Sustained invisibility through crowding and continuous flash suppression: a comparative review

Abstract

The study of non-conscious vision benefits from several alternative methods that allow the suppression of an image from awareness. Here, we present and compare two of them that are particularly well-suited for creating sustained periods of invisibility, namely visual crowding and continuous flash suppression (CFS). In visual crowding, a peripheral image surrounded by similar flankers becomes impossible to discriminate. In CFS, an image presented to one eye becomes impossible to detect when rapidly changing patterns are presented to the other eye. After discussing the experimental specificities of each method, we give a comparative overview of the main empirical results derived from them, from the mere analysis of low-level features to the extraction of semantic contents. We conclude by proposing practical guidelines and future directions to obtain more quantitative and systematic measures of non-conscious processes under prolonged stimulation.

Keywords: awareness; consciousness; continuous flash suppression; unconscious processing; visual crowding.

FIGURE 1

Illustration of typical crowding and CFS displays. (A) Schematic representation. In crowding, participants stare at a fixation cross (which here appears at the top of the screen), while a stimulus of interest (here the letter S) is presented in the periphery of the visual field, surrounded by similar flankers (here, the letters X). Note that the display is constant across eyes. In CFS, the stimulus of interest is usually presented foveally to the non-dominant eye, while the dominant eye receives a stream of rapidly changing pattern called mondrians. (B) Subjective percept. In crowding, observers report seeing a jumbled array of letters in the periphery (i.e., discrimination, but not detection is impeded). In CFS, observers typically report seeing only the mondrians (i.e., both discrimination and detection are impeded). (C) Distribution map. In crowding, eye fixation (depicted here by a red square) and attentional focus (depicted here by an orange area) are always dissociated (i.e., endogenous attention), while they are usually conjoint in CFS.