Imagine Jane and Identify John: Face Identity Aftereffects Induced by Imagined Faces

It is not known whether prolonged exposure to perceived and imagined complex visual images produces similar shifts in subsequent perception through selective adaptation. This question is important because a positive finding would suggest that perception and imagery of visual stimuli are mediated by shared neural networks. In this study, we used a selective adaptation procedure designed to induce high-level face-identity aftereffects—a phenomenon in which extended exposure to a particular face facilitates recognition of subsequent faces with opposite features while impairing recognition of all other faces. We report here that adaptation to either real or imagined faces produces a similar shift in perception and that identity boundaries represented in real and imagined faces are equivalent. Together, our results show that identity information contained in imagined and real faces produce similar behavioral outcomes. Our findings of high-level visual aftereffects induced by imagined stimuli can be taken as evidence for the involvement of shared neural networks that mediate perception and imagery of complex visual stimuli

Adapting effects of emotional expression in anxiety: evidence for an enhanced late positive potential

An adaptation paradigm was used to investigate the influence of a previously experienced visual context on the interpretation of ambiguous emotional expressions. Affective classification of fear-neutral ambiguous expressions was performed following repeated exposure to either fearful or neutral faces. There was a shift in the behavioural classification of morphs towards ‘fear’ following adaptation to neutral compared to adaptation to fear with a non-significant trend towards the high anxiety group compared to the low being more influenced by the context. The event-related potential (ERP) data revealed a more pronounced late positive potential (LPP), beginning at ~400 ms post-stimulus onset, in the high but not the low anxiety group following adaptation to neutral compared to fear. In addition, as the size of the behavioural adaptation increased there was a linear increase in the magnitude of the late-LPP. However, context-sensitivity effects are not restricted to trait anxiety, with similar effects observed with state anxiety and depression. These data support the proposal that negative moods are associated with increased sensitivity to visual contextual influences from top-down elaborative modulations, as reflected in an enhanced late positive potential deflection

Face adaptation aftereffects reveal anterior medial temporal cortex role in high level category representation

AbstractPrevious studies have shown reductions of the functional magnetic resonance imaging (fMRI) signal in response to repetition of specific visual stimuli. We examined how adaptation affects the neural responses associated with categorization behavior, using face adaptation aftereffects. Adaptation to a given facial category biases categorization towards non-adapted facial categories in response to presentation of ambiguous morphs. We explored a hypothesis, posed by recent psychophysical studies, that these adaptation-induced categorizations are mediated by activity in relatively advanced stages within the occipitotemporal visual processing stream. Replicating these studies, we find that adaptation to a facial expression heightens perception of non-adapted expressions. Using comparable behavioral methods, we also show that adaptation to a specific identity heightens perception of a second identity in morph faces. We show both expression and identity effects to be associated with heightened anterior medial temporal lobe activity, specifically when perceiving the non-adapted category. These regions, incorporating bilateral anterior ventral rhinal cortices, perirhinal cortex and left anterior hippocampus are regions previously implicated in high-level visual perception. These categorization effects were not evident in fusiform or occipital gyri, although activity in these regions was reduced to repeated faces. The findings suggest that adaptation-induced perception is mediated by activity in regions downstream to those showing reductions due to stimulus repetition

Modulation of neural responses in inferotemporal cortex during the interpretation of ambiguous photographs

Ambiguous images are interpreted in the context of biases about what they might be; these biases and the behavioral consequences induced by them may influence the processing of images. In this report, we examine neural responses in inferotemporal cortex (IT) during the interpretation of ambiguous photographs created by morphing between two photographs. Monkeys classified different images as being one of two choices and learned to classify most of the samples correctly. For one image (the ambiguous sample) reward was administered randomly for either possible choice, and the monkeys were free to classify that image based on their own interpretation, with no learning possible. The ambiguous samples were not classified randomly: the monkey interpreted the samples differently during different sessions. The interpretation of the ambiguous sample was, in turn, highly correlated with the normalized response of individual neurons in IT to the ambiguous sample. If an ambiguous sample was interpreted as a particular choice during a session, the response to that ambiguous sample more closely resembled the response to that choice. Identical ambiguous images were interpreted differently during different sessions, and neural responses reflected the differing interpretations of the image during that session. The relationship between the interpretation of the image and neural responses strengthened over the course of a session because neural responses shifted to more closely resemble the response to the initial interpretation of the image. The data support a flexible representation of visual stimuli in higher visual areas

The temporal context of face perception: behavioural, electrophysiological and neuroimaging correlates

Adaptation-related aftereffects (AEs) and repetition priming (PR) are two phenomena when recent experiences alter face perception. Although the behavioural reflections of AEs and PR are quite different, both phenomena share some similarities regarding their functional properties and neural correlates. As most of the previous studies focused on either one or the other phenomenon, little is known about the relationship between AEs and PR. The present studies attempted to fill this gap. Study I investigated face identity AEs and PR within the same stimulus repetition paradigm, keeping timing and task constant. Following face or Fourier phase randomized (noise) stimuli, participants classified test faces varying on a morph continuum between two famous identities. Study I showed that AEs and PR can be observed within the same paradigm and subjects, behaviourally and in event-related potentials (ERPs). Interestingly, we found identity-specific as well as category-specific ERP modulations. Study II further investigated the factors underlying face identity AEs in a similar paradigm. The results confirmed and extended the findings of Study I, e.g., there were again different ERP modulations by stimulus category and face identity. In Study III, AEs and image-specific PR were investigated in the perception of face gender using functional magnetic resonance imaging (fMRI). Study III suggested dissociations between 1) gender-specific AEs and image-specific PR in behaviour, 2) brain areas associated with AEs and PR, and 3) brain areas associated with gender-specific and categorical processes. In conclusion, the present studies showed that similarity between adaptor and test faces and ambiguity of the test face both determine whether AE or PR is observed, and suggested that exclusive mechanisms might underlie both phenomena. Our results also revealed that the processing of face identity or gender runs in parallel to object-category processing during the earlier processing stages

Face adaptation: behavioural and electrophysiological studies on the perception of eye gaze and gender

Whereas the investigation of perceptual aftereffects has a very long tradition in studies on low-level vision, the report and analysis of face-related high-level aftereffects is a very recent line of research. Webster et al. (2004, Nature) first showed a visual aftereffect on the perception of face gender by demonstrating that adaptation to male faces biased the subsequent classification of androgynous faces towards female gender. Similar adaptation effects have also been observed for one of the most important social signals: human eye gaze. Jenkins et al. (2006, Psychological Science) found that adaptation to gaze in one direction virtually eliminated participants’ ability to perceive smaller gaze deviations in the same direction. The present thesis further examined these high-level face aftereffects by determining the temporal characteristics of gaze adaptation and by analysing the neural correlates of eye gaze and gender adaptation. A behavioural study on the temporal decay of gaze adaptation effects shed further light on their basic characteristics: not only was the aftereffect surprisingly long-lasting, but its exponential decay revealed remarkable similarity with the time course of low-level adaptation effects. Further, in a series of event-related potential (ERP) studies it was found that the N170 was only marginally affected by both eye gaze and gender adaptation, whereas pronounced effects of both kinds of adaptation emerged in the P3 component with smaller amplitudes in response to test stimuli similar to the adaptation condition. Finally, gaze adaptation was found to affect ERPs in an earlier time interval ~250-350 ms which appeared to be sensitive to the discrimination between direct vs. averted gaze even when this was only an illusionary percept induced by adaptation. Together, these studies extend previous knowledge of the temporal parameters and the neural correlates of high-level face adaptation

The effect of familiarity on face adaptation

Face adaptation techniques have been used extensively to investigate how faces are processed. It has even been suggested that face adaptation is functional in calibrating the visual system to the diet of faces to which an observer is exposed. Yet most adaptation studies to date have used unfamiliar faces: few have used faces with real world familiarity. Familiar faces have more abstractive representations than unfamiliar faces. The experiments in this thesis therefore examined face adaptation for familiar faces. Chapters 2 and 3 explored the role of explicit recognition of familiar faces in producing face identity after-effects (FIAEs). Chapter 2 used composite faces (the top half of a celebrity's face paired with the bottom half of an unfamiliar face) as adaptors and showed that only recognised composites produced significant adaptation. In Chapter 3 the adaptors were cryptic faces (unfamiliar faces subtly transformed towards a celebrity's face) and faces of celebrity's siblings. Unrecognised cryptic and sibling faces produced FIAEs for their related celebrity, but only when adapting and testing on the same viewpoint. Adaptation only transferred across viewpoint when a face was explicitly recognised. Chapter 4 demonstrated that face adaptation could occur for ecologically valid, personally familiar stimuli, a necessary pre-requisite if adaptation is functional in calibrating face processing mechanisms. A video of a lecturer's face produced FIAEs equivalent to that produced by static images. Chapters 5 and 6 used a different type of after-effect, the face distortion after-effect (FDAE), to explore the stability of our representations for personally familiar faces, and showed that even representations of highly familiar faces can be affected by exposure to distorted faces. The work presented here shows that it is important to take facial familiarity into account when investigating face adaptation effects, as well as increasing our understanding of how familiarity affects the representations of faces

An integrated analysis of the extended hippocampal system across species

The objective of this thesis was to investigate functional differences within the extended hippocampal system by 1. analysing its connectional topography and 2. looking at evidence for differential functions within its component structures. The main areas under examination were A. the subiculum and B. its diencephalic targets, along with C. the fornix, the principle white matter tract connecting these structures. Retrograde tracer experiments in rodents and primates revealed consistent topographies in the subiculum projections to these diencephalic target sites, with distinctions occurring primarily along the proximal-distal and laminar subicular axes in rodents and primarily along the anterior-posterior and laminar subicular axes in primates. Based on different input patterns to the proximal subiculum (principally from sites processing object information) and distal subiculum (principally from sites processing spatial/context information) it was predicted that this proximal-distal axis would show functional activation differences in rodents for matched object:spatial tasks. Immediate early gene imaging (using zif268 expression) did not, however, reveal clear-cut gradient differences, although there were indications of the expected bias to object memory in the proximal subiculum. Diffusion MRI was used to study the fornix by separating its precommissural and postcommissural connections in a healthy older and cognitively impaired human population. Reliable topographic differences were found for the precommissural and postcommissural fornix in each group but cognitive function proved difficult to differentiate between the tracts for the tasks used. Lastly, fornix reconstructions were also found to be separable according to their links with either the anterior or posterior hippocampus in a healthy population. These distinctions provide another way of studying the fornix in terms of relating different functional properties with different sets of hippocampal connections. It is assumed that different populations of fornical fibres should underlie different aspects of memory/ cognitive tasks involving the fornix, making their segregation informative in future studies researching this tract. detailing the nature of the connections within the extended hippocampal system, this thesis lays the groundwork for future studies investigating the relative roles of its component structures in cognitive functio