A Unified Comparison of Stimulus-Driven, Endogenous Mandatory and ‘Free Choice’ Saccades

It has been claimed that saccades arising from the three saccade triggering modes–stimulus-driven, endogenous mandatory and ‘free choice’–are driven by distinct mechanisms. We tested this claim by instructing observers to saccade from a white or black fixation disc to a same polarity (white or black) disc flashed for 100 or 200 ms presented either alone (Exo), or together with an opposite (Endo) or same (EndoFC) polarity disc (blocked and mixed sessions). Target(s) and distractor were presented at three inter-stimulus intervals (ISIs) relative to the fixation offset (ISI: −200, 0, +200 ms) and were displayed at random locations within a 4°-to-6° eccentricity range. The statistical analysis showed a global saccade triggering mode effect on saccade reaction times (SRTs) with Endo and EndoFC SRTs longer by about 27 ms than Exo-triggered ones but no effect for the Endo-EndoFC comparison. SRTs depended on both ISI (the “gap-effect”), and target duration. Bimodal best fits of the SRT-distributions were found in 65% of cases with their count not different across the three triggering modes. Percentages of saccades in the ‘fast’ and ‘slow’ ranges of bimodal fits did not depend on the triggering modes either. Bimodality tests failed to assert a significant difference between these modes. An analysis of the timing of a putative inhibition by the distractor (Endo) or by the duplicated target (EndoFC) yielded no significant difference between Endo and EndoFC saccades but showed a significant shortening with ISI similar to the SRT shortening suggesting that the distractor-target mutual inhibition is itself inhibited by ‘fixation’ neurons. While other experimental paradigms may well sustain claims of distinct mechanisms subtending the three saccade triggering modes, as here defined reflexive and voluntary saccades appear to differ primarily in the effectiveness with which inhibitory processes slow down the initial fast rise of the saccade triggering signal.

有研究主张，三种眼跳触发模式——刺激驱动型（stimulus-driven）、内源性强制型（endogenous mandatory）与自由选择型（free choice）——所产生的眼跳由不同的神经机制驱动。为验证这一主张，我们要求被试将视线从白色或黑色注视圆盘，移至与注视圆盘极性相同（白色或黑色）、闪现时长为100或200ms的目标圆盘：目标可单独呈现（外源性触发，Exo），或与反极性（内源性触发，Endo）、同极性（自由选择触发，EndoFC）的圆盘同时呈现，采用区组与混合实验范式。目标与干扰项的呈现相对于注视点偏移的刺激间间隔（ISI）分为三类：−200、0、+200 ms，且呈现位置随机分布于4°至6°的偏心度范围内。统计分析结果显示，眼跳触发模式对眼跳反应时（SRTs）存在整体效应：内源性强制型与自由选择型的SRT较外源性触发组长约27 ms，但内源性强制型与自由选择型之间无显著差异。SRT同时受刺激间间隔（即“间隙效应”）与目标闪现时长的影响。在65%的被试数据中发现了眼跳反应时分布的双峰最优拟合，且三种触发模式下的拟合数量无显著差异。双峰拟合中“快速”与“慢速”眼跳的占比同样不受触发模式的影响。双峰性检验未发现三种触发模式间存在显著差异。对干扰项（内源性触发组）或重复目标（自由选择触发组）的推测性抑制时序进行分析，结果显示内源性强制型与自由选择型眼跳间无显著差异，但随刺激间间隔缩短，眼跳反应时显著缩短，这与此前的结果一致，表明干扰项与目标间的相互抑制可被“注视”神经元所抑制。尽管其他实验范式或许能够支持三种眼跳触发模式对应不同神经机制的主张，但正如本研究所示，反射性与自主性眼跳的核心差异，主要在于抑制过程延缓眼跳触发信号初始快速上升的有效性。