No abstract

The rate of longitudinal flow of fluid in scala tympani (ST) has been quantified under a number of experimental conditions. The method used to measure flow involved using a tracer ion (trimethylphenylammonium: TMPA) as a volume flow marker. Movement of marked perilymph was monitored by ion-selective microelectrodes which were capable of detecting exceedingly low concentrations of TMPA. Our results show that when the cochlea is perforated at the apex, flow rates of 400-500 nl/min are induced in ST, compared to the normal very slow rate of 2 nl/min when the cochlea is sealed. This artifactual flow of CSF through the perforated cochlea can be reduced to 6.9 nl/min by releasing the hydrostatic pressure of cerebrospinal fluid (CSF) or further reduced to 1.8 nl/min by surgically obstructing the cochlear aqueduct. In addition, we observed no basally-directed flow in ST when the round window (RW) was perforated, demonstrating that perilymph is not produced in volume as previously assumed. This study demonstrates the importance of separating artifactual flows, induced by the experimental procedures required to access the cochlear fluids, from the low flow rates which occur in normal, physiologic conditions.

No abstract

No abstract

It is well documented that the amplitude of an averaged cochlear action potential decreases as the rate of stimulus presentation is increased. In the present study, frequency regions contributing to the AP elicited by tone burst stimuli at rates from 4 to 80/s have been investigated in guinea pigs using a tone masking procedure. Derived responses corresponding to specific regions were obtained by subtracting AP responses in the presence of a low-level continuous masking tone from those in the absence of masker. This is a modification of the technique described by Hood et al. [J. Acoust. Soc. Am. Suppl. 1 81, S8 (1987)]. When stimulation rate is increased, the derived response amplitudes decreased near the probe frequency region, while amplitudes at outlying frequencies were less markedly changed. These data suggest the contribution to the AP from the region corresponding to the probe frequency becomes less at high stimulation rates. With an 80/s stimulation rate the contribution from this region is negligible. The degradation of frequency specificity of responses is presumed to arise from each stimulus acting as a “forward masker” for the following stimulus. To optimize the frequency specificity of AP responses to tone burst stimuli, the rate of presentation must therefore be considered. [Work supported by NIH.]