Low-frequency neural and cochlear-microphonic tuning curves in the gerbil.

Average tuning curves of single auditory-nerve fibers are compared with average cochlear-microphonic (CM) tuning curves corrected for electrical filtering of the cochlea. Both the neural and CM data were obtained from Mongolian gerbils (Meriones unguiculatus) with the same acoustic system and similar corrections for middle-ear effects. Under these conditions the CM tuning in the second and third cochlear turns is similar to the tuning of fibers whose characteristic frequencies (CFs) correspond to the CM best frequencies (2.5 and 0.5 dHz). Thus, little sharpening seems to take place for low CF fibers. CM tuning at the most apical electrode position is sharper than expected for frequencies below the best frequency--a result that may be due to the shunting effect of the helicotrema at low frequencies- Previous modeling results have confirmed that apical basilar-membrane tuning may be appreciable affected by the mechanical impedance of the helicotrema. This helicotrema effect may account for the nearly symmetrical shapes of neural tuning curves of low-CF fibers.