LEVEL-DEPENDENT REPRESENTATION OF STIMULUS FREQUENCY IN CAT PRIMARY AUDITORY-CORTEX

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that fie ld. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a sing le tonal frequency delivered to the contralateral ear. Distributions o btained at each of several stimulus levels were then compared to asses s the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extrac ellular recording methods. Stimuli were 40-ms tone pulses presented th rough calibrated, sealed stimulating systems. In each animal, the CF ( stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were dete rmined for isolated neurons (usually one per track) in 60-90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetiti ons of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we genera ted maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of t he stimulus. There was a clear spatial segregation of neurons accordin g to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm(2), seemed to contain only mono tonic or only nonmonotonic units. In three cortices, a patch of nonmon otonic cells was bounded ventrally by a patch of monotonic cells, and in one of these cases, a second patch of monotonic cells was found dor sal to the nonmonotonic patch. Contralateral tones of any given SPL ev oked excitatory responses in discontinuous cortical territories. At lo w SPLs (10, 20 dB), small foci of activity occurred along the isofrequ ency line representing the test frequency. Many of these cells had non monotonic response/level functions. At mid- and high SPLs, the CFs of neurons activated by a pure tone varied across 3 octaves. At the highe st SPL used (80 dB), most of the neurons with nonmonotonic response/le vel functions were inactive, or responded poorly; the active neurons w ere widely spread across the cortex, and the distribution of activity had a pattern bearing little relationship to the threshold CF contour map. These data indicate that only isolated patches of units within th e relevant isofrequency contour are activated by a given suprathreshol d contralateral tone. At suprathreshold stimulus levels, the region of cortex containing active patches extends widely beyond the threshold isofrequency contour region corresponding to the test stimulus frequen cy. The spatial representation of a stimulus delivered to the contrala teral ear appears, therefore, to be highly level dependent and discont inuous. These observations suggest that in the cat's AI, tonotopicity and isofrequency contours are abstractions which bear little resemblan ce to the spatial representation of tonal signals.