The Tinnitus and Hyperacusis Centre

Description

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 30000 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 17000 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 its 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 dont 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 daytoday 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 quotemotional labelquot 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 neighbours 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 fearannoyance and increase of body functions to prepare for danger the conditioned aversive response