Delay-tuned neurons in auditory cortex of mustached bat are not suited for processing directional information

1. The mustached bat, Pteronotus parnellii parnellii, emits biosonar pulses each consisting of eight components: CF_1-4 and FM_1-4. In the auditory cortex of the bat there are arrays of FM-FM neurons that are tuned to particular delays of echo FM(n) (n = 2, 3, or 4) from pulse FM₁. They are specialized for the processing of target-range information. The FM signal is suited for ranging and also for target localization. Therefore we studied the directional sensitivity of FM-FM neurons with pulse FM₁ and echo FM(n). One of the FM₁-FM(n) pair was moved around the bat's head while the other was fixed in front of the bat. 2. FM-FM neurons are sharply tuned in echo delay and are broadly tuned in echo amplitude. That is, they are tuned to a target that has a particular cross-sectional area and that is located at a particular distance from the bat. Their best amplitudes for echoes range between 8 and 73 dB sound pressure level (SPL). The best amplitude is ~3 dB higher than minimum threshold in the majority of neurons. 3. The higher the best amplitude is relative to minimum threshold, the larger the receptive field is the best amplitude. The receptive field of FM-FM neurons at 30 dB above minimum threshold is always so large that it covers the entire contralateral auditory field or the entire contralateral field and the medial half of the ipsilateral auditory field. The large size of the receptive field and the uniform distribution of response magnitudes within the receptive field indicate that FM-FM neurons are not suited for sound localization. Directional information is probably processed in parallel by a separate population of neurons other than FM-FM neurons. 4. The receptive field of FM-FM neurons at 10 dB above minimum threshold is much smaller than that at 30 dB above minimum threshold, but it is still large. The mean azimuthal and elevational widths for echo FM(n) are >70° in all directions. There is no sign that FM-FM neurons are more directional than peripheral neurons. Furthermore, there is neither an azimuthal nor an elevational axis within the FM-FM area. 5. Mean best azimuths of FM-FM neurons are different for each echo FM harmonic: lateral 35° for FM₂ and lateral 19° for FM₃ and FM₄. These best azimuths are similar to those of peripheral neurons. However, the best azimuth of FM-FM neurons for pulse FM₁ (lateral 2°) is quite different from that of peripheral neurons (lateral 25°). It is most likely that FM-FM neurons are binaurally excited by FM₁ but are contralaterally excited by FM(n). Because FM-FM neurons are conditioned by self-vocalized pulse FM₁ to process target-range information, their directional sensitivity to pulse FM₁ is probably an adaptation of these neurons for ranging. 6. Because FM-FM neurons are broadly tuned to auditory space, the best delay (best range) of these neurons changes little with changes in the direction of an echo source. There is only a 30- to 50-μs (5.2- to 8.7-mm distance) change for shifting the echo source by 25° lateral or 25° medial from the best azimuth. This change is 2-4 times smaller than the representation of delay (range) information per cortical column in the FM-FM area: 116 μs (20 mm)/cortical column.