Paper on ISI variability

[Boris Gutkin]

Dear Colleagues, we wanted to bring to your attention our recently
published paper on spike generating dynamics and ISI varibility in
cortical neurons, which is available in postscript from:

cnbc.cmu.edu in pub/user/gutkin the file is type1noise.ps

Cheers,
Boris Gutkin

_______________________________
Neural Computation 10(5)

Dynamics of membrane excitability determine inter-spike interval
variability:a link between spike generation mechanisms and cortical 
spike trainstatistics.

Boris S. Gutkin and G. Bard Ermentrout
Program in Neurobiology and Dept. of Mathematics
University of PIttsburgh
Pittsburgh PA

We propose a biophysical mechanism for the high interspike interval
variability observed in cortical spike trains. The key lies in the
non-linear dynamics of cortical spike generation which are consistent
with type I membranes where saddle-node dynamics underlie
excitability. [Rinzel '89]. We present a canonical model for type I
membranes, the $\theta$-neuron. The $\theta$-neuron is a phase model
whose dynamics reflect salient features of type I membranes. This
model generates highly var iable spike trains (coefficient of
variation (cv) above 0.6) when brought to firing by noisy inputs. This
happens because the timing of spikes for a type I excitable cell is
exquisitely sensitive to the amplitude of the supra-threshold stimulus
pulses. A noisy input current, giving random amplitude "kicks" to the
cell, evokes highly irregular firing across a wide range of firing
rates. On the other hand, an intrinsically oscillating cell gives
regular spike trains.  We corroborate the above results with
simulations of Morris-Lecar (M-L) neural model with random synaptic
inputs. Type I M-L yields high cv's. When this model is modified to
have type II dynamics (periodicity arises via a Hopf bifurcation), it
gives regular spike trains (cv below 0.3).  Our results suggest that
the high cv values such as those observed in cortical spike trains are
an intrinsic characteristic of type Imembrane driven to firing by 
"random" inputs. In contrast, neural oscillators or neurons 
exhibiting type II excitability shouldproduce regular spike trains.