Abstract
Populations of cortical neurons respond to common input within a millisecond. Morphological features and active ion channel properties were suggested to contribute to this astonishing processing speed. Here we report an exhaustive study of ultrafast population coding for varying axon initial segment (AIS) location, soma size, and axonal current properties. In particular, we studied their impact on two experimentally observed features 1) precise action potential timing, manifested in a wide-bandwidth dynamic gain, and 2) high-frequency boost under slowly fluctuating correlated input. While the density of axonal channels and their distance from the soma had a very small impact on bandwidth, it could be moderately improved by increasing soma size. When the voltage sensitivity of axonal currents was increased we observed ultrafast coding and high-frequency boost. We conclude that these computationally relevant features are strongly dependent on axonal ion channels' voltage sensitivity, but not their number or exact location. We point out that ion channel properties, unlike dendrite size, can undergo rapid physiological modification, suggesting that the temporal accuracy of neuronal population encoding could be dynamically regulated. Our results are in line with recent experimental findings in AIS pathologies and establish a framework to study structure-function relations in AIS molecular design.
Highlights
Humans, monkeys, and other mammals can perform complicated cognitive tasks that engage deep cortical hierarchies with processing times as brief a few hundred milliseconds [1, 2]
Experiments suggest that the ion channels at the axon initial segment strongly contribute to the ultra-fast response, but recent
100Hz, this additional damping effect was weak for xNa = 20μm, 40μm, 80μm. These results suggest that the dynamic gain values are enhanced for larger xNa because the larger axonal resistance reduces the lateral current while the input experiences only minor damping
Summary
Monkeys, and other mammals can perform complicated cognitive tasks that engage deep cortical hierarchies with processing times as brief a few hundred milliseconds [1, 2]. Numerous experimental studies tested different populations of cortical neurons for their capability to perform ultrafast population coding [3,4,5,6,7,8,9,10,11,12,13,14,15]. These studies used time domain and frequency domain analyses to assess the dynamics of the population’s mean firing rate in response to a shared input superimposed on a continuously fluctuating background, mimicking ongoing synaptic input. Confirming a seminal theoretical prediction [17], when the correlation time of the background input is increased, the dynamic gain in the high frequency regime is substantially boosted [7], a phenomenon previously dubbed the Brunel effect [7]
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