This paper describes two examples of perceptual disturbance and shows how they relate to the patients' mode of dealing with sensory experience. In the first instance the patient complained of a severe photophobia during an analytical interview when she had decided to abandon a habit of covering her eyes with her hands. In the second case there was a hyperacuity of smell and taste when the patient gave up excessive cigarette smoking. The author points out that within ordinary limits of effective ego functioning there is a direct relationship between instinctual tension and responsiveness to sensory stimuli. Increased instinctual tension heightens perception, and intensified sensory stimulation increases instinctual tension. Both patients used habitual defences whose aim was to reduce instinctual tension and so regulate the intensity of perception. When these defences were weakened alterations in perception occurred.