Valérie Goffaux

Spatio-temporal localization of the face inversion effect : an event-related potentials study

Event-related potentials (ERPs) from 58 electrodes at standard EEG sites were recorded while 14 subjects performed a delayed-matching task on normal and inverted faces. A large and single difference between normal and inverted face processing was observed at occipito-temporal sites about 160 ms following stimulus onset, mainly in the right hemisphere (RH). Although the topographies indicate that similar areas are involved at this latency in processing the two types of stimuli, the electrophysiological activity, which corresponds to the previously described N170, was larger and delayed for inverted as compared to normal face processing. These results complement and specify, at a neural level, previous behavioral and divided visual field studies which have suggested that the loss of configural face information by inversion may slow down and increase the difficulty of face processing, particularly in the RH.

Faces are "spatial"--holistic face perception is supported by low spatial frequencies

Faces are perceived holistically, a phenomenon best illustrated when the processing of a face feature is affected by the other features. Here, the authors tested the hypothesis that the holistic perception of a face mainly relies on its low spatial frequencies. Holistic face perception was tested in two classical paradigms: the whole-part advantage (Experiment 1) and the composite face effect (Experiments 2-4). Holistic effects were equally large or larger for low-pass filtered faces as compared to full-spectrum faces and significantly larger than for high-pass filtered faces. The disproportionate composite effect found for low-pass filtered faces was not observed when holistic perception was disrupted by inversion (Experiment 3). Experiment 4 showed that the composite face effect was enhanced only for low spatial frequencies, but not for intermediate spatial frequencies known be critical for face recognition. These findings indicate that holistic face perception is largely supported by low spatial frequencies. They also suggest that holistic processing precedes the analysis of local features during face perception.

Expertise Training with Novel Objects Leads to Left-Lateralized Facelike Electrophysiological Responses

Scalp event-related potentials (ERPs) in humans indicate that face and object processing differ approximately 170 ms following stimulus presentation, at the point of the N170 occipitotemporal component. The N170 is delayed and enhanced to inverted faces but not to inverted objects. We tested whether this inversion effect reflects early mechanisms exclusive to faces or whether it generalizes to other stimuli as a function of visual expertise. ERPs to upright and inverted faces and novel objects (Greebles) were recorded in 10 participants before and after 2 weeks of expertise training with Greebles. The N170 component was observed for both faces and Greebles. The results are consistent with previous reports in that the N170 was delayed and enhanced for inverted faces at recording sites in both hemispheres. For Greebles, the same effect of inversion was observed only for experts, primarily in the left hemisphere. These results suggest that the mechanisms underlying the electrophysiological face-inversion effect extend to visually homogeneous nonface object categories, at least in the left hemisphere, but only when such mechanisms are recruited by expertise.

The respective role of low and high spatial frequencies in supporting configural and featural processing of faces

One distinctive feature of processing faces, as compared to other categories, is thought to be the large dependence on configural cues such as the metric relations among features. To test the role of low spatial frequencies (LSFs) and high spatial frequencies (HSFs) in configural and featural processing, subjects were presented with triplets of faces that were filtered to preserve either LSFs (below 8 cycles per face width), HSFs (above 32 cycles per face width), or the full frequency spectrum. They were asked to match one of two probe faces to a target face. The distractor probe face differed from the target either configurally, featurally, or both featurally and configurally. When the difference was at the configural level, performance was better with LSF faces than with HSF faces. In contrast, with a featural difference, a strong performance advantage was found for HSF faces as compared to LSF faces. These results support the dominant role that LSFs play in the configural processing of faces, whereas featural processing is largely dependent on HSFs.

Local discriminability determines the strength of holistic processing for faces in the fusiform face area

Recent evidence suggests that the Fusiform Face Area (FFA) is not exclusively dedicated to the interactive processing of face features, but also contains neurons sensitive to local features. This suggests the existence of both interactive and local processing modes, consistent with recent behavioral findings that the strength of interactive feature processing (IFP) engages most strongly when similar features need to be disambiguated. Here we address whether the engagement of the FFA into interactive versus featural representational modes is governed by local feature discriminability. We scanned human participants while they matched target features within face pairs, independently of the context of distracter features. IFP was operationalized as the failure to match the target without being distracted by distracter features. Picture-plane inversion was used to disrupt IFP while preserving input properties. We found that FFA activation was comparably strong, irrespective of whether similar target features were embedded in dissimilar contexts(i.e., inducing robust IFP) or dissimilar target features were embedded in the same context (i.e., engaging local processing). Second, inversion decreased FFA activation to faces most robustly when similar target features were embedded in dissimilar contexts, indicating that FFA engages into IFP mainly when features cannot be disambiguated at a local level. Third, by means of Spearman rank correlation tests, we show that the local processing of feature differences in the FFA is supported to a large extent by the Occipital Face Area, the Lateral Occipital Complex, and early visual cortex, suggesting that these regions encode the local aspects of face information. The present findings confirm the co-existence of holistic and featural representations in the FFA. Furthermore, they establish FFA as the main contributor to the featural/holistic representational mode switches determined by local discriminability.

Spatial scale contribution to early visual differences between face and object processing.

Event-related potential (ERP) studies have highlighted an occipito-temporal potential, the N170, which is larger for faces than for other categories and delayed by stimulus inversion of faces, but not of other objects. We examined how high-pass and low-pass filtering modulate such early differences between the processing of faces and objects. Sixteen grey-scale pictures of faces and cars were filtered to preserve only relatively low (LSF) or high (HSF) spatial frequencies and were presented upright or upside-down. Subjects reported the orientation of the faces and cars in broad-pass and filtered conditions. In the broad-pass condition, we replicated typical N170 face-specific effects of amplitude and delay with inversion. These effects were also present in the LSF condition. However, a completely different pattern was revealed by the HSF condition: (1). a similar N170 amplitude for cars as compared to faces and (2). an absence of N170 latency delay with face inversion. These results show that the source of early processing differences between faces and objects is related to the extraction of information contained mostly in the LSF, which conveys coarse configuration cues particularly salient for face processing.

Face Inversion Disproportionately Impairs the Perception of Vertical but not Horizontal Relations Between Features

Upside-down inversion disrupts the processing of spatial relations between the features of a face, while largely preserving local feature analysis. However, recent studies on face inversion failed to observe a clear dissociation between relational and featural processing. To resolve these discrepancies and clarify how inversion affects face perception, the authors monitored inversion effects separately for vertical and horizontal distances between features. Inversion dramatically declined performance in the vertical-relational condition, but it impaired featural and horizontal-relational performance only moderately. Identical observations were made whether upright and inverted trials were blocked or randomly interleaved. The largest performance decrement was found for vertical relations even when faces were rotated by 90 degrees. Evidence that inversion dramatically disrupts the ability to extract vertical but not horizontal feature relations supports the view that inversion qualitatively changes face perception by rendering some of the processes activated by upright faces largely ineffective.

Is the early modulation of brain activity by fearful facial expressions primarily mediated by coarse low spatial frequency information

Rapidly decoding the emotional content of a face is an important skill for successful social behavior. Several Event Related brain Potential (ERP) have indicated that emotional expressions already influence brain activity as early as 100 ms. Some studies hypothesized that this early brain response to fear depends on coarse-magnocellular inputs, which are primarily driven by Low Spatial Frequency (LSF) cues. Until now however, evidence is inconclusive probably due to the divergent methods used to match luminance and contrast across spatial frequencies and emotional stimuli. In the present study, we measured ERPs to LSF and HSF faces with fearful or neutral expressions when contrast and luminance was matched across SF or not. Our findings clearly show that fearful facial expressions increases the amplitude of P1 (only for contrast-luminance equated images) and N170 in comparison to neutral faces but only in LSF faces, irrespective of contrast or luminance equalization, further suggesting that LSF information plays a crucial role in the early brain responses to fear. Furthermore, we found that, irrespective of luminance or contrast equalization, N170 occurred faster when perceiving LSF faces than HSF faces, again emphasizing the primacy of LSF processing in early face perception.

Horizontal information drives the behavioral signatures of face processing

Recent psychophysical evidence indicates that the vertical arrangement of horizontal information is particularly important for encoding facial identity. In this paper we extend this notion to examine the role that information at different (particularly cardinal) orientations might play in a number of established phenomena each a behavioral “signature” of face processing. In particular we consider (a) the face inversion effect (FIE), (b) the facial identity after-effect, (c) face-matching across viewpoint, and (d) interactive, so-called holistic, processing of face parts. We report that filtering faces to remove all but the horizontal information largely preserves these effects but conversely, retaining vertical information generally diminishes or abolishes them. We conclude that preferential processing of horizontal information is a central feature of human face processing that supports many of the behavioral signatures of this critical visual operation.

From Coarse to Fine? Spatial and Temporal Dynamics of Cortical Face Processing

Primary vision segregates information along 2 main dimensions: orientation and spatial frequency (SF). An important question is how this primary visual information is integrated to support high-level representations. It is generally assumed that the information carried by different SF is combined following a coarse-to-fine sequence. We directly addressed this assumption by investigating how the network of face-preferring cortical regions processes distinct SF over time. Face stimuli were flashed during 75, 150, or 300 ms and masked. They were filtered to preserve low SF (LSF), middle SF (MSF), or high SF (HSF). Most face-preferring regions robustly responded to coarse LSF, face information in early stages of visual processing (i.e., until 75 ms of exposure duration). LSF processing decayed as a function of exposure duration (mostly until 150 ms). In contrast, the processing of fine HSF, face information became more robust over time in the bilateral fusiform face regions and in the right occipital face area. The present evidence suggests the coarse-to-fine strategy as a plausible modus operandi in high-level visual cortex.