Limits of linear rate coding of dynamic stimuli by electroreceptor afferents
Daniel Gussin, Jan Benda & Leonard Maler
Journal of Neurophysiology 97: 2917-2929 (2007)
Abstract
We estimated the frequency-intensity (f-I) curves of P-unit
electrore- ceptors using 4-Hz random amplitude modulations (RAMs)
and using the covariance method (50-Hz RAMs). Both methods showed
that P-units are linear encoders of stimulus amplitude with
additive noise; the gain of the f-I curve was, on average, 0.32
and 2.38 spikes s-1 µV-1 for the low- and high-frequency cutoffs,
respectively. There were two sources of apparent noise in the
encoding process: the first was the variability of baseline P-unit
discharge and the second was the variation of receptor discharge
due to variability of the stimulus slope independent of its
intensity. The covariance method showed that a linear combination
of eigenvectors representing the time-weighted stimulus intensity
(E1) and its derivative (E2) could account for, on average, 92% of
the total response variability; E1 by itself accounted for 76% of
the variability. The low gain of the low-frequency f-I curve
implies that detection of small (1 µV) signals would require
integration over many receptors (1,200) and time (200 ms); even
then, signals that elicit behavioral responses could not be
detected using rate coding with the estimated gain and noise
levels. Weak signals at the limit of behavioral thresholds could
be detected if the animal were able to extract E1 from the
population of responding P units; we propose a tentative mechanism
for this operation although there is no evidence as to whether it
is actually implemented in the nervous system of these fish.
Last modified: Thu Feb 7 10:57:44 CET 2008