DIFFERENCES IN RESPONSE PROPERTIES OF NEURONS BETWEEN 2 DELAY-TUNED AREAS IN THE AUDITORY-CORTEX OF THE MOUSTACHED BAT

1. The orientation sound (pulse) of the mustached bat, Pteronotus parn ellii parnellii, consists of four harmonics (H1-4), each containing a long constant-frequency component (CF1-4) followed by a short frequenc y-modulated component (FM1-4). The auditory cortex of this species con tains several ''combination-sensitive'' areas: FM-FM, dorsal fringe (D F), ventral fringe (VF), CF/CF, and H-1-H-2. The FM-FM, DF, and VF are as each consist of neurons tuned to particular delays of echo FM(n)(n = 2, 3, or 4) from pulse FM1, and have an echo-delay (target-range) ax is. This delay axis is from 0.4 to approximately 18 ms in the FM-FM ar ea, to approximately 9 ms in the DF area, and to approximately 5 ms in the VF area. Therefore we hypothesized that the VF area was more spec ialized for the processing of range information in the terminal phase of echolocation than was the FM-FM area. The aim of our present studie s was to find differences in response properties between neurons with best delays shorter than 6 ms in the VF and FM-FM areas and thus to te st our hypothesis. 2. In the terminal phase of target-directed flight, the rate of pulse emission becomes higher, pulse duration (in particu lar, CF duration) becomes shorter, echo delay becomes shorter, and ech oes (both the CF and FM components) are less Doppler shifted. Therefor e, a ''temporal-pattern-simulating (TPS) '' stimulus was designed to m imic the train of pulse-echo pairs that would be heard by the bat duri ng the terminal phase, and responses of single neurons to the TPS stim ulus and other types of stimuli were recorded from the VF and FM-FM ar eas of the auditory cortex of unanesthetized bats with a tungsten-wire microelectrode. 3. Best delays of the neurons studied range between 0 .9 and 5.5 ms (2.64 +/- 0.72 ms, N = 181) for the VF area, and between 0.6 and 6.0 ms (3.64 +/- 1.14, N = 144) for the FM-FM area. More neur ons in the VF area than those in the FM-FM area showed no response or a poor response to the TPS stimulus. Therefore VF neurons are less sui ted than neurons in the FM-FM area for processing target ranges in the terminal phase of target-directed flight. Facilitative delay-tuning c urves were commonly sandwiched between inhibitory delay-tuning curves. The lack of response or poor response to the TPS stimulus can be expl ained by this inhibition. 4. The widths of delay-tuning curves of VF n eurons tend to be slightly narrower than those of neurons in the FM-FM area. The widths of delay-tuning curves measured with double-echo sti muli were narrower than those measured with single-echo stimuli. The s harpening of the curves by the secondary echo was larger for neurons i n the VF area than for those in the FM-FM area. Therefore neurons in t he VF area are potentially better than neurons in the FM-FM area for r anging in a cluttered environment. 5. Unlike neurons in the FM-FM area , those in the VF area showed fast adaptation to repetitive stimuli co nsisting of pulse-echo pairs. Therefore neurons in the VF area are inf erior to those in the FM-FM area in encoding range information in the terminal phase of echolocation. 6. The facilitative responses of neuro ns in the FM-FM area to FM components in the pulse-echo pair were litt le affected by CF components in the pair. However, those in the VF are a were often inhibited when the CF components were short as those of t he biosonar signals in the terminal phase. Therefore they are less spe cialized for processing echoes in the terminal phase than neurons in t he FM-FM area. 7. The responses of neurons in the FM-FM and VF areas w ere influenced by Doppler shifts. Different from velocity-tuned CF/CF neurons, the neurons in the FM-FM and VF areas were broadly tuned to r elative velocities of 2.64 +/- 2.80 and 0.89 +/- 3.44 m/s, respectivel y. The flight speed of the bat in the terminal phase is probably 0-3 m /s. Therefore neurons in the VF area are more suited than those in the FM-FM area for ranging targets that are either stationary or moving w ithout pursuit. 8. Neurons in the VF area are less sensitive to the di rection of frequency sweep than those in the FM-FM area. However, thei r responses are greater to FM-FM stimuli than to CF-CF stimuli. 9. The data obtained do not support the hypothesis that the VF area is more specialized for processing range information in the terminal phase of target-directed flight than the FM-FM area. The VF area is probably in volved in ranging nearby objects when the bat is hanging down from the ceiling of a cave or branch of a tree rather than in ranging while fl ying.