14Dynamic Range and Electrode-to-Neuron Distance

TA narrow electrical window

Electrical dynamic range is the gap between a user's threshold (T-level) and most-comfortable upper level (C/M/MCL). Everyday sound spans ~100 dB acoustic, but this must be compressed into a narrow electrical range — often only ~20 dB once clinical units are converted — driving the compression mapping covered in Ch.16 Programming.[2017]

CWhy the range is narrow

The narrowness stems from the deaf ear's loss of spontaneous activity and hypersynchronised firing, which steepen loudness growth. Threshold definitions differ by manufacturer: Nucleus = detected 100% of the time, AB = detected with 50% accuracy, MED-EL = the highest level with NO response — a subtlety that matters when comparing maps.[2008]

TDistance shapes threshold

Electrode-to-neuron distance directly shapes threshold, loudness growth and selectivity: bringing contacts closer to the spiral-ganglion neurons (perimodiolar arrays such as the Contour Advance) lowers thresholds and improves selectivity because less current is wasted spreading through fluid.[2020]

CGrowth functions and distance

Evoked-potential growth functions (EABR/ECAP) are shallower and thresholds lower when contacts sit closer to neural elements; wider bipolar spacing and greater distance flatten growth and worsen resolution — the device-level basis for the objective-measure findings in Ch.23.[2003]

TClosing the loop with proximity

This closes the loop with Module 12: the perimodiolar efficiency argument is fundamentally a distance argument, and it explains why measured impedance, ECAP thresholds and dynamic range all shift with scalar position and modiolar proximity.

CEvery microampere counts

Because the achievable range is so narrow, every microampere of unnecessary current spread costs both selectivity and battery; reducing electrode-to-neuron distance is one of the few levers that improves efficiency and resolution simultaneously (cross-ref Module 11, Ch.16).