AZIMUTHAL SENSITIVITY OF NEURONS IN PRIMARY AUDITORY-CORTEX OF CATS .1. TYPES OF SENSITIVITY AND THE EFFECTS OF VARIATIONS IN STIMULUS PARAMETERS

Preliminary to studying the organization of azimuthal sensitivity of neurons along frequency-band strips in the primary auditory cortex (AI) of cat (see companion paper), this study examined the sensitivity of 251 units in cat AI to variations in the azimuthal location of sound sources in the frontal hemifield. Most units (231) were tested with tones at the characteristic frequency (CF; frequency to which the unit had the lowest threshold). Unit CFs ranged from 5 to 36 kHz. A large number of units (91) were tested with broadband noise stimuli, and a few units were also tested at other frequencies within the cell's tuning response area. When tested at stimulus intensities 20-30 dB above CF or noise threshold, the different forms of azimuthal sensitivity exhibited by AI neurons could be divided into (1) contra-field azimuth functions; (2) ipsi-field functions; (3) central-field functions; (4) omnidirectional functions, and (5) multipeaked functions. Contra-field azimuth functions were the most prevalent, with 45.9% of units tested with CF tones and 42.9% of units tested with noise exhibiting this type of azimuthal sensitivity. Ipsi-field azimuthal sensitivity was found in 16.9% of units tested with CF tones and 19.8% of units tested with noise. Central-field azimuthal sensitivity was seen in 10.8% of units tested with CF tones and 17.6% of units tested with noise. Omnidirectional azimuthal sensitivity was seen in 19.9% of units tested with CF tones and 17.6% of units tested with noise, whereas multipeaked azimuthal sensitivity was found in 6.5% of units tested with CF tones and 5.5% of units tested with noise. The effects of increasing stimulus intensity on azimuthal sensitivity were examined in 185 units tested with CF tones and 67 units tested with noise. For four major classes of azimuthal sensitivity (contra-field, ipsi-field, central-field and omnidirectional), the most common effect (~60% of each class) was for the azimuth function to remain constant in form by the defining criteria for these classes. The next most common effect for all classes except omnidirectional azimuth functions was for an expansion of the azimuthal range eliciting responses. (The definition of omnidirectionality precluded any expansion of the response range in this class of azimuth function.) A smaller number of units in some classes showed a compression of the azimuth function to a smaller response range, and others showed more complex expansive and compressive effects with increasing stimulus intensity. In 30 units tested with CF tones and 12 units tested with noise, azimuth functions were also recorded at near-threshold intensities (within ~10 dB of stimulus threshold). At these intensities azimuthal response ranges were restricted. When recorded from several units of similar CF in the one animal, these restricted response ranges were located in the same region of space, consistent with the notion that they were produced by the pinna's acoustic axis. With increasing stimulus intensity the azimuthal response range always expanded, with the type and extent of expansion varying between units. These pinna-axiality effects were not seen in the small number of central-field units tested at near-threshold intensities. Various incidental observations were also made on the forms of azimuthal sensitivity displayed by AI units. 1) Some contra-field and ipsi-field azimuth functions to CF tones at ~12 kHz, as well as occasionally at other higher frequencies, showed secondary response foci at ipsilateral azimuths close to the midline. These effects were explicable on the basis of the interaural intensity difference (IID)-azimuth relationship in cat at frequencies ~12 kHz for speaker displacements away from the midline. 2) Six units showed equal threshold sensitivity to two widely separated frequencies. Azimuth functions were obtained at the two frequencies in each of the units, and only one unit was found to display different types of azimuthal sensitivity at the two frequencies to which it was equally sensitive. The effect of spectral content on azimuth functions was examined in detail in 71 units in which azimuth functions were obtained to CF tones and to noise stimuli. For most units (~75%) azimuth functions to both stimuli were similar. In ~17% of the sample, noise stimuli produced more restricted and constant azimuth functions than did CF tones, whereas the reverse was true in ~8.5% of the sample. Thus, in general, the azimuthal sensitivity of AI units in cat appears either to be shaped solely by the input in or around the CF, or to be of the same form at CF and the other frequencies within the neuron's tuning response area.