Wever and Lawrence revisited: effects of nulling basilar membrane movement on concomitant whole-nerve action...

It has been assumed for decades that mechanically stimulating hair cells, both inner and outer (IHC, OHC), leads to CM and subsequent neural activity. A test of that assumption was attempted in this experiment. Tone-pips of 300 msec duration at 4 or 5 kc/s with fast rise times were simultaneously presented to the cochleae of 10 chinchillas, through the external meatus and a hole drilled into the scala tympani. A round-window electrode allowed the recording of CM and computer-averaged whole-nerve action potentials (CAP). Stimulus levels and relative phase could be adjusted to yield CAPs of similar amplitude and shape to either stimulus alone. When the two stimuli were combined, the vectorial CM could be changed by about 30 db between maximum and minimum levels when delta phi was changed by 180 degrees. However, the combined CAP was relatively insensitive to delta phi. If basilar membrane motion was minimized at CM minimum, the data mean that some other principle than basilar membrane motion must underlie or generate neural activity. These data are not consistent with the traditional view that basilar membrane motion underlies sensitivity and frequency discrimination, and are congruent with theories of sensitivity of hair cells or their stereocilia to direct acoustic or electric stimulation, with basilar membrane mechanical stimulation assigned some secondary role. The author offers an electromodel comprising one system of basilar membrane motion of supramolecular dimensions leading to mechanical stimulation of OHCs and their large CM, and a second parallel system excited by the same stapes displacements but of submolecular dimensions leading to a propagated acoustic wave through the cochlear partition and to acoustic----electric transduction by the tectorial membrane; the output of that membrane is picked up in the fluids of the subtectorial space by the electro-sensitive IHCs and analyzed by them in some unknown manner for frequency. These IHCs are then the sole direct precursors of neural activity. A seeming anomaly was found in that at delta phi = CM minimum, when the traditional model would predict reduced basilar membrane movement, a reduced CM, consequent reduction in neural activity, and an increase in the latency of the N1 component of the CAP, latency was in fact slightly but uniformly decreased. It was suggested that in this phase condition the larger CM may have been correlated with the suppressive action of the OHCs upon the IHCs.