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Thoughts difficult to understand, but HHMI scientists are making headway

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  • Dick
    AUGUST 01, 2005 Cracking the Perception Code The brain may interpret the information it receives from sensory neurons using a code more complicated than
    Message 1 of 1 , Aug 1, 2005
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      AUGUST 01, 2005
      Cracking the Perception Code
      The brain may interpret the information it receives from sensory
      neurons using a code more complicated than scientists previously
      thought, according to new research from the National Autonomous
      University of Mexico and Cold Spring Harbor Laboratory. By studying
      how monkeys perceive a vibrating object when it touches the skin,
      scientists found that changes in an animal's attention over time
      influence how a sensory signal is interpreted.
      Howard Hughes Medical Institute (HHMI) international research scholar
      Ranulfo Romo of the Institute of Cellular Physiology at the National
      Autonomous University of Mexico and his colleagues—Rogelio Luna and
      Adrián Hernández, also of the National Autonomous University of
      Mexico, and Carlos D. Brody of Cold Spring Harbor Laboratory in New
      York—report their results in the September 2005 issue of the journal
      Nature Neuroscience, published online July 31, 2005.

      "It's been tricky to determine which component of neuronal activity
      is more likely associated with behavioral performance."
      Ranulfo Romo
      Neuroscientists already knew that touching the skin with a vibrating
      object causes specialized sensory neurons in the brain to fire, and
      that firing of these neurons, which are found in a region of the
      brain known as the primary somatosensory cortex, is directly related
      to monkeys' ability to tell how fast something is vibrating, Romo
      said. But the neurons' firing patterns are complex, and it's been
      tricky to tease out "which component of the neuronal activity was
      more likely associated with behavioral performance," he explained.
      Theoretically, there are many ways in which neurons could relay
      information about stimulus frequency, Romo said. Frequency
      information might be encoded in the time between consecutive neuronal
      firings, the overall rate of firing, or the number of times a neuron
      fires.
      To distinguish among these possibilities, Romo and his colleagues
      designed an experiment in which they touched the monkeys' fingertips
      with a vibrating but painless probe for varying lengths of time. The
      monkeys were first taught to respond to varying vibration
      frequencies; in a training session, the scientists touched the
      monkeys twice in a row, with the probe vibrating at a different
      frequency each time. The monkeys signaled to the experimenters which
      stimulus was vibrating faster, and, when they were correct, they were
      rewarded with a treat.
      The standard stimulus that the scientists trained the monkeys to
      respond to lasted 500 milliseconds (half a second). They found that
      when they used a stimulus that lasted 750 milliseconds instead, the
      monkeys consistently thought the probe was vibrating with a higher
      frequency than it actually was. The same thing happened in reverse;
      if a stimulus was given for only 250 milliseconds, the monkeys
      thought it was vibrating at a lower frequency. The effect was
      stronger for the shortened stimulus than for the lengthened stimulus,
      Romo noted.
      Based on this experiment, it seemed most likely that the monkeys were
      determining the vibration frequency by the number of times the
      neurons fired, Romo said, since the firing rate and time between
      firings wouldn't change just because the stimulus duration changed.
      The scientists knew they hadn't quite cracked the neural code,
      though, because the magnitude effects weren't right; the monkeys
      thought that a stimulus that was 50 percent shorter was vibrating at
      just a slightly lower frequency than it was—not 50 percent lower.
      To find the cause of this discrepancy, they recorded electrical
      activity in single neurons of the primary somatosensory cortex.
      Since the shortened stimulus had produced a greater effect than the
      lengthened stimulus, the researchers wondered if the first part of
      the response might be more significant in determining vibration
      frequency.
      They explored two possible mechanisms of action: the neural firing
      response could adapt to the stimulus over time, making the neurons
      more sensitive at the beginning than at the end, or a perceptual
      process after neuronal firing could give more subjective weight to
      the beginning of the response.
      Looking at the electrical responses from single neurons, Romo and his
      colleagues determined that, if all the neuronal firings were treated
      equally, these responses could not explain the monkeys' perception of
      the signal. If the researchers assumed that the monkeys paid more
      attention to the beginning of the response, however, the neural
      activity perfectly explained the monkeys' errors when judging
      different durations of stimuli.
      Romo suggested that the best explanation for the behavioral data was
      to assume that the monkeys pay the most attention to the first 250
      milliseconds of neural firing, and that their attention falls off
      exponentially from there. The longer the stimulus, the less important
      additional neuronal firings become to the monkeys' perception of how
      fast the stimulus is vibrating, even though they continue to pay some
      attention throughout.
      Figuring out how the brain codes sensory information into neuronal
      firing and how the firing patterns are interpreted by perceptual
      areas of the brain is a huge challenge in neurophysiology, one that's
      often overlooked, said Romo.
      "The neuronal correlates reported in most of the neurophysiological
      studies in the different sensory modalities simply do not pay
      attention to this," he noted. "They assume that variation in firing
      rate is enough as a measure."
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