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

The emotional adaptation aftereffect discriminates between individuals with high and low levels of depressive symptoms

The adaptation aftereffect plays a critical role in human development and survival. Existing studies have found that, compared with general individuals, individuals with learning disability, autism and dyslexia show a smaller amount of non-affective-based cognitive adaptation aftereffect. Nevertheless, it is unclear whether individuals with depression or depression tendency show similar phenomenon in the adaptation aftereffect, and whether such depression tendency occurs in the non-affective-based cognitive or emotional adaptation aftereffect. To address this question, the present study conducted two experiments. Experiments 1A and 1B used the emotional facial expression adaptation paradigm to examine whether Chinese participants showed the emotional adaptation aftereffect and whether the emotional adaptation aftereffect was influenced by physical features of faces, respectively. Experiment 2 recruited two groups of participants, with high and low depression, respectively, to examine whether they showed differences in the emotional or cognitive adaptation aftereffect. Results showed that Chinese participants showed the typical emotional adaptation aftereffect, which was not influenced by physical features of faces. More importantly, compared to the low-depression group, the high-depression group showed a smaller emotional adaptation aftereffect, but the two groups showed a similar cognitive adaptation aftereffect. These results suggest that level of depressive symptoms is associated with the emotional adaptation aftereffect.

Duration Selectivity in Right Parietal Cortex Reflects the Subjective Experience of Time

The perception of duration in the subsecond range has been hypothesized to be mediated by the population response of duration-sensitive units, each tuned to a preferred duration. One line of support for this hypothesis comes from neuroimaging studies showing that cortical regions, such as in parietal cortex exhibit duration tuning. It remains unclear whether this representation is based on the physical duration of the sensory input or the subjective duration, a question that is important given that our perception of the passage of time is often not veridical, but rather, biased by various contextual factors. Here we used fMRI to examine the neural correlates of subjective time perception in human participants. To manipulate perceived duration while holding physical duration constant, we used an adaptation method, in which, before judging the duration of a test stimulus, the participants were exposed to a train of adapting stimuli of a fixed duration. Behaviorally, this procedure produced a pronounced negative aftereffect: A short adaptor biased participants to judge stimuli as longer and a long adaptor-biased participants to judge stimuli as shorter. Duration tuning modulation, manifest as an attenuated BOLD response to stimuli similar in duration to the adaptor, was only observed in the right supramarginal gyrus (SMG) of the parietal lobe and middle occipital gyrus, bilaterally. Across individuals, the magnitude of the behavioral aftereffect was positively correlated with the magnitude of duration tuning modulation in SMG. These results indicate that duration-tuned neural populations in right SMG reflect the subjective experience of time.SIGNIFICANCE STATEMENT The subjective sense of time is a fundamental dimension of sensory experience. To investigate the neural basis of subjective time, we conducted an fMRI study, using an adaptation procedure that allowed us to manipulate perceived duration while holding physical duration constant. Regions within the occipital cortex and right parietal lobe showed duration tuning that was modulated when the test stimuli were similar in duration to the adaptor. Moreover, the magnitude of the distortion in perceived duration was correlated with the degree of duration tuning modulation in the parietal region. These results provide strong physiological evidence that the population coding of time in the right parietal cortex reflects our subjective experience of time.

A hierarchical model of social perception: Psychophysical evidence suggests late rather than early integration of visual information from facial expression and body posture

Facial expressions are one of the most important sources of information about another’s emotional states. More recently, other cues such as body posture have been shown to influence how facial expressions are perceived. It has been argued that this biasing effect is underpinned by an early integration of visual information from facial expression and body posture. Here, we replicate this biasing effect, but, using a psychophysical procedure, show that adaptation to facial expressions is unaffected by body context. The integration of face and body information therefore occurs downstream of the sites of adaptation, known to be localised in high-level visual areas of the temporal lobe. Contrary to previous research, our findings thus provide direct evidence for late integration of information from facial expression and body posture. They are consistent with a hierarchical model of social perception, in which social signals from different sources are initially processed independently and in parallel by specialised visual mechanisms. Integration of these different inputs in later stages of the visual system then supports the emergence of the integrated whole-person percept that is consciously experienced.

Structural connections support emotional connections: Uncinate Fasciculus microstructure is related to the ability to decode facial emotion expressions

The Uncinate Fasciculus (UF) is an association fibre tract connecting regions in the frontal and anterior temporal lobes. UF disruption is seen in several disorders associated with impaired social behaviour, but its functional role is unclear. Here we set out to test the hypothesis that the UF is important for facial expression processing, an ability fundamental to adaptive social behaviour. In two separate experiments in healthy adults, we used high-angular resolution diffusion-weighted imaging (HARDI) and constrained spherical deconvolution (CSD) tractography to virtually dissect the UF, plus a control tract (the corticospinal tract (CST)), and quantify, via fractional anisotropy (FA), individual differences in tract microstructure. In Experiment 1, participants completed the Reading the Mind in the Eyes Task (RMET), a well-validated assay of facial expression decoding. In Experiment 2, a different set of participants completed the RMET, plus an odd-emotion-out task of facial emotion discrimination. In both experiments, participants also completed a control odd-identity-out facial identity discrimination task. In Experiment 1, FA of the right-, but not the left-hemisphere, UF was significantly correlated with performance on the RMET task, specifically for emotional, but not neutral expressions. UF FA was not significantly correlated with facial identity discrimination performance. In Experiment 2, FA of the right-, but not left-hemisphere, UF was again significantly correlated with performance on emotional items from the RMET, together with performance on the facial emotion discrimination task. Again, no significant association was found between UF FA and facial identity discrimination performance. Our findings highlight the contribution of right-hemisphere UF microstructure to inter-individual variability in the ability to decode facial emotion expressions, and may explain why disruption of this pathway affects social behaviour.

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We studied white matter microstructure correlates of facial emotion decoding skills.

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Focused on the role of a key limbic tract, the Uncinate Fasciculus (UF).

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Right UF microstructure linked to facial expression decoding skills.

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UF microstructure not related to facial identity discrimination skills.

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Right UF has a distinct role in the processing of facial expressions of emotion.

When noise is beneficial for sensory encoding: Noise adaptation can improve face processing

The presence of noise usually impairs the processing of a stimulus. Here, we studied the effects of noise on face processing and show, for the first time, that adaptation to noise patterns has beneficial effects on face perception. We used noiseless faces that were either surrounded by random noise or presented on a uniform background as stimuli. In addition, the faces were either preceded by noise adaptors or not. Moreover, we varied the statistics of the noise so that its spectral slope either matched that of the faces or it was steeper or shallower. Results of parallel ERP recordings showed that the background noise reduces the amplitude of the face-evoked N170, indicating less intensive face processing. Adaptation to a noise pattern, however, led to reduced P1 and enhanced N170 amplitudes as well as to a better behavioral performance in two of the three noise conditions. This effect was also augmented by the presence of background noise around the target stimuli. Additionally, the spectral slope of the noise pattern affected the size of the P1, N170 and P2 amplitudes. We reason that the observed effects are due to the selective adaptation of noise-sensitive neurons present in the face-processing cortical areas, which may enhance the signal-to-noise-ratio.

The neural mechanisms for the recognition of face identity in humans

Every day we encounter dozens of people, and in order to interact with them appropriately we need to recognize their identity. The face is a crucial source of information to recognize a person’s identity. However, recognizing the identity of a face is challenging because it requires distinguishing between very similar images (e.g., the front views of two different faces) while categorizing very different images (e.g., a front view and a profile) as the same person. Neuroimaging has the whole-brain coverage needed to investigate where representations of face identity are encoded, but it is limited in terms of spatial and temporal resolution. In this article, we review recent neuroimaging research that attempted to investigate the representation of face identity, the challenges it faces, and the proposed solutions, to conclude that given the current state of the evidence the right anterior temporal lobe is the most promising candidate region for the representation of face identity.

Dissociating the neural bases of repetition-priming and adaptation in the human brain for faces

The repetition of a given stimulus leads to the attenuation of the functional magnetic resonance imaging (fMRI) signal compared with unrepeated stimuli, a phenomenon called fMRI adaptation or repetition suppression (RS). Previous studies have related RS of the fMRI signal behaviorally both to improved performance for the repeated stimulus (priming) and to shifts of perception away from the first stimulus (adaptation-related aftereffects). Here we used identical task (sex discrimination), trial structure [ stimulus 1 (S1): 3,000 ms, interstimulus interval: 600 ms, stimulus 2 (S2): 300 ms], and S2 stimuli (androgynous faces) to test how RS of the face-specific areas of the occipito-temporal cortex relates to priming and aftereffects. By varying S1, we could induce priming (significantly faster reaction times when S1 and S2 were identical compared with different images) as well as sex-specific aftereffect [an increased ratio of male responses if S1 was a female face compared with ambiguous faces or to Fourier-randomized noise (FOU) images]. Presenting any face as S1 led to significant RS of the blood oxygen level-dependent signal in the fusiform and occipital face areas as well as in the lateral occipital cortex of both hemispheres compared with FOU, reflecting stimulus category-specific encoding. Additionally, while sex-specific adaptation effects were only observed in occipital face areas, primed trials led to a signal reduction in both face-selective regions. Altogether, these results suggest the differential neural mechanisms of adaptation and repetition priming.

View dependencies in the visual recognition of social interactions

Recognizing social interactions, e.g., two people shaking hands, is important for obtaining information about other people and the surrounding social environment. Despite the visual complexity of social interactions, humans have often little difficulties to visually recognize social interactions. What is the visual representation of social interactions and the bodily visual cues that promote this remarkable human ability? Viewpoint dependent representations are considered to be at the heart of the visual recognition of many visual stimuli including objects (Bülthoff and Edelman, 1992), and biological motion patterns (Verfaillie, 1993). Here we addressed the question whether complex social actions acted out between pairs of people, e.g., hugging, are also represented in a similar manner. To this end, we created 3-D models from motion captured actions acted out by two people, e.g., hugging. These 3-D models allowed to present the same action from different viewpoints. Participants' task was to discriminate a target action from distractor actions using a one-interval-forced-choice (1IFC) task. We measured participants' recognition performance in terms of reaction times (RT) and d-prime (d'). For each tested action we found one view that led to superior recognition performance compared to other views. This finding demonstrates view-dependent effects of visual recognition, which are in line with the idea of a view-dependent representation of social interactions. Subsequently, we examined the degree to which velocities of joints are able to predict the recognition performance of social interactions in order to determine candidate visual cues underlying the recognition of social interactions. We found that the velocities of the arms, both feet, and hips correlated with recognition performance.

The implicit processing of categorical and dimensional strategies: an fMRI study of facial emotion perception

OUR UNDERSTANDING OF FACIAL EMOTION PERCEPTION HAS BEEN DOMINATED BY TWO SEEMINGLY OPPOSING THEORIES: the categorical and dimensional theories. However, we have recently demonstrated that hybrid processing involving both categorical and dimensional perception can be induced in an implicit manner (Fujimura etal., 2012). The underlying neural mechanisms of this hybrid processing remain unknown. In this study, we tested the hypothesis that separate neural loci might intrinsically encode categorical and dimensional processing functions that serve as a basis for hybrid processing. We used functional magnetic resonance imaging to measure neural correlates while subjects passively viewed emotional faces and performed tasks that were unrelated to facial emotion processing. Activity in the right fusiform face area (FFA) increased in response to psychologically obvious emotions and decreased in response to ambiguous expressions, demonstrating the role of the FFA in categorical processing. The amygdala, insula and medial prefrontal cortex exhibited evidence of dimensional (linear) processing that correlated with physical changes in the emotional face stimuli. The occipital face area and superior temporal sulcus did not respond to these changes in the presented stimuli. Our results indicated that distinct neural loci process the physical and psychological aspects of facial emotion perception in a region-specific and implicit manner.

Adaptor identity modulates adaptation effects in familiar face identification and their neural correlates

Adaptation-related aftereffects (AEs) show how face perception can be altered by recent perceptual experiences. Along with contrastive behavioural biases, modulations of the early event-related potentials (ERPs) were typically reported on categorical levels. Nevertheless, the role of the adaptor stimulus per se for face identity-specific AEs is not completely understood and was therefore investigated in the present study. Participants were adapted to faces (S1s) varying systematically on a morphing continuum between pairs of famous identities (identities A and B), or to Fourier phase-randomized faces, and had to match the subsequently presented ambiguous faces (S2s; 50/50% identity A/B) to one of the respective original faces. We found that S1s identical with or near to the original identities led to strong contrastive biases with more identity B responses following A adaptation and vice versa. In addition, the closer S1s were to the 50/50% S2 on the morphing continuum, the smaller the magnitude of the AE was. The relation between S1s and AE was, however, not linear. Additionally, stronger AEs were accompanied by faster reaction times. Analyses of the simultaneously recorded ERPs revealed categorical adaptation effects starting at 100 ms post-stimulus onset, that were most pronounced at around 125-240 ms for occipito-temporal sites over both hemispheres. S1-specific amplitude modulations were found at around 300-400 ms. Response-specific analyses of ERPs showed reduced voltages starting at around 125 ms when the S1 biased perception in a contrastive way as compared to when it did not. Our results suggest that face identity AEs do not only depend on physical differences between S1 and S2, but also on perceptual factors, such as the ambiguity of S1. Furthermore, short-term plasticity of face identity processing might work in parallel to object-category processing, and is reflected in the first 400 ms of the ERP.

The neural correlates of the face attractiveness aftereffect: a functional near-infrared spectroscopy (fNIRS) study

Extensive behavioral evidence shows that our internal representation of faces, or face prototype, can be dynamically updated by immediate experience. This is illustrated by the robust attractiveness aftereffect phenomenon whereby originally unattractive faces become attractive after we are exposed to a set of unattractive faces. Although behavioral evidence suggests this effect to have a strong neural basis, limited neuroimaging evidence exists. Here we used functional near-infrared spectroscopy methodology (fNIRS) to bridge this gap. During the pre-adaptation trials, participants judged the attractiveness of three sets of faces: normal/undistorted faces, compressed faces (the internal features and distances between them were compressed), and expanded faces (the internal features and distances between them were stretched). Then, participants were shown extremely compressed faces for 5 min as adaptation stimuli, after which participants judged the same three sets of faces in post-adaptation trials. Behaviorally, after the adaptation trials, participants rated the compressed faces more attractive whereas they judged the other two sets of faces as less attractive, replicating the robust adaptation effect. fNIRS results showed that short-term exposure to compressed faces led to significant decreases in neural activity to all face types, but in a more extended network of cortical regions in the frontal and occipital cortexes for undistorted faces. Taken together, these findings suggest that the face attractiveness aftereffect mainly reflects changes in the neural representation of the face prototype in response to recent exposures to new face exemplars.

Processing of fear and anger facial expressions: the role of spatial frequency

Spatial frequency (SF) components encode a portion of the affective value expressed in face images. The aim of this study was to estimate the relative weight of specific frequency spectrum bandwidth on the discrimination of anger and fear facial expressions. The general paradigm was a classification of the expression of faces morphed at varying proportions between anger and fear images in which SF adaptation and SF subtraction are expected to shift classification of facial emotion. A series of three experiments was conducted. In Experiment 1 subjects classified morphed face images that were unfiltered or filtered to remove either low (<8 cycles/face), middle (12-28 cycles/face), or high (>32 cycles/face) SF components. In Experiment 2 subjects were adapted to unfiltered or filtered prototypical (non-morphed) fear face images and subsequently classified morphed face images. In Experiment 3 subjects were adapted to unfiltered or filtered prototypical fear face images with the phase component randomized before classifying morphed face images. Removing mid frequency components from the target images shifted classification toward fear. The same shift was observed under adaptation condition to unfiltered and low- and middle-range filtered fear images. However, when the phase spectrum of the same adaptation stimuli was randomized, no adaptation effect was observed. These results suggest that medium SF components support the perception of fear more than anger at both low and high level of processing. They also suggest that the effect at high-level processing stage is related more to high-level featural and/or configural information than to the low-level frequency spectrum.

FIAEs in Famous Faces are Mediated by Type of Processing

An important question regarding face aftereffects is whether it is based on face-specific or lower-level mechanisms. One method for addressing this is to explore how adaptation in upright or inverted, photographic positive or negative faces transfers to test stimuli that are either upright or inverted and normal or negated. A series of studies are reported in which this is tested using a typical face identity aftereffect paradigm in unfamiliar and famous faces. Results showed that aftereffects were strongest when the adaptor matched the test stimuli. In addition, aftereffects did not transfer from upright adaptors to inverted test images, but did transfer from inverted adaptors to upright test images in famous faces. However, in unfamiliar faces, a different pattern was observed. The results are interpreted in terms of how identity adaptation interacts with low-level adaptation and highlight differences in the representation of famous and unfamiliar faces.

Neural adaptation to thin and fat bodies in the fusiform body area and middle occipital gyrus: an fMRI adaptation study

Visual perception can be strongly biased due to exposure to specific stimuli in the environment, often causing neural adaptation and visual aftereffects. In this study, we investigated whether adaptation to certain body shapes biases the perception of the own body shape. Furthermore, we aimed to evoke neural adaptation to certain body shapes. Participants completed a behavioral experiment (n = 14) to rate manipulated pictures of their own bodies after adaptation to demonstratively thin or fat pictures of their own bodies. The same stimuli were used in a second experiment (n = 16) using functional magnetic resonance imaging (fMRI) adaptation. In the behavioral experiment, after adapting to a thin picture of the own body participants also judged a thinner than actual body picture to be the most realistic and vice versa, resembling a typical aftereffect. The fusiform body area (FBA) and the right middle occipital gyrus (rMOG) show neural adaptation to specific body shapes while the extrastriate body area (EBA) bilaterally does not. The rMOG cluster is highly selective for bodies and perhaps body parts. The findings of the behavioral experiment support the existence of a perceptual body shape aftereffect, resulting from a specific adaptation to thin and fat pictures of one's own body. The fMRI results imply that body shape adaptation occurs in the FBA and the rMOG. The role of the EBA in body shape processing remains unclear. The results are also discussed in the light of clinical body image disturbances.

Selectivity of face aftereffects for expressions and anti-expressions

Adapting to a facial expression can alter the perceived expression of subsequently viewed faces. However, it remains unclear whether this adaptation affects each expression independently or transfers from one expression to another, and whether this transfer impedes or enhances responses to a different expression. To test for these interactions, we probed the basic expressions of anger, fear, happiness, sadness, surprise, and disgust, adapting to one expression and then testing on all six. Each expression was varied in strength by morphing it with a common neutral facial expression. Observers determined the threshold level required to correctly identify each expression, before or after adapting to a face with a neutral or intense expression. The adaptation was strongly selective for the adapting category; responses to the adapting expression were reduced, while other categories showed little consistent evidence of either suppression or facilitation. In a second experiment we instead compared adaptation to each expression and its anti-expression. The latter are defined by the physically complementary facial configuration, yet appear much more ambiguous as expressions. In this case, for most expressions the opposing faces produced aftereffects of opposite sign in the perceived expression. These biases suggest that the adaptation acts in part by shifting the perceived neutral point for the facial configuration. This is consistent with the pattern of renormalization suggested for adaptation to other facial attributes, and thus may reflect a generic level of configural coding. However, for most categories aftereffects were stronger for expressions than anti-expressions, pointing to the possible influence of an additional component of the adaptation at sites that explicitly represent facial expressions. At either level our results are consistent with other recent work in suggesting that the six expressions are defined by dimensions that are largely independently normalized by adaptation, possibly because the facial configurations conveying different expressions vary in independent ways.

Visual adaptation and face perception

The appearance of faces can be strongly affected by the characteristics of faces viewed previously. These perceptual after-effects reflect processes of sensory adaptation that are found throughout the visual system, but which have been considered only relatively recently in the context of higher level perceptual judgements. In this review, we explore the consequences of adaptation for human face perception, and the implications of adaptation for understanding the neural-coding schemes underlying the visual representation of faces. The properties of face after-effects suggest that they, in part, reflect response changes at high and possibly face-specific levels of visual processing. Yet, the form of the after-effects and the norm-based codes that they point to show many parallels with the adaptations and functional organization that are thought to underlie the encoding of perceptual attributes like colour. The nature and basis for human colour vision have been studied extensively, and we draw on ideas and principles that have been developed to account for norms and normalization in colour vision to consider potential similarities and differences in the representation and adaptation of faces.

Identity-specific face adaptation effects: evidence for abstractive face representations

The effects of selective adaptation on familiar face perception were examined. After prolonged exposure to photographs of a celebrity, participants saw a series of ambiguous morphs that were varying mixtures between the face of that person and a different celebrity. Participants judged fewer of the morphs to resemble the celebrity to which they had been adapted, implying that they were now less sensitive to that particular face. Similar results were obtained when the adapting faces were highly dissimilar in viewpoint to the test morphs; when they were presented upside-down; or when they were vertically stretched to three times their normal height. These effects rule out explanations of adaptation effects solely in terms of low-level image-based adaptation. Instead they are consistent with the idea that relatively viewpoint-independent, person-specific adaptation occurred, at the level of either the "Face Recognition Units" or "Person Identity Nodes" in Burton, Bruce and Johnston's (1990) model of face recognition.

Emotional perception: meta-analyses of face and natural scene processing

Functional imaging studies of emotional processing typically contain neutral control conditions that serve to remove simple effects of visual perception, thus revealing the additional emotional process. Here we seek to identify similarities and differences across 100 studies of emotional face processing and 57 studies of emotional scene processing, using a coordinate-based meta-analysis technique. The overlay of significant meta-analyses resulted in extensive overlap in clusters, coupled with offset and unique clusters of reliable activity. The area of greatest overlap is the amygdala, followed by regions of medial prefrontal cortex, inferior frontal/orbitofrontal cortex, inferior temporal cortex, and extrastriate occipital cortex. Emotional face-specific clusters were identified in regions known to be involved in face processing, including anterior fusiform gyrus and middle temporal gyrus, and emotional scene studies were uniquely associated with lateral occipital cortex, as well as pulvinar and the medial dorsal nucleus of the thalamus. One global result of the meta-analysis reveals that a class of visual stimuli (faces vs. scenes) has a considerable impact on the resulting emotion effects, even after removing the basic visual perception effects through subtractive contrasts. Pure effects of emotion may thus be difficult to remove for the particular class of stimuli employed in an experimental paradigm. Whether a researcher chooses to tightly control the various elements of the emotional stimuli, as with posed face photographs, or allow variety and environmental realism into their evocative stimuli, as with natural scenes, will depend on the desired generalizability of their results.

Neural correlates of high-level adaptation-related aftereffects

Prolonged exposure to complex stimuli, such as faces, biases perceptual decisions toward nonadapted, dissimilar stimuli, leading to contrastive aftereffects. Here we tested the neural correlates of this perceptual bias using a functional magnetic resonance imaging adaptation (fMRIa) paradigm. Adaptation to a face or hand stimulus led to aftereffects by biasing the categorization of subsequent ambiguous face/hand composite stimuli away from the adaptor category. The simultaneously observed fMRIa in the face-sensitive fusiform face area (FFA) and in the body-part-sensitive extrastriate body area (EBA) depended on the behavioral response of the subjects: adaptation to the preferred stimulus of the given area led to larger signal reduction during trials when it biased perception than during trials when it was less effective. Activity in two frontal areas correlated positively with the activity patterns in FFA and EBA. Based on our novel adaptation paradigm, the results suggest that the adaptation-induced aftereffects are mediated by the relative activity of category-sensitive areas of the human brain as demonstrated by fMRI.

Network changes in the transition from initial learning to well-practiced visual categorization

Visual categorization is a remarkable ability that allows us to effortlessly identify objects and efficiently respond to our environment. The neural mechanisms of how visual categories become well-established are largely unknown. Studies of initial category learning implicate a network of regions that include inferior temporal cortex (ITC), medial temporal lobe (MTL), basal ganglia (BG), premotor cortex (PMC) and prefrontal cortex (PFC). However, how these regions change with extended learning is poorly characterized. To understand the neural changes in the transition from initially learned to well-practiced categorization, we used functional MRI and compared brain activity and functional connectivity when subjects performed an initially learned categorization task (100 trials of training) and a well-practiced task (4250 trials of training). We demonstrate that a similar network is implicated for initially learned and well-practiced categorization. Additionally, connectivity analyses reveal an increased coordination between ITC, MTL, and PMC when making category judgments during the well-practiced task. These results suggest that category learning involves an increased coordination between a distributed network of regions supporting retrieval and representation of categories.

Learning to attend: effects of practice on information selection

Though practice can lead to improved performance in many domains, it is currently unknown how practice affects the deployment of selective attention to filter distracting information. We conducted a series of experiments to address this issue by examining how performance on a task changed after repeated exposure to distractors. Distraction initially slowed response time during task performance, an effect that diminished with repeated exposure to the distractors. When the distractors were consistent in appearance, the practice effect developed quickly but was stimulus-specific. When the distractors were more variable in appearance, the practice effect developed slowly but transferred more readily to other conditions. These data indicate that practice with overcoming distraction leads to improvements in information filtering mechanisms that generalize beyond the training regimen when variable distractor stimuli are experienced.

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.

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.

Morphing rhesus monkey vocalizations

The capability to systematically morph between different types of animal vocalizations will give us insights into how the features of vocal sounds are perceived by listening individuals. Following behavioral study, neurophysiological recordings in nonhuman animals, could reveal how neurons support the perception of communication signals. Signal processing algorithms are now available for creating sophisticated morphs between complex sounds, like human speech. However, most morphing approaches have been applied to harmonic sounds whose frequency components can be readily identified. We show that auditory morphing can be more generally applied by describing a procedure for using the STRAIGHT signal processing package to gradually morph between: (1) vocalizations from different macaque monkeys, (2) acoustically dissimilar types of monkey vocalizations, such as a 'coo' and a 'grunt', and (3) monkey and human vocalizations. We then evaluated the quality of the morphs and obtained classification curves from human listeners who seemed to categorize the monkey vocalizations much like the ones produced by humans. The outlined procedures prepare macaque-monkey vocalizations for neuroethological study and the approach establishes basic principles that will assist in creating suitable morphs of other natural sounds and animal vocalizations.