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The Journal of Neuroscience, October 24, 2007, 27(43):11687-11699; doi:10.1523/JNEUROSCI.1486-07.2007

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Behavioral/Systems/Cognitive
The Neural Coding of Stimulus Intensity: Linking the Population Response of Mechanoreceptive Afferents with Psychophysical Behavior

Michael A. Muniak,¹ Supratim Ray,² Steven S. Hsiao,^1,3 J. Frank Dammann,³ and Sliman J. Bensmaia^1,3

Departments of ¹Neuroscience and ²Biomedical Engineering and ³Krieger Mind/Brain Institute, Johns Hopkins University, Baltimore, Maryland 21218

Correspondence should be addressed to Sliman J. Bensmaia, Krieger MindBrain Institute, Johns Hopkins University, Baltimore, MD 21218. Email: sliman{at}jhu.edu

How specific aspects of a stimulus are encoded at different stages of neural processing is a critical question in sensory neuroscience. In the present study, we investigated the neural code underlying the perception of stimulus intensity in the somatosensory system. We first characterized the responses of SA1 (slowly adapting type 1), RA (rapidly adapting), and PC (Pacinian) afferents of macaque monkeys to sinusoidal, diharmonic, and bandpass noise stimuli. We then had human subjects rate the perceived intensity of a subset of these stimuli. On the basis of these neurophysiological and psychophysical measurements, we evaluated a series of hypotheses about which aspect(s) of the neural activity evoked at the somatosensory periphery account for perception. We evaluated three types of neural codes. The first consisted of population codes based on the firing rate of neurons located directly under the probe. The second included population codes based on the firing rate of the entire population of active neurons. The third included codes based on the number of active afferents. We found that the response evoked in the localized population is logarithmic with stimulus amplitude (given a constant frequency composition), whereas the population response across all neurons is linear with stimulus amplitude. We conclude that stimulus intensity is best accounted for by the firing rate evoked in afferents located under or near the locus of stimulation, weighted by afferent type.

Key words: neural coding; neurophysiology; afferent; population coding; psychophysics; somatosensory; touch; vibration

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Received April 2, 2007; revised Aug. 2, 2007; accepted Sept. 9, 2007.

Correspondence should be addressed to Sliman J. Bensmaia, Krieger MindBrain Institute, Johns Hopkins University, Baltimore, MD 21218. Email: sliman{at}jhu.edu

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