The scientists' findings about how the brain regulates sensory inputs and motor outputs to accomplish those tasks are detailed in the June 13 issue of the journal Neuron. Their discovery could have a direct application to future efforts to help victims of stroke or spinal injury with "neuroprosthetics" that might move artificial limbs or other procedures that might lead to the reestablishment of motor control in stroke victims.
According to the researchers, how we filter out much of the of sensory inputs we're not interested in to focus on a specific smell, taste or sound is a consequence of the way the sensory and motor cortices of our brain are hardwired to handle sensory inputs. This subconscious mechanism enables us to immediately send motor signals to our eyes, ears, nose and hands to enhance our perception of the sensory information that we are interested in.
"The act of sensation is inexplicably tied to that of motor control," says David Kleinfeld, a professor of physics at UCSD and one of the leaders of the research collaboration. "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."
In their experiments, the scientists discovered the specific mechanisms by which sensory signals are converted into motor control signals. They relied on an eclectic blend of physical and psychological tools to probe the transformation of transient sensory inputs into a smooth motor control signal for the position of the tactile whiskers on the snouts of laboratory rats.
"A central theme in sensory perception is how movement influences sensor information processing," says Ford F. Ebner, a professor of psychology and cell biology at Vanderbilt and the other leader of the research team. "Our results show that the sensory cortex performs a complicated extraction of information about what the whiskers touch, while the motor cortex output 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."
The other researchers involved in the study, financed by the National Institute of Mental Health and the National Institute of Neurological Disease and Stroke, were Lynne M. Merchant of UCSD, Robert N. S. Sachdev of Vanderbilt, and Murray R. Jarvis of the California Institute of Technology.
The researchers used the rat whisker system, known to scientists as the "vibrissa sensorimotor system," because the normal motion of the whiskers, or vibrissa, is a relatively simple back and forth rhythmic movement. This allowed the scientists to apply a variety of experimental techniques originally developed to study speech and other sound waves. What they discovered was that the signal processing mechanisms between the sensory and motor cortex of the rats extracted only the fundamental part of the complex rhythmic sensory signals that entered the brain so that the rats could optimize their sensory perception.
"This is like the determination of pitch when members of an orchestra tune their instruments," says Kleinfeld. "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."
"Our results may have applications in both biomedicine and robotics," he adds. "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. This could one day allow researchers to formulate control signals for the reestablishment of motor control in stroke patients with sensory deficits. In addition, the signals we describe teach us more about the general issue of how biology might solve problems in engineering. Take bipedal walking for example. Humans can accomplish this easily, but machines fail at all but the most stereotypic versions of task. Experiments such as this one provide additional clues to the solution of some of these computational problems."
Comment: David Kleinfeld (858) 822-0342
Ford F. Ebner (615) 343-0239
Media Contact: Kim McDonald (858) 534-7572