Response characteristics of inferior colliculus neurons of the awake CF-FM batRhinolophus ferrumequinum: I. Single-tone stimulation

Recovery cycle of inferior collicular neurons in Hipposideros pratti under behavior-related sound stimulus and the best Doppler-shift compensation conditions

The Doppler-shift compensation (DSC) behavior of constant frequency - frequency modulation (CF-FM) bat (Hipposideros pratti) is vital for extraction and analysis of echo information. This type of behavior affects the recovery cycles of sound-sensitive neurons, but their precise relationship remains unclear. In this study, we investigated the effects of DSC on the recovery cycles of inferior collicular (IC) neurons in H. pratti. We simulated the pulse-echo pair in bats by changing the emitted pulse frequency and keeping the echo frequency constant during DSC in echolocation. The neuronal recovery cycles of IC neurons are categorized into four types: unrecovered, monotonic, single-peak, and multi-peak. The recovery cycle of IC neurons shortens after DSC; moreover, the amount of neurons with multi-peak recovery cycle increases and concentrates in the short recovery area. This paper also discusses the possible neural mechanisms and their biological relevance to different phases of bat predation behavior.

Encoding of sound motion by binaural brainstem units in a Horseshoe bat

In order to study how and if single brainstem units respond to moving compared with stationary sounds, radially moving sound sources were presented to the bat, Rhinolophus ferrumequinum. This time-variant binaural stimulation was simulated dichotically through earphones (closed-acoustic-field for the virtual azimuth range of +/-40 degrees from the midline). Neurophysiologically recorded responses primarily showed a function of interaural intensity difference (IID) which is considered a direct correlate of the sound source's azimuth angle. However, this is only true for the stationary case. Unit's response did not remain unaffected by the dynamic stimulus cues of sound source movement (velocity and direction). Maximal discharge rate became a function of motion velocity as well as the slopes of the response profiles. Hence, coding of IID became ambiguous as, depending on the unit, the response profiles and therefore a unit's receptive field, became spatially shifted with respect to one another when the direction of the sound source movement was reversed. Shifts within the movement direction (hysteresis) as well as against it (termed here 'advance') were observed: hysteresis is typical for units with non-monotonic, stationary rate/intensity functions, whereas those units with monotonic functions predominantly show advances. Further dynamic response features in form of transient peaks and troughs, superimposed on the response profiles, were registered. It appears that the ongoing firing rate no longer represents azimuth position alone, but vigorously reproduces the dynamic cues (velocity and movement direction), too. With respect to the neural mechanisms leading to dynamic response features, it is proposed that, as long excitation and inhibition act with similar short time constants, neural activity can rapidly and faithfully follow changing IIDs. Different time constants for excitation, inhibition, facilitation, and depression may be responsible for the dynamic 'features' such as transient responses and hysteresis/advance. They may provide biologically relevant information for nocturnally hunting bats to efficiently guide their flight maneuvers.

Frequency tuning properties of neurons in the inferior colliculus of an FM bat

We examined frequency tuning characteristics of single neurons in the inferior colliculus of the echolocating bat, Eptesicus fuscus, in order to determine whether there are different classes of spectral selectivity at this level and to relate frequency tuning properties to the design of the echolocation signal. In unanesthetized but tranquilized animals, we recorded responses from 363 single units to pure tones, frequency-modulated (FM) sweeps, or broad-band noise. Most units were selective for stimulus type; 50% responded only to pure tones, 14% responded only to FM sweeps, and 5% responded only to noise. The remainder responded to two or more types of stimuli. Tuning curves could be classified as follows: 1) V-shaped tuning curves (57%) were the most common type; 2) closed tuning curves (20%) had thresholds at both low and high sound levels; 3) narrow filters (14%) had Q values above 20 at 10 dB and 30 dB above threshold or 10 dB and 40 dB above threshold; 4) frequency-opponent tuning (6%) was found in units with high spontaneous activity; within a center range of frequencies, firing rate increased above spontaneous level, but at higher or lower frequencies, firing rate decreased below spontaneous level; 5) double-tuned units (3%) had two best frequencies (BF). The most clear evidence of topographic distribution was seen for filter units, which were only found in the dorsal part of the 20-30 kHz isofrequency contour. Filter units were also the most clearly related to the echolocation signal of the bat. Their BFs were all within the range of the dominant frequency (approximately 20-30 kHz) that Eptesicus uses during the searching phase of echolocation.

Auditory Pontine Grey: Connections and Response Properties in the Horseshoe Bat

This study investigates the role of the pontine grey as a link between the auditory system and the cerebellum in the bat, Rhinolophus rouxi. We recorded response properties of single neurons in the pontine grey and, in the same preparation, injected wheat germ agglutinin - horseradish peroxidase (WGA - HRP) in areas responsive to sound. Thus the functional evidence was correlated with retrograde and anterograde transport. The main results are: (i) all auditory neurons in the pontine grey are tuned within one of two harmonically related frequency ranges of the echolocation call. The upper range corresponds to the constant frequency and frequency modulated components of the second harmonic, but the lower range corresponds only to the frequency modulated component of the first harmonic. There is no systematic tonotopic organization; (ii) discharge patterns are extremely variable, latencies cover a wide range, and about half of the neurons are binaurally responsive with excitation from both ears; (iii) most pontine auditory neurons respond preferentially to frequency modulated stimuli; (iv) there is massive input to the pontine grey from the central nucleus of the inferior colliculus; (v) cortical input to the pontine grey does not originate in tonotopically organized auditory cortex. The input is from a dorsal belt area that is specialized for processing combinations of sounds with specific frequency ratios and delays; (vi) projections from the auditory region of the pontine grey are widespread within the cerebellar cortex. The data suggest that the pontine grey transmits to the cerebellum information contained in specific components of the bat's biosonar signal.

Complex sound analysis (frequency resolution, filtering and spectral integration) by single units of the inferior colliculus of the cat

The central nucleus of the inferior colliculus (ICC) is a center of convergence of brainstem input and is critical for auditory information processing. Here, the analysis of complex sound spectra by single neurons in the ICC is investigated. Several measures of frequency resolution (excitatory/inhibitory tuning curves, effective bandwidths, critical ratio bands, critical bands derived using narrowband masking and two-tone separation paradigms) have been obtained from the responses of these neurons at sound pressure levels (SPL) up to 80 dB above the units' response thresholds (nearly 110 dB SPL). Among our results are the following: (1) Narrowband masking measures of critical bands from ICC neurons closely parallel behavioral measures using the same stimulus paradigm. (2) Frequency resolution power as measured by critical bandwidths varies little as a function of stimulus intensity. (3) Tuning curves of ICC neurons provide no simple basis for predicting the frequency filtering of the same neurons excited by complex sound spectra. (4) There is a frequency dependence of all measures of frequency resolution similar to that found in psychophysical determinations of critical bandwidths. That is, spatial frequency resolution in the cochlea is the origin for the resolution found in the ICC and in behavioral tests. (5) Lateral inhibition at the level of the ICC clearly plays a role in frequency resolution. (6) Frequency resolution is encoded by response rate changes of ICC neurons and is independent of tone response threshold, response latency, spontaneous activity, tone response type, binaural response type. It is concluded that spectral analysis of sound is established by processes, including lateral inhibition, independent of other basic response properties of neurons at the level of the ICC.

The nuclei of the lateral lemniscus in the rufous horseshoe bat, Rhinolophus rouxi. A neurophysiological approach

In the rufous horseshoe bat, Rhinolophus rouxi, responses to pure tones and sinusoidally frequency modulated (SFM) signals were recorded from 289 single units and 241 multiunit clusters located in the nuclei of the lateral lemniscus (NLL). The distribution of best frequencies (BFs) of units in all three nuclei of the lateral lemniscus showed an overrepresentation in the range corresponding to the constant-frequency (CF) part of the echolocation signal ('filter frequency' range): in the ventral nucleus of the lateral lemniscus (VNLL) 'filter neurons' represented 43% of all units encountered, in the intermediate nucleus (INLL) 33%, and in dorsal nucleus (DNLL) 29% (Fig. 2a). Neurons with best frequencies in the filter frequency range had highest Q10dB-values (maxima up to 400, Fig. 2c) and only in low-frequency units were values comparable to those found in other mammals. On the average, filter neurons in ventral nucleus had higher Q10dB-values (about 220) than did those in intermediate and dorsal nucleus (both about 160, Fig 2d). Response patterns and tuning properties showed higher complexity in the dorsal and intermediate nucleus than in the ventral nucleus of the lateral lemniscus (Figs. 4 and 6). Multiple best frequencies were found in 12 neurons, nine of them with harmonically related excitation maxima (Fig. 5c, d). Best frequencies of six of these harmonically tuned units could not be correlated with any harmonic components of the echolocation signal. Half of all multiple tuned neurons were located in the caudal dorsal nucleus the other half in the caudal intermediate nucleus. Synchronization of responses to sinusoidally frequency modulated (SFM) signals occurred in VNLL-units in the average up to modulation frequencies of 515 Hz (maximum about 800 Hz) whereas in the intermediate and dorsal nucleus of the lateral lemniscus responses were synchronized in the average only up to modulation frequencies of about 300 Hz (maximum about 600 Hz) (Figs. 7 and 8). A tonotopic arrangement of units was found in the intermediate nucleus of the lateral lemniscus with units having high best frequencies located medially and those with low best frequencies laterally. In the dorsal nucleus the tonotopic distribution was found to be fairly similar to that in the intermediate nucleus but much less pronounced. In more rostral parts of the dorsal nucleus additionally higher best frequencies predominated whereas in caudal areas of that nucleus and also of the intermediate nucleus low BFs were found more regularly.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional organization of the cochlear nucleus of rufous horseshoe bats (Rhinolophus rouxi): frequencies and internal connections are arranged in slabs

The functional organization of the cochlear nucleus (CN) was studied with physiological recording and anatomical tracing techniques. Recordings were made from single CN neurons to examine their temporal firing patterns to tone burst stimuli and their frequency tuning characteristics. Recording loci of individual neurons were carefully monitored in order to understand how the functional properties of a cell relate to its location within the CN. We found that tonal frequencies were systematically represented in each of the three CN divisions (anteroventral, AVCN; posteroventral, PVCN; dorsal, DCN). Eight temporal response patterns were observed in CN neurons when stimulated at units' best excitatory frequencies (BF). With a few exceptions, neurons in each CN division could generate all eight firing patterns with different distributions for the three division. A focal injection of horseradish peroxidase (HRP), at the end of the physiological study, to a group of neurons possessing a similar BF in one CN division resulted in anterograde labeling of nerve terminals in the other two divisions at precisely the areas where the same frequency band was processed in these divisions. Labeled terminals in each division were closely congregated in the form of a thin slab. The slab orientation was division specific whereas its location was frequency specific, which could be predicted on the basis of physiological data. HRP injections into the DCN also resulted in retrograde labeling of somata in the AVCN and PVCN. On the other hand, only DCN neurons were retrogradely labeled when HRP was injected into the AVCN or the PVCN. These data showed how the three CN divisions are internally connected. Furthermore, retrogradely labeled cells occupied the same slabs where we found anterogradely labeled nerve terminals. Additionally, in a group of bats, HRP was injected into various functionally (i.e., BF) identified regions of the central nucleus of the inferior coliculus (IC) to clarify the type and location of CN projecting neurons. Retrogradely labeled cells in individual CN divisions likewise were arranged in slabs whose locations in the CN nuclei depended on the BFs of neurons at the injection site in the IC. These results show that slabs represent units of functional organization (i.e., tonal frequency, local connection and central projection) in the CN.

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

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 bat's 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.

Noise-induced hearing loss can alter neural coding and increase excitability in the central nervous system

Responses of auditory neurons in the inferior colliculi of mice were studied longitudinally before and shortly after each animal was exposed to intense noise. Noise exposure caused expected losses in auditory sensitivity, but in 31 percent of the neurons studied, unexpected alterations of temporal patterns of action potentials were observed: certain suprathreshold stimuli that had evoked only transient "onset" responses or inhibition of spontaneous discharges prior to noise exposure came to elicit sustained excitation after exposure. Thus, noise-induced hearing loss can be associated with increases in neural responsivity and alterations of normal neural coding processes.

Time and Frequency domain processing in the inferior colliculus of echolocating bats

Tone bursts and frequency-modulated (FM) signals were presented to Mexican free-tailed bats and tuning curves, discharge patterns, and discharge latencies of single units in the inferior colliculus were recorded. Cells were broadly tuned to tone bursts, with most Q 10 values ranging from 3 to 20. However, in response to FM stimulation the discharges of neurons were closely synchronized to the time of occurrence of restricted frequency components within the FM sweep. These excitatory frequencies (EFs) were generally unaffected by changes in the starting frequency or intensity of the stimulus. Thus, in response to FM signals, the cells exhibited a much greater frequency selectivity than that observed following tone burst stimulation. Across the population of neurons sampled, EFs covering a wide frequency range were found, and the different EFs were represented in a systematic fashion within the colliculus. The frequencies in an FM biosonar signal or echo will thus be neurally represented both by the time of occurrence of neuronal discharges and by the location of the discharging cells within the nucleus. The potential role of this dual frequency coding in spectral and temporal processing of biosonar signals and echoes is discussed, with emphasis on the neural coding of target range.

Processing of noise by single units of the inferior colliculus of the bar Rhinolophus ferrumequinum

For inferior colliculus units the response patterns and the thresholds for pure tones and noise of variable bandwidth were determined. In a threshold-bandwidth plot the noise thresholds usually fell along two regression lines whose point of intersection established the size of the neuronal critical bandwidth (nCB). The relevance of the small nCBs (0.2-0.4 kHz) obtained for the frequency range of the constant frequency part of the orientation call is discussed. No fixed relation was found either between the nCBs and the neuronal critical ratios or between the size of nCB and the width of the tuning curve 3 dB above threshold of the best frequency.

Cochlear innervation in the greater horseshoe bat: demonstration of an acoustic fovea

The innervation of the cochlea of the greater horseshoe bat was investigated by different methods. The regional densities of the spiral ganglion neurons and of the inner and outer receptors were determined from surface specimens and histological sections. The pattern of the unmyelinated fibers was reconstructed in EM serial sections and the efferent pattern separately by localization of cholinesterase activity. The study reveals three regions each adapted to different auditory functions: (1) The region 1.3--5.4 mm from the basal end where the constant frequency segment of the orientation signal (around 83 kHz) is analysed. The neuronal structures of this region are similar to other mammals studied. Since, however, this region has widely expanded frequency mapping, the innervation density per octave is very high. In the region of this 'acoustic fovea' 25% of the receptors and 21% of the spiral ganglion neurons of the cochlea represent 10% of a single octave. (2) The region from 5.4 to 8 mm with frequencies from 40--80 kHz encompasses the frequency modulated segment of the orientation signal. This region is characterized by a high density of spiral ganglion neurons together with a short spiral course of the afferent fibers to the outer receptors. (3) The region from 8 mm to the apex (16 mm) represents frequencies lower than 40 kHz. Here neuronal elements, except for the efferents, are comparable to those of other mammalian cochleae. As important and surprising finding was that there is no efferent fiber to the outer hair cells in any part of the cochlea.