Variability of stimulus effect in the course of phallometric testing

To study the serial effect in phallometric experiments, several sexual stimuli (film scenes) were presented in different orders to 56 subjects in four groups. All stimuli were presented at a relatively constant basal level of penile volume after the subjects' engagement in a nonsexual cognitive task. Serial effects occurred. At the beginning of the session, the magnitude of penile response did not depend on the content of sexual stimuli, the second stimulus eliciting a higher effect than the first. The differential effect of stimuli occurred only when they were shown in the third or fourth position. Finally, it was demonstrated that the effect of the stimulus depends to some extent on the effect of the preceding stimulus.