Journal of Neurophysiology, Vol 76, Issue 4 2608-2613, Copyright © 1996 by APS
Threshold for detecting temperature changes in a spider thermoreceptor
R. Ehn and H. Tichy
Institut fur Zoologie, Universitat Wien, Biologiezentrum, Vienna, Austria.
1. The threshold for detecting a change in temperature of a warm receptor
in the wandering spider Cupiennius salei was determined by means of its
frequency-dependent noise. To accomplish this, the warm receptor was
regarded as a linear system consisting of two components, an amplifier
(gain of the frequency response) and noise at its input added to the
temperature stimulus (input noise density). 2. The frequency response was
investigated with sinusoidal temperature modulations at frequencies between
0.05 and 12.8 Hz. The gain increased by 3.5 dB/octave in the frequency
range between 0.05 and 6.4 Hz, from 0.19 to 3.1 degrees C-1. However, at
the highest frequency, 12.8 Hz, the gain was reduced. 3. The noise density
of the warm receptor was measured by the root-mean-square noise amplitude
of the gain. The output noise density of the warm receptor, which describes
the noise density of the gain, was constant at approximately 0.2 Hz-0.5 in
the 0.05 to 6.4 Hz range, and increased at higher frequencies. The input
noise density, given by the ratio of output noise density to gain,
decreased by -2.7 dB/octave between 0.05 and 6.4 Hz, from 1.1 to 0.12
degrees C*Hz-0.5. 4. To define the threshold for detection of temperature
changes from the input noise density, the energy of the threshold was
equated to the energy of the noise. Assuming a signal-to-noise ratio of 1
and an upper limiting frequency of 10 Hz, the threshold estimated for the
wandering spider Cupiennius ranges from 0.6 to 0.08 degrees C, depending on
whether the inputs from only 1 or all 70 warm receptors of the 10 tarsal
organs are combined.
