Title: Minimal mechanism for decoding temporal frequencies in somatosensory cortex-modelling and experiment
Authors: Jing Kang, Hugh P. C. Robinson, Jianfeng Feng
Extracellular recording of single neurons in primary and secondary somatosensory cortices of monkeys in vivo has shown that neuronal output firing rate can increase, decrease, or remain constant in different cells as the frequency of the vibrotactile or intracortical electrical stimulation increases. This interesting observation is a challenge for modelling of neuronal coding. Here we show, using conductance injection in vitro in rat somatosensory cortex, that the same patterns can be observed experimentally, and that the type of response depended on the amplitude of the oscillatory input. We propose a simple principle to explain these observations using a leaky integrate-and-fire model with sinusoidal current injection and Poisson noise. We found that the output firing rate is constant versus input frequency when the threshold value is positioned above the limit cycle, defined by the oscillation of the input. When the threshold intersects with the limit cycle, the neuron fires at low frequency but is silenced at high frequency, making the output firing rate a decreasing trend, and enabling decoding of input frequency information. An increasing spiking rate versus input frequency happens when an intrinsic oscillation is included, making the neuron generate most spikes when the input and the intrinsic frequency resonate. An enhancement of the gain of encoding by local network connections and a comparison neuron of higher hierarchy order, were also introduced, aiming to illustrate the necessity of these three different output firing patterns. Our work shows a minimal mechanism for discrimination of temporal input frequency by reading out the firing rate alone, supported both by modeling and experiments.
(submitted to PloS One, under revision)
Title: Weber's law implies sub-Poisson neuronal discharge
Authors: Jing Kang, Jianhua Wu, Jianfeng Feng
Understanding the mechanism by which neural activity in cerebral cortex mediates the perceptual decision is a research topic of great interests in psychophysics. Weber’s law is one of the basic laws in psychophysics, and the link between the neuronal response and this psychophysical behavior has not been established. In this paper, we carried out an analysis on the neuronal discharge rate and the coefficient of variation of the interspike interval (CVISI) when Weber’s law holds. It is found that the mean and the standard deviation of neural firing rate must have a quasi-linear relationship which requires sub-Poisson efferent spike train. For an independent neuron ensemble, its population size is unrealistic restricted to be less than 10 for the validity of Weber’s law. However, if the population neurons share a weak correlation in their discharges, as widely reported in experimental data, and individual spike train is sub-Poisson, Weber’s law is true without any restriction on the population size.
(submitted to Euro. J. Neurosci, under revision )