Abstract
The human face is the most studied object category in visual neuroscience. In a quest for markers of face processing, event-related potential (ERP) studies have debated whether two peaks of activity – P1 and N170 – are category-selective. Whilst most studies have used photographs of unaltered images of faces, others have used cropped faces in an attempt to reduce the influence of features surrounding the “face–object” sensu stricto. However, results from studies comparing cropped faces with unaltered objects from other categories are inconsistent with results from studies comparing whole faces and objects. Here, we recorded ERPs elicited by full front views of faces and cars, either unaltered or cropped. We found that cropping artificially enhanced the N170 whereas it did not significantly modulate P1. In a second experiment, we compared faces and butterflies, either unaltered or cropped, matched for size and luminance across conditions, and within a narrow contrast bracket. Results of Experiment 2 replicated the main findings of Experiment 1. We then used face–car morphs in a third experiment to manipulate the perceived face-likeness of stimuli (100% face, 70% face and 30% car, 30% face and 70% car, or 100% car) and the N170 failed to differentiate between faces and cars. Critically, in all three experiments, P1 amplitude was modulated in a face-sensitive fashion independent of cropping or morphing. Therefore, P1 is a reliable event sensitive to face processing as early as 100 ms after picture onset.
Highlights
The human face is probably the most biologically significant stimulus encountered by humans in the environment because it provides critical information about other individuals
Double transcranial magnetic stimulation (TMS) pulses have been shown to disrupt visual processing selectively for faces when stimulation is delivered over the occipital face area (OFA) 60 and 100 ms after picture presentation but no measurable disruption is observed for double TMS pulses applied at later latencies (Pitcher et al, 2007), nor when applied to nearby extrastriate areas
To our knowledge, the effect of cropping inner parts of faces and objects on visual categorization has never been studied directly using event-related potential (ERP) and the potential effects of this manipulation have not been discussed (Eimer, 2000b; Duchaine and Nakayama, 2004). This leads to the possibility that category-effects previously reported in the N170 range may have been due to uncontrolled perceptual variance between conditions (Thierry et al, 2007a) and reduction in the amount of information afforded by artificially impoverished stimuli
Summary
The human face is probably the most biologically significant stimulus encountered by humans in the environment because it provides critical information about other individuals (e.g., identity, age, sex, mood, direction of attention, intention, etc.). Despite the fact that P1 category-sensitivity has been repeatedly challenged (Bentin et al, 2007b; Rossion and Jacques, 2008; Kuefner et al, 2010, but see Thierry et al, 2007b; Dering et al, 2009), converging evidence from MEG, ERP, and transcranial magnetic stimulation (TMS) have highlighted face-sensitive processes occurring around 100 ms post-stimulus onset (Liu et al, 2002; Herrmann et al, 2005; Pitcher et al, 2007). Double TMS pulses have been shown to disrupt visual processing selectively for faces when stimulation is delivered over the occipital face area (OFA) 60 and 100 ms after picture presentation but no measurable disruption is observed for double TMS pulses applied at later latencies (Pitcher et al, 2007), nor when applied to nearby extrastriate areas
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