LETTER Communicated by Christopher Moore Temporal Decoding by Phase-Locked Loops: Unique Features of Circuit-Level Implementations and Their Significance for Vibrissal Information Processing Miriam Zacksenhouse memz@tx.technion.ac.il Sensory-Motor Integration Laboratory, Technion Institute of Technology, Haifa, Israel Ehud Ahissar ehud.ahissar@weizmann.ac.il Department of Neurobiology, Weizmann Institute, Rehovot, Israel Rhythmic active touch, such as whisking, evokes a periodic reference spike train along which the timing of a novel stimulus, induced, for ex- ample, when the whiskers hit an external object, can be interpreted. Previ- ous work supports the hypothesis that the whisking-induced spike train entrains a neural implementation of a phase-locked loop (NPLL) in the vibrissal system. Here we extend this work and explore how the entrained NPLL decodes the delay of the novel, contact-induced stimulus and fa- cilitates object localization. We consider two implementations of NPLLs, which are based on a single neuron or a neural circuit, respectively, and evaluate the resulting temporal decoding capabilities. Depending on the structure of the NPLL, it can lock in either a phase- or co-phase-sensitive mode, which is sensitive to the timing of the input with respect to the beginning of either the current or the next cycle, respectively. The co- phase-sensitive mode is shown to be unique to circuit-based NPLLs. Concentrating on temporal decoding in the vibrissal system of rats, we conclude that both the nature of the information processing task and the response characteristics suggest that the computation is sensitive to the co-phase. Consequently, we suggest that the underlying thalamocortical loop should implement a circuit-based NPLL. 1 Introduction One of the major computational tasks facing the vibrissal somatosensory system is to determine the angle of the vibrissa on contact with an ex- ternal obstacle. The vibrissal system receives external sensory input from the trigeminal neurons whose response patterns include both whisking locked spikes and contact-induced spikes (Szwed, Bagdasarian, & Ahissar, 2003). The whisking locked spike train provides a periodic reference input at the whisking frequency. The contact-induced activity represents the Neural Computation 18, 1611–1636 (2006) C  2006 Massachusetts Institute of Technology