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 subjec
Source:Howard Hughes Medical Institute