How would you cope if you couldn’t hear, see or feel anything? How do sensory systems react when they have no information to process? Such questions may seem rather bizarre, but they are in fact the topic of sensory deprivation research. Sensory deprivation involves systematically preventing information from reaching one or more sensory modalities. As a research methodology it has a long and chequered history.
Some early sensory deprivation studies were funded by the CIA with the purpose of identifying effective methods of interrogation (1, 2). Such experiments involved attempts to completely remove all sensory information from an individual. Participants were left for long periods in secluded, sound-proofed rooms, wearing cardboard sleeves to reduce tactile stimulation and goggles to reduce visual stimulation. For the unfortunate participants, the consequences of being exposed to these conditions were often hallucinations, anxiety and mental deterioration. The extent of the volunteers suffering was such that they were (eventually) paid compensation in recognition of their maltreatment. Indeed the controversy surrounding these sorts of experiments led to the introduction of stricter ethical guidelines to prevent unwitting participants of behavioural research being exposed to potential harm or distress (1).
Although these early, total sensory deprivation experiments were clearly unacceptable, more subtle forms of sensory deprivation are still used in human research. Sensory deprivation can, if implemented correctly, reveal important information about the functioning of the nervous system without causing any harm to those participating in the research. One example of the positive use of sensory deprivation concerns its role in improving our understanding of tinnitus. Back in the 1950s it was discovered that people with normal hearing often began to experience hallucinatory sounds similar to tinnitus if they were left in a silent, sound-proofed room (3). This finding led to the idea that hearing loss may contribute to the development of tinnitus. However, most tinnitus sufferers do not exhibit the sort of total hearing loss that is mimicked by a silent, sound-proofed room. Indeed some tinnitus sufferers demonstrate very good hearing! It appears therefore that processes other than hearing loss must be involved in the development of tinnitus.
Modern theories of tinnitus point the finger at the process of homeostatic plasticity; the mechanism by which the equilibrium of neurological systems is maintained through adjustments made to physiological processes (4). In the same way that a thermostat alters the activity of a heating system to maintain a certain temperature, it is thought that the brain modifies the degree of spontaneous firing within neuronal populations in order to maintain a consistent level of activity. In response to damage to the auditory nerve, homeostatic plasticity may cause the brain to implement a ‘gain’ to ongoing spontaneous activity within the auditory system. While this gain may serve to reduce, or even remove, the impact that nerve damage has on hearing ability, it may also induce tinnitus by elevating baseline neural activity to a level that is similar to that evoked by genuine sounds (4).
Scientists funded by the British Tinnitus Association recently used a more refined sensory deprivation methodology to test whether homeostatic plasticity may contribute to tinnitus (5). 18 participants were asked to wear an earplug in one ear for a week. The earplug was specially designed to mimic the sort of high-frequency hearing loss that commonly occurs due to old age or noise damage. Altering the input in just one ear not only minimised the inconvenience for participants, it also allowed the effects of the auditory deprivation to be ethically tested over a far longer period than would otherwise be possible with more substantial deprivation. The methodology therefore provided a far more naturalistic assessment of the effects of hearing loss on the auditory system.
14 of the 18 participants reported experiencing hallucinatory sounds during the week, with most of the sounds taking a form similar to those experienced in tinnitus. This confirmed that real-life forms of hearing loss are capable of inducing tinnitus-like symptoms. Crucially, the pitch of the hallucinated sounds matched the frequency spectrum of the deprivation induced by the earplugs; selective loss of hearing for high frequencies produced mainly high frequency hallucinatory sounds. As the type of hearing loss induced in this study should only provoke homeostatic changes in neuronal populations that process high frequency sounds, this finding supports the idea that homeostatic plasticity contributes to the development of tinnitus.
Despite its somewhat inglorious history, sensory deprivation remains an extremely important methodological tool. By removing the influence of external stimuli, sensory deprivation provides a clearer view of the workings of internally-driven neurological processes such as homeostatic plasticity. As neurological disorders are often characterised by dysfunctions in these internal processes, sensory deprivation studies can provide invaluable insight into the causes of such disorders.
More information about research into the causes of tinnitus is available at http://www.tinnitus.org.uk/the-search-for-a-cure-1
(1) McCoy, A.W. (2007) Science in Dachau’s shadow: Hebb, Beecher and the development of CIA Psychological torture and modern medical Ethics. Journal of the History of the Behavioral Sciences, Vol. 43(4), 401–417. <Link>
(2) Klein, N. (2007). The shock doctrine : the rise of disaster capitalism (1st ed. ed.). New York: Metropolitan Books/Henry Holt. <Link>
(3) Heller, M.F. & Bergman, M. (1953). Tinnitus Aurium in normally hearing persons. The Annals of Otology, Rhinology and Laryngology, 62 (1), 73-83 <Link>
(4) Schaette R, Kempter R (2006) Development of tinnitus-related neuronal hyperactivity through homeostatic plasticity after hearing loss: a computational model. Eur J Neurosci 23: 3124–3138. <Link>
(5) Schaette R, Turtle C, Munro KJ (2012) Reversible Induction of Phantom Auditory Sensations through Simulated Unilateral Hearing Loss. PLoS ONE 7(6): e35238. doi:10.1371/journal.pone.0035238 <Link>