Levels of processing during non-conscious perception: a critical review of visual masking

Abstract

Understanding the extent and limits of non-conscious processing is an important step on the road to a thorough understanding of the cognitive and cerebral correlates of conscious perception. In this article, we present a critical review of research on subliminal perception during masking and other related experimental conditions. Although initially controversial, the possibility that a broad variety of processes can be activated by a non-reportable stimulus is now well established. Behavioural findings of subliminal priming indicate that a masked word or digit can have an influence on perceptual, lexical and semantic levels, while neuroimaging directly visualizes the brain activation that it evokes in several cortical areas. This activation is often attenuated under subliminal presentation conditions compared to consciously reportable conditions, but there are sufficiently many exceptions, in paradigms such as the attentional blink, to indicate that high activation, per se, is not a sufficient condition for conscious access to occur. We conclude by arguing that for a stimulus to reach consciousness, two factors are jointly needed: (i) the input stimulus must have enough strength (which can be prevented by masking) and (ii) it must receive top-down attention (which can be prevented by drawing attention to another stimulus or task). This view leads to a distinction between two types of non-conscious processes, which we call subliminal and preconscious. According to us, maintaining this distinction is essential in order to make sense of the growing neuroimaging data on the neural correlates of consciousness.

Figure 1

Schematic description of three masked priming methods for which objective direct measures have demonstrated prime invisibility or inaudibility. (a) Visual word repetition priming across case (Dehaene et al. 2001). (b) Repetition priming for faces (Kouider et al. submitted). Masks are made of overlaid and reversed faces and the prime size is reduced by 80% when compared with the target. (c) Repetition priming for spoken words (Kouider & Dupoux 2005). Masks made from backward speech and the prime are attenuated (−15 dB) and time compressed to 35% of their original duration.

Figure 2

Examples of brain imaging studies showing subliminal processing at orthographic, motor and semantic levels of processing for words and numbers. (a) The left occipitotemporal region is sensitive to repetition priming from masked words, independently of the case in which words are presented, and with a sensitivity to orthographic similarity (Dehaene et al. 2001; Devlin et al. 2004). (b) Subliminal digits can prime a motor response during a number comparison task, as revealed by the LRP measured with ERPs (Dehaene et al. 1998). (c, i) The bilateral intraparietal sulcus is sensitive to subliminal repetition priming of numbers, independently of whether they are presented as words or as digits (Naccache & Dehaene 2001b); other experiments indicate a dependency on numerical distance, suggesting that this region may encode the semantic dimension of numerical magnitude. (c, ii) The left middle temporal gyrus (with blown-up inset shown) is sensitive to priming by synonym words (Devlin et al. 2004) as well as priming by repetition of words presented in the Kanji and Kana Japanese writing systems (Nakamura et al. 2005), suggesting that this region encodes words at a semantic level.

Figure 3

Taxonomy between conscious, preconscious and subliminal processing, based on the theoretical proposal by Dehaene et al. (2006). This distinction stipulates the existence of three types of brain states associated with conscious report, non-conscious perception due to inattention (preconscious state; Kouider et al. in press) and non-conscious perception due to masking (subliminal perception; Dehaene et al. 2001, 2006).