The dog’s ear is composed of an outer ear (pinna), auditory canal, and various structures designed to convert sound waves into auditory information. The pinna gathers and directs sound into the auditory canal, where it is carried to the tympanic membrane or eardrum. The eardrum is an extremely sensitive and elastic membrane reacting to the slightest vibrations on its surface: movement of less than one-tenth the diameter of a hydrogen atom can produce an audible sensation. The vibrations caused by the pressure of sound waves on the eardrum are mechanically conducted to the cochlea through the mediation of three tiny bones or ossicles: the malleus, the incus, and the stapes. The cochlea is a snail-like tubular structure that is innervated by the auditory nerve. Sound vibrations are passed into the cochlea at the oval window. These vibrations cause a fluid wave in the cochlear fluid, which causes a rippling effect against the surrounding basilar membrane. The vibratory displacement of the basilar membrane stimulates auditory receptors (called hair cells) to bend rhythmically, thereby evoking a nerve potential that is carried by individual fibers into the auditory nerve. Different sounds are distinguished by the specific pattern of wave motion that they generate. The vibratory wave movements in the cochlear fluid selectively activate certain groups of receptor cells while passing over others as they flow against the surrounding basilar membrane. Audibly different sensations are produced by the distinctive pattern and topography of the wave involved. Auditory sensations are conducted by the auditory nerve to the cochlea nuclei located in the medulla oblongata before being relayed to the thalamus.
Frequency Range of Hearing
The dog’s range of hearing has been shown to be superior to human audition in many respects. Dogs can easily hear sounds outside the human range of audibility [20,000 cycles per second (cps)]. Estimates vary from 26,000 cps,41,000 to 47,000, 30,000 to 40,000, and 60,000 to 65,000 cps; whatever the case may be, dogs do hear ultrasound — sound that is imperceptible to normal human ears. Dogs can also hear sounds of very low frequencies at 15 cps. The general range of hearing estimated by Fox and Bekoff (1975) is 15 to 60,000 cps. To place these numbers into perspective, 28 cps is the frequency of the lowest key on the piano and 4180 cps is the frequency of sound produced by the highest key. Apparently, dogs hear best at around 4000 cps, compared with humans at 1000 to 2000 cps. Lipman and Grassi (1942) compared human hearing with that of dogs and found that under comparable sound intensities (decibels) dogs and humans did about equally well with regard to the perception of low frequencies (125 to 250 cps). They observed, however, that dogs do progressively better as the frequencies increase with “markedly superior” abilities between 4000 and 8000 cps, and concluded that “the dog lives in a broader and deeper acoustic world, thus gaining direct contact with natural events which are imperceptible to man” (1942:88).
Another way that the dog’s sense of hearing is better than ours is its ability to locate the origin of sounds coming from a distance. A variable ability to localize the origin of sounds is evident in puppies as early as 16 days of age. Adult dogs are able to pinpoint the origin of sound with a great accuracy with the aid of their movable earflaps (pinnas). Locating the origin of sound, however, involves much more than facile movement of the ears. Sound location depends on complex brain calculations that rely on the dog’s ability to register narrow time differences between the sound reaching each of its opposing ears. The ear closest to the source of sound is struck slightly sooner than the opposite ear. Determining the direction of the sound’s origin depends on a cooperative mediation of information between the cochlear nuclei and time-sensitive neurons located on either side of the brain stem in a structure known as the superior olive. These neurons can detect delays of stimulation between one ear and the other on the level of microseconds (a millionth of a second). The slightest movement of the head toward the source of stimulation provides additional information about distance. A change in the dog’s head position relative to the sound provides spatial information that can then be used by the brain to triangulate and compute the sound’s distance. The common tendency for a dog to cock its head to one side when listening carefully to an unusual sound is probably a reflexive effort to pinpoint a more exact location, perhaps involving a dimension of height relative to the ears when positioned parallel to the ground.
Ultrasound and Training
A potential application utilizing the dog’s ability to hear in the ultrasonic range is to use high-frequency sounds in dog training. Of course, the Galton or “silent” whistle has been used for many years as a signaling device, especially for recall. Recently, however, many battery-powered ultrasonic devices have come onto the market for use in behavioral training as “humane” forms of punishment for nuisance barking and other behavior problems. An assumption underlying the use of such devices is that the ultrasound stimulation produced by them is aversive to dogs — that is, that it hurts their ears. This assumption, however, has not been borne out by personal experience or experimental testing. For example, Blackshaw and coworkers (1990) tested the auditory reaction of several dogs, ranging widely in size and breed type, to variable frequencies of ultrasound under controlled conditions. They found that ultrasonic devices producing high-frequency sounds between 14 to 36 kHz resulted in remarkably little apparent aversion in the dogs, mostly yielding a “no effect” response or minimum signs of interest as indicated by brief pricking of the subjects’ ears. A few dogs reacted aversively to the sound by turning away from it. Small dogs appeared to be slightly more sensitive to ultrasound than are medium or large dogs. This latter finding is consistent with Galton’s early observation that small dogs responded to his silent whistle while large dogs did not. However, this apparent difference between small and large dogs does not appear to depend on the size of the dog’s head or auditory apparatus. Heffner (1983) found that the upper limits for high-frequency hearing are remarkably similar from breed to breed, regardless of their size and ear shape: Chihuahua, 47 kHz; dachshund, 41 kHz; poodle, 46 kHz; pointer, 45 kHz; and St. Bernard, 47 kHz. Perhaps smaller dogs are simply more behaviorally reactive to ultrasound than are large breeds.
These devices have not proven to be very reliable, effective, or aversive to most dogs. I have been disappointed by my own experiences with the products, finding them unreliable or ineffective beyond an initial “What’s that?” or a mild annoying effect that dogs readily habituate to after a few trials. One popular bark-activated model that I tested actually jammed on a continuous mode and would have continued producing the ultrasound until it “fried” or the batteries ran down. Fortunately, it was not on a dog; unfortunately, the product is still on the market and widely used.
A possible explanation for the relative ineffectiveness of ultrasonic devices may be the result of insufficient power to drive the ultrasonic pulse. In other words, the small battery-powered models may not be strong enough to produce an aversive auditory effect. Ultrasound requires relatively more energy and amplitude than sound generated at lower frequencies. Frequencies above a dog’s optimal range of hearing require progressively more amplitude boosting to be heard. For instance, to obtain an orienting response to the sound of a silent whistle, the effort needed to blow the whistle adjusted at a high frequency is much more forceful than required when it is adjusted to a lower one.
Ultrasound has two other distinct characteristics limiting its usefulness: narrow field of directionality and limited effective range. Unless the device is pointed directly at a dog’s head at a close range, its effectiveness is drastically diminished. In the case of bark-activated collars, the ultrasonic burst may be blocked by the dog’s neck and jaw, requiring that the sound stimulus reach the dog’s ears by way of echoes from surrounding objects rather than from the collar itself. This further mitigates its usefulness in situations where nearby objects are not present, such as outdoors. Lastly, dogs may not be biologically prepared to readily associate ultrasound stimuli (even at high levels of stimulation) with threat without additional aversive conditioning. Ultrasound may possess some innate significance as a directional indicator for detecting and locating small prey animals, whose distress vocalizations are expressed at ultrasonic frequencies.
No adequate learning studies (that I am familiar with) have been carried out that demonstrate the effectiveness of ultrasound as a punisher or negative reinforcer in dogs. Considering the cost of the devices, and the ready availability of consistently more effective alternatives, one should resort to their use only in rare cases of special need, for example, with especially sensitive dogs or where auditory-mediated punishment needs to be silent. Although the use of ultrasound as an auditory punisher is not recommended, low-intensity ultrasound can be usefully employed in dog training as a means for signaling and controlling learned behavior (e.g., the silent whistle). Pairing unobtrusive ultrasonic cues with trained behaviors proves very effective in place of verbal commands in certain situations where silent control is desirable. Unfortunately, ultrasound is currently most often applied as a punitive device rather than a potentially valuable training tool for the delivery of discriminative signals and secondary reinforcement.
An additional problem raised by the dog’s sensitivity to ultrasound is the advisability of ultrasonic flea deterrents. Ultrasonic collars are frequently used by pet owners to control flea infestation. This is unfortunate, both because they do not work and because the sounds produced by such devices are audible to dogs and cats exposed to them. Obviously, the possibility exists that ultrasonic flea collars may produce significant annoyance to dogs with sensitive hearing abilities. Ultrasonic flea collars produce frequencies well within the range of a dog’s hearing capability, at approximately 40,000 cps (92 dB amplitude at a distance of 10 cm). A serious question must be raised regarding the impact of daily exposure to pulsing ultrasound stimulation at these levels to a dog’s quality of life. This is especially pressing since ultrasonic collars have been proven uniformly ineffective against flea infestation. While the directionality of ultrasound at the aforementioned frequencies probably prevents it from directly reaching the dog’s ears while wearing the device, it does not prevent echoes from reaching the dog’s ears indirectly or prevent the ultrasound from reaching resident dogs or cats exposed to its unobstructed output.
Deafness occurs in dogs as a congenital disorder or may be acquired as the result of disease or physical damage to the auditory mechanism. Congenital deafness appears to be linked to pigmentation, with the likelihood of deafness increasing with the amount of white pigmentation present in the dog, especially in dogs that exhibit an absence of normal iris pigmentation. The merle gene (e.g., the Australian shepherd, Harlequin Great Dane, Old English sheepdog, and others) and piebald gene (e.g., bullterrier, Dalmatian, Great Pyrenees, and others) have been associated with an increased incidence of deafness. Dalmatians commonly exhibit congenital deafness, with as many as 30% of the puppies born exhibiting the disorder in one or both ears. Temporary hearing loss (elevated thresholds) may result from exposure to intense auditory stimulation exceeding 100 dB. For example, hunting dogs exposed to repeated close-range gunfire may experience significant noise-related hearing loss.
Determining whether a dog is deaf is best accomplished by the brain stem auditory evoked response (BAER) test, which detects electrical activity in the cochlea and other auditory nervous pathways in the brain. The test is conducted by directing a brief auditory stimulus (a click) into both ears and measuring the evoked electrical patterns produced by the stimulation. Deaf dogs will present a flat-line appearance. A major advantage of the BAER test is that it can isolate deafness in one ear (unilateral) or both (bilateral) ears. Bilateral deafness can also be detected by the absence of an appropriate response to loud startling noises or a failure to acquire various conditioned associations that depend on hearing to learn (e.g., the dog’s name and other verbal/auditory cues used in training).
According to the reports of many deaf dog owners, deaf dogs can make a good adjustment to domestic life. Success with such dogs depends on careful training and other management efforts needed to protect the hearing-impaired dogs from injuries that might be sustained as the result of their inability to hear, especially the threat of vehicular injury. Like blind dogs, deaf dogs learn to rely on other sensory modalities to obtain environmental information, including vision, touch, and olfaction. Consequently, deaf dogs can be taught to respond to a wide variety of visual cues and hand signals, as well as various common forms of tactile stimulation used routinely in dog training. Obviously, training a deaf dog poses many unique problems, such as securing and maintaining the dog’s attention, especially when the handler is out of the dog’s field of vision. Campbell (1992) recommends the use of beanbags to condition dogs to keep their attention on the handler. When a dog’s attention wavers, the beanbag is tossed at the dog’s legs. The handler then turns and walks in an opposite direction while encouraging the dog to follow along. As the dog approaches, the handler crouches down and reinforces the following behavior with petting. Other ways to hold a deaf dog’s attention include consistently reinforcing attention with treats, tossing toys into the dog’s field of vision, stomping on the floor, and using flashlights and lasers. Remote-activated electronic collars are sometimes used for the control of undesirable behavior and to train dogs to come. A rather unique application of such devices set at a very low level is to pair the mild stimulation produced by the collar with food and other rewards. As a consequence, the stimulation can then be used to reinforce desirable behaviors conditionally in much the same manner as applying other common conditioned rein-forcers (e.g., “Good”). Remote-activated vibratory devices are also used for such purposes. Finally, even though dogs cannot hear, Tanner has emphasized that trainers should still speak to dogs as though the dogs can hear, since “we transmit our feelings and desires to our dogs through facial expressions as well as oral commands” (1970:23).
Deaf puppies are routinely euthanized, in part, as the result of the widespread belief that congenital bilateral deafness represents a significant risk factor for the development of a variety of behavior problems, including aggression — presumably developing as the result of repeated and unpredictable startle. The linkage between deafness and aggression is a highly controversial topic, with little current evidence available other than anecdotal reports and clinical impressions supporting the assumption that deaf dogs are more prone to bite. What most authorities do agree on is that deaf dogs require considerably more focused care and training than hearing dogs — a factor that the prospective owner of a deaf dog should realistically assess before making the decision to adopt.
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