Biomedical subjects

P H Jen

Publications and source records attributed to P H Jen.

At least 55 records · Page 3Linked to original sources

PubMed1993

Responses of superior collicular neurons of the big brown bat, Eptesicus fuscus, to stationary and moving sounds.

Responses of 59 superior collicular neurons (SC) of the big brown bat, Eptesicus fuscus, to stationary and moving sounds were studied under free-field stimulation conditions. Neurons responding to sound stimulation were recorded from all three layers of the SC; however, responses of neurons in the superficial and upper intermediate layers generally show poor signal to noise ratio. Spatial sensitivity of each SC neuron was studied by determining variation of the number of impulses and minimum threshold (MT) of the neuron with a sound broadcast from a stationary loudspeaker at several selected azimuthal angles. More than 95% of SC neurons showed a lowest MT and maximal number of impulses to sound broadcast from a contralateral angle. However, the angle of lowest MT and the angle of maximal number of impulses of a given neuron were not always the same. Responses of 40 SC neurons to moving sounds were studied by determining the number of impulses of each neuron to a sound broadcast from a moving loudspeaker at selected azimuthal angles. Most (n = 27, 67%) neurons were sensitive to a sound with low moving velocity (36 degrees/s). There were 9 (23%) neurons which discharged maximally to a sound moved in a preferred direction but poorly when moved in the opposite direction. In addition, 5 (13%) neurons discharged maximally to a sound moving away from the midline and across 20 degrees lateral of the bat's frontal auditory space. These findings suggest that SC neurons may help a bat acoustically "gaze" at insect during hunting.

Acoustic Stimulation↗

PubMed1993

Directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, to sounds delivered from selected horizontal and vertical angles.

Under free field stimulation conditions, directional sensitivity of 336 inferior collicular (IC) neurons of 6 Eptesicus fuscus was studied by recording the minimum threshold (MT) and number of impulses of each neuron to a best frequency (BF) sound delivered from selected horizontal and vertical angles. An auditory response center was determined for each neuron by delivering a BF sound from a specific point of the bat's frontal auditory space. At the response center, the neuron had maximal directional sensitivity and lowest MT. Auditory response centers of all 336 IC neurons studied were located in the contralateral frontal auditory space and they shifted from lateral angles toward the midline with increasing BF. Sound direction affected the sharpness of frequency tuning (i.e. Q10-dB value) but not the BF and overall shape of the frequency threshold curve of a neuron. Sound direction also affected the MTs, number of impulses and intensity-rate functions but not the discharge patterns of IC neurons. High BF neurons generally had sharper directional sensitivity than low BF neurons. Bending downward the pinna contralateral to the recording site increased a neuron's MT and shifted its response center downward and toward the midline of the bat's frontal auditory space. The effect of pinna bending on a neuron's MT and impulse directional curves can be explained on the basis of variation of its auditory response area and its MT.

Acoustic Stimulation↗

PubMed1992

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.

Acoustic Stimulation↗

PubMed1992

Pulse repetition rate and duration affect the responses of bat auditory cortical neurons.

Under free field stimulation conditions, we studied the effect of pulse repetition rate and duration on the responses of auditory cortical neurons of the big brown bat, Eptesicus fuscus. The best frequency (BF) and minimum threshold (MT) of each recorded neuron were first determined with a 4 ms pulse delivered from a specific point of the bat's frontal auditory space at which the neuron had maximal spatial sensitivity. Then a 4 ms pulse was delivered repetitively at different rates with an intensity in 10 dB increments above the neuron's MT and its number of impulses and response latency to each combination of repetition rate and intensity were recorded. From the series of isorepetition rate-intensity functions, we identified the neuron's best intensity (BI) which elicited a maximal number of impulses. By using a 4 ms BF and BI pulse delivered at a wide range of repetition rates, we then determined the neuron's best repetition rate (BRR) to which the neuron discharged maximally. The effect of pulse duration on the neuron's responses was subsequently studied by recording its number of impulses and latency to a BF, BI and BRR pulse delivered at a wide range of durations. A neuron's best duration (BD) to which the neuron discharged maximally was also identified. We found that cortical neurons sequentially isolated with an orthogonally penetrated electrode have comparable BFs, MTs and latencies. In addition, high BF neurons tend to have higher MTs and shorter latencies than low BF neurons. Most intensity-rate functions of cortical neurons were nonmonotonic but they can be affected by pulse repetition rate. Under repetitive single pulse stimulation, all cortical neurons show low pass characteristic to pulse repetition rate such that their number of impulses reduced drastically with increasing pulse repetition rate regardless of pulse intensity. The BRRs of most cortical neurons studied are between 1 and 3 pulses/s and their BDs are between 0.5 and 10 ms. Response latencies of cortical neurons generally shortened to a plateau level with increasing pulse intensity. In contrast, they increased with pulse repetition rate and varied in different manner with pulse duration. Nevertheless, a cortical neuron generally responds with a shortest latency to a pulse delivered at its BF, BI, BRR and BD. Thus in auditory signal processing, a neuron's responses to a wide combination of pulse intensity, duration and repetition rate enhance its versatility in signal detection. Its maximal response to a specific combination of signal duration, repetition rate and intensity provides a neural basis for fine analysis of signal features.

Acoustic Stimulation↗

PubMed1991

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.

Acoustic Stimulation↗

PubMed1991

Directional sensitivity of bat inferior collicular neurons determined under normal and monaurally plugged ear conditions.

In an effort to further understand the combined effect of binaural intensity difference and pinna position on the directional sensitivity of an auditory neuron, we used free field stimulation to study the directional sensitivity of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, under the following three ear conditions: normal ears, monaurally plugged ear and monaurally plugged plus pinna bending backward. The best frequency (BF) and minimum threshold (MT) of each neuron were first determined for a sound (4 ms duration, 0.5 ms rise-decay times) delivered from 40 degrees contralateral in azimuth, 0 degrees in elevation relative to the recording site. Then, the neuron's MT to a BF sound delivered from 7 selected azimuthal angles with respect to the bat's head were determined. The number of impulses to a BF sound delivered at two intensities (10 and 20 dB re MT determined at 40 degrees contralateral) from each of 7 azimuthal angles was also subsequently determined. Although different IC neurons showed different variations in MT and number of impulses with sound source azimuth, most had a lowest MT and/or maximal number of impulses to BF sounds delivered from contralateral azimuthal angles regardless of ear conditions. However, among 117 neurons studied, there were 12 neurons in which the number of impulses did not vary significantly with sound direction thus showing nondirectional characteristics. Monaural plugging (a wet cotton ball) with or without backward bending of the pinna not only increased the MT of a recorded neuron but also greatly reduced the number of impulses of the neuron so that the directional sensitivity of the neuron was also modified. The effect of monaural plugging on the threshold of a neuron and the degree of modification on directional sensitivity varied among individual IC neurons. Thus, while the directional sensitivity of some IC neurons became sharper, for other neurons the same monaural plugging reduced the sharpness of their directional sensitivity curve. Directional sensitivity of IC neurons is dependent upon stimulus frequency regardless of ear conditions. High frequency neurons generally have sharper directional sensitivity than low frequency neurons. The angle of lowest MT and maximal number of impulses tended to move toward the middle portion of a bat's frontal auditory space with increasing BF.

Acoustic Stimulation↗

PubMed1991

Encoding of acoustic stimulus intensity by inferior collicular neurons of the big brown bat, Eptesicus fuscus.

Using bats as a model system, we studied the isointensity and isofrequency discharge rate functions of inferior collicular (IC) neurons under free field stimulation conditions in order to understand how the midbrain auditory neurons encode stimulus intensity. For each encountered IC neuron, the best frequency (BF), minimum threshold (MT), tuning curve and intensity rate function for the BF were first determined. Then at a fixed stimulus intensity, the number of impulses was measured for several frequencies which were incrementally chosen across the entire range of the neuron's tuning curve. Such an isointensity discharge rate function was determined for at least three different intensities above each neuron's MT. A series of isofrequency discharge rate functions were also obtained by determining the intensity rate function for each of several frequencies chosen. Among 110 intensity rate functions measured at the BF of each IC neuron, 98 (89%) were non-monotonic and 12 (11%) were monotonic. The profiles of the intensity rate functions measured at several chosen frequencies for each neuron varied greatly. Thus, within the same series of isofrequency intensity rate functions, a neuron showed both monotonic and non-monotonic functions when measured at different frequencies. Similarly, the profile of each intensity rate curve within a series of isointensity rate functions might also vary with stimulus intensity. The isointensity discharge rate functions of most IC neurons (62 neurons, 61%) was triangular shaped reaching a peak value when stimulated with BF. The remaining (39 neurons, 39%) IC neurons had isointensity functions in which different curves peaked at different stimulus frequencies. In some extreme cases, some curves had two peaks or fluctuated within a moderate range of discharge rate throughout the whole range of stimulus frequency. Our studies suggest that encoding of a wide range of acoustic stimulus intensity in the IC involves more than the discharge rate of any one individual IC neuron. Different populations of IC neurons with different BFs will have to work coordinately in order to overcome the limitation of the dynamic range of individual neurons and to solve the ambiguity created by the same discharge rate of each individual neuron to different combinations of frequency and intensity within its auditory response area.

Acoustic Stimulation↗

PubMed1990

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.

Animals↗

PubMed1990

The influence of the auditory cortex on acoustically evoked cerebellar responses in the CF-FM bat, Rhinolophus pearsonic chinesis.

1. Acoustically evoked responses of 284 neurons isolated from the cerebellar vermis, hemispheres and paraflocculus of Rhinolophus pearsonic chinesis were studied under free field acoustic stimulation conditions. 2. The BFs of these cerebellar auditory neurons ranged from 24 to 76 kHz but they mostly fall either between 48 and 64 kHz or between 65 and 76 kHz. However, the BF distribution varies among vermal, hemispheric and parafloccular neurons. 3. Threshold curves of cerebellar neurons are generally broad but those tuned to the frequency of the predominant CF component are extremely narrow. 4. Response latencies of cerebellar neurons ranged from 2 to 48 ms suggesting multiple auditory cerebellar pathways. The latency distribution also varies among vermal, hemispheric and parafloccular neurons. 5. Although both the vermis and hemispheres contain a disproportionate number of 65-74 kHz neurons, the response latencies of those neurons isolated from the vermis are scattered over a wide range of 2.2-28 ms while those neurons isolated from the hemispheres are generally stabilized between 5 and 12 ms. 6. Electrical stimulation of the auditory cortex evokes discharges from a recorded cerebellar auditory neuron. Cortical stimulation also facilitates the response of an acoustically evoked cerebellar neuron by increasing its number of impulses. The degree of facilitation is dependent upon the amplitude of the acoustic stimulus. 7. For a given electrical and acoustic stimulation condition, the facilitative latency and the degree of facilitation varied with the interstimulus interval. Among 23 neurons studied, most of them (19 neurons, 82.6%) had a maximal facilitative latency between 2 and 10 ms. 8. By examining the difference in the facilitative effect in each isolated cerebellar auditory neuron before and after a topical application of local anesthetic, procaine, onto the point of electrical stimulation in the auditory cortex, we found that the facilitative pathways to vermal and hemispheric neurons may be different from the pathway to parafloccular neurons. 9. Possible auditory pathways to different parts of the cerebellum are discussed in relation to the wide range of recorded response latencies. 10. The facilitative influence of the auditory cortex on the cerebellar auditory neurons is assumed to enhance the cerebellar role in acoustic motor orientation.

Acoustic Stimulation↗

PubMed1990

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)

Acoustic Stimulation↗

PubMed1990

Influence of monaural plugging on postnatal development of auditory spatial sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus.

Postnatal development of auditory sensitivity in the big brown bat, Eptesicus fuscus, was studied by determining the auditory spatial sensitivity of inferior collicular (IC) neurons of juvenile bats which were raised under normal (control) or monaurally plugged conditions. The auditory spatial sensitivity of each IC neuron was determined by measuring its variation in number of impulses and minimum threshold to a best frequency sound delivered from different azimuthal angles under free field stimulation conditions. Monaural plugging was performed on different juvenile bats at 7, 14, and 35 days of age. Auditory spatial sensitivity was then determined for IC neurons of monaurally plugged bats at 58-69 days after birth. Monaural plugging did not alter the discharge pattern, distribution of best frequency and latency of IC neurons, but it raised the minimum threshold of IC neurons of plugged juvenile bats by an average of 21-24 dB. Monaural plugging also appeared to modify postnatal development of auditory spatial sensitivity. The auditory spatial sensitivity of IC neurons was sharper when the earplug was intact than when the earplug was removed. Thus, the distribution of response center and the spatial sensitivity of IC neurons of the monaurally plugged bats was more comparable to IC neurons of the control bats before the earplug was removed. These findings suggest that the neural circuits underlying auditory spatial sensitivity of IC neurons of the monaurally plugged juvenile bats have undergone modifications to compensate for the unnatural binaural disparity during postnatal development. The auditory spatial sensitivity was also determined for two control juvenile bats at 49 and 60 days of age and for an adult bat (more than 1 year). Our data show that auditory spatial sensitivity of IC neurons sharpens with postnatal age. Furthermore, the sharpening process appears to be still in progress even at 60 days after birth.

Acoustic Stimulation↗

PubMed1989

Electrical stimulation of bat superior colliculus influences responses of inferior collicular neurons to acoustic stimuli.

The influence of electrical stimulation of the superior colliculus (SC) on acoustically evoked responses of inferior collicular (IC) neurons was examined in 24 barbiturate-anesthetized Rufous horseshoe bats, Rhinolophus rouxi. Acoustic stimuli (50 ms, 0.5 ms rise-decay times) were delivered from a loudspeaker placed 68 cm in front of each bat and a total of 354 IC neurons were isolated. The response latencies of these neurons were mainly between 7.5 and 17.5 ms. When the ipsilateral SC was electrically stimulated, responses of 227 (64%) neurons were not affected, but responses of the remaining (127 neurons, 36%) were either inhibited (102 neurons, 29%) or facilitated (25 neurons, 7%). The degree of inhibition and the response latency of the inhibited neurons increased with the amplitude of electrical stimulation. Inhibition of a neuron's activity was also dependent upon the time interval between acoustic and electrical stimuli. The best inhibitory latency measured at maximal inhibition was between 12 and 20 ms. Conversely, facilitation shortened the response latency of IC neurons and the degree of facilitation increased with the amplitude of the acoustic stimulus. Since the SC plays an essential role in orienting an animal's responses toward sensory stimuli, our findings suggest that the SC may affect the processing of acoustic signals in the auditory system during acoustically guided orientation.

Acoustic Stimulation↗

PubMed1989

Corticofugal influences on the responses of bat inferior collicular neurons to sound stimulation.

Corticofugal influences on the responses of inferior collicular neurons (IC) to acoustic stimulation were studied by electrical stimulation of the auditory cortex. Among 471 IC neurons isolated, about 26% were affected by cortical stimulation. Responses of 103 (22%) IC neurons were inhibited and 17 (3.6%) were facilitated. The degree of inhibition was dependent upon the amplitude of both auditory and electrical stimuli. Corticofugal inhibition of the response of an IC neuron was likely due to an increase in the neuron's minimum threshold. Inhibitory latency varied with the interstimulus interval. The shortest inhibitory latency of most IC neurons was between 1 and 2 ms. The localization of the point of cortical stimulation was crucial in determining the responses of IC neurons. It is assumed that corticofugal influences on IC neurons are a part of regulatory mechanism in the centrifugal pathway for frequency analysis and acoustic orientation.

Acoustic Stimulation↗

PubMed1989

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

1. 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. 2. The majority of the auditory cortical neurons were recorded at a depth of less than 500 microns with a response latency between 8 and 20 ms. They generally discharged phasically and had nonmonotonic intensity-rate functions. 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 V-shaped and can be described as broad, intermediate, or narrow. 3. 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. An overwhelming majority of neurons were sensitive to a frequency range between 30 and 75 kHz. 4. 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. High-frequency-sensitive neurons tended to have smaller spatial response areas than low-frequency-sensitive neurons. 5. Response centers of all 223 neurons were located between 0 degrees and 50 degrees in azimuth, 2 degrees up and 25 degrees down in elevation of the contralateral frontal auditory space. Response centers of auditory cortical neurons tended to move toward the midline and slightly downward with increasing best frequency. 6. 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. Space representation, however, is less systematic in the vertical direction. 7. 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.

Acoustic Stimulation↗

PubMed1988

Auditory spatial sensitivity of inferior collicular neurons of echolocating bats.

The sensitivity of 94 inferior collicular (IC) neurons of Eptesicus fuscus and Myotis lucifugus to spatial location of the acoustic stimulus were studied under free-field stimulus conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were determined with sound delivered in front of the bat. Then the variation in discharge rate of the neuron was measured with a BF sound broadcast from a moving loudspeaker at different angular positions along the horizontal, vertical or diagonal plane of the frontal auditory space. A wide range of stimulus intensities above the MT of the neuron was used. Neurons were classified into 3 classes on the basis of their spatial sensitivity: (1) omnisensitive neurons (15%) were broadly tuned to sound delivered in the frontal auditory space and their responses did not show any correlation with sound location; (2) stimulus intensity-dependent neurons (28%) varied their discharge rates with sound location and intensity so that the peak of their spatial response profiles also varied with stimulus intensity; and (3) stimulus intensity-independent neurons (57%) varied their discharge rates only with sound location over a wide range of stimulus intensities so that their peak discharge always appeared at the same or a small range of angle. In most cases, the medial limbs of the spatial sensitivity curve for these neurons were extremely sharp and congruent. By moving the loudspeaker along the horizontal, vertical and diagonal planes, it was possible to approximate the boundary of the spatial response area of a neuron. Most IC neurons responded to sound delivered within 20 degrees ipsilateral, 60 degrees contralateral, 45 degrees up and 40 degrees down of the frontal auditory space, confirming previous similar studies. In general, an increasing stimulus repetition rate appeared to sharpen the spatial sensitivity curve of a neuron. Conversely, an increasing moving velocity of the stimulus decreased its response. The possible role of these 3 classes of neurons in echolocation and neural mechanisms underlying the spatial sensitivity of these neurons is discussed.

Acoustic Stimulation↗

PubMed1988

Directionality of sound pressure transformation at the pinna of echolocating bats.

The directionality of sound pressure transformation at the pinna of three species of bats was studied by measuring the sound pressure level of a tone (25 45 65 and 85 kHz) at the tympanic membrane as a function of azimuth and elevation of the sound source under free-field conditions. The tympanic sound pressure level varied with location of the sound source. The directionality of sound pressure transformation pattern of the pinna of each bat was studied by plotting isopressure contours. The area within each isopressure contour decreased with increasing tonal frequency. For each tonal frequency, the point of maximal sound pressure was always located in the frontal ipsilateral sound field. This point shifted medially with increasing tonal frequency along the horizontal plane in all species tested, but it shifted in a species-specific manner along the vertical plane. Removal or distortion of the pinna and tragus resulted in either uncircumscribed or irregular isopressure contours for all tonal frequencies tested. Acoustic pressure gain of the external ear reached 16-23 dB for frequencies at 15-18 kHz. The importance of the external ear to the directionality of the bat's echolocation system is discussed.

Acoustics↗

PubMed1987

Auditory space representation in the inferior colliculus of the FM bat, Eptesicus fuscus.

The auditory spatial response areas of 333 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free-field acoustic stimulus conditions. A stimulus was delivered from a loudspeaker placed 14 cm in front of a bat and the best frequency of an encountered neuron was determined. Then a best frequency (BF) stimulus was delivered as the loudspeaker was moved across the frontal auditory space to determine the response center of the neuron. At the response center, the neuron had the lowest minimum threshold. The stimulus was then raised 3-15 dB above the lowest minimum threshold of the neuron and the spatial response area for each stimulus intensity was measured. The response center and spatial response area of a neuron measured with a one-octave downward-sweep FM stimulus were similar to those measured with the pure tone pulse. The spatial response area of a neuron expanded asymmetrically with the stimulus intensity. High BF neurons generally had smaller spatial response areas than low BF neurons had. All 333 response centers were located in the contralateral auditory space. Response centers of low BF neurons tended to be located laterally while those of high BF neurons were located medially. Although each neuron had a point of lowest minimum threshold in the contralateral auditory space, the point-to-point representation of the auditory space was not systematically organized. This representation was not correlated with the recording sites of the neurons in the mediolateral, posteroanterior and dorsoventral axes of the IC.

Acoustic Stimulation↗

PubMed1987

Anatomical study of neural projections to the superior colliculus of the big brown bat, Eptesicus fuscus.

Auditory inputs to the intermediate and deep layers of the superior colliculus of the bat, Eptesicus fuscus, were studied by iontophoretic injection of horseradish peroxidase (HRP) into the superior colliculus. HRP was injected into the recording sites of superior collicular neurons that responded to acoustic stimuli (4 ms duration, 0.5 ms rise-decay times). The results showed that the superior colliculus received its auditory projections mainly from the inferior colliculus bilaterally, but with ipsilateral projections prevailing. A few projections came from the dorsal nucleus of the lateral lemniscus. HRP-labeled neurons were also found in 11 other brain structures.

Acoustic Stimulation↗