Dp. Phillips et al., LEVEL-DEPENDENT REPRESENTATION OF STIMULUS FREQUENCY IN CAT PRIMARY AUDITORY-CORTEX, Experimental Brain Research, 102(2), 1994, pp. 210-226
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.