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Evidence of varying hearing sensitivity according to the target distance was obtained in a false killer whale Pseudorca crassidens during echolocation. Auditory evoked potentials (AEPs) triggered by echolocation clicks were recorded. The target distance varied from 1 to 6 m. The records contained AEPs to the self-heard emitted click and AEPs to the echoes. Mean level of echolocation clicks depended on distance (the longer the distance, the higher the click level), however, the effect of click level on AEP amplitude was eliminated by extracting AEPs to clicks of certain particular levels. The amplitude of the echo-provoked AEP was almost independent of distance, however, the amplitude of the AEP to the emitted click, did depend on distance within a range from 1 to 4 m: the longer the distance, the higher the amplitude. The latter result is interpreted as confirmational evidence that the animal is capable of varying hearing sensitivity according to target distance. The variation of hearing sensitivity may help to compensate for the echo attenuation with distance; as a secondary effect, this variation manifested itself in a variation of the amplitude of the AEP to emitted clicks.

The total number, size, topographic distribution and peak density of ganglion cells were studied in retinal wholemounts of the fur seal, Callorhinus ursinus. The cell distribution showed a distinct zone of high ganglion cell density. It was located in the temporal retinal quadrant, near the horizontal meridian, 10-12 mm (25-31 degrees) from the optic disk. The peak cell density in this zone was 812-1332 cells/mm2 (mean 1053 cells/mm2), i.e. 125-205 cells/deg2 (mean 162 cells/deg2). These data predict a retinal resolution of 5.6-7.1 cycle/deg. The ganglion cell soma size ranged from 10 to 50 microns. Cell size histograms were bimodal in shape with modes below and above 30 microns.

Auditory evoked potentials (AEPs) were recorded during echolocation in a false killer whale Pseudorca crassidens. An electronically synthesized and played-back ("phantom") echo was used. Each electronic echo was triggered by an emitted biosonar pulse. The echo had a spectrum similar to that of the emitted biosonar clicks, and its intensity was proportional to that of the emitted click. The attenuation of the echo relative to the emitted click and its delay was controlled by the experimenter. Four combinations of echo attenuation and delay were tested (-31 dB, 2 ms), (-40 dB, 4 ms), (-49 dB, 8 ms), and (-58 dB, 16 ms); thus, attenuation and delay were associated with a rate of 9 dB of increased attenuation per delay doubling. AEPs related to emitted clicks displayed a regular amplitude dependence on the click level. Echo-related AEPs did not feature amplitude dependence on echo attenuation or emitted click levels, except in a few combinations of the lowest values of these two variables. The results are explained by a hypothesis that partial forward masking of the echoes by the preceding emitted sonar pulses serves as a kind of automatic gain control in the auditory system of echolocating odontocetes.

Using rippled noise probes, spectrum-pattern resolution was measured with and without a narrow-band noise masker. Diotic presentation of both the probe and masker (S(0)N(0) mode) resulted in decreased spectrum resolution as compared to the control (no masker) conditions. The effects of the low- and on-frequency maskers differed quantitatively, however in both cases the ability to discriminate the probe spectrum pattern was suppressed completely when the masker/probe level ratio exceeded 10dB (on-frequency masker) or 10-25dB, depending on the probe level (low-frequency masker). The effect of the high-frequency masker was negligible. Slight but noticeable releasing of the spectrum-pattern resolution was found when the probe was presented to both ears in-phase and the masker counter-phase (S(0)N(pi) mode). In conditions of the probe delivered to one ear and the masker to the other ear (S(L)N(R) mode), the effect on the spectrum-pattern resolution was slight or negligible within a wide range of the noise/probe ratio.

Hearing sensitivity was measured in a false killer whale during echolocation. Sensitivity was measured using probe stimuli as sinusoidally amplitude modulated signals with a 22.5-kHz carrier frequency and recording auditory evoked potentials as envelope-following responses. The probes were presented and responses were recorded during short 2-s periods when the animal echolocated to detect the presence or absence of a target in a go/no-go paradigm. In the target-absent trials, a hearing threshold of 90.4 dB re 1 muPa was found; in the target-present trials, the threshold was 109.8 dB. Thus, a 19.4-dB difference was found between thresholds in the target-present and target-absent trials. To check the possibility that this difference was the result of different masking degree of the probe by the emitted sonar clicks, click statistics were investigated in similar trials. No indication was found that the energy of the emitted clicks was higher in the target-present than in target-absent trials; on the contrary, mean click level, mean number of clicks per train, and overall train energy was slightly higher in the target-absent trials. Thus the data indicate that the hearing sensitivity of the whale varied depending on target presence or absence.

While there has been recent concern about the effects of sound on marine mammals, including polar bears, there are no data available measuring the hearing of any bear. The in-air hearing of three polar bears was measured using evoked auditory potentials obtained while tone pips were played to three individually anaesthetized bears at the Kolmården Djurpark. Hearing was tested in half-octave steps from 1 to 22.5 kHz. Measurements were not obtainable at 1 kHz and best sensitivity was found in the range from 11.2–22.5 kHz. Considering the tone pips were short and background noise measurements were available, absolute measurements were estimated based on an assumed mammalian integration time of 300 ms. These data show sensitive hearing in the polar bear over a wide frequency range and should cause those concerned with the introduction of anthropogenic noise into the polar bear's environment to operate with caution.

Transmitted biosonar pulses, and the brain auditory evoked potentials (AEPs) associated with those pulses, were synchronously recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP amplitude was investigated as a function of the transmitted biosonar pulse source level. For that, a few thousand of the individual AEP records were sorted according to the spontaneously varied amplitude of synchronously recorded biosonar pulses. In each of the sorting bins (in 5-dB steps) AEP records were averaged to extract AEP from noise; AEP amplitude was plotted as a function of the biosonar pulse source level. For comparison, AEPs were recorded to external (in free field) sound pulses of a waveform and spectrum similar to those of the biosonar pulses; amplitude of these AEPs was plotted as a function of sound pressure level. A comparison of these two functions has shown that, depending on the presence or absence of a target, the sensitivity of the whale's hearing to its own transmitted biosonar pulses was 30 to 45 dB lower than might be expected in a free acoustic field.

Hearing thresholds as a function of sound-source azimuth were measured in bottlenose dolphins using an auditory evoked potential (AEP) technique. AEP recording from a region next to the ear allowed recording monaural responses. Thus, a monaural directivity diagram (a threshold-vs-azimuth function) was obtained. For comparison, binaural AEP components were recorded from the vertex to get standard binaural directivity diagrams. Both monaural and binaural diagrams were obtained at frequencies ranging from 8 to 128 kHz in quarter-octave steps. At all frequencies, the monaural diagram demonstrated asymmetry manifesting itself as: (1) lower thresholds at the ipsilateral azimuth as compared to the symmetrical contralateral azimuth and (2) ipsilateral shift of the lowest-threshold point. The directivity index increased with frequency: at the ipsilateral side it rose from 4.7 to 17.8 dB from 11.2 to 128 kHz, and from 10.5 to 15.6 dB at the contralateral side. The lowest-threshold azimuth shifted from 0 degrees at 90-128 kHz to 22.5 degrees at 8-11.2 kHz. The frequency-dependent variation of the lowest-threshold azimuth indicates the presence of two sound-receiving apertures at each head side: a high-frequency aperture with the axis directed frontally, and a low-frequency aperture with the axis directed laterally.

Auditory brainstem responses (ABRs) were recorded in a false killer whale while the animal echolocated a target. The ABR collection was triggered by echolocation clicks of the animal. In these conditions, the recorded ABR pattern contained a duplicate set of waves. A comparison of ABR wave delays recorded during echolocation with those recorded during regular external stimulation with experimenter generated clicks showed that the first set of waves may be a response to the emitted click whereas the second one may be a response to the echo. Both responses, to the emitted click and to the echo, were of comparable amplitude in spite of the intensity difference of these two sounds that may differ by more than 40 dB near the animal’s head. This finding indicates the presence of some mechanism of releasing responses to echoes from masking by loud emitted clicks. The evoked-potential method may be productive to investigate these mechanisms.

Invariant target recognition by sonar requires assessment of the target strength invariably of distance despite wide variation of the echo level. Information on the echo delay and the level of the emitted pulse allows computing of the target strength. The question is: which particular mechanisms perform this computation in the biosonar of odontocetes? Investigations of the auditory evoked potentials (AEPs) during echolocation in several odontocetes have shown that comparison of the emitted pulse level, echo delay, and echo level is based on the forward masking of the echo-response by the preceding self-heard emitted click. Prolongation of the echo delay results in releasing of the echo-related AEP from masking. This release from masking compensated for the echo attenuation with distance. As a result, the echo-related AEP amplitude depended on the target strength and little depended on the target distance. Moreover, the forward-masking duration depended on the level of the emitted pulse. As a result, the echo-related AEP featured amplitude little depended on the level of the emitted pulse. The constancy of the echo-related AEP amplitude indicates the dependence of sensation level of the echo only on the target strength and the independence of both emitted click level and target distance.

Work on hearing during echolocation has demonstrated that a whale was capable of changing its hearing sensitivity while it echolocated, perhaps to protect its hearing from its own intense emitted pulses. Would a whale similarly change hearing sensitivity when warned prior to receiving a loud sound? Hearing was measured using auditory evoked potentials (AEP). The whale had been trained to station within a hoop while wearing surface electrodes. Baseline AEP dependence on test-sound level and an auditory threshold were first established for a 20 kHz tone. In a second phase, the test sound was followed by a sudden increase in amplitude up to 170 dB re 1 μPa. Thus, the faint test sounds took on the role of a warning signal for the ensuing loud (unconditioned) sound. After a few trials, the test stimuli revealed a substantial reduction of hearing sensitivity before the loud sound. When the delay between the warning tone onset and loud tone was short (varied randomly from 1 to 9 s), the whale increased its hearing thresholds (reduced sensitivity) by around 13 dB. The data indicate that: (1) the whale learned to change hearing sensitivity when warned that the loud sound was about to arrive, and (2) the learning acted only when warnings were immediate.

The visual system of aquatic mammals is of special interest because of its mostly amphibious mode of action. Although truely aquatic mammals (cetaceans, sirenians) spend their entire lives in water, their aerial mode of breathing confines them to the superficial water layer. Semiaquatic mammals (pinnipeds) spend a significant proportion of time on land. Therefore, the visual system in most of aquatic mammals functions both in air and water, though these two media have very different optical features and thus impose different requirements on the visual system.

Pinnipeds and sirenians have not been subjects of as extensive investigation of hearing abilities and mechanisms as cetaceans have been. The studies performed on these animals did not reveal extreme hearing capabilities or echolocation. Nevertheless, their hearing system presents a great opportunity for understanding the various ways they have adapted to an aquatic environment.

Somatic sense in cetaceans did not attract as much attention of investigators as the auditory and visual ones. So available data in this field are restricted. Morphological studies have shown the presence of both incapsulated and free nerve endings in the dolphin’s skin, especially numerous at the head and snout, around the blowhole, and around the anus and genital slit (Palmer and Weddel, 1964; Harrison and Thurley, 1974; Ling, 1974; Herman and Tavolga, 1980). This is an indication of developed tactile sensitivity of these regions. In addition, some river dolphins have vibrissae along the upper lip area (Layne and Caldwell, 1964; Ling, 1977). The trigeminal nerve and the trigeminal nucleus are very well developed; the trigeminal nerve is exceeded in size only by the auditory nerve (Breathnach, 1960), which is also an indication of a predominating role of tactile sensitivity in the head region as compared to other body regions.

The auditory system of cetaceans, since they are capable of underwater hearing and adapted for echo location, has attracted major interest for many years. More precisely speaking, one of the two cetacean suborders, Odontoceti (toothed whales, dolphins, and porpoises) was a subject of a particular interest. Probably all of them (at least, all species investigated to date) are capable of active echolocation. For echolocation, they use ultrasonic signals ranging to higher than 100 kHz (Kellogg, 1959; Norris et al., 1961; Norris, 1969; Au, 1993). Some information on auditory perception of odontocetes obtained in behavioral conditioning studies is presented in reviews by Popper (1980), Fobes and Smock (1981), Watkins and Wartzok (1985), and Au (1993).

Auditory brainstem responses (ABR) to clicks and noise bursts of various frequency bands and intensities were recorded in two bottlenosed dolphins, Tursiops truncatus. The purpose was to assess contributions of various parts of the cochlear partition to ABR and travelling wave velocity in the cochlea. At band-pass filtered stimuli (1-0.25 oct wide), ABR amplitude increased with increasing stimulus frequency, thus indicating higher contribution of basal cochlear parts. At high-pass and low-pass filtered stimuli, ABR amplitude increased with passband widening. However, the sum of all narrow-band contributions was a waveform of higher amplitude than the real ABR evoked by the wide-band stimulus. Applying a correction based on an assumption that the 'internal spectrum' is about 0.4 oct wider than the nominal stimulus spectrum resulted in the sum of narrow-band contributions equal to the wide-band ABR. The travelling wave velocity was computed based on ABR latencies and assigned a frequency of 128 kHz to the basal end of the cochlea. The computation gave values from 38.2 oct/ms at the proximal end of the basilar membrane to 4.0 oct/ms at a distance of 3.25 oct (13.5 kHz).

Depth resolution of spectral ripples was measured in normal humans using a phase-reversal test. The principle of the test was to find the lowest ripple depth at which an interchange of peak and trough position (the phase reversal) in the rippled spectrum is detectable. Using this test, ripple-depth thresholds were measured as a function of ripple density of octave-band rippled noise at center frequencies from 0.5 to 8 kHz. The ripple-depth threshold in the power domain was around 0.2 at low ripple densities of 4–5 relative units (center-frequency-to-ripple-spacing ratio) or 3–3.5 ripples/oct. The threshold increased with the ripple density increase. It reached the highest possible level of 1.0 at ripple density from 7.5 relative units at 0.5 kHz center frequency to 14.3 relative units at 8 kHz (5.2 to 10.0 ripple/oct, respectively). The interrelation between the ripple depth threshold and ripple density can be satisfactorily described by transfer of the signal by frequency-tuned auditory filters.

The ability of auditory evoked potentials to follow sound pulse (click or pip) rate was studied in bottlenosed dolphins. Sound pulses were presented in 20-ms rhythmic trains separated by 80-ms pauses. Rhythmic click or pip trains evoked a quasi-sustained response consisting of a sequence of auditory brainstem responses. This was designated as the rate-following response. Rate following response peak-to-peak amplitude dependence on sound pulse rate was almost flat up to 200 s−1, then displayed a few peaks and valleys superimposed on a low-pass filtering function with a cut-off frequency of 1700 s−1 at a 0.1-amplitude level. Peaks and valleys of the function corresponded to the pattern of the single auditory brain stem response spectrum; the low-pass cut-off frequency was below the auditory brain stem response spectrum bandwidth. Rate-following response frequency composition (magnitudes of the fundamental and harmonics) corresponded to the auditory brain stem response frequency spectrum except for lower fundamental magnitudes at frequencies above 1700 Hz. These regularities were similar for both click and pip trains. The rate-following response to steady-state rhythmic stimulation was similar to the rate-following response evoked by short trains except for a slight amplitude decrease with the rate increase above 10 s−1. The latter effect is attributed to a long-term rate-dependent adaptation in conditions of the steady-state pulse stimulation.

Notch-noise masking was used to measure frequency tuning in a dolphin (Tursiops truncatus) in a simultaneous-masking paradigm in conjunction with auditory brain-stem evoked potential recording. Measurements were made at probe frequencies of 64, 76, 90, and 108 kHz. The data were analyzed by fitting the rounded-exponent model of the auditory filters to the experimental data. The fitting parameter values corresponded to the filter tuning as follows: QER (center frequency divided by equivalent rectangular bandwidths) of 35 to 36.5 and Q10 dB of 18 to 19 at all tested frequencies.

A paradoxical phenomenon was found in the auditory system of dolphins: weak sounds suppressed the brain responses to much stronger sounds. This occurred when the brain evoked potentials to rhythmic sound amplitude modulations were recorded. The response was markedly suppressed by addition of another sound of higher frequency and down to 40 dB lower intensity than the amplitude-modulated signal. Only the sustained rhythmic response was suppressed while transient on-response was not, thus indicating that the suppression influenced the ability of evoked potentials to follow rapid amplitude modulations. This prevents weak sounds from being masked by stronger ones. It may help a dolphin to perceive weaker echo-signals in the background of stronger emitted pulses.

Temporal resolution of hearing was studied in bottlenosed dolphins by recording the auditory brain-stem response (ABR) evoked by gap in noise. Gaps shorter than 0.5 ms evoked a response combining both off- and on-components; longer gaps evoked separate off- and on-responses. Both the response to a short gap and on-response to the end of a long gap increased with increasing gap duration. On-response recovered completely at gap duration of 5–10 ms. Small but detectable response arose at gap duration as short as 0.1 ms. Contrary to the on-response after a long silence, the response to a short gap was less dependent on noise intensity. From these data, the temporal transfer function of the supposed integrator was derived assuming nonlinear transform of the integrator output to ABR amplitude. Equivalent rectangular duration of the found temporal transfer function was 0.27 ms.

Simultaneous tone-tone masking in conjunction with the envelope-following response (EFR) recording was used to obtain tuning curves in dolphins (Turslops truncatus). The EFR was evoked by amplitude-modulated probes of various frequencies. A modulation rate of 600 Hz was found to fit the requirement to have a narrow spectrum and evoke EFR of large amplitude. Tuning curves were obtained within the frequency range from 11.2 to 110 kHz. The Q10 values of the obtained tuning curves varied from 12–14 at the 11.2 kHz center frequency to 17–20 at the 64–90 kHz frequencies.

Temporal resolution of hearing in two bottlenosed dolphins was estimated by measuring auditory brain-stem response (ABR) recovery in conditions of double-click stimuli. From these data, temporal transfer function of the supposed integrator was derived assuming nonlinear transform of the integrator output to ABR amplitude. The obtained temporal transfer function showed a nearly constant level up to 200 microseconds. then decay to approximately -3 dB at 300 microseconds (as presented in the sound intensity domain), and subsequent decay of 10-11 dB per time doubling (about 35 dB/decade).

We have been examining the hearing of both the outgoing clicks and the returning echoes of actively echolocating odontocetes using evoked auditory potential techniques. In order to protect themselves from the loud outgoing sound while still maximizing the hearing of the acoustic echo return, odontocete echolocators appear to have developed both passive and active control of hearing. Passive control has been demonstrated by comparing hearing of their own outgoing signals to similar signals presented to them from the outside. Clicks produced by the animal itself are heard about 40 dB down. Active control has been demonstrated by a comparison of hearing outgoing clicks during target present and target absent trials. During target absent trials, when searching for targets, hearing is 20 dB more sensitive than during target present trials. The current critical question is: If the animal is warned that a loud sound is about to arrive, does it possess a mechanism of self-mitigation that will allow it to control ...

Frequency resolving power (FRP) was measured in normal humans using rippled noise with a phase-reversal test. The principle of the test was to find the highest ripple density at which an interchange of mutual peak and trough position (the phase reversal) in the rippled spectrum is detectable. In the frequency range below 0.5 kHz FRP was found to be about 21 ripples per kHz when tested by both broad-band and narrow-band rippled noise. In the frequency range above 2 kHz, FRP measured by the narrow-band rippled noise was 22 to 23 relative units (relation of the noise central frequency to the ripple frequency spacing).

Frequency resolving power (FRP) of the human's hearing was measured using the rippled noise as a probe. To examine the ripple discrimination, a phase-reversal test was used: the rippled noise was replaced by that with the opposite peak and through positions. This switch can be detected only when rippled structure of the noise spectrum is discriminated. The highest ripple density when the switch was detectable was taken as a FRP measure. Narrow-band rippled spectra were used to measure the FRP within a frequency range of 0.175 to 11 kHz. The highest resolvable ripple density in absolute measure (ripples number per kHz) was about 21/KHz at frequencies below 0.5 kHz and fell down at higher frequencies. Resolvable ripple density in relative measure (central frequency divided by ripple spacing) was about 22 relative units at frequencies above 2.8 kHz and fell down at lower frequencies.

Noise that interferes with active echolocation can come from either outside sources or be self‐generated. Our work on temporary threshold shifts has shown that either lower levels of anthropogenic noise presented for long time periods or intense sonar pings for short time periods can produce temporary reduction in sensitivity and temporary threshold shifts. Intense anthropogenic sounds intended to disrupt echolocation can also reduce echolocation performance. Our measurements of hearing during echolocation have shown that self‐generated intense outgoing signals are managed by the whale’s auditory system by (1) the use of forward masking as an automatic gain control system, (2) anatomical (or neurological) damping of outgoing signals, and (3) active manipulation of sensitivity levels of over all hearing. These processes of hearing allow the whale to adjust its hearing during echolocation to maximize the hearing of echoes. Measurements of hearing while a false killer whale actively echolocated, by measuring auditory evoked potentials, showed that the whale heard returns from a small cylinder better than it heard its own outgoing 180‐dB clicks that produced the echo returns from the cylinder. Interactions between outside anthropogenic noise and the hearing of self‐generated intense sounds during echolocation remain largely unexplored.

Point-to-point representation of sensory systems, including the somatosensory system, into the cerebral cortex provides important information on the system organization and peculiarities of the input signals processing. As a rule, the most important parts of sensory surface have the most extensive representation in sensory centers.

Ordinary mechanisms of sound conduction acting in terrestrial mammals do not seem to function in dolphins due to features of the aquatic media (the acoustical impedance of body tissues is closer to that of water than air) and peculiarities of the outer ear (absence of a pinna, etc.).

Two main mechanisms are known to be involved in the binaural spatial hearing in terrestrial mammals: interaural intensity difference (IID) and interaural time-delay (ITD): see reviews of Erulkar (1972) and Blauert (1983). Aquatic mammals, particularly dolphins, are known to have a high ability for spatial sound localization (Renaud and Popper, 1975; Dubrovskiy, 1990), but it remains uncertain whether it is supported by the intensity and/or time-delay cues.

An important characteristic of the auditory system, used for fine discrimination of sounds, is its frequency selectivity. In past years, attempts were made to measure the pass bands of the auditory filters (critical bands) in dolphins. These measurement were carried-out by the tone-tone and noisetone masking behavioral method (Johnson, 1968; Moore and Au, 1983). But results were contradictory and the true ability of the dolphin’s auditory system to resolve sound frequencies in a complex of sounds remains uncertain.

Binaural hearing mechanisms were measured in dolphins (Inia geoffrensis) by recording the auditory nerve evoked response from the body surface. The azimuthal position of a sound source at 10-15 degrees from the longitudinal axis elicited interaural intensity disparity up to 20 dB and interaural latency difference as large as 250 microseconds. The latter was many times greater than the acoustical interaural time delay. This latency difference seems to be caused by the intensity disparity. The latency difference seems to be an effective way of coding of intensity disparity.

In a false killer whale Pseudorca crassidens, auditory evoked potentials (AEPs) were recorded in conditions of echolocation using both real targets and electronically synthesized and played‐back (phantom) echoes. The electronic echoes were triggered by emitted sonar pulses, their spectra were similar to that of the emitted sonar clicks of the subject, and their intensities were proportional to the level of the emitted sonar pulse. AEPs to both the emitted pulse and real or electronic echoes were well detectable. AEPs related to emitted sonar clicks displayed the regular amplitude dependence on click level: The higher the level, the higher the amplitude. The echo‐related AEP amplitudes depended on both echo attenuation and delay with a trade of around 9‐dB attenuation per delay doubling. When the echo attenuation was varied in conjunction with delay keeping a rate of 9‐dB deeper attenuation per delay doubling, the echo‐related AEPs were nearly invariant. This result is well explainable by a hypothesis impl...

While much has been previously learned about the echolocation performance and characteristics of the outgoing signals of echolocating dolphins and small whales, we have measured hearing using evoked potentials during echolocation. We have found that: (1) the whale may hear her loud outgoing clicks and much quieter returning echoes at comparable levels, (2) the whale has protective mechanisms and hears her outgoing signals at a level about 40 dB lower than similar signals presented directly in front of her, (3) when echo return levels are lowered either by making the targets smaller or by placing the targets farther away ‐ without changing the levels of her outgoing signals, the hearing of those echoes remains at almost the same level, (4) if targets are made much smaller and harder to echolocate, the animal will increase what she hears of her outgoing signal ‐ as if to heighten overall hearing sensitivity to keep the echo level hearable, and (5) the animal has an active 'automatic gain control' mechanism ...

Positions of sound‐receiving apertures were searched‐for in bottlenose dolphins using the ABR technique. The receiving‐area position was computed basing on ABR delays at various sound‐source positions. Two acoustic apertures were revealed in such a way: for frequencies of 32 kHz and higher, the receiving area was located near a proximal part of the lower jaw (the mandibular acoustic window); for lower frequencies, the receiving area was located near the tympanic bulla. In another experimental series, AEP thresholds to near‐field stimuli were measured with transducer positioning next to various points of the dolphin's head. Again, at stimulus frequencies of 32 kHz and higher, the lowest threshold area was next to the mandibilar acoustic window; at lower frequencies, the lowest threshold area was next to the bulla. The conclusion is that dolphins have at least two acoustic apertures differing in their frequency sensitivity. Directional sensitivity of these two apertures was investigated by measuring ABR thr...

The envelope‐following response (EFR) in odontocetes is composed of overlapping ABRs produced by each cycle of rhythmic sound stimulus at a rate of a few hundred cycles per sec. It has been shown recently (Finneran et al., JASA 2007, 121: 1775) that a complex stimulus consisting of a few carriers modulated by different rates produces a complex EFR composed of components reproducing all the modulation rates. Using this technique, interactions between different components of complex EFR were investigated in a beluga whale Delphinapterus leucas. When all carriers of the complex stimulus were equalized by SPL, the interaction depended on both the SL of carriers (their level relative the threshold) and inter‐carrier frequency spacing. Addition of components of higher SL (at low‐threshold frequencies) dramatically reduced the amplitudes of other EFR components, especially at short frequency spacing. The leading factor of this reduction was decreasing of the modulation depth of each of the carriers when the overall power of the stimulus increased by additional components. The threshold estimate of each carrier little depended on the number of components; however precision of threshold determination fell down with increasing the number of stimulus components due to the reduced amplitudes of EFR components.

In a false killer whale Pseudorca crassidens, echo perception thresholds were measured using a go/no‐go psychophysical paradigm and one‐up‐one‐down staircase procedure. Electronically synthesized echoes were triggered by whale‐emitted echolocation pulses, and the echo amplitudes were presented proportional to the triggering biosonar pulse amplitudes, thus imitating the basic feature of real target echoes. Echo thresholds were measured in terms of attenuation relative to the source level of the biosonar pulses. The echo threshold dependence on echo delay was investigated. With increasing echo delay, the threshold echo attenuation decreased from ‐49.3 dB at 2 ms to ‐79.5 dB at 16 ms, with a regression slope of ‐31.5 dB per delay decade. At delays longer than 16 ms, the threshold remained nearly constant around ‐80.4 dB. This echo threshold dependence on delay is interpreted as a release from forward masking by the preceding emitted pulse. This release may well compensate for the echo level decrease with distance which is from 20 to 40 dB per distance decade, depending on target properties. This compensation may keep the echo sensation level independent of, or little dependent on, target distance. Supported by Office of Naval Research (USA), Ministry of Science and Education (Russian Federation).

Auditory brain stem responses (ABR) were recorded from the head surface of non-anesthetized and non-relaxed bottle-nosed dolphins, Tursiops truncatus. The region of best ABR recording was shown to be located 6-9 cm caudal to the blowhole. The threshold values were about 1 mPa for noise bursts and -3 dB re 1 mPa for tone bursts of the optimal frequency (80 kHz). The maximum frequency at which ABR could be evoked was 140 kHz. The duration of temporal summation reached 0.5 ms at intensities near the threshold and decreased with an increase in intensity. When the stimuli were paired clicks of the same intensity, the time to complete recovery from the second response was about 5 ms, while that to its 50% recovery was 0.7 ms. When the conditioning click exceeded the testing one in intensity, prolongation of the recovery period was observed. A 40-dB intensity difference led to an approximately 10-fold prolongation of this period.

Auditory brain stem response (ABR) is used extensively for studies of hearing. The possibility to record activity of auditory nerve centers without any surgery makes this evoked response useful for investigation of auditory physiology and pathology, particularly, for comparative studies of hearing in various groups of animals.

Organization of the visual system in Cetacea still remains unclear in many respects. Of special interest are data on ganglion cell distribution in the retina. It is related closely with visual acuity (Van Buren, 1963; Frisen and Frisen, 1976).

The mechanism of sound transmission to the ear in aquatic mammals, particularly in dolphins, remains obscure. It has been suggested that sound reaches the inner ear directly via head tissues, or sound transmission involves the middle ear and the closed auditory meatus (Fraser and Purves, 1959; Fleischer, 1978). This assumption was supported by calculations indicating that the closed auditory meatus can serve as an effective acoustic transformer for the sounds to be transmitted to the middle ear (Lipatov, 1978).