Poster Session 1 - B17
1,2Dhekra Al-Basha, 1-3Milad Lankarany, 1-3Stephanie Ratté, 1-3Steven A. Prescott
1 Neurosciences and Mental Health, The Hospital for Sick Children, Toronto, ON, Canada; 2 Department of Physiology, University of Toronto, Toronto, ON, Canada; 3 Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada
Sensory information is represented by different neural responses, or spike patterns. Information carried by the spike count in a neural response is known as a rate code while information carried by the spike timing is called a temporal code. While some neurons may exclusively show rate or temporal coding, some neurons may exhibit both codes. The ability of neurons to use both rate and temporal codes simultaneously is known as multiplexing. Multiplexing enables neurons to increase their encoding capacity, thus becoming more efficient.
Based on previous theoretical work in the Prescott Lab, we had demonstrated that the rate of asynchronous spikes can encode the intensity of a slow signal while the timing of synchronous spikes simultaneously encodes abrupt changes in stimulus intensity. This suggests that a single set of neurons can represent distinct features of an external stimulus using differentially synchronized spikes. Here we tested in vivo whether this occurs in real neurons.
To this end, we recorded extracellularly from single units in the primary somatosensory cortex of sedated rats. Computer-controlled mechanical stimuli were applied to the whisker pad as discrete steps of increasing force. We asked whether we could decode (1) the force during the sustained phase of the step based on the rate of asynchronous spiking and (2) the timing of the onset and offset of the step based on the timing of synchronous spikes. Using responses from 17 neurons deemed to be responsive to our mechanosensory input, we constructed a firing rate histogram (FRH) in two ways. By constructing the FRH with a broad Gaussian kernel (500 ms), we found that the magnitude of the firing rate tracks the intensity of the force. By constructing the FRH with a narrow Gaussian kernel (5 ms), we found that abrupt changes in the force were reflected in abrupt increases in the firing rate caused by transient synchronization. Applying a synchrony threshold to the latter FRH yielded 86% sensitive and 100% specific detection of the stimulus transients.
These findings definitively demonstrate that synchrony-division multiplexing occurs in somatosensory cortex. Specifically, we have shown that stimulus intensity (a first-order stimulus feature) is encoded by the rate of asynchronous spiking while abrupt variations in stimulus intensity (a second-order stimulus feature) are simultaneously encoded by the timing of synchronous spikes. The results help reconcile apparently contradictory evidence for rate or temporal coding by showing how these coding strategies can operate together.