Further, studies on saccadic inhibition have also provided temporal evidence consistent with the parallel programming hypothesis. When a visual change, such as a briefly presented blank scene (33 ms), was displayed during the visual search of a target in a natural scene, saccade executions after the visual change were sometimes delayed. This effect, known as saccadic inhibition, is characterized by a “dip” in the distribution of saccade onset times ∼100 ms after the visual change (Bompas & Sumner,
2011; Henderson & Pierce,
2008; Henderson & Smith,
2009; Pannasch, Schulz, & Velichkovsky,
2011; Reingold & Stampe,
1999,
2000,
2002,
2004; Stampe & Reingold,
2002). The distribution has been interpreted to reflect two types of saccades: Prior to the dip in the distribution, saccade executions were unaffected by the visual change, and after the dip, saccade executions were affected by the visual change (Pannasch & Velichkovsky,
2009; Reingold & Stampe,
1999,
2000). Studies have proposed that saccade planning is carried out in two consecutive stages: an initial, labile stage when a saccade plan can be cancelled or modified and a later, nonlabile stage when the saccade plan can no longer be changed and must be executed (Becker & Jürgens,
1979; Engbert, Longtin, & Kliegl,
2002; Engbert, Nuthmann, Richter, & Kliegl,
2005; Morrison,
1984; Nuthmann, Smith, Engbert, & Henderson,
2010; Reichle, Pollatsek, Fisher, & Rayner,
1998; Reichle, Rayner, & Pollatsek,
2003; Salvucci,
2001). The demarcation between the labile and nonlabile stages of a saccade plan has been referred to as the point of no return, PONR (e.g., Engbert et al.,
2005; Nuthmann et al.,
2010; Reichle et al.,
1998). Nuthmann et al. (
2010) have interpreted unaffected saccades to reflect cases in which the visual change occurred after saccade planning had proceeded past the PONR. Based on this interpretation, affected saccades would then reflect cases in which the visual change occurred before the PONR. This could include cases in which the earlier plan was still in the labile stage as well as cases in which a saccade plan has yet to be made active. The subsequent saccade execution could be delayed due to processes involved in cancelling any ongoing saccade, processing the new visual scene, and forming a new saccade plan. This account implies that it is possible for the later saccade plan to overlap the earlier plan; i.e., saccades can be programmed in parallel.