Boltzmann analysis of CM waveforms using virtual instrument software

We describe a modification to our technique for the rapid analysis of low-f requency cochlear microphonic (CM) waveforms in the basal turn of the guine a pig cochlea (Patuzzi and Moleirinho, 1998). The transfer curve relating i nstantaneous sound pressure in the ear canal to instantaneous receptor curr ent through the outer hair cells (OHCs) is determined from the distorted mi crophonic waveform generated in the extracellular fluid near the hair cells , assuming a first-order Boltzmann activation curve. Previously, the analys is was done in real time using custom-built electronic circuitry. Here, the same task is performed numerically using virtual instrument software (Nati onal Instruments LabVIEW 4.1) running on a personal computer. The assumed t heoretical function describing the CM waveform is V-cm = V-off + V-sat/{1 exp[(E-o+Z.P-o.sin(2 pi(f)+(tot))}/kT]}, where the six parameters are (i) a DC offset voltage (V-off); (ii) the frequency of the sinusoidal stimulus ( f); (iii) the phase of the sinusoidal stimulus (Z); (iv) the maximal amplit ude of the distorted microphonic signal (V-sat); (v) the sensitivity of the transduction process (Z); and (vi) the operating point on the sigmoidal tr ansfer curve (E-o). The software obtains the least-squares fit to the CM wa veforms by continuously deriving the six parameters at a speed of about one determination per second. The independent fitting of the frequency and pha se allows the data to be analysed off-line from data previously recorded to tape (i.e. the frequency and phase of the microphonic response need not be known accurately beforehand). We present here an outline of the software w e have used, and give an example of the changes which can be monitored usin g the technique (transient asphyxia). The method's advantages and limitatio ns have been discussed in our previous paper. The virtual instrument descri bed here is available from the authors on request. (C) 1999 Elsevier Scienc e B.V. All rights reserved.