Greenwood frequency-position relationship in the primary auditory cortex in guinea pigs.

Although orderly representation of sound frequency over space is a hallmark feature of the primary auditory cortex (A1), the quantitative relationship between sound frequency and cortical position is unclear. We examined this relationship in the guinea pig A1 by presenting a series of stimulus tones with a wide frequency range, and recording the evoked cortical responses using an optical imaging technique with high spatial resolution. We identified the cortical positions of three best-frequency indices for each tone: the onset response position, the peak amplitude position, and the maximum rise rate position of the response. We found a nonlinear log frequency-position relationship for each of the three indices, and the frequency-position relationship was always well described by a Greenwood equation, with correlation coefficients greater than 0.98. The cortical magnification factor, measured in octave/mm, was found to be a function of frequency, i.e. not a constant. Our results are novel in that they demonstrate a quantitative relationship between sound frequency and cortical position in the guinea pig A1, as described by the Greenwood equation.