Ipek Oruc

The anatomic basis of the right face-selective N170 IN acquired prosopagnosia: A combined ERP/fMRI study

The N170 waveform is larger over posterior temporal cortex when healthy subjects view faces than when they view other objects. Source analyses have produced mixed results regarding whether this effect originates in the fusiform face area (FFA), lateral occipital cortex, or superior temporal sulcus (STS), components of the core face network. In a complementary approach, we assessed the face-selectivity of the right N170 in five patients with acquired prosopagnosia, who also underwent structural and functional magnetic resonance imaging. We used a non-parametric bootstrap procedure to perform single-subject analyses, which reliably confirmed N170 face-selectivity in each of 10 control subjects. Anterior temporal lesions that spared the core face network did not affect the face-selectivity of the N170. A face-selective N170 was also present in another subject who had lost only the right FFA. However, face-selectivity was absent in two patients with lesions that eliminated the occipital face area (OFA) and FFA, sparing only the STS. Thus while the right FFA is not necessary for the face-selectivity of the N170, neither is the STS sufficient. We conclude that the face-selective N170 in prosopagnosia requires residual function of at least two components of the core face-processing network.

It doesn't matter how you feel. The facial identity aftereffect is invariant to changes in facial expression

Previous studies have shown that facial expression aftereffects are modulated by the identity of the adapting face, suggesting both identity-dependent and identity-independent representations of facial expression. In this study, we asked whether facial identity aftereffects were similarly modulated by expression. In Experiment 1, the congruency of expression between adapting and test faces did not affect the identity aftereffect for novel faces, suggesting that the neural representations activated by novel identities are independent of expression. In Experiment 2, we examined whether expression dependency might be found with more familiar faces but still did not find any modulation of identity aftereffects by the congruency of expression. In Experiment 3, we measured the similarity between faces used to probe expression and identity adaptation, using both an ideal observer and human subjects, to determine if the discrepancy between the results of these two studies is related to greater similarity between faces from the same person with different expressions than between faces of different people with the same expression. However, the contrast thresholds required to discriminate between faces of differing expression were similar to those for faces with differing identity. We conclude that, in contrast to the significant identity-dependent component seen in representations of expression, representations of facial identity are independent of variations in expression.

Factors contributing to the adaptation aftereffects of facial expression

Previous studies have demonstrated the existence of adaptation aftereffects for facial expressions. Here we investigated which aspects of facial stimuli contribute to these aftereffects. In Experiment 1, we examined the role of local adaptation to image elements such as curvature, shape and orientation, independent of expression, by using hybrid faces constructed from either the same or opposing expressions. While hybrid faces made with consistent expressions generated aftereffects as large as those with normal faces, there were no aftereffects from hybrid faces made from different expressions, despite the fact that these contained the same local image elements. In Experiment 2, we examined the role of facial features independent of the normal face configuration by contrasting adaptation with whole faces to adaptation with scrambled faces. We found that scrambled faces also generated significant aftereffects, indicating that expressive features without a normal facial configuration could generate expression aftereffects. In Experiment 3, we examined the role of facial configuration by using schematic faces made from line elements that in isolation do not carry expression-related information (e.g. curved segments and straight lines) but that convey an expression when arranged in a normal facial configuration. We obtained a significant aftereffect for facial configurations but not scrambled configurations of these line elements. We conclude that facial expression aftereffects are not due to local adaptation to image elements but due to high-level adaptation of neural representations that involve both facial features and facial configuration.

Scale dependence and channel switching in letter identification.

Letters are broadband visual stimuli with information useful for discrimination over a wide range of spatial frequencies. Yet, recent evidence suggests that observers use only a single, fixed spatial-frequency channel to identify letters and that the scale of that channel, in units of letter size, is determined by the size of the letter (scale dependence). We report two letter-identification experiments using critical-band masking. With sufficiently high-amplitude, low- or high-pass masking noise, observers switched to a different range of spatial frequencies for the task. Thus, letter channels are not fixed for a given letter size. When an additional white-noise masker was added to the stimulus to flatten the contrast-sensitivity function, the letter channel used by the observer still depended on letter size, further supporting the hypothesis that letter identification is scale dependent.

Bootstrap analysis of the single subject with event related potentials

Neural correlates of cognitive states in event-related potentials (ERPs) serve as markers for related cerebral processes. Although these are usually evaluated in subject groups, the ability to evaluate such markers statistically in single subjects is essential for case studies in neuropsychology. Here we investigated the use of a simple test based on nonparametric bootstrap confidence intervals for this purpose, by evaluating three different ERP phenomena: the face-selectivity of the N170, error-related negativity, and the P3 component in a Posner cueing paradigm. In each case, we compare single-subject analysis with statistical significance determined using bootstrap to conventional group analysis using analysis of variance (ANOVA). We found that the proportion of subjects who show a significant effect at the individual level based on bootstrap varied, being greatest for the N170 and least for the P3. Furthermore, it correlated with significance at the group level. We conclude that the bootstrap methodology can be a viable option for interpreting single-case ERP amplitude effects in the right setting, probably with well-defined stereotyped peaks that show robust differences at the group level, which may be more characteristic of early sensory components than late cognitive effects.

Adaptation improves discrimination of face identity

Whether face adaptation confers any advantages to perceptual processing remains an open question. We investigated whether face adaptation can enhance the ability to make fine discriminations in the vicinity of the adapted face. We compared face discrimination thresholds in three adapting conditions: (i) same-face: where adapting and test faces were the same, (ii) different-face: where adapting and test faces differed, and (iii) baseline: where the adapting stimulus was a blank. Discrimination thresholds for morphed identity changes involving the adapted face (same-face) improved compared with those from both the baseline (no-adaptation) and different-face conditions. Since adapting to a face did not alter discrimination performance for other faces, this effect is selective for the facial identity that is adapted. These results indicate a form of gain control to heighten perceptual sensitivity in the vicinity of a currently viewed face, analogous to forms of adaptive gain control at lower levels of the visual system.

The role of skin texture and facial shape in representations of age and identity.

Faces have both shape and skin texture, but the relative importance of the two in face representations is unclear. Our goals were first, to determine the contribution of shape versus texture to aftereffects for facial age and identity and second, to assess whether adaptation transferred between shape and texture, suggesting integration in a single representation. In our first experiment we examined age aftereffects. We obtained young and old images of two celebrities and created hybrid images, one combining the structure of the old face with the skin texture of the young face, the other combining the young structure with the old skin texture. This allowed us to create adaptation contrasts where the two adapting faces had the same facial structure but different skin texture, and vice versa. In the second experiment, we performed a similar study but this time examining identity aftereffects between two people of a similar age. We found that both skin texture and facial shape generated significant age aftereffects, but the contribution was greater from texture than from shape. Both texture and shape also generated significant identity aftereffects, but the contribution was greater from shape than from texture. In the last experiment, we used the normal and hybrid images to determine if adaptation to one property (i.e., texture) could create aftereffects in the perception of age in the other property (i.e., shape). While there was significant within-component adaptation for texture and shape, there was no evidence of cross-component adaptation. We conclude that shape and texture contribute differently to different face representations, with texture dominating for age. The lack of cross-component adaptation transfer suggests independent encoding of shape and texture, at least for age representations.

Size determines whether specialized expert processes are engaged for recognition of faces

Many influential models of face recognition postulate specialized expert processes that are engaged when viewing upright, own-race faces, as opposed to a general-purpose recognition route used for nonface objects and inverted or other-race faces. In contrast, others have argued that empirical differences do not stem from qualitatively distinct processing. We offer a potential resolution to this ongoing controversy. We hypothesize that faces engage specialized processes at large sizes only. To test this, we measured recognition efficiencies for a wide range of sizes. Upright face recognition efficiency increased with size. This was not due to better visibility of basic image features at large sizes. We ensured this by calculating efficiency relative to a specialized ideal observer unique to each individual that incorporated size-related changes in visibility and by measuring inverted efficiencies across the same range of face sizes. Inverted face recognition efficiencies did not change with size. A qualitative face inversion effect, defined as the ratio of relative upright and inverted efficiencies, showed a complete lack of inversion effects for small sizes up to 6°. In contrast, significant face inversion effects were found for all larger sizes. Size effects may stem from predominance of larger faces in the overall exposure to faces, which occur at closer viewing distances typical of social interaction. Our results offer a potential explanation for the contradictory findings in the literature regarding the special status of faces.

A novel face aftereffect based on recognition contrast thresholds

Previously, repulsive perceptual-shift face aftereffects have been reported. Here, we introduce a novel face adaptation method involving changes in contrast thresholds for face recognition. We find non-monotonic changes for adapted faces, with facilitation at short and suppression at long durations. Thresholds for unadapted faces were unaffected at short but elevated at long durations, more than those for the adapted face. A population-coding model showed that selective suppression of adapted representations cannot explain repulsive perceptual-bias aftereffects. The findings indicate greater complexity to adaptation, with facilitation, suppression, lateral inhibition of unadapted representations, and additional perceptual factors at long durations.

Cross-orientation transfer of adaptation for facial identity is asymmetric: a study using contrast-based recognition thresholds.

Recent studies suggest that adaptation effects for face shape and gender transfer from upright to inverted faces more than the reverse. We investigated whether a similar asymmetry occurred for face identity, using a recently developed adaptation method based on contrast-recognition thresholds. When adapting and test stimuli shared the same orientation, aftereffects were similar for upright and inverted faces. When orientation differed, there was significant transfer of aftereffects from upright adapting to inverted test faces, but none from inverted to upright faces. We show that asymmetric cross-orientation transfer of face aftereffects generalize across two distinct face adaptation paradigms: the previously used perceptual-bias methodology and the recently introduced contrast-threshold based adaptation paradigm. These results also represent a generalization from aftereffects for face shape and gender to aftereffects for face identity. While these results are consistent with the dual-mode hypothesis, they can also be accounted for by a single population of units of varying orientation selectivity.