When sensation becomes perception

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When sensation becomes perception

http://www.medicalnewstoday.com/medicalnews.php?newsid=33281

Perceiving a simple touch may depend as much on memory, attention,

and expectation as on the stimulus itself, according to new research

from Medical Institute (HHMI) international research

scholar Ranulfo Romo and his colleague Victor de Lafuente. The

scientists found that monkeys' perceptions of touch match brain

activity in the frontal lobe, an area that assimilates many types of

neural information.

Romo and de Lafuente, both of the Institute of Cellular Physiology at

the National Autonomous University of Mexico, report their results in

the December 2005 issue of the journal Nature Neuroscience, published

early online on November 6, 2005.

One of neuroscience's most difficult questions concerns how the brain

converts simple sensory inputs to complete perceptual experiences.

Many neuroscientists assume that perceptions arise in the sensory

cortices, which are the first areas of the brain to process

information coming in from sense organs, Romo said. Some recent

research, however, has hinted that activity in other parts of the

brain may also contribute to sensory perception.

When it comes to the sense of touch, a stimulus at the skin triggers

an impulse that travels first to an area at the top of the brain

called the primary somatosensory cortex (S1). The information then

moves to other parts of the brain, where it can contribute to memory,

decision-making, and motor outputs.

To explore what regions of the brain contribute to sensory

perception, Romo and de Lafuente analyzed neural activity associated

with the sense of touch in macaque monkeys. The researchers touched

the monkeys' fingertips with a painless stimulus that sometimes

vibrated and sometimes did not. The intensity of the vibration

varied, so sometimes it was easy for the monkeys to tell that the

vibration was on, while other times the vibrations were so weak that

the monkeys couldn't always detect them. The monkeys were trained to

indicate to the researchers whether the stimulus was vibrating or

still, and they were rewarded with treats when they were correct.

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 monkeys' fingertips. Romo believes this

is because the monkey is expecting the stimulus and the neurons fire

in anticipation.

" I think that we do not feel with our sensory cortices, " Romo said.

Perceptions instead arise in higher-order brain areas from a

combination of sensation, attention, and expectation. " The sensory

representation is [just] to confirm something that you have already

thought. "

Medical Institute

http://www.hhmi.org