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Optimal eye-gaze fixation position for face-related neural responses.

Zerouali Y, Lina JM, Jemel B - PLoS ONE (2013)

Bottom Line: It is generally agreed that some features of a face, namely the eyes, are more salient than others as indexed by behavioral diagnosticity, gaze-fixation patterns and evoked-neural responses.However, because previous studies used unnatural stimuli, there is no evidence so far that the early encoding of a whole face in the human brain is based on the eyes or other facial features.We found that the N170 indexing the earliest face-sensitive response in the human brain was the largest when the fixation position is located around the nasion.

View Article: PubMed Central - PubMed

Affiliation: Ecole de Technologie Supérieure, Montreal, Canada ; Hôpital Riviere des Prairies, Montreal, Canada. youness.zerouali-boukhal.1@ens.etsmtl.ca

ABSTRACT
It is generally agreed that some features of a face, namely the eyes, are more salient than others as indexed by behavioral diagnosticity, gaze-fixation patterns and evoked-neural responses. However, because previous studies used unnatural stimuli, there is no evidence so far that the early encoding of a whole face in the human brain is based on the eyes or other facial features. To address this issue, scalp electroencephalogram (EEG) and eye gaze-fixations were recorded simultaneously in a gaze-contingent paradigm while observers viewed faces. We found that the N170 indexing the earliest face-sensitive response in the human brain was the largest when the fixation position is located around the nasion. Interestingly, for inverted faces, this optimal fixation position was more variable, but mainly clustered in the upper part of the visual field (around the mouth). These observations extend the findings of recent behavioral studies, suggesting that the early encoding of a face, as indexed by the N170, is not driven by the eyes per se, but rather arises from a general perceptual setting (upper-visual field advantage) coupled with the alignment of a face stimulus to a stored face template.

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Illustration of the experimental protocol.(a) Schematic view of a trial progression during the eye-gaze contingent paradigm used in this study. On the beginning of each trial, a fixation-cross is presented randomly on the screen, and is replaced by a face image as soon as the eye-tracker detects that the observer's line of sight lays on the fixation-cross. The yellow circles represent the observer's gaze position on the screen. Eye-fixation landing position on the seven predefined fROI (b) is controlled by the presentation of a fixation-cross located randomly in one of nine invisible quadrants on the screen, that were spatially mapped with the location of the fROIs (top). Actual gaze positions compiled over all subjects and trials for each face orientation (bottom). Gaze position for each trial is depicted with a black dot. (c) Saliency map and spatial resolution of face images and visual field map coverage when the centre of gravity of the face is fixated.
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pone-0060128-g001: Illustration of the experimental protocol.(a) Schematic view of a trial progression during the eye-gaze contingent paradigm used in this study. On the beginning of each trial, a fixation-cross is presented randomly on the screen, and is replaced by a face image as soon as the eye-tracker detects that the observer's line of sight lays on the fixation-cross. The yellow circles represent the observer's gaze position on the screen. Eye-fixation landing position on the seven predefined fROI (b) is controlled by the presentation of a fixation-cross located randomly in one of nine invisible quadrants on the screen, that were spatially mapped with the location of the fROIs (top). Actual gaze positions compiled over all subjects and trials for each face orientation (bottom). Gaze position for each trial is depicted with a black dot. (c) Saliency map and spatial resolution of face images and visual field map coverage when the centre of gravity of the face is fixated.

Mentions: Several lines of evidence suggest that face processing is optimized when gaze fixation focuses on the eyes. Indeed, the eyes are the most salient features of the face based on its energy density spectrum [5], [6] (see fig. 1c). When presented alone, the eyes evoke a N170 response with higher amplitude than any other face parts, and even higher than to whole faces [7]. Meanwhile, face pictures evoke similar N170 amplitudes whether the eyes are present or removed [8], [9]. To reconcile these findings, some authors suggested that the N170 results from an interplay between a holistic face-processing system and specialized eye-processing system. According to this interpretation, the N170 reflects the holistic processing of faces; however, when holistic information is not available, either using isolated faces parts or inverted faces, the eye-processor is brought to play [8]. However the existence of an eye-specialized brain structure remains speculative and was contradicted by recent findings from McPartland and colleagues [10], who found that gaze fixations between the eyes (i.e., the nasion) or on the mouth evoke N170 responses with similar amplitudes.


Optimal eye-gaze fixation position for face-related neural responses.

Zerouali Y, Lina JM, Jemel B - PLoS ONE (2013)

Illustration of the experimental protocol.(a) Schematic view of a trial progression during the eye-gaze contingent paradigm used in this study. On the beginning of each trial, a fixation-cross is presented randomly on the screen, and is replaced by a face image as soon as the eye-tracker detects that the observer's line of sight lays on the fixation-cross. The yellow circles represent the observer's gaze position on the screen. Eye-fixation landing position on the seven predefined fROI (b) is controlled by the presentation of a fixation-cross located randomly in one of nine invisible quadrants on the screen, that were spatially mapped with the location of the fROIs (top). Actual gaze positions compiled over all subjects and trials for each face orientation (bottom). Gaze position for each trial is depicted with a black dot. (c) Saliency map and spatial resolution of face images and visual field map coverage when the centre of gravity of the face is fixated.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3675086&req=5

pone-0060128-g001: Illustration of the experimental protocol.(a) Schematic view of a trial progression during the eye-gaze contingent paradigm used in this study. On the beginning of each trial, a fixation-cross is presented randomly on the screen, and is replaced by a face image as soon as the eye-tracker detects that the observer's line of sight lays on the fixation-cross. The yellow circles represent the observer's gaze position on the screen. Eye-fixation landing position on the seven predefined fROI (b) is controlled by the presentation of a fixation-cross located randomly in one of nine invisible quadrants on the screen, that were spatially mapped with the location of the fROIs (top). Actual gaze positions compiled over all subjects and trials for each face orientation (bottom). Gaze position for each trial is depicted with a black dot. (c) Saliency map and spatial resolution of face images and visual field map coverage when the centre of gravity of the face is fixated.
Mentions: Several lines of evidence suggest that face processing is optimized when gaze fixation focuses on the eyes. Indeed, the eyes are the most salient features of the face based on its energy density spectrum [5], [6] (see fig. 1c). When presented alone, the eyes evoke a N170 response with higher amplitude than any other face parts, and even higher than to whole faces [7]. Meanwhile, face pictures evoke similar N170 amplitudes whether the eyes are present or removed [8], [9]. To reconcile these findings, some authors suggested that the N170 results from an interplay between a holistic face-processing system and specialized eye-processing system. According to this interpretation, the N170 reflects the holistic processing of faces; however, when holistic information is not available, either using isolated faces parts or inverted faces, the eye-processor is brought to play [8]. However the existence of an eye-specialized brain structure remains speculative and was contradicted by recent findings from McPartland and colleagues [10], who found that gaze fixations between the eyes (i.e., the nasion) or on the mouth evoke N170 responses with similar amplitudes.

Bottom Line: It is generally agreed that some features of a face, namely the eyes, are more salient than others as indexed by behavioral diagnosticity, gaze-fixation patterns and evoked-neural responses.However, because previous studies used unnatural stimuli, there is no evidence so far that the early encoding of a whole face in the human brain is based on the eyes or other facial features.We found that the N170 indexing the earliest face-sensitive response in the human brain was the largest when the fixation position is located around the nasion.

View Article: PubMed Central - PubMed

Affiliation: Ecole de Technologie Supérieure, Montreal, Canada ; Hôpital Riviere des Prairies, Montreal, Canada. youness.zerouali-boukhal.1@ens.etsmtl.ca

ABSTRACT
It is generally agreed that some features of a face, namely the eyes, are more salient than others as indexed by behavioral diagnosticity, gaze-fixation patterns and evoked-neural responses. However, because previous studies used unnatural stimuli, there is no evidence so far that the early encoding of a whole face in the human brain is based on the eyes or other facial features. To address this issue, scalp electroencephalogram (EEG) and eye gaze-fixations were recorded simultaneously in a gaze-contingent paradigm while observers viewed faces. We found that the N170 indexing the earliest face-sensitive response in the human brain was the largest when the fixation position is located around the nasion. Interestingly, for inverted faces, this optimal fixation position was more variable, but mainly clustered in the upper part of the visual field (around the mouth). These observations extend the findings of recent behavioral studies, suggesting that the early encoding of a face, as indexed by the N170, is not driven by the eyes per se, but rather arises from a general perceptual setting (upper-visual field advantage) coupled with the alignment of a face stimulus to a stored face template.

Show MeSH
Related in: MedlinePlus