Researchers from the UK and France have found that the protein stereocilin, which is found in fine hair-like projections of the ear, is essential for discriminating relevant sounds from surrounding noise. Their findings are a result of the Eurohear project, which is funded under the 'Life sciences, genomics and biotechnology for health' Thematic area of the Sixth Framework Programme (FP6). The cochlea is the part of the ear responsible for sensing, amplifying and filtering sounds before conveying them to the brain for interpretation. 'Masking', whereby relevant sounds are picked out from background noise, is pivotal to the hearing process because it makes understanding speech possible. The masking effect is caused by distortions produced in the cochlea that allow one tone to suppress another. These distortions generated by the cochlea are audible from the ear canal, and the emissions can be easily detected using a microphone about the size of a hearing aid; this is why physicians can screen for hearing loss at birth. The authors of the current study, which is published in the journal Nature, sought to understand the origins of these beneficial distortions on a molecular level. The cochlea houses two types of sensory receptor cells: inner and outer 'hair cells'. Outer hair cells (OHCs) amplify and sharpen sound and inner hair cells transmit the information to the brain. The OHCs house stereocilia, hair-like projections which move in response to sound waves. These movements trigger a series of reactions that generate a nerve impulse, and play an important role in generating distortions. Stereocilia are arrayed in three rows of increasing height and are joined by links and connectors along their tips. A protein called stereocilin is associated with one of these: a zipper-like 'top connector' that joins the tops of adjacent stereocilia within and between rows. The scientists bred mice that did not produce stereocilin, and found that the mice did not form top connectors between stereocilia. As a result, the mice had severely reduced masking effects early on and became progressively deaf; no acoustic or electrical waveform distortions were observed. Interestingly, they were able to amplify and filter sounds normally. 'Because both cochlear amplification and distortion originate from the outer hair cells,' the study explains, 'it has been speculated that they stem from a common mechanism. Here we show that the nonlinearity underlying cochlear waveform distortions relies on the presence of stereocilin, a protein defective in a recessive form of human deafness.' The study's results contribute greatly to our understanding of the molecular mechanisms of hearing. They provide information crucial to the development of therapeutic tools, a challenge undertaken by the Eurohear project. Eurohear comprises 250 scientists from 22 academic institutions and 3 small companies in 10 countries. The 5-year project began in December 2004 and received EUR 12.5 million in EC funding.