Using Single-Trial Analysis to Elucidate the Neural Correlates of Conscious Face Perception

JOAQUIN NAVAJAS; MARYAM AHMADI; RODRIGO QUIAN QUIROGA

Congreso; British Neuroscience Association (BNA): Festival of Neuroscience; 2013

British Neuroscience Association (BNA)

When a face is flashed to an observer, a large negative component is elicited in the occipitotemporal cortex at around 170 ms from the onset of presentation (N170). While the selectivity of the N170 to different stimuli has been extensively studied, it remains unclear whether this activation depends on conscious processing or, alternatively, if conscious experience emerges at a later stage. To address this question, we used briefly flashed faces, coupled with backward masking, and varying degrees of Gaussian noise. Using a double-staircase procedure, for each subject we determined the threshold of face detection, where participants failed to detect faces in approximately half of the trials. In the average evoked responses, we observed that supraliminal faces elicited stronger occipitotemporal activity than subliminal faces. This activation was also sensitive to the amount of noise added to the stimulus. Moreover, we show that the single-trial N170 can decode the participants' report (face seen or unseen) with a performance significantly larger than chance and consistent across subjects. In contrast, the single-trial N170 was uninformative of the noise-level suggesting that the modulation observed in the average was explained by latency-jitters, as confirmed with latency-corrected averages. Therefore, the uncertainty introduced by noise is correlated with a larger time variability of the evoked responses, whereas the conscious detection of faces is correlated to an amplitude increase in single-trial responses. Altogether, these results suggest that activity in the occipitotemporal cortex play a decisive role in the dynamical processing of the stimulus leading to conscious perception.