Ad Aertsen
Neurobiology and Biophysics, Institute of
Biology III, Albert-Ludwigs-University
Sch\"{a}nzlestrasse 1, D-79104 Freiburg,
Germany.
E-mail: aertsen@biologie.uni-freiburg.de
Studies of cortical function on the basis
of multiple single-neuron recordings have
revealed neuronal interactions which depend
on stimulus and behavioral context. These
interactions exhibit dynamics on different
time scales, with time constants
down to the millisecond range. Mechanisms
underlying such dynamic organization of the
cortical network are investigated by experimental
and theoretical approaches. Our recent work
focuses on two interrelated aspects: {\it
precision} and {\it variability} of cortical
activity.
In a series of studies we investigated conditions
for the occurrence of precise joint- spiking
events in cortical activity [1]. Specifically,
we tested the hypothesis that precise synchronization
of action potentials among groups of neurons
is supported by
cortical network activity, in spite of the
fluctuating background [2]. Thus, we found
evidence [3] that volleys of precisely synchronized
spikes can propagate through the cortical
network in a stable fashion with a temporal
precision down to $\pm$1 ms,
consistent with experimental observations.
These findings suggest that a combinatorial
neural code, based on rapid associations
of groups of neurons co- ordinating their
activity at the single spike level, is biologically
feasible.
In a separate study we assessed the trial-by-trial
variability of spike trains from neurons
in the monkey primary motor cortex. We found
that Fano factors (ratio of variance and
mean of spike counts across trials) of motor
cortical discharges are widely distributed,
covering a range from 0.2 to 6, and in rare
cases reach even beyond [4]. More detailed
analyses taking into account the time-resolved
firing rate profiles [5] revealed that many
of the recorded neurons exhibit systematic
changes of variability throughout the trial.
Our findings show that the spiking process
of primary motor cortical neurons is not
captured by a rate-modulated Poisson or gamma
process. Instead, they indicate that a more
elaborate point process model, possibly with
time-dependent statistics, is required to
adequately describe motor cortical unit activity.
How the findings from these two approaches
can be reconciled into a single model of
cortical function is still an open question,
and the subject of current work.
1. Riehle A, Gr\"{u}n S, Diesmann M,
Aertsen A (1997) Spike synchronization and
rate modulation
differentially involved in motor cortical
function. Science 278:1950-1953
2. Arieli A, Sterkin A, Grinvald A, Aertsen
A (1996) Dynamics of ongoing activity: Explanation
of
the large variability in evoked cortical
responses. Science 273:1868-1871
3. Diesmann M, Gewaltig M-O, Aertsen A (1999)
Stable propagation of synchronous spiking
in cortical
neural networks. Nature 402: 529-533
4. Nawrot MP, Riehle A, Aertsen A, Rotter
S (2000) Spike count variability in motor
cortical neurons.
In: {\it Abstr Forum Europ Neurosci 2000}
(Brighton), Europ J Neurosci 12, Suppl 11,
p 506
5. Nawrot M, Aertsen A, Rotter S (1999) Single-trial
estimation of neuronal firing rates - From
single-ne
uron spike trains to population activity.
J Neurosci Meth 94: 82-92
Funded by DFG, GIF and HFSP. Further information
at http://www.brainworks.uni-freiburg.de