Vision is degraded during saccades, as shown by decreased sensitivity to visual events (saccadic suppression of vision) and by elimination of the perception of the visual blur which should be seen during saccades (saccadic omission). These phenomena have been shown to be due largely to visual masking effects from the clear fixations before and after a saccade. A small contribution to the effect is due to extraretinal influences, however. This is probably a 'corollary discharge' of the efferent oculomotor commands. In contrast to other forms of suppression, saccadic suppression of displacement (SSD, a decrease in sensitivity to visual displacements during saccades) has often been considered to be due to this efferent component rather than to visual masking.

The aim of this experiment was to explicitly assess the importance of visual conditions in SSD. In two allied experiments a small computer-generated stimulus made random horizontal jumps. An infrared limbus tracker (IRIS, Skalar Medical) was used to detect the saccade toward the new position, triggering a smaller centripetal displacement. Subjects reported awareness of these intrasaccadic displacements by pressing a key. Each subject performed the task in both a well lit environment and in complete darkness.

In Experiment 1, 8 normal control subjects performed 180 trials in both lighting conditions, with initial target jumps of 8 - 24 deg and intrasaccadic displacements of 0 - 4 deg. In Experiment 2, 14 normal control subjects performed 44 trials in both lighting conditions, with initial target jumps of 15 - 24 deg. Rather than a range of absolute sizes, intrasaccadic displacements were 0 - 30% of the initial target jump.

Sensitivity to target displacements was measured by the area under the curve (AUC) of the receiver operating characteristic curve. AUC ranges from 0.5 (chance performance) to 1.0 (perfect sensitivity and specificity). In Experiment 1, sensitivity was slightly lower in the dark (AUC = 0.73 vs. 0.80, Wilcoxon matched pairs test, p < 0.05). In Experiment 2, sensitivity was also lower in the dark, but not significantly so (AUC = 0.70 vs. 0.75, ns).

The absence of clear fixations before and after a saccade eliminates other forms of suppression (Campbell & Wurtz, 1978, Vision Research, 18: 1297-1303). If visual masking was responsible for causing SSD as it is for other forms of suppression, then a large increase in sensitivity would be expected in the dark. Although visual conditions had some effect on sensitivity to intrasaccadic displacements, this effect was small and in the opposite direction.

Thus visual masking is not responsible for SSD in the way that it is for other forms of suppression. Other authors have provided evidence that SSD is not due to a simple efferent signal either. Our results are consistent with the view that SSD results from a high level pre/post saccade cognitive comparison process (e.g. Blackmore et al., 1995, Perception, 24: 1075-1081).