Cochlear Adaptation, Central Influences

Abstract

Cochlear adaptation can be divided into a peripheral and a central form; the latter influences the cochlea via the crossed and uncrossed olivary bundle. The adaptation can be studied by recording the AP generated in cochlea or cochlear nucleus by presenting clicks to the ear of the test animal. In order to differentiate between peripheral and central influence on cochlear adaptation, the cochlea is isolated from its central part of the auditory tract by ablation techniques. As long as the cochlea is connected to its central pathways the relationship between click intensity and AP magnitude is linear from threshold up till 30–40 dB above threshold, whence it levels off asymptotically. If, however, the cochlea is isolated, this relationship becomes linear. After cutting of the crossed olivo-cochlear bundle an influence is only found at threshold levels; the threshold is lowered, thus cochlear sensitivity is increased and the signal/noise ratio is diminished. Apparently the cochlea is not under any adaptational influence, either of central or of peripheral origin, in the intensity range from just above threshold to 30–40 dB.