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1 Anatomy and Neurobiology & Program in Neuroscience, University of Maryland School of Medicine, Baltimore, Maryland, United States
2 National Eye Institute, National Institute of Health, United States
* To whom correspondence should be addressed. E-mail: akeller{at}umaryland.edu.
In all sensory systems, information is processed along several parallel streams. In the vibrissa-to-barrel cortex system, these include the lemniscal system, and the lesser-known paralemniscal system. The posterior medial nucleus (POm) is the thalamic structure associated with the latter pathway. Previous studies suggested that POm response latencies are positively correlated with stimulation frequency, and negatively correlated with response duration, providing a basis for a phase locked loop-temporal decoding of stimulus frequency (Ahissar et al. 2000). We tested this hypothesis by analyzing response latencies of POm neurons, in both awake and anesthetized rats, to vibrissae deflections at frequencies between 0.3 to 11 Hz. We found no significant, systematic correlation between stimulation frequency and the latency or duration of POm responses. We obtained similar findings from recording in awake rats, in rats under different anesthetics, and in anesthetized rats in which the reticular activating system was stimulated. These findings suggest that stimulus frequency is not reliably reflected in response latency of POm neurons. We also tested the hypothesis that POm neurons respond preferentially to sensor motion, that is, they respond to whisking in air, without contacts (Yu et al. 2006). We recorded from awake, head-restrained rats, while monitoring vibrissae movements. All POm neurons responded to passive whisker deflections, but none responded to non-contact whisking. Thus, like their counterparts in the trigeminal ganglion, POm neurons may not reliably encode whisking kinematics. These observations suggest that POm neurons might not faithfully encode vibrissae inputs to provide reliable information on vibrissae movements or contacts.
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