Early ERP differences for faces could be the result of uncontrolled low-level differences across object categories (Johnson & Olshausen,
2003; Rousselet, Macé, Thorpe, & Fabre-Thorpe,
2007; VanRullen & Thorpe,
2001). For instance, stimulus characteristics such as overall luminance, contrast, or spatial frequency and orientation components have not been equated across object categories. It has been suggested that the N170 varies with spatial frequency content of the stimulus (Goffaux, Gauthier, & Rossion,
2003; see also MEG results from Tanskanen, Näsänen, Montez, Päällysaho, & Hari,
2005), and it is equally likely that differences in early ERP components could be due to low-level differences in stimuli rather than indicating differences in category-related processing
per se. This is not to say that higher level and task factors do not influence ERP components. Indeed, top-down factors do influence early visual evoked responses (Bentin & Golland,
2002; Luck, Woodman, Vogel,
2000). However, the brain responses to various foveated stimuli like objects, words, animal, and human faces are hardly modulated by task factors before 200 ms after stimulus onset in a large majority of studies, as demonstrated by surface EEG (Carmel & Bentin,
2002; Cauquil, Edmonds, & Taylor,
2000; Lueschow et al.,
2004; Schendan et al.,
1998; Rousselet et al.,
2004; Rousselet, Macé, et al.,
2007; but see two exceptions in Eimer,
2000a,
2000b), intracranial recordings (Nobre, Allison, & McCarthy,
1998; Puce, Allison, & McCarthy,
1999), and MEG (Furey et al.,
2006, Lueschow et al.,
2004). In the present study, we used well-controlled stimuli (faces, houses, and noise textures, presented upright and inverted,
Figure 1), with identical amplitude spectra, to evaluate the timing of face and object processing. Because form information is largely carried by phase rather than amplitude (Oppenheim & Lim,
1981; Sekuler & Bennett,
1996), individual houses and faces remained easily discriminable after this manipulation. However, this manipulation ensured that any difference in the EEG was not simply a function of differences in luminance or contrast or in the relative strength of specific frequency or orientation components across categories. Despite the important ERP literature on object and face processing, very few studies have used stimuli with equated amplitude spectra: (1) some of these studies have used noise textures (e.g., Allison, Puce, Spencer, & McCarthy,
1999; Jacques & Rossion,
2004; see also MEG data in Tanskanen, Näsänen, Ojanpää, & Hari,
2007); (2) and to the best of our knowledge only our previous study used objects like houses (Rousselet, Husk, Bennett, & Sekuler,
2005; Rousselet, Husk, Bennett, & Sekuler,
2007).