1.Reference:“Handbook of clinical audiology. 7th edition by Katz, Chasin, English, et al.” The Science of Sound Perception As a physician who deals primarily with hearing health and ear related disease, I am frequently asked to review patients who are having trouble with their hearing despite what would seem to be adequate amplification. A brief review of how the ear works will help illustrate why these patients are having difficulties with per- ception of hearing clarity. We are very complex biological machines. The ear and how the biology processes sound is no less com- plex. When considering this, it may help to break the biology down into two components; the ear and the brain. Think of the ear as the receiver and the brain as the computer that processes information passed to it by the ear. The ear is composed of mechanical and electrical components. The eardrum and ear bones form the mechanical components. The cochlea, which looks like a snail shell and is embedded in the base of your skull, houses the hair cells and those hair cells electrically connect to the hearing nerve. The hearing nerve electrically transmits the information to the brain – the computer. Generally, a defect of the mechanical component of your ear can be addressed efficiently either with surgical correction or a hearing aid. The volume is increased with amplification. Then the processing of that information will be efficiently and effectively done by the electrical elements of the inner ear and the brain (assuming these are functioning normally). When the electrical elements of the inner ear, that is, the hair cells and or the nerve that transmits the information from the hair cells to the brain becomes defective we now began to see problems with speech understanding. Unlike some animals such as birds and reptiles, which replace their hair cells if damaged, humans do not. We are born with about 40,000 hair cells per ear and if damaged by trauma such as noise or natural loss through aging, they are not replaced. These hair cells begin the process of analyzing sound. These hair cells are arranged in two distinct anatomi- cal arrays. There are three rows of outer hair cells and one row of inner hair cells. Without becoming too technical let us just say that the inner hair cells transmit the majority of neural in- formation to the hearing nerve and then to the brain. The outer hair cells amplify this process. Hair cells are organized within the cochlea in such a way that hair cells at the apex transmit low frequency information to the brain and hair cells at the base transmit high frequency information to the brain. In the event of a loss of hair cells at the base of the co- chlea, the individual will experience a loss of clarity due to the loss of high frequency information. Conversely, if hair cells are damaged at the apex of the cochlea, the individual will perceive a loss of volume due to the loss of low-frequency information.1 The audiogram is the hearing test performed by your audiologist used to measure the transmission of information from your ear to your brain. During the hearing test you are presented with tones. Where you respond on the scale that measures frequency identifies the intensity of sound that you can perceive. You also will be presented words to respond to. This helps identify how well the auditory system processes speech and gives informa- tion of how you may respond to amplifica- tion. If the word recognition score (WRS) or discrimination score is below 92% then this suggests some loss of the ability to process, with clarity, the auditory information. This loss of clarity, therefore, indicates defective inner ear, nerve or brain processing. The consequence, therefore, in such circum- stances is that you will perceive that there may be adequate volume and noise coming through the hearing aid but despite this there is some loss of clarity related to what you are listening to. How to overcome these issues? Some of the newer hearing aid technology allows the frequency spectrum of the hearing aid to shift into areas were the cochlea is still functioning more effectively. Hearing aid technology can also help with Bluetooth™ microphones and directional microphones within the hearing aids and with advanced processing of the sound. This technology is very helpful but when word recognition becomes more impaired – even with the use of hearing aids, it is important to keep background noise minimized. Also the facial cues of the person speaking can be used to help interpret the auditory information. Science is suggesting that even in instances where an individual finds hearing aids dis- appointing, the added stimulation of sound energy provided by the hearing aids is, in fact, beneficial to the brain’s speech processing centre and to global brain health. Dr. Fraser Noel is an Otologist/Neuro-otologist in practice in Victoria, British Columbia. He completed his Otolaryngology residency at the University of British Columbia in 1986. In 1988 he completed a Fellowship in Otology/Neurotology at the Otologic Medical Group/House Ear Institute in Los Angeles, California. He practiced at St. Pauls Hospital in Vancouver, British Columbia until 1997 at which time he moved his practice to Victoria. He has been published in peer reviewed journals. He is a past president of the British Columbia Otolaryngology Society. He has an appointment as Clinical Instructor with the University of British Columbia. Dr. Fraser Noel M.D., F.R.C.S.C. — Contributed by Dr. Fraser Noel M.D., F.R.C.S.C. 33 | HEARING TODAY