Crosstalk is a phenomenon in stereoscopy where an image becomes blurry due to leakage of the left image into the right eye and vice versa, and is considered one of the serious problems impairing stereoscopic experience. The current study examines mental/cognitive activity under a various levels of crosstalk through heart activity and forehead blood flow. In the experiment that presented three still natural images and one graphical video with a various crosstalk levels, heart rate showed a decelerative-accelerative-decelerative pattern for all the stimuli up to the intolerably severe level. The result suggests changes in mental state in accordance to the crosstalk level: i.e. orientation response under no perceived crosstalk, active mental elaboration upon noticing crosstalk, and reduced level of elaboration as crosstalk progressed. The pattern, however, did not always agree amongst the physiological measures and the crosstalk ratios. This suggests that the mental state under crosstalked image viewing could be more complex than a simple combination of orientation response and active mental elaboration.