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The neural representations of person identity information: fMRI and neuropsychological investigations

Final Report Summary - PINS (The neural representations of person identity information: fMRI and neuropsychological investigations)

Project description:
This project investigated the cognitive and neural bases of person identity nodes (PINs). According to an influential model of face perception (Bruce & Young, 1986), faces are processed in stages. In this model, visual information derived from faces is first used to encode facial structure, after which the structural information is passed on to face recognition units (FRUs), where structural codes of previously seen faces are stored. Faces are successfully recognised when visual information matches a structural code stored in the FRUs. Similar modality-dependent recognition units can be conceptualised for voices, names, or other cues typically used for person recognition (e.g. Belin et al., 2011). FRUs then send the face information to PINs, which contain or provide access to semantic, modality-independent information associated with the person. However, despite the model’s intuitive appeal, there is limited evidence supporting the existence of these abstract, modality-independent modes. Our project aimed to fill this critical gap by investigating human brain representations of abstract (i.e. extracted away from sensory information) identity information. Furthermore, we examined the modularity of the person identity recognition system (whether face recognition units are independent of the voice recognition units) and whether PINS are affected when domain-specific recognition systems are compromised due to atypical neurodevelopment. During the project, it also became apparent that a re-examination of presumed face-specific behavioural and neural markers was necessary.

Work performed and main results:
We have performed three fMRI experiments aimed at identifying and validating brain areas that can reliably distinguish between person identities across dramatically different sensory inputs: natural photographs, line drawings (black and white minimalistic sketches true to the real facial structure), and caricatures (sketches exaggerating particular facial features). The line drawings and caricatures were produced by a hired artist (see Figure 1 for all stimuli). Note that natural photos of different individuals (e.g. Brad Pitt and George Clooney) typically share more physical similarities than a photo and a caricature of the same individual. Therefore, brain areas responding more similarly to person identity (e.g. photo and caricature of Brad Pitt) rather than image format are likely to encode a more abstract representation of identity, not dependant on a realistic depiction of facial features.
In all experiments, participants underwent a short training session until they could reliably recognise the three actors from any image, and then underwent fMRI scanning. Our aim was to identify the brain areas where neural activation patterns could reliably predict whether participants were looking at a particular identity regardless of image type. Linear support vector machine (SVM) classifiers were trained on neural activation patterns in regions of interest and performance was measured by how well the classifier predicted category membership of new data. In fMRI analysis, successful classifiers indicate that a particular region is sensitive to the experimental conditions. The brain regions of interest, identified with standard face localizer tasks, were: the fusiform face area (FFA), the occipital face area (OFA), the superior temporal sulcus (STS) and the face-selective anterior temporal lobe (fATL). We used the early visual cortex (EVC) as a control region of interest.

Figure 1. Frontal, side and profile face shots of three actors and the corresponding line drawings and caricatures created by an artist.

In the first study (n=13), we found that image type (i.e. photos, line drawings or caricatures) can be reliably distinguished in EVC, an area known to respond to the physical attributes of visual information. OFA, FFA, and STS were also sensitive to the image format. Most importantly, when we looked at brain areas encoding identity regardless of image type (e.g. Brad Pitt from photos, line drawings, and caricatures), we found classification accuracy significantly above chance in STS (when we considered regions of interest with 6mm and 9mm radii) and FFA (when we considered regions of interest with 6mm radius) (Figure 2). Two follow-up studies (each with n=15 participants) were performed to validate the results of the first study, with increased statistical power. The results provided a strong confirmation of our initial findings that the right posterior STS is involved in abstract representations of person identity. This finding is consistent with recent data (Anzellotti et al., 2015) finding multimodal representations of identity (using faces and voices) in posterior STS. Furthermore, we have obtained some evidence (though not as strong as for the right posterior STS) that the right FFA and ATL may also be involved in encoding abstract representations of person identities.

Figure2. Accuracy of person identity classification in our regions of interest. Significant results are marked with a red asterisk. rV1=right early visual cortex, rOFA=right occipital face area, rSTS=right superior temporal sulcus, rFFA=right fusiform face area, rATL=right anterior temporal lobe.

Our next research objective was to examine the modularity of the person identity recognition system. To this end, first we have carefully screened a relatively large number of potential prosopagnosics (more than 1000) to identify the most “pure” cases of developmental prosopagnosia (ie. individuals with no other perceptual or cognitive deficits apart from face recognition). Second, to test the normal development of the abstract person identity nodes and of the other modality-specific recognition units in developmental prosopopagnosia, we created and extensively validated two new tests of voice recognition to match the face recognition tests typically used to classify prosopagnosia. These new tests were necessary because the existing tests of voice recognition were not appropriate for our purposes. Third, we started testing voice perception and recognition abilities in 100 individuals with developmental prosopagnosia selected at the first step. We estimate this work will be completed in the next couple of months. Preliminary results point towards normal voice recognition in most of these individuals, suggesting that developmental recognition deficits for identity can be contained to the visual domain.
The current investigation into the specificity of the recognition deficits exhibited by individuals with prosopagnosia also led us to pursue a much-needed, thorough and robust investigation of holistic processing, the notion that there are special cognitive mechanisms that process faces as a perceptual whole or gestalt. Holistic processing has been suggested to produce face-specific behavioral and neural effects. We have provided compelling evidence that the behavioural effects widely believed to tap the same holistic mechanisms are in fact measuring largely distinct phenomena. Furthermore, we showed that the most robust of these behavioural effects, the large inversion effect, can be obtained with non-face stimuli, challenging previous landmark findings reported to support face specificity. A further neuroimaging study (the fourth fMRI study included in this project) confirmed that the neural markers of face-specific mechanisms (i.e. the existence of face-selective brain areas) are not affected by this challenge, but similar investigations are required for the other behavioural, electrophysiological and neuropsychological markers of face-specificity.

Final results and impact:
Our three main fMRI experiments represent a comprehensive and robust investigation of abstract representations of person identity. Although photographs of different people are more similar in terms of low-level visual information than a photograph and a caricature of the same person, we found some brain regions (most consistently the posterior STS, but also possibly the FFA and the ATL) that care more about the identity of the person in the image than the low-level visual properties of the image. This is a first indication about the existence of abstract person identity nodes.
Our preliminary data on the modularity of the person identity recognition system, showing normal voice recognition in individuals with developmental prosopagnosia, suggest that: i) the face recognition units are independent of the voice recognition units, and: ii) atypical neurodevelopment of domain-specific recognition systems (FRUs) did not affect the normal development of the abstract identity nodes (PINS).
Our results will inform contemporary theories of person identity recognition. Basic knowledge about the neural mechanisms of person recognition may lay the groundwork for translational research into rehabilitative strategies for individuals with atypical person recognition abilities.