5.5 Dynamic range and fiber types

Each fiber’s instantaneous rate is bounded. Spontaneous rates range from 0.1 to about 100 spikes/sec; saturating rates at high stimulus levels are typically 200–300 spikes/sec. A single fiber therefore has a dynamic range of about 30 dB — between threshold and saturation. But the auditory system as a whole has a dynamic range of 120 dB (from the quietest detectable sound to the loudest tolerable). The factor-of-1000 gap is bridged by fiber heterogeneity.

Spiral-ganglion neurons fall into three populations distinguished by their spontaneous rate:

• High-spontaneous-rate fibers (about 60% of the total): high spontaneous activity (~60 spikes/s), low threshold (~5 dB SPL above human absolute threshold), saturate at moderate levels.
• Medium-spontaneous-rate fibers (about 20%): intermediate properties.
• Low-spontaneous-rate fibers (about 20%): nearly silent at rest, high threshold (~30 dB above the high-spont fibers’ threshold), do not saturate until very high levels.

The three populations tile the dynamic range. At quiet levels, the high-spont fibers are doing the encoding; at loud levels, those fibers are saturated and the low-spont fibers take over.

A clinical note worth flagging: the low-spont fibers are the ones most vulnerable to noise-induced damage. They synapse onto IHCs through synapses that are preferentially lost in noise exposure, even when hair cells themselves survive. This is cochlear synaptopathy, sometimes called “hidden hearing loss” because standard audiograms (which measure threshold, dominated by high-spont fibers) do not show it. A person with synaptopathy may have a normal audiogram but struggle to follow speech in noise — the high-level dynamic range that the low-spont fibers carry has been compressed. This is one of the most active areas of hearing research today.