Journal ofNeurology, Neurosurgery, and Psychiatry 1988;51:1455-1456 Short report A technique to detect psychogenic sensory loss

suMMARY Tactile thresholds were measured in six patients with psychogenic hypaesthesia using a forced choice procedure. The response to stimuli close to threshold differed in a characteristic manner from that seen in patients with peripheral or central nervous system damage. Psychogenic sensory deficits can be detected by forced choice testing. ' A patient who claims he cannot perceive a certain stimulus is asked to "guess " whether it is present or not; significantly fewer correct respon-ses are obtained than expected from the binomial distribution. This paper describes an alternative method. It is a modified version of a previously described and validated procedure to measure tactile thresholds in organic diseases.2 Patien and method The study comprised two men and four women aged 23-47 years. They all presented with typical psychogenic weakness of the dominant upper extremity.34 In addition they had hypaesthesia ofnon-anatomical distribution. To test for light touch calibrated von Frey hairs were used. Each patient was first tested on the normal homologue area and this was again examined at the end of the experimental session to exclude a failure of general attention. A short pretest indicated the appropriate stimulus strengths to be used in further testing. Patients were told they would hear a sound and then either be touched by a hair or not, which they should report as "yes " or "no". Testing comprised a random series of blocks, within each ofwhich the same von Frey hair was used for 10 stimuli randomly interspersed with 10 null stimuli. A total of40 trials were made with each von Frey hair and on each side. From observed frequencies of "yes " responses on stimuli ("hits") and on null stimuli ("false alarms"), A ' was calculated. This is a measure of sensitivity which is independent of patient's response criterion2 " (such as, how eager the subject is to make a hit or how reluctant he is to make a false alarm)