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

Auditory response properties and spatial response areas of single neurons in the pontine nuclei of the big brown bat, Eptesicus fuscus

Using free-field acoustic stimulation conditions, we studied the response properties and spatial sensitivity of 146 pontine neurons of the big brown bat, Eptesicus fuscus. The best frequency (BF) and minimum threshold (MT) of a pontine neuron were first determined with a sound broadcast from a loudspeaker placed ahead of the bat. A BF sound was delivered from the loudspeaker as it moved across the frontal auditory space in order to locate the response center at which the neuron had its lowest MT. Then the basic response properties of the neuron to a sound delivered from the response center were studied. As in inferior collicular and auditory cortical neurons, pontine neurons can be characterized as phasic responders, phasic bursters and tonic responders. They have both monotonic and non-monotonic intensity-rate functions. However, most of them are broadly tuned as are cerebellar neurons. Auditory spatial sensitivity was studied for 144 pontine neurons. In 9 neurons, variation of MT with a BF sound delivered from several azimuthal and elevational angles along the horizontal and vertical planes crossing the neuron's response center was measured. In addition, variation in the number of impulses with several stimulus intensities at 10 dB increments above a neuron's MT delivered from each angle was also studied. The auditory spatial sensitivity of other pontine neurons was studied by measuring the response area of each neuron with stimulus intensities at 3, 5, 10, 15 or 40 dB above its lowest MT. The response areas of pontine neurons expanded asymmetrically with stimulus intensity, but the size of the response area was not correlated with either MT or BF. In half of the pontine neurons studied, the response area expanded greatly and eventually covered almost the entire frontal auditory space. The response areas of the other half of the pontine neurons only expanded to a restricted area of frontal auditory space. Two possible neural mechanisms underlying these two types of response areas are hypothesized. The response centers of all 144 neurons were located within a small area of the frontal auditory space. The locations of response centers of these neurons are not correlated with their BFs. The distribution pattern of these response centers is comparable to that of superior collicular and cerebellar neurons but is different from that of inferior collicular and auditory cortical neurons. The results of our study suggest that auditory information is integrated in the pontine nuclei before being further sent into the cerebellum.

Encoding repetition rate and duration in the inferior colliculus of the big brown bat, Eptesicus fuscus

1. Encoding of temporal stimulus parameters by inferior collicular (IC) neurons of Eptesicus fuscus was studied by recording their responses to a wide range of repetition rates (RRs) and durations at several stimulus intensities under free field stimulus conditions. 2. The response properties of 424 IC neurons recorded were similar to those reported in previous studies of this species. 3. IC neurons were classified as low-pass, band-pass, and high-pass according to their preference for RRs and/or durations characteristic of, respectively, search, approach, or terminal phases of echolocation. These neurons selectively process stimuli characteristic of the various phases of hunting. 4. Best RRs and best durations were not correlated with either the BFs or recording depths This suggests that each isofrequency lamina is capable of processing RRs and durations of all hunting phases. 5. Responses of one half of IC neurons studied were correlated with the stimulus duty cycle. These neurons may preferentially process terminal phase information when the bat's pulse emission duty cycle increases. 6. While the stimulus RR affected the dynamic range and overall profile of the intensity rate function, only little effect was observed with different stimulus durations.

Auditory response properties and directional sensitivity of cerebellar neurons of the echolocating bat, Eptesicus fuscus

Auditory response properties and directional sensitivity of cerebellar neurons of Eptesicus fuscus were studied under free-field stimulation conditions. The best frequency (BF) and minimum threshold (MT) of a recorded neuron were first determined with a sound delivered in front of the bat. Discharge pattern and MT were studied with both BF stimuli and one-octave downward and upward sweep FM (frequency-modulated) stimuli. The directional sensitivity of cerebellar neurons was then studied by determining the variation of MT and response latency with BF and FM stimuli broadcast from each of 15 loudspeakers attached to a semicircular wooden track in front of the bat. All 85 cerebellar neurons recorded discharged phasically to acoustic stimuli. Only 20 were spontaneously active. Cerebellar neurons were generally more sensitive to FM stimuli than to pure tone pulses. Thus, they discharged more vigorously and had a lower MT to the former than the latter stimulus. Directional sensitivity of 47 neurons (BF = 23.4-81.1 kHz) was studied. All neurons varied their MTs with sound direction. Most neurons (n = 37, 79%) showed a lowest MT to a frontal sound. Directional sensitivity of cerebellar neurons appears to be sharper when determined with BF tone pulses than with FM stimuli. Thus the directional slope and the difference in MT between the best and worst angles of these neurons were larger when determined with the BF stimulus. Directional sensitivity of cerebellar neurons is not dependent upon stimulus frequency, unlike that of the inferior and cortical neurons of the same bat. Cerebellar neurons also varied their response latency with sound direction. Such a variation may provide the bat with another neural code for sound localization.

Convergence of temporal and spectral information into acoustic images of complex sonar targets perceived by the echolocating bat, Eptesicus fuscus

1. FM echolocating bats (Eptesicus fuscus) were trained to discriminate between a two-component complex target and a one-component simple target simulated by electronically-returned echoes in a series of experiments that explore the composition of the image of the two-component target. In Experiment I, echoes for each target were presented sequentially, and the bats had to compare a stored image of one target with that of the other. The bats made errors when the range of the simple target corresponded to the range of either glint in the complex target, indicating that some trace of the parts of one image interfered with perception of the other image. In Experiment II, echoes were presented simultaneously as well as sequentially, permitting direct masking of echoes from one target to the other. Changes in echo amplitude produced shifts in apparent range whose pattern depended upon the mode of echo presentation. 2. Eptesicus perceives images of complex sonar targets that explicitly represent the location and spacing of discrete glints located at different ranges. The bat perceives the target's structure in terms of its range profile along a psychological range axis using a combination of echo delay and echo spectral representations that together resemble a spectrogram of the FM echoes. The image itself is expressed entirely along a range scale that is defined with reference to echo delay. Spectral information contributes to the image by providing estimates of the range separation of glints, but it is transformed into these estimates. 3. Perceived absolute range is encoded by the timing of neural discharges and is vulnerable to shifts caused by neural amplitude-latency trading, which was estimated at 13 to 18 microseconds per dB from N1 and N4 auditory evoked potentials in Eptesicus. Spectral cues representing the separation of glints within the target are transformed into estimates of delay separations before being incorporated into the image. However, because they are encoded by neural frequency tuning rather than the time-of-occurrence of neural discharges, the perceived range separation of glints in images is not vulnerable to amplitude-latency shifts. 4. The bat perceives an image that is displayed in the domain of time or range. The image receives no evident spectral contribution beyond what is transformed into delay estimates. Although the initial auditory representation of FM echoes is spectrogram-like, the time, frequency, and amplitude dimensions of the spectrogram appear to be compressed into an image that has only time and amplitude dimensions.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency and space representation in the inferior colliculus of the FM bat, Eptesicus fuscus

The tonotopic organization and spatial sensitivity of 217 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free field stimulation conditions. Acoustic stimuli were delivered from a loudspeaker placed 21 cm ahead of the bat to determine the best frequency (BF) and minimum threshold (MT) of isolated IC neurons. A BF stimulus was then delivered as the loudspeaker was moved horizontally across the frontal auditory space of the bat to locate the best azimuthal angle (BAZ) at which the neuron had its lowest MT. The stimulus was then raised 3 dB above the lowest MT to determine the horizontal extent of the auditory space within which a sound could elicit responses from the neurons. This was done by moving the loudspeaker laterally at every 5 degrees or 10 degrees until the neuron failed to respond. These measurements also allowed us to redetermine the BAZ at which the neuron fired maximal number of impulses. Electrodes were placed evenly across the whole IC surface and IC neurons were sampled as many as possible within each electrode penetration. Tonotopic organization and spatial sensitivity were examined among all 217 IC neurons as a whole as well as among IC neurons sequentially sampled within individual electrode penetrations. The whole population of 217 IC neurons is organized tonotopically along the dorsoventral axis of the IC. Thus, low frequency neurons are mostly located dorsally and high frequency neurons ventrally with median frequency neurons intervening in between. The BAZ of these 217 IC neurons tend to shift from lateral to medial portions of the contralateral frontal auditory space with increasing BF.(ABSTRACT TRUNCATED AT 250 WORDS)