Different Signal Enhancement Pathways of Attention and Consciousness Underlie Perception in Humans

Growing evidence for separate neural mechanisms for attention and consciousness

Our conscious experience of the world seems to go in lockstep with our attentional focus: We tend to see, hear, taste, and feel what we attend to, and vice versa. This tight coupling between attention and consciousness has given rise to the idea that these two phenomena are indivisible. In the late 1950s, the honoree of this special issue, Charles Eriksen, was among a small group of early pioneers that sought to investigate whether a transient increase in overall level of attention (alertness) in response to a noxious stimulus can be decoupled from conscious perception using experimental techniques. Recent years saw a similar debate regarding whether attention and consciousness are two dissociable processes. Initial evidence that attention and consciousness are two separate processes primarily rested on behavioral data. However, the past couple of years witnessed an explosion of studies aimed at testing this conjecture using neuroscientific techniques. Here we provide an overview of these and related empirical studies on the distinction between the neuronal correlates of attention and consciousness, and detail how advancements in theory and technology can bring about a more detailed understanding of the two. We argue that the most promising approach will combine ever-evolving neurophysiological and interventionist tools with quantitative, empirically testable theories of consciousness that are grounded in a mathematically formalized understanding of phenomenology.

The SSVEP tracks attention, not consciousness, during perceptual filling-in

Research on the neural basis of conscious perception has almost exclusively shown that becoming aware of a stimulus leads to increased neural responses. By designing a novel form of perceptual filling-in (PFI) overlaid with a dynamic texture display, we frequency-tagged multiple disappearing targets as well as their surroundings. We show that in a PFI paradigm, the disappearance of a stimulus and subjective invisibility is associated with increases in neural activity, as measured with steady-state visually evoked potentials (SSVEPs), in electroencephalography (EEG). We also find that this increase correlates with alpha-band activity, a well-established neural measure of attention. These findings cast doubt on the direct relationship previously reported between the strength of neural activity and conscious perception, at least when measured with current tools, such as the SSVEP. Instead, we conclude that SSVEP strength more closely measures changes in attention.

Neural correlates of goal-directed enhancement and suppression of visual stimuli in the absence of conscious perception

An observer's current goals can influence the processing of visual stimuli. Such influences can work to enhance goal-relevant stimuli and suppress goal-irrelevant stimuli. Here, we combined behavioral testing and electroencephalography (EEG) to examine whether such enhancement and suppression effects arise even when the stimuli are masked from awareness. We used a feature-based spatial cueing paradigm, in which participants searched four-item arrays for a target in a specific color. Immediately before the target array, a nonpredictive cue display was presented in which a cue matched or mismatched the searched-for target color, and appeared either at the target location (spatially valid) or another location (spatially invalid). Cue displays were masked using continuous flash suppression. The EEG data revealed that target-colored cues produced robust N2pc and N_T responses-both signatures of spatial orienting-and distractor-colored cues produced a robust P_D-a signature of suppression. Critically, the cueing effects occurred for both conscious and unconscious cues. The N2pc and N_T were larger in the aware versus unaware cue condition, but the P_D was roughly equivalent in magnitude across the two conditions. Our findings suggest that top-down control settings for task-relevant features elicit selective enhancement and suppression even in the absence of conscious perception. We conclude that conscious perception modulates selective enhancement of visual features, but suppression of those features is largely independent of awareness.

Conscious access in the near absence of attention: critical extensions on the dual-task paradigm

Whether conscious perception requires attention remains a topic of intense debate. While certain complex stimuli such as faces and animals can be discriminated outside the focus of spatial attention, many simpler stimuli cannot. Because such evidence was obtained in dual-task paradigms involving no measure of subjective insight, it remains unclear whether accurate discrimination of unattended complex stimuli is the product of automatic, unconscious processing, as in blindsight, or is accessible to consciousness. Furthermore, these paradigms typically require extensive training over many hours, bringing into question whether this phenomenon can be achieved in naive subjects. We developed a novel dual-task paradigm incorporating confidence ratings to calculate metacognition and adaptive staircase procedures to reduce training. With minimal training, subjects were able to discriminate face-gender in the near absence of top-down attentional amplification, while also displaying above-chance metacognitive accuracy. By contrast, the discrimination of simple coloured discs was significantly impaired and metacognitive accuracy dropped to chance-level, even in a partial-report condition. In a final experiment, we used blended face/disc stimuli and confirmed that face-gender but not colour orientation can be discriminated in the dual task. Our results show direct evidence for metacognitive conscious access in the near absence of attention for complex, but not simple, stimuli.This article is part of the theme issue 'Perceptual consciousness and cognitive access'.