The appearance of a salient stimulus rapidly inhibits saccadic eye movements. Curiously, this "oculomotor freezing" reflex is triggered only by stimuli that the participant reports seeing (White & Rolfs, 2016). But is oculomotor freezing linked to the participant's sensory experience, or their decision that a stimulus was present? If it were decision-related, oculomotor freezing should become less prevalent when the participant is induced to have a conservative decision criterion and reports seeing a stimulus less often. Here we manipulated decision criterion in two ways: by adjusting monetary payoffs and stimulus probability in a detection task. These bias manipulations greatly affected participants' explicit reports but did not affect the degree to which microsaccades were inhibited by stimulus presence. In addition, the link between oculomotor freezing and explicit reports was stronger when the decision criterion was conservative rather than liberal. The simplest explanation is that conservative reports of stimulus presence are more often based on a strong sensory signal that also inhibits microsaccades. We conclude that the sensory threshold for oculomotor freezing is independent of decision bias. To the extent that conscious experience is also unaffected by such bias, oculomotor freezing provides an involuntary, implicit indication that a stimulus has entered awareness.
Adaptivity of fixational saccadic eye movements in a visual detection task
