Abstract
Involuntary eye movements during fixation of gaze are typically transiently inhibited following stimulus onset. This oculomotor inhibition (OMI), which includes microsaccades and spontaneous eye blinks, is modulated by stimulus saliency and anticipation, but it is currently unknown whether it is sensitive to familiarity. To investigate this, we measured the OMI while observers passively viewed a slideshow of one familiar and 7 unfamiliar facial images presented briefly at 1 Hz in random order. Since the initial experiments indicated that OMI was occasionally insensitive to familiarity when the facial images were highly visible, and to prevent top-down strategies and potential biases, we limited visibility by backward masking making the faces barely visible or at the fringe of awareness. Under these conditions, we found prolonged inhibition of both microsaccades and eye-blinks, as well as earlier onset of microsaccade inhibition with familiarity. These findings demonstrate, for the first time, the sensitivity of OMI to familiarity. Because this is based on involuntary eye movements and can be measured on the fringe of awareness and in passive viewing, our results provide direct evidence that OMI can be used as a novel physiological measure for studying hidden memories with potential implications for health, legal, and security purposes.
Highlights
During fixation of gaze, our eyes move involuntarily in a random-walk like movement and occasional small saccades or microsaccades[1,2]
We found that the Microsaccade RT (msRT), presumably related to the onset of inhibition, was ~30 ms faster for the familiar compared with the non-familiar faces (~90 ms and ~120 ms, respectively); this was true for both the group average (Fig. 3c left panel) and for the different face images
Our results revealed that the microsaccade inhibition time course was modulated by familiarity, where briefly flashed and masked familiar faces evoked earlier and longer microsaccade inhibition compared with unfamiliar faces (Figs 2a and 3)
Summary
Our eyes move involuntarily in a random-walk like movement and occasional small saccades or microsaccades[1,2]. When consciously perceiving a stimulus onset, even when predictable or illusory[10], microsaccades are first inhibited for a short period of time, disinhibited and the rate increases before returning to baseline (see review in[11]) This oculomotor inhibition (OMI) effect, which was found for microsaccades as well as for spontaneous eye blinks[12,13], depends on the properties of the stimulus, attention, and expectation[11,13,14,15,16]; the saliency of the stimulus systematically affects the inhibition time course[13]. Shorter fixation durations on familiar faces were recently reported, when observers had to examine and memorize 4 faces for later reporting[31], presumably because the familiar face requires a shorter memorizing process These studies, do not shed light on OMI because they refer to free viewing of static stimuli and not to the involuntary oculomotor response to visual onsets, which we investigated in the current study. Given the evidence for co-variation of OMI and consciousness for simple stimuli at threshold[37], and the evidence for familiar faces breaking suppression more readily than unfamiliar faces[18,19,20], face stimuli on the verge of awareness could create an ideal condition for exaggerating the difference in OMI response between the familiar and unfamiliar, if it exists
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