Quantitative analysis and two-dimensional reconstruction of the tonotopic organization of the auditory field L in the chick from 2-deoxyglucose data.

The tonotopic organization of the input layer L2 of the auditory neostriatum (field L) of chicks was analyzed using 2DG autoradiographs of serial transverse sections through this structure. In the experiments neonatal chicks were stimulated with pure tones in the frequency range between 0.1 and 2.5 kHz. The spacing of isofrequency contours, which are represented as stripes of 2DG labeling across the field, were measured on autoradiographs and on computer generated densitometric profiles. Positions of contours were determined along both the dorsolateral-ventromedial axis (tonotopic gradient) and along the rostrocaudal axis (parallel to the isofrequencies) of the field. Using these data a two-dimensional top view of the L2-layer and its tonotopic organization was reconstructed. The results demonstrate that isofrequency contours do not run parallel to each other but rather diverge from rostral to caudal. Whereas in the rostral part the spatial resolution is about 0.3 mm per octave this factor increases to 0.4 mm per octave towards the caudal border of the L2-layer. Regression analyses along the tonotopic gradient and mathematical extrapolations revealed that the tonotopic organization can be equally well described by logarithmic and power functions. The frequency range which is represented in L2 converges from rostral to caudal. In that way the rostral part of L2 is characterized by the representation of a wide frequency range with low spatial resolution, whereas in the caudal part a more confined frequency range with a higher spatial resolution is represented.