Suzanne de Bruijn

13 General introduction INTRODUCTION Hearing, smell, taste, touch and vision are considered the primary senses of the human body. Sensation is defined as the ability to transduce, encode and perceive information that is generated from our external and internal environment. A significant portion of the brain is devoted to this task; the organ is continuously distinguishing relevant input from background signals, a process called perception. This is a critical process, and the output serves as the most important determinant for behavior, cognition and thought. Vision and hearing are by many considered as the most important of the five senses. Already since the old days, they have been critical for the survival and evolution of mankind. Vision allows the early observation of potential threats, while hearing allows detection of enemies that are not even within sight yet. Today, these senses do not only warn us for danger; they have also become more and more important in social (e.g. communication) and cultural (e.g. music and art) aspects of life. With this in mind, it is not surprising that vision and hearing disabilities have severe consequences for the quality of life. 1 Individuals with a visual disability are severely handicappedintoday’ssociety,whichincreasinglyreliesonrapid(visual)communication. Also the impact of hearing loss (HL) should not be underestimated; disabling HL has been linked with several mental conditions including depression. 2,3 Especially when HL manifests before the development of speech, it can have serious negative consequences for the cognitive and socio-emotional development. 4 Therefore, both the auditory system and the visual system are critical for a person’s wellbeing. For both visual and hearing disabilities, genetic etiologies play a crucial role in the development of disease and are the topic of this thesis. In this chapter, the basic principles of the visual and auditory systems are discussed. The first part addresses the visual system, starting with the anatomy of the eye followed by an overview of inherited retinal dystrophies (RD). Next, the anatomy of the ear is discussed and followed by the different aspects of (inherited) HL. In the final section of this chapter, the general aim and contents of the thesis will be described. ANATOMY AND PHYSIOLOGY OF THE EYE The human eye is a highly specialized sensory organ, responsible for the capture and detection of light signals. It allows for the visualization of objects and to distinguish them based on their color, shape, size, and texture. Visual perception is mediated by the retina, a light sensitive layer of highly-specialized cells that lines the back of the