Abstract A central question in consciousness research is whether perceptual awareness is primarily reflected in early posterior sensory activity or in later frontal processes associated with access and report. Empirical tests of these alternatives are challenging because most paradigms rely on explicit perceptual reports, introducing motor and decisional confounds. Here, we used an electroencephalography design combining two independent tasks to decode stimulus location across report and no-report conditions and to examine how these task-general stimulus-location signals vary with subjective visibility. Participants viewed lateralized gratings in a backward-masking task and rated their subjective visibility, whereas in a separate no-report task, they viewed unmasked gratings without reporting. Classifiers were trained on stimulus location in the no-report task and tested on masked trials sorted by subjective visibility in the masking task. This cross-decoding approach identifies stimulus-location signals that generalize across tasks and vary with subjective visibility, making them less likely to reflect reporting or decision-related processes. Decoding revealed stronger stimulus-location signals for subjectively visible than invisible stimuli in an early time window from 130 to 170 msec, most prominently over posterior occipital, temporal, and parietal electrode groups. These results indicate that subjective visibility enhances early posterior EEG decoding of stimulus location across report contexts.
Abstract In the first part of this three-part series, we reviewed the mechanisms that cause nuisance signals in fMRI data. Here, in the second part of the series, we review “human factors” that modulate these confounding mechanisms. The modulators may be trait characteristics such as age or bodyweight, or they may be state dependencies that alter the relative contributions of nuisance signals in the fMRI data. A wide array of psychological (e.g., motivation/arousal, cognitive strategy, task engagement), lifestyle (e.g., exercise, sleep, and dietary habits), experimental (e.g., scanner anxiety, task-related movement), and situational (e.g., prandial state, time of day, menses) factors can affect the brain state directly as well as via changes in systemic physiology mediated by autonomic nervous responses.
Abstract Visual search is facilitated when targets reappear within invariant distractor layouts (contextual cueing). Although contextual cueing can bias attention within 150 msec, it remains unclear whether this guidance relies on global spatial configuration or requires object identity. We recorded ERPs while participants searched for targets in repeated versus novel arrays. Critically, each trial began with a placeholder display that conveyed only spatial configuration, preceding the search display by 500 msec. Following learning, targets were relocated and swapped their locations with a distractor in the opposite visual field, such that the overall search layouts remained unchanged. ERPs time-locked to placeholder onset revealed an early poster contralateral negativity (N1pc, 140–160 msec) for repeated contexts, despite the absence of target-specific object information. After relocation, this effect reversed polarity, indicating persistent orienting to the learned target location. A second N1pc, followed by N2pc and CDA, emerged only after search onset. These findings show that learned spatial structure rapidly guides attention, whereas effective selection requires additional processing of object information.
Abstract In this review, the first in a three-part series, we consider the origins of confounding in fMRI. We review the signal properties of an fMRI experiment and the intended sensitivity to neurovascular coupling, the hemodynamic response function, and the spatial–temporal biases imposed by the heterogeneous anatomy of the vascular system. We then explore the various mechanisms that can leave nuisance signals, often termed physiologic noise, in fMRI data. All mechanisms having a biological dependence are considered, including mechanical effects from head motion and respiration, blood gases, brain motion from the cardiac pressure wave, blood pressure, and scanner instabilities that couple with the participant's body.
A single glance at a face can evoke a sense of familiarity, yet how this percept unfolds over time remains unclear. We adopted a frequency-tagging EEG paradigm to investigate neural responses to face familiarity across different SOAs. Streams of unfamiliar faces were presented at base frequencies of 8, 12, 16, and 20 Hz (SOAs: 125, 83, 62.5, and 50 msec), with personally familiar faces inserted periodically at 2 Hz. Significant oddball responses were observed at all four SOAs over occipito-temporal regions, indicating reliable discrimination between familiar and unfamiliar faces even at the shortest SOA of 50 msec. Time domain analysis revealed a significant N170 component in all four conditions, with amplitudes increasing progressively as SOA lengthened. In contrast, a reliable P300 component emerged only at SOAs of 83 msec and longer. These findings suggest two temporally distinct stages of familiar face processing: an early stage indexed by the N170 that scales with available processing time and likely reflects familiarity-sensitive perceptual processing and a later stage indexed by P300 that requires a longer processing window and likely reflects higher-order evaluative processing.
