Z. Xu et al., LOGARITHMIC TIME-COURSE OF SENSORY ADAPTATION IN ELECTROSENSORY AFFERENT NERVE-FIBERS IN A WEAKLY ELECTRIC FISH, Journal of neurophysiology, 76(3), 1996, pp. 2020-2032
1. We recorded single unit activity from individual primary electrosen
sory afferent axons in the posterior branch of the anterior lateral li
ne nerve of gametoid weakly electric fish, Apteronotus leptorhynchus.
We analyzed the responses of P-type (probability-coding afferent fiber
s to externally applied amplitude step changes in the quasi-sinusoidal
transdermal potential established by the fish's own electric organ di
scharge (EOD). 2. In response to AM step increases in transdermal pote
ntial, the firing rate of P-type afferents exhibited an abrupt increas
e followed by an initially rapid and subsequently more gradual decay b
ack toward the baseline level. Afferent responses continued to adapt s
lowly throughout the duration of prolonged step stimuli lasting > 100
s. The time course of sensory adaptation was similar for all units tes
ted. 3. We introduce a new functional form for describing the time cou
rse of sensory adaptation in which the change in firing rate Delta r d
ecays logarithmically with time: Delta r(t) = A/[B In (t) + I]. This l
ogarithmic form accurately describes the adaptation time course of P-t
ype afferents over five decades in time, from milliseconds to hundreds
of seconds, with only two free parameters. Using a nonlinear least-sq
uares fitting technique, we obtained a mean value of the parameter B,
which characterizes the adaptation time course, of 0.149 +/- 0.028 (me
an +/- SD, n = 49). 4. We compare logarithmic fits with traditional mu
ltiexponential and power law forms and demonstrate that the logarithmi
c form yields a better characterization of P-type afferent responses.
This analysis helps explain the variability in previously reported ada
ptation time constants, which have ranged from 0.2 to 3.4 s, in gymnot
id P-type afferents. 5. We tested the linearity of P-type afferent res
ponses using positive and negative AM steps of varying amplitudes. Asi
de from nonlinearities associated with rectification (firing rates can
not be negative) and saturation (firing rates cannot exceed the EOD fr
equency), we found that P-type afferent responses scaled linearly with
stimulus amplitude. 6. Based on the observed linearity, we predict th
e frequency domain response characteristics of P-type afferents and fi
nd that the predicted gain and phase are in good agreement with experi
mental measurements using sinusoidal AM stimuli over a range of AM fre
quencies from 1 to 100 Hz. Thus the logarithmic parameterization of th
e step response appears to accurately capture the response dynamics of
P-type afferents over a wide range of behaviorally relevant AM freque
ncies. 7. We conclude that the temporal filtering properties of pyrami
dal cells in the medullary electrosensory nucleus, the electrosensory
lateral line lobe (ELL), need to be reevaluated in light of the logari
thmic adaptation time course in the periphery, and we discuss implicat
ions for the role of P-type afferents in driving a feedback gain contr
ol mechanism that regulates ELL pyramidal cell responsiveness.