Neural capacity limits during unconscious semantic processing

No electrophysiological evidence for semantic processing during inattentional blindness

A long-standing question concerns whether sensory input can reach semantic stages of processing in the absence of attention and awareness. Here, we examine whether the N400, an event related potential associated with semantic processing, can occur under conditions of inattentional blindness. By employing a novel three-phase inattentional blindness paradigm designed to maximise the opportunity for detecting an N400, we found no evidence for it when participants were inattentionally blind to the eliciting stimuli (related and unrelated word pairs). In contrast, participants noticed the same task-irrelevant word pairs when minimal attention was allocated to them, and a small N400 became evident. When the same stimuli were fully attended and relevant to the task, a robust N400 was observed. In addition to univariate ERP measures, multivariate decoding analyses were unable to classify related from unrelated word pairs when observers were inattentionally blind to the words, with decoding reaching above-chance levels only when the words were (at least minimally) attended. By comparison, decoding reached above-chance levels when contrasting word pairs with non-word stimuli, even when participants were inattentionally blind to these stimuli. Our results also replicated several previous studies by finding a "visual awareness negativity" (VAN) that distinguished task-irrelevant stimuli that participants noticed compared with those that were not perceived, and a P3b (or "late positivity") that was evident only when the stimuli were task relevant. Together, our findings suggest that semantic processing might require at least a minimal amount of attention.

The functions of consciousness in visual processing

Conscious experiences form a relatively diverse class of psychological phenomena, supported by a range of distinct neurobiological mechanisms. This diversity suggests that consciousness occupies a variety of different functional roles across different task domains, individuals, and species; a position I call functional pluralism. In this paper, I begin to tease out some of the functional contributions that consciousness makes to (human) visual processing. Consolidating research from across the cognitive sciences, I discuss semantic and spatiotemporal processing as specific points of comparison between the functional capabilities of the visual system in the presence and absence of conscious awareness. I argue that consciousness contributes a cluster of functions to visual processing; facilitating, among other things, (i) increased capacities for semantically processing informationally complex visual stimuli, (ii) increased spatiotemporal precision, and (iii) increased capacities for representational integration over large spatiotemporal intervals. This sort of analysis should ultimately yield a plurality of functional markers that can be used to guide future research in the philosophy and science of consciousness, some of which are not captured by popular theoretical frameworks like global workspace theory and information integration theory.

Role of visual awareness on semantic integration of sequentially presented words: An fMRI study

Reading comprehension requires the semantic integration of words across space and time. However, it remains unclear whether comprehension requires visual awareness for such semantic integration. Compared to earlier studies that investigated semantic integration indirectly from its priming effect, we used functional magnetic resonance imaging (fMRI) to directly examine the processes of semantic integration with or without visual awareness. Specifically, we manipulated participants' visual awareness by continuous flash suppression (CFS) while they viewed a meaningful sequence of four Chinese words (i.e., an idiom) or its meaningless counterpart (i.e., a random sequence). Behaviorally, participants had better recognition memory for idioms than random sequences only when their visual awareness was interfered rather than blocked by CFS. Neurally, semantics-processing areas, such as the superior temporal gyrus and inferior frontal gyrus, were significantly activated only when participants were aware of word sequences, be they meaningful or meaningless. By contrast, orthography-processing areas, such as the fusiform gyrus and inferior occipital gyrus, were significantly activated regardless of visual awareness or word sequence. Taken together, these results suggest that visual awareness modules the functioning of the semantic neural network in the brain and facilitates reading comprehension.

Neuroscientific Evidence for Processing Without Awareness

The extent to which we are affected by perceptual input of which we are unaware is widely debated. By measuring neural responses to sensory stimulation, neuroscientific data could complement behavioral results with valuable evidence. Here we review neuroscientific findings of processing of high-level information, as well as interactions with attention and memory. Although the results are mixed, we find initial support for processing object categories and words, possibly to the semantic level, as well as emotional expressions. Robust neural evidence for face individuation and integration of sentences or scenes is lacking. Attention affects the processing of stimuli that are not consciously perceived, and such stimuli may exogenously but not endogenously capture attention when relevant, and be maintained in memory over time. Sources of inconsistency in the literature include variability in control for awareness as well as individual differences, calling for future studies that adopt stricter measures of awareness and probe multiple processes within subjects.

Dividing attention influences contextual facilitation and revision during language comprehension

Although we often seem to successfully comprehend language in the face of distraction, few studies have examined the role of sustained attention in critical components of sentence processing, such as integrating information over a sentence and revising predictions when unexpected information is encountered. The current study investigated the impact of attention on sentence processing using a novel dual-task paradigm. Participants read weakly and strongly constraining sentences with expected or unexpected endings while also tracking the motion of dots in the background, and their EEG was recorded. Under full attention, the amplitude of the N400 component of the ERP, a measure of semantic access, was reduced (facilitated) in a graded fashion by contextual strength and fit. This context-based facilitation was attenuated when attention was divided, suggesting that sustained attention is important for building up message-level representations. In contrast, the post-N400 frontal positivity that has been observed to prediction violations and associated with revision processes was unaffected by dividing attention. However, under divided attention, participants also elicited posteriorly-distributed effects to these violations. Thus, predictive processes seem to be engaged even when attention is divided, but additional resources may then be required to process unexpected information.

Subliminal temporal integration of linguistic information under discontinuous flash suppression

Whether unconscious complex visual information integration occurs over time remains largely unknown and highly controversial. Previous studies have tended to use a combination of strong masking or suppression and a weak stimulus signal (e.g., low luminance), resulting in a low signal-to-noise ratio during unconscious stimulus presentation. To lengthen the stimulus exposure, we introduced intermittent presentation into interocular suppression. This discontinuous suppression allowed us to insert a word during each suppression period and deliver multiple words over time unconsciously. We found that, after participants received the subliminal context, they responded faster to a syntactically incongruent target word in a lexical decision task. We later replicated the finding in a separate experiment where participants exhibited chance performance on locating the subliminal context. These results confirmed that the sentential context was both subjectively and objectively subliminal. Critically, the effect disappeared when the context was disrupted by presenting only partial sentences or sentences with a reversed word order. These control experiments showed that the effect was not merely driven by word-word association but instead required integration over multiple words in the correct order. These findings support the possibility of unconscious high-level, complex information integration.

How does inattention affect written and spoken language processing?

The classic cocktail party effect suggests that some, but probably not all levels of language processing can proceed without attention. We used whole-brain functional MRI to investigate how modality-specific and modality-independent language areas are modulated by the withdrawal of attention to another sensory modality (e.g., attending to vision during the presentation of auditory sentences, or vice-versa). We tested the hypotheses that inattention may abolish sentence-level integration and eliminate top-down effects. In both written and spoken modalities, language processing was strongly modulated by the distraction of attention, but this inattention effect varied considerably depending on the area and hierarchical level of language processing. Under inattention, a bottom-up activation remained in early modality-specific areas, particularly in superior temporal spoken-language areas, but the difference between sentences and words lists vanished. Under both attended and unattended conditions, ventral temporal cortices were activated in a top-down manner by spoken language more than by control stimuli, reaching posteriorily the Visual Word Form Area. We conclude that inattention prevents sentence-level syntactic and semantic integration, but preserves some top-down crossmodal processing, plus a large degree of bottom-up modality-specific processing, including a ventral occipito-temporal specialization for letter strings in a known alphabet.

Windows of Integration Hypothesis Revisited

In the ongoing research of the functions of consciousness, special emphasis has been put on integration of information: the ability to combine different signals into a coherent, unified one. Several theories of consciousness hold that this ability depends on - or at least goes hand in hand with - conscious processing. Yet some empirical findings have suggested otherwise, claiming that integration of information could take place even without awareness. Trying to reconcile this apparent contradiction, the "windows of integration" (WOI) hypothesis claims that conscious access enables signal processing over large integration windows. The hypothesis applies to integration windows defined either temporally, spatially, or semantically. In this review, we explain the hypothesis and re-examine it in light of new studies published since it was suggested. In line with the hypothesis, these studies provide compelling evidence for unconscious integration, but also demonstrate its limits with respect to time, space, and semantic distance. The review further highlights open questions that still need to be pursued to demonstrate the applicability of the WOI hypothesis as a guiding principle for understanding the depth and scope of unconscious processes.

Can unconscious sequential integration of semantic information occur when the prime Chinese characters are displayed from left to right?

Recent studies have investigated whether conscious awareness is necessary for semantic integration. Although results have varied, simultaneous presentation of words have consistently led to greater semantic integration than sequential presentation in a single location. The current studies were designed to investigate whether the disadvantage of sequential presentation for unconscious semantic integration is specific to unfamiliar word-by-word presentation in one location or extends to the more natural reading conditions of viewing items sequentially from left to right. In Experiment 1, when the first three characters of Chinese idioms were presented simultaneously under masked conditions, performance on a separate two-alternative forced-choice recognition task was at chance level. Despite being unaware of the identity of prime characters, participants were faster to indicate that a subsequent item was a Chinese character when it was congruent with the beginning of the idiom, thus providing evidence of semantic integration. In contrast, when the three (Experiment 2) or two (Experiment 3) prime characters were presented sequentially in time from left to right, there was no evidence of semantic integration. These results indicate that unconscious semantic integration is more limited than previously reported, and may require simultaneous visual presentation.

Faster might not be better: Pictures may not elicit a stronger unconscious priming effect than words when modulated by semantic similarity

It has been suggested that unconscious semantic processing is stimulus-dependent, and that pictures might have privileged access to semantic content. Those findings led to the hypothesis that unconscious semantic priming effect for pictorial stimuli would be stronger as compared to verbal stimuli. This effect was tested on pictures and words by manipulating the semantic similarity between the prime and target stimuli. Participants performed a masked priming categorization task for either words or pictures with three semantic similarity conditions: strongly similar, weakly similar, and non-similar. Significant differences in reaction times were only found between strongly similar and non-similar and between weakly similar and non-similar, for both pictures and words, with faster overall responses for pictures as compared to words. Nevertheless, pictures showed no superior priming effect over words. This could suggest the hypothesis that even though semantic processing is faster for pictures, this does not imply a stronger unconscious priming effect.

No language unification without neural feedback: How awareness affects sentence processing

How does the human brain combine a finite number of words to form an infinite variety of sentences? According to the Memory, Unification and Control (MUC) model, sentence processing requires long-range feedback from the left inferior frontal cortex (LIFC) to left posterior temporal cortex (LPTC). Single word processing however may only require feedforward propagation of semantic information from sensory regions to LPTC. Here we tested the claim that long-range feedback is required for sentence processing by reducing visual awareness of words using a masking technique. Masking disrupts feedback processing while leaving feedforward processing relatively intact. Previous studies have shown that masked single words still elicit an N400 ERP effect, a neural signature of semantic incongruency. However, whether multiple words can be combined to form a sentence under reduced levels of awareness is controversial. To investigate this issue, we performed two experiments in which we measured electroencephalography (EEG) while 40 subjects performed a masked priming task. Words were presented either successively or simultaneously, thereby forming a short sentence that could be congruent or incongruent with a target picture. This sentence condition was compared with a typical single word condition. In the masked condition we only found an N400 effect for single words, whereas in the unmasked condition we observed an N400 effect for both unmasked sentences and single words. Our findings suggest that long-range feedback processing is required for sentence processing, but not for single word processing.

Probing the limits of activity-silent non-conscious working memory

Two types of working memory (WM) have recently been proposed: (i) active WM, relying on sustained neural firing, and (ii) activity-silent WM, for which firing returns to baseline, yet memories may be retained by short-term synaptic changes. Activity-silent WM in particular might also underlie the recently discovered phenomenon of non-conscious WM, which permits even subliminal stimuli to be stored for several seconds. However, whether both states support identical forms of information processing is unknown. Theory predicts that activity-silent states are confined to passive storage and cannot operate on stored information. To determine whether an explicit reactivation is required before the manipulation of information in WM, we evaluated whether participants could mentally rotate brief visual stimuli of variable subjective visibility. Behaviorally, even for unseen targets, subjects reported the rotated location above chance after several seconds. As predicted, however, at the time of mental rotation, such blindsight performance was accompanied by (i) neural signatures of consciousness in the form of a sustained desynchronization in alpha/beta frequency and (ii) a reactivation of the memorized information as indicated by decodable representations of participants' guess and response. Our findings challenge the concept of genuine non-conscious "working" memory, argue that activity-silent states merely support passive short-term memory, and provide a cautionary note for purely behavioral studies of non-conscious information processing.

A gateway to consumers' minds: Achievements, caveats, and prospects of electroencephalography-based prediction in neuromarketing

In the last decade, the field of consumer neuroscience, or neuromarketing, has been flourishing, with numerous publications, academic programs, initiatives, and companies. The demand for objective neural measures to quantify consumers' preferences and predict responses to marketing campaigns is ever on the rise, particularly due to the limitations of traditional marketing techniques, such as questionnaires, focus groups, and interviews. However, research has yet to converge on a unified methodology or conclusive results that can be applied in the industry. In this review, we present the potential of electroencephalography (EEG)-based preference prediction. We summarize previous EEG research and propose features which have shown promise in capturing the consumers' evaluation process, including components acquired from an event-related potential design, inter-subject correlations, hemispheric asymmetry, and various spectral band powers. Next, we review the latest findings on attempts to predict preferences based on various features of the EEG signal. Finally, we conclude with several recommended guidelines for prediction. Chiefly, we stress the need to demonstrate that neural measures contribute to preference prediction beyond what traditional measures already provide. Second, prediction studies in neuromarketing should adopt the standard practices and methodology used in data science and prediction modeling that is common in other fields such as computer science and engineering. This article is categorized under: Economics > Interactive Decision-Making Economics > Individual Decision-Making Psychology > Prediction Neuroscience > Cognition.