A systematic narrative review of the form, process, and function of sensory hyposensitivity in depression

As well as the key diagnostic symptoms that define Major Depressive Disorder (MDD), there is also evidence that some aspects of sensory sensitivity may decrease concurrently with MDD. However, although various individual studies have reported sensory hyposensitivity as a correlate of depression, most studies have focused upon single senses (i.e., vision, hearing, taste, touch, olfaction) and no comprehensive review of all five senses has been published to date. By applying a systematic narrative review methodology to identify research on loss of each these senses and their association with depression, a total of 51 individual studies and five reviews (of olfaction) were identified and examined for their methodology, major findings, and explanation of those findings. Following that summary of the form of sensory hyposensitivity, the neurobiological processes underlying it are described as being based in peripheral hyposensitivity or dysfunction of central cognitive processes. From these bases, an explanation is provided of how sensory hyposensitivity can be understood from the perspective of providing an 'escape' from a noxious environment, thus representing a functional benefit for the depressed individual. Translation of these findings to clinical practice, and avenues for future research, are described.