The scientists found that activity in S1 neurons, where touch information first arrives, correlated directly with the strength of the stimulus; when the strength of the vibrations was more intense, the S1 neurons' fired more rapidly. However, these neurons' activity did not correlate with the monkeys' behavioral responses. Their firing rates were directly associated with the stimulus intensity, whether the monkeys consciously felt and responded to the stimulus or not.
Romo and de Lafuente also recorded neuronal activity in the medial premotor cortex (MPC), a region of the brain's frontal lobe that is known to be involved in making decisions about sensory information. Activity here did mirror the monkeys' subjective responses to the vibrating probe. MPC neurons responded in an all-or-none manner; they fired when the monkey thought the vibrations were there--even if they weren't--and they didn't fire when the monkey thought the vibrations were absent--even if they were actually occurring.
These results indicate that the monkeys' perceptions arise not from brain activity in the sensory cortex itself, but from activity in the frontal lobe MPC, Romo said.
The MPC "is very interesting," Romo said. "Apparently, it's able to pull information from memory and from the sensory areas, and also link this activity to the motor apparatus" so that the monkeys can physically indicate what they think is happening.
To clinch the MPC's association with the monkeys' perceptions, the researchers used an electrode to apply weak electrical stimulation to MPC neurons. They found that stimulating these neurons made the monkeys more likely to respond that they perceived a vibration, whether the vibrating stimulus was occurring or not.
Romo and de Lafuente also found that MPC neurons began to fire before the stimulus even touched the mon
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Contact: Jennifer Donovan
donovanj@hhmi.org
301-215-8859
Howard Hughes Medical Institute
6-Nov-2005