Neuroimaging studies have provided compelling evidence of visual object category processing of stimuli
within awareness. For example, the perception of human faces closely correlates with greater hemodynamic responses in the ventral occipital area known as the fusiform face area (FFA; Andrews & Schluppeck,
2004; Kanwisher,
2000; Kanwisher & Yovel,
2006; McCarthy, Puce, Gore, Allison,
1997). This finding is corroborated by corresponding event-related potential (ERP; Allison, Mccarthy, Nobre, Puce, & Belger,
1994; Allison, Puce, Spencer, & McCarthy,
1999; Bentin, Allison, Puce, Perez, & McCarthy,
1996; McCarthy, Puce, Belger, & Allison,
1999; Puce, Allison, Asgari, Gore, & McCarthy,
1996; Puce, Allison, & McCarthy,
1999) and event-related magnetoencephalographic (MEG) measures (Liu, Higuchi, Marantz, & Kanwisher,
2000). These latter two measures of high temporal resolution, EEG and MEG, have succeeded in characterizing the well-studied N170 component and its MEG analog, the M170 (Bentin,
1998; Bentin et al.,
1996), as reflecting face-specific processing. The N170 is a negative-polarity ERP response to images of faces relative to images of other object categories. It typically peaks first at approximately 170 ms after stimulus onset and is often followed by an extended negative-polarity ERP wave with a similar topographic distribution over the next several hundred milliseconds, with this later effect tending to be more closely tied to behavior (i.e., delayed for longer face categorization response times; Philiastides & Sajda,
2006). Such a dual-phase spatiotemporal profile of visual evoked potentials in response to objects (Fahrenfort, Scholte, & Lamme,
2008) and faces in particular (Jemel, Schuller, & Goffaux,
2010; Luo, Feng, He, Wang, & Luo,
2010) has been characterized in several studies and supports an account of feedforward signal propagation followed by reentrant processing of the same polarity and topographic distribution. The face-specific N170 exhibits a bilateral, although typically somewhat right-weighted, occipitotemporal scalp distribution (Bentin,
1998; Bentin et al.,
1996). By tracking the intactness of the N170, along with the later recurrent face-specific ERP activity phase, in conditions of awareness and unawareness of visual object stimuli, it is possible to evaluate whether this type of object category level processing occurs in the brain in the absence of awareness.