Introduction 2002 1987b 1996 2003 2003 1 However, in all aforementioned studies, depth perception was correlated with stimulus properties, i.e. the depth cues giving rise to the perceived depth. Therefore, the distinction between an effect on vergence caused by perception and an effect due to depth cues could not be made. Thus, it may be possible that signals related to the depth cues contributed to vergence rather than perceived depth itself. 1998 1968 1992 2001 2002 2002 Using these slant stimuli, the contribution to vergence predicted by perception is different from that predicted by the depth cues. If perceived depth is sufficient to influence vergence, a difference in vergence should be observed while alternations occur between the two possibly perceived surface slants. However, if perceived slant does not influence vergence, vergence should remain stable regardless of the perceived surface slant orientation. 1989 Methods Experimental setup 1 Fig. 1 Wheatstone stereoscope. Subjects viewed one TFT display with the corresponding eye via one of the mirrors. The viewing distance (eye-mirror-display) was 57 cm. Note that the subjects were in reality much closer to the mirrors than depicted here and that there was no crossover, i.e. each eye could only see via one mirror  Eye movements were measured using the head-mounted Eyelink I system at 250 Hz. The cameras were positioned beneath the mirrors. The whole setup and experimental room were painted black matte and the room was darkened. Stimuli 2 Fig. 2 An image as shown on one display. Perspective foreshortening indicates a slant of 70°. The disparity gradient was produced by horizontally scaling the two eyes’ half images. The red fixation cross is positioned in the center of the stimulus  3 Fig. 3 a γ D P b α β 1 2  P D P D 3 P D 3 2.5 Furthermore, a standing disparity was added to all stimuli, which made all surfaces appear to be positioned in front of the display. A fixation cross was presented in the center of the stimulus. Procedure and tasks Experimental trials consisted of a sequence of five different displayed items. Subjects were first presented with a fixation dot (used for offline drift correction) in the center of the display at display depth for 1.5 s (1). This dot was replaced by a fixation cross at the location and depth corresponding to the center of the stimulus (2). After 1.5 s, the stimulus images were added (3). A beep was sounded 2.0 s after stimulus onset. A monocular arrow appeared 2 s thereafter in the left eye’s image (4). The screen was blanked after 4 s (5). 1984 Subjects had to report their percept prior to saccade onset. They were asked to report the perceived nearest side of the stimulus after the beep using a numerical keypad. By reporting which stimulus side they perceived as nearest, subjects implicitly indicated whether they perceived a slanted rectangle or a slanted trapezoid. 2002 2005b One experimental block consisted of a total of 16 trials, of which 8 were ambiguous stimuli conditions and the other 8 were unambiguous stimuli conditions. Both conditions were counter balanced for saccade direction and trials were randomized for conditions and saccade directions. Each experimental session contained three to four experimental blocks run consecutively. A calibration was performed, at the start of each block, i.e. every 5 min. Data analysis 1993 2007 Subjects Subjects (6 females and 9 males aged between 18 and 30) had normal or corrected to normal vision. Before taking part in the experiment, it was checked whether a subject was able to comply with the instructions of the experiment. Not all potential subjects could perceive the trapezoidal interpretation of the stimulus within the timeframe of the experiment (2 s), when disparity and perspective foreshortening were in conflict. These subjects either had poor stereovision or needed more than 2 s to elicit a voluntary controlled flip. In total eight subjects were excluded from further participation, because it would have been impossible to measure the effect of depth perception on vergence. Results 1988 Experiment 1: Contribution of perception of depth to vergence 2.3 Predictions 4 4 Fig. 4 a t box b b S6 top S1 bottom S6 S1 t S6 S1  Results 4 t t t 4 2.4 t 1988 4 4 2.4 2 5 5 5 Fig. 5 a 2.4 bottom top S4 red circle blue pentagon S2 S5 S4 S6 b 2.4 right side S1 F P  Experiment 2: Contribution of depth cues other than disparity to vergence The results from experiment 1 show that perceived orientation does not influence vergence. Still, as stated in the introduction, cues other than disparity may contribute to vergence. We investigated this hypothesis by analyzing the data from the ambiguous stimulus and the unambiguous stimulus presentations. Predictions 6 Fig. 6 a 4 b S6 top S1 bottom S6 S1 t  Results 6 4 6 6 7 7 F P F P F P F P F P F P F P F P F P F P Fig. 7 a 2.4 bottom top purple triangle cyan square b F P F P star  Thus, perspective, or signals related to it, significantly contributed to vergence. Discussion Our findings show that depth cues rather than depth perception itself contribute to vergence accompanying saccadic movements. Perspective being congruent or incongruent with disparity caused a difference of about 14% in vergence changes predicted by disparity alone. Although our results show that vergence remained constant in the ambiguous stimulus condition, subjects perceived alternations of surface orientation. Collectively, these results show that perspective and disparity are each weighted differently for perception and vergence. Binocular studies 2003 2003 2003 2003 Monocular studies 1987a b 1987a b 1996 However, each depth cue used in these studies was inherently ambiguous. Thus, in these studies, the contribution of perception of depth to vergence could again not be dissociated from the contribution of the depth cues themselves. Taking the present study into consideration, there is evidence that their results reflect the alternations of the depth cue related signals and not those related to perception. In view of this conclusion, the fact that the vergence changes measured by Enright in the Necker cube experiment were much smaller than those in the linear perspective experiment may be explained by a degradation of the signals related to the depth cues due to the ambiguity herein. Influence of perception on other visual phenomena 2005b a 2003 2006 Different weighting of cues for vergence and perception 1987a b 1996 1996 8 Fig. 8 Perception and vergence are based on separate processing streams. Both monocular cues (such as relative motion, linear perspective, blur and looming) and binocular cues [such as horizontal disparity (global and local) and global vertical disparity] are used for perception as well as for vergence. The weights assigned to the individual cues might be different when used for perception than when used for vergence, resulting in different outcomes. In the present study, for example, the ambiguous stimulus yields a bistable perception of surface slant, whereas it yields a stable vergence angle. Cognitive factors, such as voluntary control, do not seem to exert influence on vergence, but they do influence perception  Perception and action 1992 2004 2001 2001 2000 2001 1985 1997 2001 2001 2003 2000 2001 2001 2002 2003 2006a b 2005 2007 Conclusion Our findings show that depth cues rather than perceived depth govern vergence that accompanies saccades. Perspective being congruent or incongruent with disparity caused a 14% difference in vergence change. These results combined with other studies show that in addition to local horizontal disparity, also global disparity, relative motion and perspective are used as input by the vergence system. Furthermore, our findings suggest that monocular and binocular cues are weighted differently for perception and vergence.