Tsutomu Kamada

Analysis of orientation signals emitted by the CF-FM bat,Pteronotus p. parnellii and the FM bat,Eptesicus fuscus during avoidance of moving and stationary obstacles

Summary1.The echolocative skills ofPteronotus parnellii parnellii andEptesicus fuscus were studied by measuring their ability in avoiding stationary and moving obstacles.2.The frequency, repetition rate, duration and amplitude of the orientation signals emitted by the bat during three phases of negotiation of obstacles were studied.3.During the avoidance of obstacles, both species of bats systematically shorten the duration, increase the repetition rate, and decrease the amplitude of their emitted orientation signals as they approach, negotiate and pass the obstacles.4.Flying at different speeds inside the flight room during the avoidance of obstacles,Pteronotus parnellii parnellii appropriately adjust their emitted CF frequency according to the flight speed to accurately compensate for the positively Doppler-shifted echoes.5.The frequency of the FM signals emitted at high repetition rate byEptesicus fuscus shifts downward as the bat enters into the final phase of negotiation of the obstacles.6.The significance of the change in parameters of emitted signals in relation to echolocation is discussed. Presumably, the increase in the repetition rate of sound emission and the shortening of the sound duration is to monitor rapid changes in obstacle positions. The decrease in sound amplitude is either due to the difficulty in producing loud short sounds at the end of a long-held breath or to appropriately adjust the echo amplitude into the optimal range of sensitivity of the bats ears.

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

SummaryThe 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 neuron. This was done by moving the loudspeaker laterally at every 5° or 10° 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. Thus, the auditory space appears to have an orderly representation along the tonotopic axis of the IC. The lateral space is represented dorsally and the medial space ventrally. Nevertheless, tonotopic organization and spatial sensitivitty of sequentially isolated IC neurons within each electrode penetration may vary with the point of electrode penetration. This variation may be explained on the basis of the arrangement and thickness of each frequency lamina within the IC.

Auditory response properties and spatial response areas of superior collicular neurons of the FM bat,Eptesicus fuscus

Summary1.Electrophysiological properties of 279 single units in the superior colliculus (SC) of the big brown bat,Eptesicus fuscus, were studied by recording their responses to pure tone pulses and frequency-modulated stimuli.2.The SC units generally fired only a few impulses to acoustic stimulus. They are not tonotopically organized along the dorsoventral axis of the SC (Fig. 1).3.The response latency of 276 units was between 3.6 and 20 ms, but most (253 units) were below 12.5 ms (Fig. 2).4.Tuning curves of 164 units are either narrow, intermediate, or broad, according to the bandwidth of the tuning curves (Fig. 3). Q10 dB values range between 1.21 (best frequency (BF) =33.52 kHz) and 68.9 (BF=69.94 kHz), but the majority are below 20. The minimum thresholds (MTs) of these units were between 15 and 104 dB SPL, but most were above 40 dB SPL.5.MTs to pure tone pulses and one octave upward and downward sweeping FM stimuli were compared (Fig. 4). SC units generally had their lowest MTs to downward sweeping FM stimuli and their highest MTs to pure tone pulses (Table 1).6.Intensity-rate functions of 16 SC units were measured with both pure tone pulses and upward and downward sweeping FM stimuli. There was no correlation between the type of intensity-rate function obtained and the type of acoustic stimulus used (Fig. 5).7.Auditory spatial response areas were measured for 47 units. The size of the spatial response area increases with stimulus intensity (Fig. 6). The auditory space is not orderly represented in the SC of a bat (Fig. 7).

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 neurons response center was measured. In addition, variation in the number of impulses with several stimulus intensities at 10 dB increments above a neurons 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.

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.

Neurons in the superior colliculus of echo-locating bats respond to ultrasonic signals

Using conventional electrophysiological techniques, we demonstrate that neurons in the superior colliculus of the big brown bat (Eptesicus fuscus) respond to ultrasonic signals. Most response properties of these neurons are very similar to neurons of the inferior colliculus in the same bat.

Mapping of the auditory area in the cerebellar vermis and hemispheres of the mustache bat, Pteronotus parnellii parnellii

Microelectrode mapping of the auditory areas in the cerebellar vermis and hemispheres of mustache bats, Pteronotus parnellii parnellii, reveals that a large area of the bats cerebellum contains units responding to acoustic signals. A study of frequency tuning of isolated units shows that there are two large groups of auditory units. The units of one group are sharply tuned to a very narrow band of frequency with BFs between 60 and 64 kHz. The units of the other group are broadly tuned, with BFs between 47 and 59 kHz. These two groups of units are probably involved in processing the predominant CF and FM portions of the bats orientation sounds during echolocation.

Responses of cerebellar neurons of the CF-FM bat, Pteronotus parnellii to acoustic stimuli.

Single units (125) which faithfully discharged action potentials to acoustic stimuli (35 ms in duration with 0.5 ms rise and decay times) were recorded in the cerebellar vermis and hemispheres of the CF-FM bat, Pteronotus parnellii. These units had response latencies between 1.5 and 27 ms and minimum thresholds between 2 and 83.5 dB SPL. Best frequencies (BFs) of these units ranged from 30.32 to 79.28 kHz, but more than half (64 units, 51.2%) were between 59.73 and 63.32 kHz. While most tuning curves of these units were either broad or irregular, those curves with BFs tuned at around 61 kHz which is the frequency of the predominant CF component of the bats echolocation signals were extremely narrow with Q10-dB values as high as 153. Those units (29) with BFs tuned near the 61 kHz also showed off-responses. These data indicate that auditory specialization for processing of species-specific orientation signals also exists in the cerebellum of this bat.

Auditory representation in the cerebellum of the CF‐FM bat, Pteronotus parnellii parnellii

Auditory representation in the cerebellum of the mustache bat, Pteronotus parnellii parnellii, was studied with microelectrode mapping technique. This study reveals that a large area of the bats cerebellum contains units responding to acoustic stimuli. A study of the frequency representation and frequency tuning of the recorded units show that auditory specialization for processing of species‐specific echolocation sounds also exists in its cerebellum. There is a large number of units with best frequencies between 60 and 64 kHz. These units have extremely sharp threshold curves with a Q10‐dB value as high as 394. These auditory units are organized into a fractured pattern of small patches within which units are tonotopically organized. The units also appear to be columnarly assembled according to their best frequencies. Although different in sensory modality, such a finding is highly comparable to the fractured somatotopy in the tactile areas of cerebellar hemispheres [Shambes et al. Brain Behav. Evol. 15...

The unit responses to various complex sounds in the auditory cortex of the Japanese monkey

It is well investigated in the mammalian cortex that physiological parameters like tonotopicity and binaural response properties are represented in the primary auditory cortex, while the coding mechanism of complex sounds is not always understood in the primary as well as secondary cortex. In these circumstances it is still necessary to examine the response properties of neurons in a wide area of auditory cortices. The author has examined the neuronal responses to monkey vocalizations as well as pure tones, white noise, and synthetic sounds in the auditory cortex of the Japanese monkey in the chronic condition. More than 200 hundreds of units were recorded, most of which responded to either of the sound stimuli while the specific response to a sound was rare. Sharp tuning units with clear characteristic frequencies were located mainly in the primary cortex, while broad tuning ones were in periphery of the koniocortex. Some neurons responded, like the frequency analyzer when examined, to ‘‘coo’’ vocalizations and to synthetic harmonic components of the vocalization. Other responses were compared to acoustic structures of the stimuli. The correlations between stimuli and neural responses were discussed in association with the location of the neurons in the cortex.