Improvement of the sediment ecosystem following diversion of an intertidal sewage outfall at the Fraser river estuary, Canada, with emphasis on Corophium salmonis (Amphipoda).

Primary treated sewage effluent from the city of Vancouver, Canada was deposited directly onto the intertidal ecosystem of Sturgeon bank, Fraser river estuary between 1962 and 1988. In response to the degraded sediment conditions an azoic zone developed near the discharge outfall. Effluent discharges into the intertidal zone were almost completely stopped in 1988 with the construction of a submerged outfall. Our studies, conducted between 1994 and 1996, showed considerable improvement in the environment of the mudflat ecosystem, including increased dissolved oxygen, decreased sediment chlorophyll, decreased organic material in the sediment, reduced heavy metals in surficial sediment and increased grain size. The amphipod Corophium salmonis, important in the food web for juvenile salmon and other fish species, recolonized the previously azoic location. At reference stations, C. salmonis density was similar to that observed in previous surveys two decades earlier. Our data strongly suggest that improvement or sediment conditions near the former sewage outfall was a major factor enabling colonization by C. salmonis.

Early bilateral deafening prevents calretinin up-regulation in the dorsal cortex of the inferior colliculus of aged CBA/CaJ mice.

This study was conducted to test the hypothesis that age-related calretinin (CR) up-regulation seen in the dorsal cortex of the inferior colliculus (ICdc) of old hearing CBA mice is dependent upon neural activity within the auditory pathway. We tested this hypothesis by bilaterally deafening young CBA/CaJ mice with kanamycin, and then aging them until 24 months. This manipulation mimics the lack of sound-evoked auditory activity experienced by old C57BL/6J mice, who are deaf and do not show CR up-regulation with age. Cell counts revealed that the density of CR+ cells in the ICdc of old hearing CBA mice was statistically different from old deafened CBA mice raised under identical conditions. Old hearing CBAs possessed an average of 27.54 more CR+ cells/100 microm2 than old deafened CBAs. When old deafened CBAs were compared to young hearing CBAs, young hearing C57s, and old deaf C57s, there was no significant difference in mean CR+ cell density in ICdc. Thus, only the old normal hearing CBAs showed an increase in CR+ cells with age, supporting the hypothesis that CR up-regulation depends upon sound-evoked activity. Moreover, these results demonstrate that up-regulation of CR expression was not simply due to a mouse strain difference.

An extralemniscal component of the mustached bat inferior colliculus selective for direction and rate of linear frequency modulations.

Frequency modulations (FMs) are prevalent in human speech, and are important acoustic cues for the categorical discrimination of phonetic contrasts. For bats, FM sweeps are also important for communication and are often the only component in echolocation calls. Auditory neurons tuned to the direction and rate of FM might underlie the encoding of rapid frequency transitions. In the mustached bat, we have discovered a population of such FM selective cells in an area interposed between the central nucleus of the inferior colliculus (ICC) and the nuclei of the lateral lemniscus (NLL). We believe this area to be the ventral extent of the external nucleus of the inferior colliculus (ICXv). To describe FM selectivity of neurons in the ICXv and to compare it to other midbrain nuclei, up- and down-sweeping linear FM stimuli were presented at different modulation rates. Extracellular recordings were made from 171 single units in the ICC, ICXv, and NLL of 10 mustached bats. In the ICXv, there was a much higher degree of FM selectivity than in ICC or NLL and a consistent preference for upward over downward FM sweeps. Anterograde and retrograde transport was examined following focal injections of wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) into ICXv. The main targets of anterograde transport were the deep layers of the superior colliculus and the suprageniculate division of the medial geniculate body. The primary site of retrograde transport was the nucleus of the central acoustic tract in the brainstem. Thus, the ICXv appears to be a part of the central acoustic tract, an extralemniscal pathway linking the auditory brainstem directly to a multimodal nucleus of the thalamus.

Temporal processing across frequency channels by FM selective auditory neurons can account for FM rate selectivity.

Auditory neurons tuned to the direction and rate of frequency modulations (FM) might underlie the encoding of frequency sweeps in animal vocalizations and formant transitions in human speech. We examined the relationship between FM direction and rate selectivity and the precise temporal interactions of excitatory and inhibitory sideband inputs. Extracellular single-unit recordings were made in the auditory midbrains of eight mustached bats. Up- and down-sweeping linear FM stimuli were presented at different modulation rates in order to determine FM selectivity. Brief tone pairs with varying interstimulus delays were presented in a forward masking paradigm to examine the relative timing of excitatory and inhibitory inputs. In the 33 units for which tone pair data were collected, a correspondence existed between FM rate selectivity and the time delays between paired tones. Moreover, FM directional selectivity was strongly linked to rate selectivity, because directional preferences were expressed only at certain rates and not others. We discuss how abnormalities in the relative timing of inputs could alter or abolish the selectivity of such neurons, and how such a mechanism could account for the perceptual deficits for formant transitions seen in certain children with phonological deficits.

Auditory motion induces directionally dependent receptive field shifts in inferior colliculus neurons.

This research focused on the response of neurons in the inferior colliculus of the unanesthetized mustached bat, Pteronotus parnelli, to apparent auditory motion. We produced the apparent motion stimulus by broadcasting pure-tone bursts sequentially from an array of loudspeakers along horizontal, vertical, or oblique trajectories in the frontal hemifield. Motion direction had an effect on the response of 65% of the units sampled. In these cells, motion in opposite directions produced shifts in receptive field locations, differences in response magnitude, or a combination of the two effects. Receptive fields typically were shifted opposite the direction of motion (i.e., units showed a greater response to moving sounds entering the receptive field than exiting) and shifts were obtained to horizontal, vertical, and oblique motion orientations. Response latency also shifted as a function of motion direction, and stimulus locations eliciting greater spike counts also exhibited the shortest neural latency. Motion crossing the receptive field boundaries appeared to be both necessary and sufficient to produce receptive field shifts. Decreasing the silent interval between successive stimuli in the apparent motion sequence increased both the probability of obtaining a directional effect and the magnitude of receptive field shifts. We suggest that the observed directional effects might be explained by "spatial masking," where the response of auditory neurons after stimulation from particularly effective locations in space would be diminished. The shift in auditory receptive fields would be expected to shift the perceived location of a moving sound and may explain shifts in localization of moving sources observed in psychophysical studies. Shifts in perceived target location caused by auditory motion might be exploited by auditory predators such as Pteronotus in a predictive tracking strategy to capture moving insect prey.

Age-related alteration in processing of temporal sound features in the auditory midbrain of the CBA mouse.

The perception of complex sounds, such as speech and animal vocalizations, requires the central auditory system to analyze rapid, ongoing fluctuations in sound frequency and intensity. A decline in temporal acuity has been identified as one component of age-related hearing loss. The detection of short, silent gaps is thought to reflect an important fundamental dimension of temporal resolution. In this study we compared the neural response elicited by silent gaps imbedded in noise of single neurons in the inferior colliculus (IC) of young and old CBA mice. IC neurons were classified by their temporal discharge patterns. Phasic units, which accounted for the majority of response types encountered, tended to have the shortest minimal gap thresholds (MGTs), regardless of age. We report three age-related changes in neural processing of silent gaps. First, although the shortest MGTs (1-2 msec) were observed in phasic units from both young and old animals, the number of neurons exhibiting the shortest MGTs was much lower in old mice, regardless of the presentation level. Second, in the majority of phasic units, recovery of response to the stimulus after the silent gap was of a lower magnitude and much slower in units from old mice. Finally, the neuronal map representing response latency versus best frequency was found to be altered in the old IC. These results demonstrate a central auditory system correlate for age-related decline in temporal processing at the level of the auditory midbrain.

Calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body declines with age in C57BL/6, but not CBA/CaJ, mice.

This study compared calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body (MNTB) in young (3-4 month old) and old (24-26 month old) CBA/CaJ mice, and young (3-4 month old), middle-aged (6.5-8.5 month old), and old (24-29 month old) C57BL/6 mice. C57BL/6 mice exhibit progressively more severe peripheral (sensorineural) hearing loss between 4 and 12 months of age, whereas CBA/CaJ mice show little change in peripheral sensitivity until very late in life. We obtained auditory brainstem response audiograms on all subject mice. Old CBA mice were selected for study whose audiograms matched those of young CBA and C57 controls. Middle-aged C57 mice showed elevated thresholds indicative of peripheral degeneration. Brain sections were reacted with anti-calbindin D-28k (CB). Staining patterns in Nissl and anti-CB material were characterized and cells were counted. We found no significant change in the number of CB+ cells or the total number of cells in the MNTB of old CBA mice compared to young controls. However, the mean number of CB+ cells decreased by 11% in middle-aged, and by 14.8% in old C57 mice. Since the decline in C57 mice was significant by 6.5-8.5 months of age, the decrease could be the consequence of a loss of input from the cochlear nucleus where cell numbers are known to decline by this age in this strain. The total number of neurons in MNTB assessed from Nissl material showed a more modest 7.1% decline with age in C57 mice, implying that the greater loss of CB immunoreactive cells with age cannot be completely attributed to a reduction in the total number of cells.

Age-related changes in calbindin D-28k and calretinin immunoreactivity in the inferior colliculus of CBA/CaJ and C57Bl/6 mice.

This study examines calbindin D-28k and calretinin immunoreactivity in the inferior colliculus (IC) of young and old mice of two strains. The CBA/CaJ mouse maintains good hearing until very late in life, whereas the C57Bl/6 strain exhibits severe sensorineural hearing loss at an early age. Young and old mice of both strains were selected with matching auditory brainstem response audiograms and gap detection thresholds. Brain sections were reacted with anti-calbindin D-28k (CB) and anti-calretinin (CR). Staining patterns were characterized and cell counts performed. CB immunoreactivity was high only in the nucleus of the commissure (NCO); counts revealed a 22.3% decrease in the number of CB+ cells in old CBA mice and a 25.1% decrease in old C57 mice. Calretinin immunoreactivity was high in the pericentral regions of the IC, but the central nucleus was devoid of CR+ cells. The dorsal cortex, lateral nucleus, and NCO showed increases of 42.3, 49.0, and 61%, respectively, in the number of CR+ cells, but only in the old CBA mice. No significant change was observed in the old C57 mice. Whereas decreases in CB immunoreactivity are common with age, this study is the first to report an age-related increase in CR immunoreactivity in the auditory system. The increase in CR+ cells is a possible compensatory adaptation to the decrease in CB+ cells. That the number of CR+ cells remains constant with age in C57 mice suggests this compensation may depend upon stimulus-driven activity, but this requires further study.

Neural correlates of behavioral gap detection in the inferior colliculus of the young CBA mouse.

The gap detection paradigm is frequently used in psychoacoustics to characterize the temporal acuity of the auditory system. Neural responses to silent gaps embedded in white-noise carriers, were obtained from mouse inferior colliculus (IC) neurons and the results compared to behavioral estimates of gap detection. Neural correlates of gap detection were obtained from 78 single neurons located in the central nucleus of the IC. Minimal gap thresholds (MGTs) were computed from single-unit gap functions and were found to be comparable, 1-2 ms, to the behavioral gap threshold (2 ms). There was no difference in MGTs for units in which both carrier intensities were collected. Single unit responses were classified based on temporal discharge patterns to steady-state noise bursts. Onset and primary-like units had the shortest mean MGTs (2.0 ms), followed by sustained units (4.0 ms) and phasic-off units (4.2 ms). The longest MGTs were obtained for inhibitory neurons (x = 14 ms). Finally, the time-course of behavioral and neurophysiological gap functions were found to be in good agreement. The results of the present study indicate the neural code necessary for behavioral gap detection is present in the temporal discharge patterns of the majority of IC neurons.

Calbindin-like immunoreactivity in the central auditory system of the mustached bat, Pteronotus parnelli.

With the aid of a polyclonal antibody specific for Calbindin D-28k, we studied the distribution of this calcium-binding protein in the central auditory system of the mustached bat, Pteronotus parnelli. Components of the cochlear nucleus (CN) that were calbindin-positive (cabp(+] included the root of the auditory nerve, multipolar and globular bushy cells in the anteroventral CN, multipolar and octopus cells in the posteroventral CN, and small and medium-size cells in the dorsal CN. Not stained were spherical bushy cells of the anteroventral CN and pyramidal/fusiform cells in the dorsal CN. In the superior olivary complex, labeled cells were found in the lateral and medial nuclei of the trapezoid body, the ventral and ventromedial periolivary nuclei, and the anterolateral periolivary nucleus. No cellular labeling was seen in the lateral superior olive. In the medial superior olive, only marginal cells were cabp(+). Labeled fibers could be seen surrounding the gosts of unlabeled cells in both the latter nuclei. Most cells in the intermediate nucleus and the columnar division of the ventral nucleus of the lateral lemniscus were cabp(+). However, the dorsal nucleus was cabp(-). A group of cabp(+) cells was also seen in the paralemniscal zone. The inferior colliculus had a relatively low density of cabp(+) cells. Labeled cells were more common in the caudal half of the central nucleus, and in the external nucleus and dorsal cortex. In the auditory thalamus, nearly every cell in the medial geniculate body was cabp(+), but those in the suprageniculate nucleus and in the posterior group did not stain. Small cells in the intermediate layer and giant cells in the deep layers of the superior colliculus were densely cabp(+). In the pons, cabp(+) cells and neuropil could be seen in the medial and lateral pontine nuclei (pontine gray). In conclusion, calbindin-like immunoreactivity was found in most of the brainstem auditory system, as well as in regions associated with acoustic orientation or control of vocalization. However, except for a minority of cells of the medial superior olive, it is conspicuously absent from the nuclei receiving binaural input below the level of the inferior colliculus.

Facilitation and Delay Sensitivity of Auditory Cortex Neurons in CF - FM Bats, Rhinolophus rouxi and Pteronotus p.parnellii.

Responses of auditory neurons to complex stimuli were recorded in the dorsal belt region of the auditory cortex of two taxonomically unrelated bat species, Rhinolophus rouxi and Pteronotus parnellii parnellii, both showing Doppler shift compensation behaviour. As in P.p.parnellii (Suga et al., J. Neurophysiol., 49, 1573 - 1626, 1983), cortical neurons of R.rouxi show facilitated responses to pairs of pure tones or frequency modulations. Best frequencies for the two components lie near the first and second harmonic of the echolocation call but are in most cases not harmonically related. Neurons facilitated by pairs of pure tones show little dependence on the delay between the stimuli, whereas pairs of frequency modulations evoke best facilitated responses at distinct best delays between 1 and 10 ms. Facilitated neurons are found in distinct portions of the dorsal cortical belt region, with a segregation of facilitated neurons responding to pure tones and to frequency modulations. Non-facilitated neurons are found throughout the field. Neurons are topographically aligned with increasing best delays along a rostrocaudal axis. The best delays between 2 and 4 ms are largely overrepresented numerically, and occupy approximately 56% of the cortical area containing facilitated neurons. A functional interpretation of the large overrepresentation of best delays approximately 3 ms is proposed. Facilitated neurons are located almost entirely within layer V of the dorsal field.

The emission pattern of vocalizations and directionality of the sonar system in the echolocating bat, Pteronotus parnelli.

The radiation patterns of the first three harmonics (approx. 30, 60, 90 kHz) of the mustached bat biosonar signal were measured from vocalizations elicited by cortical microstimulation. The primary foci of the acoustic beam patterns were in front of the mouth but somewhat below the horizontal plane. The prominent second and third harmonics showed sharp cutoffs between 20 degrees and 30 degrees lateral to the midline. Sidelobes were found, suggesting the influence of some vocal tract interference. When compared with previously measured estimates of the directionality of the auditory system, the vocal emission patterns are roughly complementary: Regions of maximum auditory sensitivity are found in areas of submaximal power for the sonar pulse beam pattern. The result is that, for the two most important harmonics, the "biosonar system" (i.e., vocal beam pattern plus receiver directionality) has a broader and more uniform directionality than either component alone. Therefore, within a limited region of space, echo amplitude will vary less as a function of angular displacement. This reduces the confounding influences of absolute sound pressure level on interaural intensity differences.

ON-OFF units in the mustached bat inferior colliculus are selective for transients resembling "acoustic glint" from fluttering insect targets.

Of 311 single units studied in the central nucleus of the inferior colliculus (ICC) in 18 mustached bats (Pteronotus parnelli), a small but significant population (13%) of cells with on-off discharge patterns to tone bursts at best frequency (BF) was found in the dorsoposterior division. In contrast to units with the same BF's but other discharge patterns, the majority of ON-OFF units were unresponsive to sinusoidally amplitude-modulated tone bursts (SAM). To define the contribution of linear and nonlinear components to the responses of ICC neurons to amplitude modulation, we tested some of these neurons with a long, seamlessly repeating pseudorandom sequence of ternary amplitude-modulated tones at BF. Wiener-like kernels were subsequently derived from cross-correlation of spikes with acoustic events in the sequence. These kernels provided estimates of neural impulse responses that proved unusual in SAM-unresponsive ON-OFF units. First, their estimated impulse response had no linear component. Second, the predicted second-order impulse responses to both increments and decrements in stimulus intensity were long (about 20 ms) and nearly identical in shape: triphasic, with the positive phase bounded by leading and trailing negative periods. The similar shape of responses to increments and decrements in these neurons suggests a full-wave rectifier. The triphasic, initially negative second-order prediction of the impulse response accounted for an unusual result in experiments measuring the recovery cycle of ON-OFF units using a pair of identical stimulus pulses separated by various time delays. This recovery cycle can be related to their response to amplitude modulation. As the delay between two brief, near-threshold BF tone bursts decreased, the response to the first tone diminished, rather than to the second. The second-order prediction of this experiment derived from impulse responses obtained with pseudorandom noise suggests that, at short interpulse intervals, the initial negative phase of the response to the later stimulus cancels the positive phase of the response to the first. Such cancellation at short interpulse intervals may help explain why the majority of ON-OFF units are unresponsive to SAM. The unusual properties of these ON-OFF units make them ideally suited to respond selectively to infrequent acoustic transients superimposed on an ongoing background of modulation. Such patterns are commonly encountered by mustached bats foraging in cluttered habitats for small, fluttering insects, which generate "acoustic glints" upon a background of modulated echoes from the surroundings (Schnitzler et al. 1983; Henson et al. 1987).

Functional organization of mustached bat inferior colliculus: I. Representation of FM frequency bands important for target ranging revealed by 14C-2-deoxyglucose autoradiography and single unit mapping.

The representation in the inferior colliculus of the frequency modulated (FM) components of the first (25-30 kHz) and second (50-60 kHz) harmonic of the sonar signal of the mustached bat, which may be important for target range processing, was investigated by using the 2-deoxyglucose (2-DG) technique and single-unit mapping. In the 2-DG experiments, bats presented with second harmonic FM stimuli alone showed uptake of label in specific regions of the central nucleus and dorsal cortex of the inferior colliculus, and the nucleus of the brachium. In the central nucleus, a dorsoventrally and mediolaterally elongated slab at the caudal border of the anterolateral division was observed. Labeling in the dorsal cortex was contiguous with this band. Bats stimulated with pairs of first and second harmonic FM stimuli separated by short time delays showed similar patterns of labeling, with the addition of another dorsoventrally elongated region of uptake in the more rostral part of the anterolateral division, associated with label in the dorsal cortex. By comparison to control cases exposed to delayed pairs of first and third harmonic signals, or to a second harmonic constant-frequency tone burst at the bat's reference frequency (ca. 60 kHz), we deduced that this additional region of uptake was attributable to the first harmonic FM component. To elucidate further the details of the tonotopic organization and to correlate the frequency representation with anatomical features of the IC, fine-grained maps of single-unit best frequencies were obtained in the central nucleus. Isofrequency contours were reconstructed by computer from five bats after focal, iontophoretic injection of horseradish peroxidase to locate the penetrations and trace connections of the FM2 area. We found that the tissue volume representing FM2 frequencies (50-60 kHz) showed approximately a sixfold overrepresentation for this frequency band. This region occupied most of the caudal portion of the anterolateral division of the central nucleus. Only a single tonotopic representation was found in the central nucleus, consistent with the pattern seen in other mammals. However, isofrequency contours in the anterolateral division were oriented dorsoventrally, approximately parallel to the coronal plane. The small band of frequencies (ca. 60-62 kHz) associated with the dominant constant-frequency component of the biosonar signal was even more dramatically overrepresented (40x) and was confined to the dorsoposterior division, as previously reported by Zook et al. (1985, 530-456).(ABSTRACT TRUNCATED AT 400 WORDS)

Functional organization of mustached bat inferior colliculus: II. Connections of the FM2 region.

Echolocating bats estimate target distance by analyzing the time delay between frequency-modulated portions of their emitted ultrasonic vocalizations and the resultant echoes. In the companion paper we investigated, in the central nucleus of the inferior colliculus, the representation of the predominant second-harmonic frequency-modulated component (FM2) of the mustached bat biosonar signal (O'Neill et al.: J. Comp. Neurol. 283:000-000,'89). In the present paper we report the connections of this part of the colliculus, as determined by focal, iontophoretic injections of HRP following single-unit mapping of the FM2 representation. It was found that the major inputs to the FM2 region of the inferior colliculus come from the contralateral cochlear nucleus; ipsilaterally from the medial superior olive, periolivary nuclei, and ventral and intermediate nuclei of the lateral lemniscus; and bilaterally from the lateral superior olive and dorsal nucleus of the lateral lemniscus. This study identifies for the first time those specific regions of brainstem nuclei providing input to the central nucleus of the inferior colliculus that process FM2 information in the mustached bat. The primary outputs of the FM2 region project to the medial and dorsal divisions of the medial geniculate body. In sharp contrast to other mammals, we found little evidence of connections to the ventral division of the medial geniculate. Other regions receiving significant inputs from the FM2 area include the deep superior colliculus ipsilaterally and the ipsilateral lateral pontine nuclei. Some fibers also terminated near the midline in the dorsal midbrain periaqueductal gray. Sparse intrinsic connections were also seen to the ipsilateral dorsoposterior division of the central nucleus and to the contralateral inferior colliculus at a location homologous to the injection site in the anterolateral division. The finding that FM2 projections to the medial geniculate heavily favor the medial and dorsal divisions is consistent with the location of "FM-FM" delay-dependent facilitation neurons found by Olsen (Processing of Biosonar Information by the Medical Geniculate Body of the Mustached Bat, Pteronotus parnellii. Dissertation, Washington Univ., St. Louis, '86) in these divisions, and with thalamocortical projection patterns in this species. These findings demonstrate that for the FM portions of the biosonar signal, a transformation from a tonotopic form of processing to a more specialized, convergent pattern of organization occurs at the level of the inferior colliculus outputs.

Topographic representation of vocal frequency demonstrated by microstimulation of anterior cingulate cortex in the echolocating bat, Pteronotus parnelli parnelli.

1. A midline region of brain dorsal and anterior to the corpus callosum, presumably anterior cingulate cortex, has been explored for its role in the production of vocalization in the mustached bat, Pteronotus p. parnelli. 2. Vocalizations elicited by microstimulation were virtually indistinguishable from natural biosonar sounds. The spectral content, relative intensity of harmonic components, and durations of emitted pulses are comparable to spontaneous emissions. 3. The frequencies of elicited vocalizations were within the range typically used by the mustached bat during Doppler-shift compensation. The frequency of the second-harmonic constant-frequency component (CF2) covered the range from 57-62 kHz, but was most commonly emitted at frequencies of 59-61 kHz. 4. An increase in the frequency of vocalizations over a number of consecutive pulses towards a steady-state plateau is evident in both spontaneous vocalizations and emissions elicited by microstimulation just above threshold. Increasing the stimulus intensity caused the frequency of emissions to approach the steady state more rapidly. 5. The anterior cingulate cortex appears to be organized topographically for increasing frequency of elicited biosonar sounds along a rostrocaudal axis. The area from which biosonar emissions were elicited was overrepresented for a 2 kHz band of frequencies just below the bats' CF2 resting frequency. Audible vocalizations with a complex spectrum resembling social cries can also be elicited by microstimulation, but only in an area that is adjacent and posterior to the biosonar region. 6. Some examples of both elicited and spontaneous vocalizations contained a relative intensity pattern of the harmonic components which deviated from the typical pattern. This suggests that mustached bats are capable of actively altering the spectrum of their pulses to subserve different tasks in echolocation.

Directional sensitivity of the auditory midbrain in the mustached bat to free-field tones.

To ascertain the directional characteristics of the auditory system in the mustached bat, Pteronotus parnellii, we measured the summated neural response at the lateral lemniscus (N4) in response to pure tones at 30, 60 and 90 kHz, frequencies that are typical of the harmonics of this species' biosonar signal. Stimuli were presented at various vertical and horizontal locations in the contralateral hemifield. Intensity-response functions were measured at different horizontal locations for the second harmonic, and showed no variation in shape with variations in azimuth. There was little difference in directionality measured from either threshold or amplitude of N4 potentials. Our results show that areas of maximum sensitivity (best areas) were significantly different for each of the harmonics (P less than 0.05). The centers of the best areas were: first harmonic (30 kHz), 39 degrees azimuth and -19 degrees elevation; second harmonic, 20 degrees azimuth and 0 degrees elevation; and third harmonic, 12 degrees azimuth and -11 degrees elevation. Thus, with increasing frequency best areas shifted toward the vertical midline. Directionality to first harmonic stimuli was broader than to either of the two higher harmonics.

Responses to pure tones and linear FM components of the CF-FM biosonar signal by single units in the inferior colliculus of the mustached bat.

The responses of 682 single-units in the inferior colliculus (IC) of 13 mustached bats (Pteronotus parnellii parnellii) were measured using pure tones (CF), frequency modulations (FM) and pairs of CF-FM signals mimicking the species' biosonar signal, which are stimuli known to be essential to the responses of CF/CF and FM-FM facilitation neurons in auditory cortex. Units were arbitrarily classified into 'reference frequency' (RF), 'FM2' and 'Non-echolocation' (NE) categories according to the relationship of their best frequencies (BF) to the biosonar signal frequencies. RF units have high Q10dB values and are tuned to the reference frequency of each bat, which ranged between 60.73 and 62.73 kHz. FM2 units had BF's between 50 and 60 kHz, while NE units had BF's outside the ranges of the RF and FM2 classes. PST histograms of the responses revealed discharge patterns such as 'onset', 'onset-bursting' (most common), 'on-off', 'tonic-on','pauser', and 'chopper'. Changes in discharge patterns usually resulted from changes in the frequency and/or intensity of the stimuli, most often involving a change from onset-bursting to on-off. Different patterns were also elicited by CF and FM stimuli. Frequency characteristics and thresholds to CF and FM stimuli were measured. RF neurons were very sharply tuned with Q10dB's ranging from 50-360. Most (92%) also responded to FM2 stimuli, but 78% were significantly more sensitive (greater than 5 dB) to CF stimuli, and only 3% had significantly lower thresholds to FM2. The best initial frequency for FM2 sweeps in RF units was 65.35 +/- 2.138 kHz (n = 118), well above the natural frequency of the 2nd harmonic. FM2 and NE units were indistinguishable from each other, but were quite different from RF units: 41% of these two classes had lower thresholds to CF, 49% were about equally sensitive, and 10% had lower thresholds to FM. For FM2 units, mean best initial frequency for FM was 60.94 kHz +/- 3.162 kHz (n = 114), which is closely matched to the 2nd harmonic in the biosonar signal. Very few units (5) responded only to FM signals, i.e., were FM-specialized. The characteristics of spike-count functions were determined in 587 units. The vast majority (79%) of RF units (n = 228) were nonmonotonic, and about 22% had upper-thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)

Encoding of target range and its representation in the auditory cortex of the mustached bat.

The time course of acoustic events is a critical element for the recognition of biologically meaningful sounds. Echolocating bats analyze the time intervals between their emitted biosonar pulses and the echoes returning from objects to assess target distance (range). In this study, we have explored the auditory cortex of the mustached bat, Pteronotus parnellii rubiginosus, using pairs of acoustic stimuli mimicking the multiharmonic biosonar signals (pulses) used by this species and their echoes. A discrete field of auditory cortex dorsorostral to the tonotopically organized primary field contains neurons which are insensitive to pure tone, frequency-modulated (FM), or noise stimuli presented singly. Rather, they respond strongly to pairs of stimuli, specifically, the fundamental FM component of the pulse paired with an FM component of one of the higher harmonics of the echo. We call these neurons FM1-FMn facilitation neurons. There are three separate longitudinal clusters in this cortical area containing FM1-FM2, FM1-FM3, and FM1-FM4 neurons, respectively. Moreover, FM1-FMn neurons are specifically sensitive to the time delay between the two FM components, i.e., the time delay of the echo from the pulse. Thus, they can decode target range. Two types of delay-sensitive neurons were found. Tracking neurons, whose response to echo delay varied according to repetition rate and stimulus duration, were found rarely. Delay-tuned neurons, which were tuned to specific time delays (best delays) of the echo from the pulse, were much more evident. Both types of neurons are organized into columns with similar best delays, and the best delay of delay-tuned neurons was found to increase systematically along the cortical surface in the rostrocaudal direction. This area, therefore, contains a neural representation of target range along this best delay axis. Such an axis exists in each of the clusters of FM1-FM2, FM1-FM3, and FM1-FM4 neurons. This is a new type of cortical organization which is not tonotopic but which represents an important acoustic cue related to the time course of acoustic events.