Encoding of frequency and volley theory

1. ENCODING OF FREQUENCY AND VOLLEY PRINCIPLE Roll number 23 KEMU ‘12-’17

2. CODING/ENCODING OF FREQUENCY Sound signals are carried to the brain in terms of all-or-none action potentials by peripheral afferent nerve fibres. The term ‘CODE’ simply describes the manner in which the information about the sound is represented in such neural activity. FREQUENCY is the number of impulses or cycles per second. The frequency of discharged impulses is the function of stimulus strength; t he greater the stimulus intensity, the higher the impulse frequency of the message.

3. Auditory nerve fibres contacting each hair cell fire action potentials in response to movement of the basilar membrane at that location. Fibres on the outside of the auditory nerve bundle (those that innervate the basal hair cells) have high characteristic frequencies. Those towards the middle of the nerve bundle (those that innervate the apex of the cochlea) have low characteristic frequencies. Response of any given fibre reflects the frequency selectivity of that location on the basilar membrane from which it comes.

4. Sound of any frequency causes whole basilar membrane movement to a little extent but a specific area is maximally activated (which is dependent upon the respective frequency). So, each fibre within the nerve responds ‘BEST’ to a particular frequency.

5. Locations of activity on the basilar membrane, in case of more than one stimuli, are irrelevant. e.g. A high frequency stimulus can cause maximal movement of basilar membrane in the region near the oval window, while at the same time low frequency stimulus can cause activation near the helicotrema , and both messages will be carried to the brain through their respective fibres.

6. Characteristic Frequency CF The most sensitive sound frequency for a particular auditory fibre is called neuron’s characteristic frequency. OR The sound frequency producing greatest response in the auditory nerve fibre.

7. VOLLEY THEORY VOLLEY means ‘simultaneous discharge’. Volley Principle is an information encoding scheme used in human hearing. A population of auditory nerve fibres, all in-phase/phase-locking to the same stimulus, represent their combined discharge pattern in response to a high frequency stimulus which can’t be as such travelled by a single nerve fibre, instead travels by more than one fibres in form of low frequency stimuli.

8. When high frequency sounds are experienced too frequently for a single neuron to adequately process and fire for each sound event the organ of Corti that is found in the cochlea, combine the multiple stimuli into a "volley" in order to process the sounds. e.g. While one neuron alone can’t carry code for 20,000 Hz tone, 20 neurons with staggered firing rates can. Every neuron will respond on average to every 20 th cycle of the original tone, and the bunch of neurons will jointly carry the information of the 20,000 Hz tone.

9. Reason: Nerve cells transmit information by generating brief action potentials. Sound is encoded by producing an action potential for each cycle of the vibration, e.g. 200Hz results in a neuron producing 200 action potentials per second. BUT, neurons can only produce action potentials around 300 to 500 Hz. The human ear overcomes this problem by allowing several nerve cells to take turns performing this single task. The volley principle was proposed to deal with this apparent anomaly between the behaviour of single neurons and groups of neurons.