Effect of face-related task on rapid individual face discrimination

Human adults can typically visually discriminate the faces of unfamiliar individuals accurately, rapidly, and automatically, i.e. even without the explicit intention to do so. Recent studies have used fast periodic visual stimulation (FPVS) coupled with electroencephalography (EEG) to measure this process with objectivity and high sensitivity during simple non-face related tasks (Liu-Shuang et al., 2014). Here we consider to what extent fast individual face discrimination measured in the human brain with this approach is modulated by a direct face-related task. We recorded 128-channel EEG while participants viewed 70s sequences of a random female face identity (A) repeating at 6 Hz. Female faces of different identities (B, C…), interleaved regularly every 7th image (AAAAAABAAAAAAC…) led to significant periodic responses at 0.857 Hz (i.e., 6 Hz/7) and its harmonics, thereby indexing individual face discrimination. Participants performed two tasks: (1) an orthogonal Fixation task, monitoring random colour changes of the central fixation cross, and (2) a Face task, detecting male faces randomly replacing a female face. While the implicit Fixation task elicited robust individual face discrimination responses peaking over the (right) occipito-temporal region, the Face task led to significantly greater overall response amplitude (∼100% increase). However, this attentional boost strongly reduced response specificity by disproportionately recruiting prefrontal and central parietal regions, thereby blurring the occipito-temporal topography typical of specialized high-level face processing. The individual face discrimination response over face-selective occipito-temporal cortex was modulated by the face-sex task starting from 180 ms onset, followed by activations over prefrontal and central parietal region from 200 ms to 450 ms, respectively. Overall, these findings show that even a robust automatic individual face discrimination response can be further enhanced when explicitly searching for face-related information, albeit with a decrease in response specificity.