Late Positivity Correlates with Subjective Reports: Evidence from the Low-frequency and High-frequency Reporting Tasks.

Identifying the neural correlates of consciousness (NCCs) is an important way to understand the fundamental nature of consciousness. By recording event-related potentials (ERPs) using EEG, researchers have found three potential electrophysiological NCCs: early positive correlate of consciousness (enhanced P1), visual awareness negativity (VAN), and late positivity (LP). However, LP may reflect post-perceptual processing associated with subjective reports rather than consciousness per se. The present experiment investigated the relationship between LP and subjective reports. We adopted two subjective reporting tasks that differed in the requirement for subjective reports. In the low-frequency reporting task, participants needed to report whether they saw the target picture in 25% of trials, whereas in the high-frequency reporting task, participants needed to report whether they saw the target picture in each trial. Behavioral results showed that the hit rates were lower and false alarm rates were higher on reporting trials in low-frequency reporting tasks than on reporting trials in high-frequency reporting tasks. Unexpectedly, VAN was larger on reporting trials in the low-frequency reporting task than on reporting trials in the high-frequency reporting task. Importantly, our ERP results showed that LP was larger on reporting trials in the high-frequency reporting task than on reporting trials in the low-frequency reporting task. Thus, our findings indicated that when the frequency of reports was increased, the task relevance of the stimuli increased, which led to larger LP amplitudes. These findings suggest that LP correlates with subjective reports.

The P3 Reflects Awareness and Can Be Modulated by Confidence.

An important question in neural correlate of consciousness (NCC) studies is whether event-related potential (ERP) component P3 reflects visual awareness or the confidence with which one reports a visual experience. In the present study, participants detected visual stimuli presented at threshold-level contrast, then rated their subjective confidence with respect to their response on a four-point scale (very confident, quite confident, slightly confident, and not confident at all). Because awareness responses in trials with rating of "not confident at all" were likely noise, we analyzed the data excluding those trials. The ERP results revealed a significant positive difference in P3 amplitude between "aware" and "unaware" trials. P3 amplitude was more positive in aware trials compared to unaware trials. Importantly, this pattern was observed for trials with combined confidence ratings of "very confident" and "quite confident," and for trials with confidence ratings of "slightly confident," suggesting that awareness alone can modulate P3. A significant interaction between awareness and confidence is reported, suggesting that confidence influences P3 as well. In addition, ERP results revealed that visual awareness negativity (VAN) was observed over posterior temporal and occipital electrodes and largely not influenced by confidence. This result indicated that VAN is an early neural correlate of visual awareness.

Later Positivity Reflects Post-perceptual Processes: Evidence From Immediate Detection and Delayed Detection Tasks.

Electrophysiological recordings are usually used to study neural correlates of consciousness (NCCs). The aim of our present study was to use two detection tasks to dissociate the electrophysiological correlates of visual awareness from the post-perceptual processes. In immediate detection task, participants had to quickly report whether the stimulus was presented after stimulus, whereas in delayed detection task, participants had to put off reporting whether the stimulus was presented after stimulus. The results showed that two previously frequently observed electrophysiological NCCs were observed: visual awareness negativity (VAN) and later positivity (LP). Importantly, the amplitude of VAN over posterior temporal and occipital areas was not influenced by the task manipulation. However, the amplitude of LP over parietal, posterior temporal and occipital areas was influenced by the task manipulation during 650-850 ms. These data suggest that VAN is an early electrophysiological correlates of visual awareness, and LP reflects post-perceptual processes required in reporting perceptual awareness.