TOPOGRAPHIC REPRESENTATION OF TONE INTENSITY ALONG THE ISOFREQUENCY AXIS OF CAT PRIMARY AUDITORY-CORTEX

The sound pressure level (SPL), henceforth termed intensity, of acoust ic signals is encoded in the central auditory system by neurons with d ifferent forms of intensity sensitivity. However, knowledge about the topographic organization of neurons with these different properties an d hence about the spatial representation of intensity, especially at h igher levels of the auditory pathway, is limited. Here we show that in the tonotopically organized primary auditory cortex (AI) of the cat t here are orderly topographic organizations, along the isofrequency axi s, of several neuronal properties related to the coding of the intensi ty of tones, viz. minimum threshold, dynamic range, best SPL, and non- monotonicity of spike count - intensity functions to tones of characte ristic frequency (CF). Minimum threshold, dynamic range, and best SPL are correlated and alter periodically along isofrequency strips. The s teepness of the high-intensity descending slope of spike count - inten sity functions also varies systematically, with steepest slopes occurr ing in the regions along an isofrequency strip where low thresholds, n arrow dynamic ranges and low best SPLs are found. As a consequence, CF -tones of various intensities are represented by orderly and, for most intensities, periodic, spatial patterns of distributed neuronal activ ity along an isofrequency strip. For low - to -moderate intensities, t he mean relative activity along the entire isofrequency strip increase s rapidly with intensity, with the spatial pattern of activity remaini ng quite constant along the strip. At higher intensities, however, the mean relative activity along the strip remains fairly constant with c hanges in intensity, but the spatial patterns change markedly. As a co nsequence of these effects, low- and high-intensity tones are represen ted by complementary distributions of activity alternating along an is ofrequency strip. We conclude that in AI tone intensity is represented by two complementary modes, viz. discharge rate and place. Furthermor e, the magnitude of the overall changes in the representation of tone intensity in Al appears to be closely related to psychophysical measur es of loudness and of intensity discrimination.