TINNITUS RETRAINING THERAPY

IMPLEMENTING THE JASTREBOFF NEUROPHYSIOLOGICAL MODEL

Jonathan Hazell F.R.C.S.,Tinnitus and Hyperacusis Centre, London UK
www.tinnitus.org

How we hear
 
The conscious awareness of sound takes place near the surface of the brain, when a pattern of electrical activity traveling up the nerve of hearing from the ear reaches the auditory cortex. Figure 1 .

The hearing nerve has about 30,000 nerve fibres, and patterns of electrical activity in these fibres are matched with other patterns, which are held in the auditory, or hearing memory. The cochlea, or inner ear, which changes sound waves into these electrical patterns, is a surprisingly noisy place, where continuous mechanical and electrical activity in 17,000 hair cells can now be monitored with sensitive, computer enhanced, listening devices (otoacoustic emissions).  Most of what we hear is a sequence of different sounds, like speech or music. In infancy, new sound experiences are stored in an information hungry, but relatively empty auditory cortex. Later on, there is a continuous process of matching familiar memory patterns with those coming from the ear. Each time a pattern from the ears is matched with a pattern in the (cortical) auditory memory we have the experience of hearing, and recognizing a sound. Putting together these matched patterns starts a process of evaluation. Another part of the brain close to this awareness centre, is involved in the meaning of what we hear, and in interpreting the language. If it's a foreign language we can hear the sound, but may not understand the meaning.

The meaning of sound

Sound is of enormous importance in monitoring and understanding our environment. Hearing in animals (who are constantly in fear of their lives because of attack from a predators), has to be very sensitive and specific. The ability of animals to develop extremely acute hearing, by which they can  detect the very small sounds of an attacker, when still a long way away, contributes to the survival of that species. These warning signals produce acute anxiety, prompting

Figure 1 Nothing is heard until sound patterns, generated in the cochlea, reach the cortex of the brain, producing our first awareness of organised sound.

appropriate action to avoid attack, the so- called survival reflex.  We respond in the same way to the sound of a motorcar horn, by automatically stepping backwards, out of the road.   Some sounds can be identified as warning signals, while others can evoke a feeling of security or pleasure.  We have this experience every day with sounds that alarm us, or sounds that soothe us, such as music, or the sounds of nature. Many sounds naturally evoke strong emotions of one sort or another.

Conditioned responses

When a sound has special or critical meaning, like our baby waking at night, or the noise of a moving floorboard, or the sound of first our name, we respond to it in an automatic manner, even if the volume is very soft. This happens after a short learning period, but the responses can remain very strong, throughout our whole life. During sleep, the conscious part of the brain is 'shut down' so we don't hear, see or feel anything. However the mother still wakes to the baby stirring even though she has just slept through a thunderstorm.  This shows that even weak patterns of sound, when they have a great significance or meaning, can be detected by subconscious filters. (figure2) in the hearing pathways, between the ear and brain (auditory cortex). The conditioned response also triggers activity outside the auditory system. Here there are large numbers of connections with the limbic system (figure 3) which is concerned with emotion and learning. The autonomic nervous system is stimulated, activating the body, to get it ready for any physical activity that might be necessary.  In situations of danger, or threat, the familiar 'fight or flight' is triggered, even in a false alarm.  This results in  high levels of autonomic activity'; tense muscles, raised heart and breathing rates, sweating;  the complete opposite to the state of relaxation. They quite rightly preclude sleep, or concentration on other, less important tasks.  

Most of our day-to-day activity consists of a series of conditioned or learned responses, executed to order, like reading, writing, playing an instrument or driving the car.

Figure 2. Between ear and brain there are 2M nerve cells forming a neuronal network, capable of sophisticated pattern recognition, enhancement and suppression of auditory signals.

So, each and  every sound that we hear, and then learn its meaning, has an "emotional label" attached to it, which may change from time to time according to how we feel in ourselves, and the context in which we hear it.  For example the sound of a neighbour's television set may be acceptable, or unpleasant and intrusive, depending on whether the television belongs to a well loved friend or relation, or to somebody else, who for various reasons we dislike or distrust.

Figure 3 Neuronal networks between ear and brain detect threatening sounds and activate a reflex response involving fear/annoyance, and increase of body functions, to prepare for danger - the conditioned aversive response

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