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Joan Liu-Shuang, Justin Ales, Anthony Norcia, Bruno Rossion; Effects of inversion and contrast-reversal on objective face detection thresholds revealed by sweep steady-state visual evoked potentials. Journal of Vision 2014;14(10):131. doi: 10.1167/14.10.131.
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Human observers are able to rapidly notice the presence of a face in a natural scene. In order to better quantify the threshold of face detection, we have conducted a study using the sweep steady-state visual evoked potentials (SSVEP) method in EEG (Ales et al., 2012, JOV, 12, 118). More precisely, we parametrically varied the visibility of a face stimulus with phase-scrambling while alternating it at a fixed rate with noise stimuli (3 Hz alternation or 6 images/second). While the face gradually emerged from noise over the course of a trial sequence, EEG responses at 3 Hz increased abruptly at 30%-35% phase coherence over the right occipito-temporal region, providing an objective face detection threshold in the human brain. The purpose of the current study is to test the degree to which this response is specific to face structure rather than reflecting general shape perception. We recorded 128-channel EEG in 13 observers presented with sweep sequences containing faces that varied in orientation (upright vs. inverted) and contrast polarity (positive vs. negative). Picture-plane and contrast inversion are well-known manipulations to which faces are particularly sensitive, in contrast to other object categories. Robust responses emerged specifically at 3 Hz on occipito-temporal electrodes at 30%-35% phase coherence for upright faces, replicating our previous experiment. However, these responses were delayed (≈ 40-45% coherence) and reduced (60%-75% of the reponse) both for inverted and negative polarity faces. These findings indicate that periodic alternations between intact and scrambled faces generate frequency-locked responses that are partly selective to face structure, so that it can be used as an objective index of face detection. The sweep SSVEP method is a promising tool for measuring high-level visual perception thresholds rapidly and objectively in a variety of populations, including infants and patients.
Meeting abstract presented at VSS 2014
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