Frequency and space representation in the primary auditory cortex of the frequency modulating batEptesicus fuscus

Frequency and space representation in the auditory cortex of the big brown bat,Eptesicus fuscus, were studied by recording responses of 223 neurons to acoustic stimuli presented in the bat's frontal auditory space.

The majority of the auditory cortical neurons were recorded at a depth of less than 500 urn with a response latency between 8 and 20 ms (Fig. 1 B, C). They generally discharged phasically and had nonmonotonic intensity-rate functions (Fig. 3). The minimum threshold, (MT) of these neurons was between 8 and 82 dB sound pressure level (SPL). Half of the cortical neurons showed spontaneous activity. All 55 threshold curves are Vshaped and can be described as broad, intermediate, or narrow (Fig. 4A).

Auditory cortical neurons are tonotopically organized along the anteroposterior axis of the auditory cortex. High-frequency-sensitive neurons are located anteriorly and low-frequency-sensitive neurons posteriorly (Figs. 5, 6). An overwhelming majority of neurons were sensitive to a frequency range between 30 and 75 kHz (Fig. 1 A).

When a sound was delivered from the response center of a neuron on the bat's frontal auditory space, the neuron had its lowest MT. When the stimulus amplitude was increased above the MT, the neuron responded to sound delivered within a defined spatial area. The response center was not always at the geometric center of the spatial response area. The latter also expanded with stimulus amplitude (Fig. 8). High-frequency-sensitive neurons tended to have smaller spatial response areas than low-frequency-sensitive neurons (Figs. 7, 9).

Response centers of all 223 neurons were located between 0‡ and 50‡ in azimuth, 2‡ up and 25‡ down in elevation of the contralateral frontal auditory space (Fig. 10). Response centers of auditory cortical neurons tended to move toward the midline and slightly downward with increasing best frequency (BF; Fig. 11).

Auditory space representation appears to be systematically arranged according to the tonotopic axis of the auditory cortex. Thus, the lateral space is represented posteriorly and the middle space anteriorly (Fig. 12). Space representation, however, is less systematic in the vertical direction (Fig. 13).

Auditory cortical neurons are columnarly organized. Thus, the BFs, MTs, threshold curves, azimuthal location of response centers, and auditory spatial response areas of neurons sequentially isolated from an orthogonal electrode penetration are similar (Figs. 4, 7).