The Best-Vision Zones and Visual Resolution of the Retina of Neonatal Bottlenose Dolphins Tursiops truncatus.

The topography of ganglion cells in the retina of three neonatal bottlenose dolphins Tursiops truncatus has been studied in retinal wholemounts. Two areas of high ganglion cell density have been identified in the form of local spots in the temporal and nasal quadrants of the retina, near a horizontal diameter at a distance of 10-15 mm from the optical disk. The maximum density of ganglion cells in these areas on average for five preparations is 657 and 636 cells/mm2 in the temporal and nasal quadrants, respectively. The retinal resolution, estimated by the maximum density of ganglion cells and a posterior nodal distance of 13 mm, was 0.17° for the temporal quadrant and 0.18° for the nasal one. These data are comparable with the results for the previously studied adult dolphin and whale species.

Position of an acoustic window in a beluga whale: Computation based on auditory evoked potential latencies.

In a beluga whale, the positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound source positions. For this investigation, auditory evoked potentials (AEPs) in response to short tone pips were recorded. Latencies of the first AEP wave that presumably reflected the activity of the auditory nerve were measured at different sound source azimuths. For AEPs of equal amplitudes, the difference in AEP latencies was attributed to the difference in the acoustic delays. These delay differences were used to compute the azimuths of sound receiving points. Measurements were conducted at frequencies from 22.5 to 90 kHz in half-octave steps. At all stimulus frequencies, the receiving points were located 24-38 cm caudal of the melon tip, which is near a proximal part of the lower jaw. Thus, the results indicated the latero-mandibular acoustic window. Possible causes for not finding a lateral or ventro-mandibular window are discussed.

Influence of the Background Noise on Recognition of Signals with a Complex Spectrum Structure in the Beluga Whale (Delphinapterus leucas).

The frequency resolving power of hearing (FRP) of the beluga whale Delphinapterus leucas was studied as dependent on influence of lasting low-intensity sounds (of the ultrasonic range from -20 to +10 dB). Testing of the spectrum ripple-phase reversal was used in conjunction with a noninvasive recording of auditory evoked potentials. FRP parameters were found to depend nonmonotonically on the intensity of the background noise. The resultant adaptation effects can be explained by the fact that, in response to the high-intensity signals, the auditory system sensitivity is reduced to the level optimal for analysis of these signals.

Sharpening of the Signal Spectrum Contrast as a Result of Lateral Suppression in the Human Auditory System.

The spectrum contrast threshold for the comb-filtered signal was measured at various ripple densities in listeners with normal hearing. The lowest spectrum contrast thresholds of less than 0.1 appeared at a ripple density of 3-4 oct-1. With the ripple density falling down to 1 oct-1, the threshold increased to 0.17. At the ripple density higher than 4 oct-1, the threshold was also growing to achieve 1.0 at a density of 7-8 oct-1. An increase of threshold at high ripple densities can be explained by integration within critical bands. At low ripple densities, the threshold increase may be a result of the lateral suppression that led to sharpening of the spectral contrast.

Compressive nonlinearity of human hearing in sound spectra discrimination.

In the psychophysical experiments reported here, cochlear compression function was derived by comparing on-frequency and off-frequency masking. The signal was rippled spectrum noise. The ripple density discrimination threshold was measured in the ripple phase reversion test. An increase in masker intensity led to a decrease in a resolvable ripple density threshold. The on-frequency masker level at threshold increased proportionally to the signal intensity. The off-frequency masker level at threshold also increased proportionally to the signal at signal intensity levels below 50 dB, whereas at signal levels above 60 dB SPL, the ratio of the masker level at threshold gradient to signal level gradient was 1 : 5 dB/dB, revealing cochlear compression.

Estimates of underwater and aerial visual acuity in the European beaver Castor fiber L. based on morphological data.

The eye optics and topographic distribution of ganglion cells were studied using whole mount preparations from European beaver Castor fiber L. The beaver eye optics provides emmetropia in air and hypermetropia in water. The optometrical measurements predict retinal resolution of the beaver eye around 17' in air and 9' in water. In air, retinal resolution corresponds to the real visual acuity, whereas in water, visual acuity is below the retinal resolution because of the non-precise focusing.

Discrimination of the spectral structures of sound signals on the background of interference.

Presentation of test signals consisting of sounds with rippled spectra allowed measurements of the frequency-resolving ability (FRA) of hearing to be performed in humans without using frequency-dependent masking techniques. This allowed studies of changes in FRA in the presence of noise interference. In conditions of diotic presentation (to both ears in parallel) of the test signal and noise, FRA decreased significantly if the interference frequency was lower or equal to the test signal frequency. The relationship between this effect and the sound intensity and noise:signal ratio varied for low-frequency noise and for noise at the same frequency as the test signal, which indicates that these two types of interference have different mechanisms. However, in both cases, noise of sufficient intensity led to a complete inability to discriminate the fine spectral structure of the test signal. In dichotic presentation (test signal to one ear, noise to the other), noise had virtually no effect on FRA over a wide range of test signal and noise frequencies and noise:signal ratios. Thus, there was essentially complete dichotic release of FRA from the effects noise, which has potential to be used in constructing prosthetic hearing devices.

Analysis of auditory information in the brains of cetaceans.

A characteristic feature of the brains of toothed cetaceans is the exclusive development of the auditory neural centers. The location of the projection sensory zones, including the auditory zones, in the cetacean cortex is significantly different from that in other mammals. The characteristics of evoked potentials demonstrate the existence of several functional subdivisions in the auditory cortex. Physiological studies of the auditory neural centers of cetaceans have been performed predominantly using the evoked potentials method. Of the several types of evoked potentials available for non-invasive recording, the most detailed studies have been performed using short-latency auditory evoked potentials (SLAEP). SLAEP in cetaceans are characterized by exclusively high time resolution, with integration times of about 0.3 msec, which on the frequency scale corresponds to a cut-off frequency of 1700 Hz. This is more than an order of magnitude greater than the time resolution of hearing in terrestrial mammals. The frequency selectivity of hearing in cetaceans has been measured using several versions of the masking method. The acuity of frequency selectivity in cetaceans is several times greater than that in most terrestrial mammals (except bats). The acute frequency selectivity allows the discrimination of very fine spectral patterns of sound signals.

Some problems in the measurement of the frequency-resolving ability of hearing.

Despite the detailed development of masking methods for measurement of the frequency selectivity of hearing, these measurements are hardly used for diagnostic purposes because they are time-consuming and because of the uncertain extrapolation of the results to the perception of complex spectral patterns. A method for the direct measurement of the spectral resolving ability of hearing using test signals with rippled spectra is proposed. These measurements showed 1) that the resolving ability of the auditory system in terms of discriminating complex spectra is greater than that suggested by the acuity of auditory frequency filters; 2) that changes in the acuity of frequency auditory filters associated with sound intensity hardly affect the ability to resolve complex spectra; 3) that the effects of interference on frequency-resolving ability do not lead to decreases in the spectral contrast of signals due to superimposition of noise.