Vincent de Gardelle

How rich is consciousness? The partial awareness hypothesis

Current theories of consciousness posit a dissociation between phenomenal consciousness (rich) and access consciousness (limited). Here, we argue that the empirical evidence for phenomenal consciousness without access is equivocal, resulting either from a confusion between phenomenal and unconscious contents, or from an impression of phenomenally rich experiences arising from illusory contents. We propose a refined account of access that relies on a hierarchy of representational levels and on the notion of partial awareness, whereby lower and higher levels are accessed independently. Reframing of the issue of dissociable forms of consciousness into dissociable levels of access provides a more parsimonious account of the existing evidence. In addition, the rich phenomenology illusion can be studied and described in terms of testable cognitive mechanisms.

Rhythmic Fluctuations in Evidence Accumulation during Decision Making in the Human Brain

Categorical choices are preceded by the accumulation of sensory evidence in favor of one action or another. Current models describe evidence accumulation as a continuous process occurring at a constant rate, but this view is inconsistent with accounts of a psychological refractory period during sequential information processing. During multisample perceptual categorization, we found that the neural encoding of momentary evidence in human electrical brain signals and its subsequent impact on choice fluctuated rhythmically according to the phase of ongoing parietal delta oscillations (1-3 Hz). By contrast, lateralized beta-band power (10-30 Hz) overlying human motor cortex encoded the integrated evidence as a response preparation signal. These findings draw a clear distinction between central and motor stages of perceptual decision making, with successive samples of sensory evidence competing to pass through a serial processing bottleneck before being mapped onto action.

A Neural Marker of Perceptual Consciousness in Infants

Consciousness Arrives Neurophysiological measures in human adults correspond to the transition between very brief, “unnoticeable,” and slightly longer-lived visual stimuli that penetrate deeply enough to leave a conscious imprint that subjects report they can “see.” Kouider et al. (p. 376) have performed parallel behavioral and neurophysiological studies in infants to identify a similar neural signal that appears to mark the development of visual consciousness. The brain mechanisms underlying conscious perception are already present in infancy and improve with age. Infants have a sophisticated behavioral and cognitive repertoire suggestive of a capacity for conscious reflection. Yet, demonstrating conscious access in infants remains challenging, mainly because they cannot report their thoughts. Here, to circumvent this problem, we studied whether an electrophysiological signature of consciousness found in adults, corresponding to a late nonlinear cortical response [~300 milliseconds (ms)] to brief pictures, already exists in infants. We recorded event-related potentials while 5-, 12-, and 15-month-old infants (N = 80) viewed masked faces at various levels of visibility. In all age groups, we found a late slow wave showing a nonlinear profile at the expected perceptual thresholds. However, this late component shifted from a weak and delayed response in 5-month-olds (starting around 900 ms) to a more sustained and faster response in older infants (around 750 ms). These results reveal that the brain mechanisms underlying the threshold for conscious perception are already present in infancy but undergo a slow acceleration during development.

Human scalp electroencephalography reveals that repetition suppression varies with expectation

Repetitions of a sensory event elicit lower levels of brain activity than its initial presentation (“repetition suppression,” RS). According to one view, RS depends on the biophysics of neuronal discharge, and is thus an automatic consequence of stimulus processing (“fatigue” model). Another account suggests that RS depends on the statistical structure of the environment, and occurs when repeated stimuli are less surprising than novel stimuli (“surprise reduction” model). In support of the latter view, functional magnetic resonance imaging studies have shown that RS is modulated by the local probability of repetition. However, single-cell recordings from macaque inferotemporal area (IT) have failed to replicate this finding. Here, we recorded scalp electroencephalography from human participants viewing pairs of faces that repeated (face1–face1) or alternated (face1–face2), in contexts in which repetitions were expected or unexpected. As previously described, event-related potentials in the range of 100–400u2009ms recorded at posterior electrode sites and at the vertex differed between repetitions and alternations. Critically, at central electrodes, we observed that the difference between repeated and alternating stimuli was attenuated when repetitions were unexpected, as predicted by the surprise reduction model. These findings demonstrate that the modulation of RS by repetition probability is observable using direct neural recording methods in human participants, and that it occurs relatively late (>300u2009ms) post-stimulus. Finally, we found that theta-band (4–8u2009Hz) spectral power over central electrodes varied with the three-way interaction between of repetition, expectation, and the rate of change of the environment, consistent with recent reports that frontal theta may be a hallmark of learning processes originating in the anterior cingulate and medial prefrontal cortex.

Robust averaging during perceptual judgment

An optimal agent will base judgments on the strength and reliability of decision-relevant evidence. However, previous investigations of the computational mechanisms of perceptual judgments have focused on integration of the evidence mean (i.e., strength), and overlooked the contribution of evidence variance (i.e., reliability). Here, using a multielement averaging task, we show that human observers process heterogeneous decision-relevant evidence more slowly and less accurately, even when signal strength, signal-to-noise ratio, category uncertainty, and low-level perceptual variability are controlled for. Moreover, observers tend to exclude or downweight extreme samples of perceptual evidence, as a statistician might exclude an outlying data point. These phenomena are captured by a probabilistic optimal model in which observers integrate the log odds of each choice option. Robust averaging may have evolved to mitigate the influence of untrustworthy evidence in perceptual judgments.

Perceptual illusions in brief visual presentations.

We often feel that our perceptual experience is richer than what we can express. For instance, when flashed with a large set of letters, we feel that we can see them all, while we can report only a few. However, the nature of this subjective impression remains highly debated: while many favour a dissociation between two forms of consciousness (access vs. phenomenal consciousness), others contend that the richness of phenomenal experience is a mere illusion. Here we addressed this question with a classical partial-report paradigm now modified to include unexpected items in the unreported parts of the stimuli. We show that even in the presence of unexpected pseudo-letters, participants still felt that there were only letters. Additionally, we show that this feeling reflects an illusion whereby participants reconstruct letters using partial letter-like information. We propose that the feeling of seeing emerges from the interplay between partially accessible information and expectations.

Concurrent Repetition Enhancement and Suppression Responses in Extrastriate Visual Cortex

Visual cortical responses are usually attenuated by repetition, a phenomenon known as repetition suppression (RS). Here, we use multivoxel pattern analyses of functional magnetic resonance imaging (fMRI) data to show that RS co-occurs with the converse phenomenon (repetition enhancement, RE) in a single cortical region. We presented human volunteers with short sequences of repeated faces and measured brain activity using fMRI. In an independently defined face-responsive extrastriate region, the response of each voxel to repetition (RS vs. RE) was consistent across scanner runs, and multivoxel patterns for both RS and RE voxels were stable. Moreover, RS and RE voxels responded to repetition with dissociable latencies and exhibited different patterns of connectivity with lower and higher visual regions. Computational simulations demonstrated that these effects must be due to differences in repetition sensitivity, and not feature selectivity. These findings establish that 2 classes of repetition responses coexist within 1 visual region and support models acknowledging this distinction, such as predictive coding models where perception requires the computation of both predictions (which are enhanced by repetition) and prediction errors (which are suppressed by repetition).

Cerebral bases of subliminal speech priming

While the neural correlates of unconscious perception and subliminal priming have been largely studied for visual stimuli, little is known about their counterparts in the auditory modality. Here we used a subliminal speech priming method in combination with fMRI to investigate which regions of the cerebral network for language can respond in the absence of awareness. Participants performed a lexical decision task on target items preceded by subliminal primes, which were either phonetically identical or different from the target. Moreover, the prime and target could be spoken by the same speaker or by two different speakers. Word repetition reduced the activity in the insula and in the left superior temporal gyrus. Although the priming effect on reaction times was independent of voice manipulation, neural repetition suppression was modulated by speaker change in the superior temporal gyrus while the insula showed voice-independent priming. These results provide neuroimaging evidence of subliminal priming for spoken words and inform us on the first, unconscious stages of speech perception.

Subliminal speech perception and auditory streaming.

Current theories of consciousness assume a qualitative dissociation between conscious and unconscious processing: while subliminal stimuli only elicit a transient activity, supraliminal stimuli have long-lasting influences. Nevertheless, the existence of this qualitative distinction remains controversial, as past studies confounded awareness and stimulus strength (energy, duration). Here, we used a masked speech priming method in conjunction with a submillisecond interaural delay manipulation to contrast subliminal and supraliminal processing at constant prime, mask and target strength. This delay induced a perceptual streaming effect, with the prime popping out in the supraliminal condition. By manipulating the prime-target interval (ISI), we show a qualitatively distinct profile of priming longevity as a function of prime awareness. While subliminal priming disappeared after half a second, supraliminal priming was independent of ISI. This shows that the distinction between conscious and unconscious processing depends on high-level perceptual streaming factors rather than low-level features (energy, duration).

How Spatial Frequencies and Visual Awareness Interact During Face Processing

In vision, high and low spatial frequencies have been dissociated at the cognitive and neural levels. Usually, high spatial frequency (HSF) is associated with slow analysis along the ventral cortical stream, and low spatial frequency (LSF) is associated with fast and automatic processing. These findings suggest a specific relation between spatial-frequency processing and visual awareness. We investigated this issue using masked-face priming with hybrid prime images of variable visibility. We found subliminal priming for both LSF and HSF information, along with a strong interaction between spatial frequency and visibility: HSF-related priming increased with stimulus visibility, whereas LSF influences remained unchanged. We argue that the results limit the validity of the coarse-to-fine model of vision and of models equating ventral-stream activity with perceptual awareness. Interpreting our results in light of the diagnostic approach suggests a close relation between awareness and diagnosticity.