves. They determined that the signal-processing mechanisms between the rats' sensory and motor cortex extracted only the fundamental part of the complex rhythmic sensory signals that entered the brain. In this way the rats could optimize their sensory perception.

Says Ford F. Ebner, professor of psychology and cell biology at Vanderbilt and the other leader of the research team, "A central theme in sensory perception is how movement influences sensor information processing." He adds that the results show that the sensory cortex performs a complicated extraction of information about what a rat's whiskers touch. At the same time, the motor cortex moves the whiskers to actively synchronize the rate at which the whiskers are being stimulated. "This is thought to be analogous to the fact that, during tactile object recognition, people require their fingers to be actively moved over surfaces of different roughness at a rate that is optimized by the motor cortex," says Ebner.

Kleinfeld explains that "this is like the determination of pitch when members of an orchestra tune their instruments. In the case of an orchestra, a fundamental 'C' note is extracted from a mixture of fundamental and harmonics produced by a bassoon. In the case of the vibrissa system, this fundamental frequency may serve to synchronize the motion of the vibrissa."

The researcher adds, "Our results may have applications in both biomedicine and robotics. As scientists and clinicians push to build neuroprosthetics to control artificial limbs as an aid to victims of stroke or spinal injury, it is essential to understand the nature of signal transformation by the nervous system." Kleinfeld emphasizes that, "Any attempt to build a prosthetic device for motor control, such as walking or hand movement in a spinalized patient, requires knowledge of the feedback signal used to fine tune the position on the limb." He adds, "Our results suggest the form of this feedback signal. This is one ingredient in an algorithm for the correct control of a neuroprosthetic."

Kleinfield also notes that recent work by Andrew Schwartz, research professor of bioengineering at Arizona State University (Tucson), supports the study results, suggesting that "the brain uses feedback to adapt its control of a prosthetic limb."

The group intends to continue its research efforts. Says Kleinfeld, "We are looking for the areas of the brain that function as adaptive filters—that is, filters that adapt to the incoming signal. These provide the substrate for our reported findings. So far, so good."

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