Validity of messages from quadriplegic persons with cerebral palsy.

Interpreting gestures from retarded, nonvocal subjects is scientifically risky. Investigators must construe the subject's meaning without reference to an external validity measure. A procedure was devised in which message content was provided to nonvocal, severely palsied quadriplegic subjects in advance. Subjects' responses were limited to yes/no gestures. Another investigator elicited the messages without prior knowledge of their content. Results, which indicated reasonably high correspondence between stimulus messages and messages elicited, suggest that such subjects can present the content of their own phenomenal field accurately and that investigators' interpretations need not be considered imaginary.