Minimal exposures reveal visual processing priorities

Abstract

The human visual system is capable of detecting extremely small light signals, as little as one single photon. But what is the visual system's minimal exposure threshold to detect meaningful stimuli? Due to hardware limitations, studies examining fast visual processing typically present stimuli for suprathreshold durations and disrupt processing with a mask. Here, we employed a newly developed tachistoscope that enables sub-millisecond presentations. Combining measures of perceptual sensitivity and recordings of neural activity, we find that both the holistic configuration and the emotional expression of a human face have the same minimal required exposure as the minimal required exposure for faces to reach awareness. These findings indicate that conscious awareness is required for processing meaningful visual information.

The importance of awareness in face processing: A critical review of interocular suppression studies

“…A simpler and more intuitive approach would be to simply present (unmasked) face images for brief exposure durations to determine their detection thresholds, but as argued in Section 2.1, even the shortest exposure durations that can be presented with computer screens may be sufficient for observers to detect faces, identify their expression, and elicit the corresponding physiological activation of these processes. However, very recently, Lanfranco et al [92] used a newly developed LCD tachistoscope that enables visual presentations with sub-millisecond precision. They presented participants with intact and scrambled human faces with fearful or neutral expressions for predefined exposure durations ranging from 0.8 to 6.2 ms and measured participants' sensitivity to the location of the face and to their expression, while also measuring their EEG data, to determine minimal required exposures of these processes.…”

“…For instance, recently Stein & Peelen [63] combined tasks of detection and discrimination of faces shown for predefined exposure durations in different orientations to test whether observers are able to detect the location of a face (and exhibit a FIE) while being unable to identify its orientation. By teasing apart these two processes, this method allows measuring detection and discrimination sensitivity, separately, and their corresponding decision criteria (also see [61,62,92]).…”

“…A newer approach that is gaining traction is using non-speeded tasks where experimenters control how long stimuli are presented and assess participants by combining detection and identification tasks. This paradigm enables control and quantification of both decision criterion (in presence/absence detection tasks) and response bias (in forcedchoice tasks) under a signal detection theoretic framework [61][62][63]92]. This approach is promising because it allows ruling out or at least accounting for post-perceptual factors.…”

The importance of awareness in face processing: A critical review of interocular suppression studies

“…A simpler and more intuitive approach would be to simply present (unmasked) face images for brief exposure durations to determine their detection thresholds, but as argued in section 2.1, even the shortest exposure durations that can be presented with computer screens may be sufficient for observers to detect faces, identify their expression, and elicit the corresponding physiological activation of these processes. However, very recently, Lanfranco et al [79] used a newly developed LCD tachistoscope that enables visual presentations with sub-millisecond precision. They presented participants with intact and scrambled human faces with fearful or neutral expressions for predefined exposure durations ranging from 0.8 to 6.2 ms of exposure and measured participants' sensitivity to the location of the face and to their expression while also measuring their EEG data; all this to determine minimal required exposures of these processes.…”

“…A newer approach that is gaining traction is using non-speeded tasks where experimenters control for the presentation displays and combining tasks of detection and identification (or discrimination) while both controlling and/or quantifying response bias (e.g., in a detection task) and decision criterion (e.g., in an identification task) under a signal detection theoretic framework [49][50][51]79]. This approach is promising because it allows ruling out or at least accounting for post-perceptual factors.…”