A low-level-only dip in tuning curves reveals the ear’s active frequency-enhancing processes

Evidence

The special dip indicated by plus signs in part (d) occurs at low levels only and is a consequence of the active processes discussed in the next section and which lead to much more pronounced frequency selectivity. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 224

Categorization

• QUESTION: Worth chasing: what exactly are the active processes — outer haircell electromotility and otoacoustic emissions — that sharpen frequency selectivity? Later sections presumably detail this.