Where each pitch sits
Each hair cell sits where its pitch belongs. High pitches are picked up at the wide base of the spiral and low ones at the narrow tip, and every cell is placed by Greenwood's place–frequency function for the human cochlea.1
What lights up
How brightly a cell glows is the sound reaching it, from the live audio, multiplied by how healthy the profile says it is at that pitch. Where hair cells have been lost, they shrink to dim dots, outer rows first. For a conductive loss, every cell stays in place and the whole spiral dims evenly, because the middle ear turns sound down before it reaches them.
From 250 Hz to 8 kHz
The spiral runs the full length of the cochlea, which covers human hearing from about 20 Hz to 20 kHz.1 The hearing loss on it goes from 250 Hz to 8 kHz, because that's the range an audiogram tests. Beyond it, toward the tip and the base, the loss holds at its 250 Hz and 8 kHz values, because hearing doesn't return to normal just past the edge of the test. The close-up opens only between 250 Hz and 8 kHz for the same reason.
Sources
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D. D. Greenwood, "A cochlear frequency-position function for several species—29 years later," Journal of the Acoustical Society of America 87, no. 6 (1990): 2592–2605. ↩ ↩2