Spectral hyperacuity in the cat: neural response to frequency modulated tone pairs

When two tones are presented to the ear, distortion products are generated which are lower in frequency than the presented (primary) tones. We studied the responses of neurons from the inferior colliculus of the cat to small frequency modulations (FM) of primary tone combinations which gave rise to distortion products within the neuron's response area. Neural discharges were modulated in response to the FM of the distortion product in a similar manner to modulation of discharges by FM of a pure tone to which these neurons were sensitive. However, very shallow, neurally-insignificant FM of high-frequency primaries could be transposed into significant FM of lower frequency distortion products. Because the sensitivity of a low-frequency neuron to a transposed FM exceeds that of neurons sensitive to a single tone with the same FM, the effect is termed hyperacuity.