Intertrial stimuli and differentiation of the conditioned eyelid response

One hundred thirty-six human Ss participated in two differential eyelid conditioning experiments designed to demonstrate the inhibitory properties of intertriai stimuli. In Experiment I when the intertriai stimulus was a tone (IT) lying midway between CS+ and CS− on the auditory frequency dimension differentiation was enhanced compared with a group which received only noise during the ITI, but this enhancement occurred only when CS+ and CS− were highly similar tones. In Experiment II differentiation was poorer where the order of stimuli along the frequency dimension was IT, CS+, CS− compared with IT, CS−, CS+.