Linear systems analysis of cutaneous Type I mechanoreceptors

Linear system transfer functions of Type I mechanoreceptor domes in hairy skin are extracted for study. The system input, defined in units of indentation depth, is a punctate, 85-Hz bandlimited colored noise indenting stimulus. The system output is a dome´s afferent action potential (AP) even stream. After digitally low-pass filtering the AP event stream to prevent aliasing, transfer and coherence function estimates are generated from spectral and cross spectral estimates of the sampled input and output signals. Transfer function magnitudes (receptor sensitivity) beyond 10 Hz are well fit by a fractional exponent (0.35+or-0.10) of frequency, indicating fractional order differentiation. Transfer function phases are near 120 degrees for frequencies less than 10 Hz and decrease progressively beyond 10 Hz. Coherence function estimates are low as a result of inherent nonlinearities, primarily rectification. Single dome results closely match previously reported results for the tactile spine of the cockroach, suggesting that, in terms of spectral sensitivity, the receptors have similar transduction mechanisms.