Perceptual learning modifies inversion effects for faces and textures

Adaptively triggered comparisons enhance perceptual category learning: evidence from face learning

Categorical learning is important and often challenging in both specialized domains, such as medical image interpretation, and commonplace ones, such as face recognition. Research has shown that comparing items from different categories can enhance the learning of perceptual classifications, particularly when those categories appear highly similar. Here, we developed and tested novel adaptively triggered comparisons (ATCs), in which errors produced during interactive learning dynamically prompted the presentation of active comparison trials. In a facial identity paradigm, undergraduate participants learned to recognize and name varying views of 22 unknown people. In Experiment 1, single-item classification trials were compared to a condition in which ATC trials were generated whenever a participant repeatedly confused two faces. Comparison trials required discrimination between simultaneously presented exemplars from the confused categories. In Experiment 2, an ATC condition was compared to a non-adaptive comparison condition. Participants learned to accuracy and speed criteria, and completed immediate and delayed posttests. ATCs substantially enhanced learning efficiency in both experiments. These studies, using a novel adaptive procedure guided by each learner's performance, show that adaptively triggered comparisons improve category learning.

The time course of stimulus-specific perceptual learning

Practice on perceptual tasks can lead to long-lasting, stimulus-specific improvements. Rapid stimulus-specific learning, assessed 24 hours after practice, has been found with just 105 practice trials in a face identification task. However, a much longer time course for stimulus-specific learning has been found in other tasks. Here, we examined 1) whether rapid stimulus-specific learning occurs for unfamiliar, non-face stimuli in a texture identification task; 2) the effects of varying practice across a range from just 21 trials up to 840 trials; and 3) if rapid, stimulus-specific learning persists over a 1-week, as well as a 1-day, interval. Observers performed a texture identification task in two sessions separated by one day (Experiment 1) or 1 week (Experiment 2). Observers received varying amounts of practice (21, 63, 105, or 840 training trials) in session 1 and completed 840 trials in session 2. In session 2, one-half of the observers in each group performed the task with the same textures as in session 1, and one-half switched to novel textures (same vs. novel conditions). In both experiments we found that stimulus-specific learning - defined as the difference in response accuracy in the same and novel conditions - increased as a linear function of the log number of session 1 training trials and was statistically significant after approximately 100 training trials. The effects of stimulus novelty did not differ across experiments. These results support the idea that stimulus-specific learning in our task arises gradually and continuously through practice, perhaps concurrently with general learning.

Profiles of visual perceptual learning in feature space

Visual perceptual learning (VPL), experience-induced gains in discriminating visual features, has been studied extensively and intensively for many years, its profile in feature space, however, remains unclear. Here, human subjects were trained to perform either a simple low-level feature (grating orientation) or a complex high-level object (face view) discrimination task over a long-time course. During, immediately after, and one month after training, all results showed that in feature space VPL in grating orientation discrimination was a center-surround profile; VPL in face view discrimination, however, was a monotonic gradient profile. Importantly, these two profiles can be emerged by a deep convolutional neural network with a modified AlexNet consisted of 7 and 12 layers, respectively. Altogether, our study reveals for the first time a feature hierarchy-dependent profile of VPL in feature space, placing a necessary constraint on our understanding of the neural computation of VPL.

The effect of training on sensitivity and stability of double fusion in Panum's limiting case

Panum's limiting case is a phenomenon of monocular occlusion in binocular vision. This occurs when one object is occluded by the other object for one eye, but the two objects are both visible for the other eye. Although previous studies have found that vertical gradient of horizontal disparity and cue conflict are two important factors for double fusion, the effect of training on the sensitivity and stability of Panum's limiting case remains unknown. The current study trained 26 participants for 5 days with several of Panum's configurations (Gilliam, Frisby, and Wang series). The latency and duration of double fusion were recorded to examine the effects of training on sensitivity and stability of double fusion in Panum's limiting case. For each level of vertical gradient of horizontal disparity and cue conflict, the latency of double fusion decreased and the duration of double fusion increased with each additional training session. The results showed that vertical gradient of horizontal disparity and cue conflict interacted, and the duration of high cue conflict was significantly shorter than that of medium and low cue conflict for each level of vertical gradient of horizontal disparity. The findings suggest that there is an effect of training for vertical gradient of horizontal disparity and cue conflict in Panum's limiting case, and that the three factors jointly affect the sensitivity and stability of double fusion.

The development of upright face perception depends on evolved orientation-specific mechanisms and experience

Here we examine whether our impressive ability to perceive upright faces arises from evolved orientation-specific mechanisms, our extensive experience with upright faces, or both factors. To do so, we tested Claudio, a man with a congenital joint disorder causing his head to be rotated back so that it is positioned between his shoulder blades. As a result, Claudio has seen more faces reversed in orientation to his own face than matched to it. Controls exhibited large inversion effects on all tasks, but Claudio performed similarly with upright and inverted faces in both detection and identity-matching tasks, indicating these abilities are the product of evolved mechanisms and experience. In contrast, he showed clear upright superiority when detecting "Thatcherized" faces (faces with vertically flipped features), suggesting experience plays a greater role in this judgment. Together, these findings indicate that both evolved orientation-specific mechanisms and experience contribute to our proficiency with upright faces.

Behavioral and neural markers of visual configural processing in social scene perception

Research on face perception has revealed highly specialized visual mechanisms such as configural processing, and provided markers of interindividual differences -including disease risks and alterations- in visuo-perceptual abilities that traffic in social cognition. Is face perception unique in degree or kind of mechanisms, and in its relevance for social cognition? Combining functional MRI and behavioral methods, we address the processing of an uncharted class of socially relevant stimuli: minimal social scenes involving configurations of two bodies spatially close and face-to-face as if interacting (hereafter, facing dyads). We report category-specific activity for facing (vs. non-facing) dyads in visual cortex. That activity shows face-like signatures of configural processing -i.e., stronger response to facing (vs. non-facing) dyads, and greater susceptibility to stimulus inversion for facing (vs. non-facing) dyads-, and is predicted by performance-based measures of configural processing in visual perception of body dyads. Moreover, we observe that the individual performance in body-dyad perception is reliable, stable-over-time and correlated with the individual social sensitivity, coarsely captured by the Autism-Spectrum Quotient. Further analyses clarify the relationship between single-body and body-dyad perception. We propose that facing dyads are processed through highly specialized mechanisms -and brain areas-, analogously to other biologically and socially relevant stimuli such as faces. Like face perception, facing-dyad perception can reveal basic (visual) processes that lay the foundations for understanding others, their relationships and interactions.

Neural sensitivity to faces is increased by immersion into a novel ethnic environment: Evidence from ERPs

Previous reports suggest that East-Asians may show larger face-elicited N170 components in the ERP as compared to Caucasian participants. Since the N170 can be modulated by perceptual expertise, such group differences may be accounted for by differential experience, for example, with logographic versus alphabetic scripts (script system hypothesis) or by exposure to abundant novel faces during the immersion into a new social and/or ethnic environment (social immersion hypothesis). We conducted experiments in Hong Kong and Berlin, recording ERPs in a series of one-back tasks, using same- and other-ethnicity face stimuli in upright and inverted orientation and doodle stimuli. In Hong Kong we tested local Chinese residents and foreign guest students who could not read the logographic script; in Berlin we tested German residents who could not read the logographic script and foreign Chinese visitors. In both experiments, we found significantly larger N170 amplitudes to faces, regardless of ethnicity, in the foreign than in the local groups. Moreover, this effect did not depend on stimulus orientation, suggesting that the N170 group differences do not reflect differences in configural visual processing. A group of short-term German residents in Berlin did not differ in N170 amplitude from long-term residents. Together, these findings indicate that the extensive confrontation with novel other-ethnicity faces during immersion in a foreign culture may enhance the neural response to faces, reflecting the short-term plasticity of the underlying neural system.

Neurocognitive effects of a training program for poor face recognizers using shape and texture caricatures: A pilot investigation

Recent research suggested disproportional usage of shape information by people with poor face recognition, although texture information appears to be more important for familiar face recognition. Here, we tested a training program with faces that were selectively caricatured in either shape or texture parameters. Forty-eight young adults with poor face recognition skills (1 SD below the mean in at least 2/3 face processing tests: CFMT, GFMT, BFFT) were pseudo-randomly assigned to either one of two training groups or a control group (n = 16 each). Training comprised six sessions over three weeks. Per session, participants studied ten unfamiliar facial identities whose shape or texture characteristics were caricatured. Before and after training (or waiting in the control group), all participants completed EEG experiments on face learning and famous face recognition, and behavioral face processing tests. Results showed small but specific training-induced improvements: Whereas shape training improved face matching (training tasks, and to some extent GFMT), texture training elicited marked improvements in face learning (CFMT). Moreover, for the texture training group the N170 ERP was enhanced for novel faces post-training, suggesting training-induced changes in early markers of face processing. Although further research is necessary, this suggests that parameter-specific caricature training is a promising way to improve performance in people with poor face recognition skills.

Effects of High-Definition Transcranial Direct Current Stimulation Over the Left Fusiform Face Area on Face View Discrimination Depend on the Individual Baseline Performance

A basic human visual function is to identify objects from different viewpoints. Typically, the ability to discriminate face views based on in-depth orientation is necessary in daily life. Early neuroimaging studies have identified the involvement of the left fusiform face area (FFA) and the left superior temporal sulcus (STS) in face view discrimination. However, many studies have documented the important role of the right FFA in face processing. Thus, there remains controversy over whether one specific region or all of them are involved in discriminating face views. Thus, this research examined the influence of high-definition transcranial direct current stimulation (HD-tDCS) over the left FFA, left STS or right FFA on face view discrimination in three experiments. In experiment 1, eighteen subjects performed a face view discrimination task before and immediately, 10 min and 20 min after anodal, cathodal and sham HD-tDCS (20 min, 1.5 mA) over the left FFA in three sessions. Compared with sham stimulation, anodal and cathodal stimulation had no effects that were detected at the group level. However, the analyses at the individual level showed that the baseline performance negatively correlated with the degree of change after anodal tDCS, suggesting a dependence of the change amount on the initial performance. Specifically, tDCS decreased performance in the subjects with better baseline performance but increased performance in those with poorer baseline performance. In experiments 2 and 3, the same experimental protocol was used except that the stimulation site was the left STS or right FFA, respectively. Neither anodal nor cathodal tDCS over the left STS or right FFA influenced face view discrimination in group- or individual-level analyses. These results not only indicated the importance of the left FFA in face view discrimination but also demonstrated that individual initial performance should be taken into consideration in future research and practical applications.

The Effect of Perceptual Learning on Face Recognition in Individuals with Central Vision Loss

Purpose

To examine whether perceptual learning can improve face discrimination and recognition in older adults with central vision loss.

Methods

Ten participants with age-related macular degeneration (ARMD) received 5 days of training on a face discrimination task (mean age, 78 ± 10 years). We measured the magnitude of improvements (i.e., a reduction in threshold size at which faces were able to be discriminated) and whether they generalized to an untrained face recognition task. Measurements of visual acuity, fixation stability, and preferred retinal locus were taken before and after training to contextualize learning-related effects. The performance of the ARMD training group was compared to nine untrained age-matched controls (8 = ARMD, 1 = juvenile macular degeneration; mean age, 77 ± 10 years).

Results

Perceptual learning on the face discrimination task reduced the threshold size for face discrimination performance in the trained group, with a mean change (SD) of –32.7% (+15.9%). The threshold for performance on the face recognition task was also reduced, with a mean change (SD) of –22.4% (+2.31%). These changes were independent of changes in visual acuity, fixation stability, or preferred retinal locus. Untrained participants showed no statistically significant reduction in threshold size for face discrimination, with a mean change (SD) of –8.3% (+10.1%), or face recognition, with a mean change (SD) of +2.36% (–5.12%).

Conclusions

This study shows that face discrimination and recognition can be reliably improved in ARMD using perceptual learning. The benefits point to considerable perceptual plasticity in higher-level cortical areas involved in face-processing. This novel finding highlights that a key visual difficulty in those suffering from ARMD is readily amenable to rehabilitation.

Training face perception in developmental prosopagnosia through perceptual learning

BACKGROUND

Recent work has shown that perceptual learning can improve face discrimination in subjects with acquired prosopagnosia.

OBJECTIVE

In this study, we administered the same program to determine if such training would improve face perception in developmental prosopagnosia.

METHOD

We trained ten subjects with developmental prosopagnosia for several months with a program that required shape discrimination between morphed facial images, using a staircase procedure to keep training near each subject's perceptual threshold. To promote ecological validity, training progressed from blocks of neutral faces in frontal view through increasing variations in view and expression. Five subjects did 11 weeks of a control television task before training, and the other five were re-assessed for maintenance of benefit 3 months after training.

RESULTS

Perceptual sensitivity for faces improved after training but did not improve after the control task. Improvement generalized to untrained expressions and views of these faces, and there was some evidence of transfer to new faces. Benefits were maintained over three months. Training also led to improvements on standard neuropsychological tests of short-term familiarity, and some subjects reported positive effects in daily life.

CONCLUSION

We conclude that perceptual learning can lead to persistent improvements in face discrimination in developmental prosopagnosia. The strong generalization suggests that learning is occurring at the level of three-dimensional representations with some invariance for the dynamic effects of expression.

Enhancement of face-sensitive ERPs in older adults induced by face recognition training

A common cognitive problem reported by older people is compromised face recognition, which is often paralleled by age-related changes in face-sensitive and memory-related components in event-related brain potentials (ERPs). We developed a new training using photorealistic caricatures based on evidence that caricatures are beneficial for people with compromised face processing. Twenty-four older participants (62-75 yrs, 13 female) completed 12 training sessions (3 per week, 60 min each) and 24 older participants (61-76 yrs, 12 female) acted as controls. Before and after training (or waiting), participants took part in a diagnostic test battery for face processing abilities, and in ERP experiments on face learning and recognition. Although performance improvements during the training provided little evidence for generalization to other face processing tasks, ERPs showed substantial training-related enhancements of face-sensitive ERPs. Specifically, we observed marked increases of the N170, P200 and N250 components, which may indicate training-induced enhancement of face detection and activation of identity-specific representations. Thus, neuronal correlates of face processing are plastic in older age, and can be modulated by caricature training.

The Bandwidth of Diagnostic Horizontal Structure for Face Identification

Horizontally oriented spatial frequency components are a diagnostic source of face identity information, and sensitivity to this information predicts upright identification accuracy and the magnitude of the face-inversion effect. However, the bandwidth at which this information is conveyed, and the extent to which human tuning matches this distribution of information, has yet to be characterized. We designed a 10-alternative forced choice face identification task in which upright or inverted faces were filtered to retain horizontal or vertical structure. We systematically varied the bandwidth of these filters in 10° steps and replaced the orientation components that were removed from the target face with components from the average of all possible faces. This manipulation created patterns that looked like faces but contained diagnostic information in orientation bands unknown to the observer on any given trial. Further, we quantified human performance relative to the actual information content of our face stimuli using an ideal observer with perfect knowledge of the diagnostic band. We found that the most diagnostic information for face identification is conveyed by a narrow band of orientations along the horizontal meridian, whereas human observers use information from a wide range of orientations.

Perceptual Learning of Facial Expressions

Perceptual learning is a phenomenon in which intensive training for a perceptual task may lead to significant improvement in the task performance. So far, the characteristics of the perceptual learning of facial expressions have not been investigated. In the current study, we trained subjects to distinguish facial expressions. With eight days of training, the subjects' discrimination performance improved significantly, and this improvement was generalized to faces with the same expression but different gender as the trained face. In the second experiment, we further examined the transfer of the learning effect between faces with different expression intensities. We found that the learning effect of happiness can be transferred from the high-intensity face to the low-intensity face, but the reverse was not true. Importantly, in all experiments, we measured the performance immediately after training and one month after training. The results showed that all learning effects and transfers were able to persist for at least one month, which implied that these findings revealed the long-term mechanisms of training. These results revealed the characteristics of facial expression learning and shed light on the mechanisms of perceptual learning for high-level vision.

The perception of a familiar face is no more than the sum of its parts

Why do faces become easier to recognize with repeated exposure? Previous research has suggested that familiarity may induce a qualitative shift in visual processing from an independent analysis of individual facial features to analysis that includes information about the relationships among features (Farah, Wilson, Drain, & Tanaka Psychological Review, 105, 482-498, 1998; Maurer, Grand, & Mondloch Trends in Cognitive Science, 6, 255-260, 2002). We tested this idea by using a "summation-at-threshold" technique (Gold, Mundy, & Tjan Psychological Science, 23, 427-434, 2012; Nandy & Tjan Journal of Vision, 8, 3.1-20, 2008), in which an observer's ability to recognize each individual facial feature shown independently is used to predict their ability to recognize all of the features shown in combination. We find that, although people are better overall at recognizing familiar as opposed to unfamiliar faces, their ability to integrate information across features is similar for unfamiliar and highly familiar faces and is well predicted by their ability to recognize each of the facial features shown in isolation. These results are consistent with the idea that familiarity has a quantitative effect on the efficiency with which information is extracted from individual features, rather than a qualitative effect on the process by which features are combined.

The consolidation of a motor skill in young adults with ADHD: Shorter practice can be better

Practice on a given sequence of movements can lead to robust procedural memory (skill). In young adults, in addition to gains in performance accrued during practice, speed and accuracy can further improve overnight; the latter, delayed, 'offline', gains are thought to emerge when procedural memory consolidation processes are completed. A recent study suggested that female college students with ADHD show an atypical procedural memory consolidation phase, specifically, gaining speed but losing accuracy, overnight. Here, to test if this accuracy loss reflected a cost of overlong training in adults with ADHD, we compared the performance of female college students with (N=16) and without (N=16) ADHD, both groups given a shorter training protocol (80 rather than the standard 160 task repetitions). Speed and accuracy were recorded before training, immediately after, and at 24-h and 2 weeks post-training. The shortened practice session resulted in as robust within-session gains and additional overnight gains in speed at no costs in accuracy, in both groups. Moreover, individuals with ADHD showed as robust speed gains and retention as in the longer training session, but the costs in accuracy incurred in the latter were eliminated. The shortening of practice sessions may benefit motor skill acquisition in ADHD.

Learning faces from variability

Research on face learning has tended to use sets of images that vary systematically on dimensions such as pose and illumination. In contrast, we have proposed that exposure to naturally varying images of a person may be a critical part of the familiarization process. Here, we present two experiments investigating face learning with "ambient images"-relatively unconstrained photos taken from internet searches. Participants learned name and face associations for unfamiliar identities presented in high or low within-person variability-that is, images of the same person returned by internet search on their name (high variability) versus different images of the same person taken from the same event (low variability). In Experiment 1 we show more accurate performance on a speeded name verification task for identities learned in high than in low variability, when the test images are completely novel photos. In Experiment 2 we show more accurate performance on a face matching task for identities previously learned in high than in low variability. The results show that exposure to a large range of within-person variability leads to enhanced learning of new identities.

Face learning with multiple images leads to fast acquisition of familiarity for specific individuals

Matching unfamiliar faces is a difficult task. Here we ask whether it is possible to improve performance by providing multiple images to support matching. In two experiments we observe that accuracy improves as viewers are provided with additional images on which to base their match. This technique leads to fast learning of an individual, but the effect is identity-specific: Despite large improvements in viewers’ ability to match a particular person's face, these improvements do not generalize to other faces. Experiment 2 demonstrated that trial-by-trial feedback provided no additional benefits over the provision of multiple images. We discuss these results in terms of familiar and unfamiliar face processing and draw out some implications for training regimes.

Task-irrelevant emotion facilitates face discrimination learning

We understand poorly how the ability to discriminate faces from one another is shaped by visual experience. The purpose of the present study is to determine whether face discrimination learning can be facilitated by facial emotions. To answer this question, we used a task-irrelevant perceptual learning paradigm because it closely mimics the learning processes that, in daily life, occur without a conscious intention to learn and without an attentional focus on specific facial features. We measured face discrimination thresholds before and after training. During the training phase (4 days), participants performed a contrast discrimination task on face images. They were not informed that we introduced (task-irrelevant) subtle variations in the face images from trial to trial. For the Identity group, the task-irrelevant features were variations along a morphing continuum of facial identity. For the Emotion group, the task-irrelevant features were variations along an emotional expression morphing continuum. The Control group did not undergo contrast discrimination learning and only performed the pre-training and post-training tests, with the same temporal gap between them as the other two groups. Results indicate that face discrimination improved, but only for the Emotion group. Participants in the Emotion group, moreover, showed face discrimination improvements also for stimulus variations along the facial identity dimension, even if these (task-irrelevant) stimulus features had not been presented during training. The present results highlight the importance of emotions for face discrimination learning.

Feedback training for facial image comparison

People are typically poor at matching the identity of unfamiliar faces from photographs. This observation has broad implications for face matching in operational settings (e.g., border control). Here, we report significant improvements in face matching ability following feedback training. In Experiment 1, we show cumulative improvement in performance on a standard test of face matching ability when participants were provided with trial-by-trial feedback. More important, Experiment 2 shows that training benefits can generalize to novel, widely varying, unfamiliar face images for which no feedback is provided. The transfer effect specifically benefited participants who had performed poorly on an initial screening test. These findings are discussed in the context of existing literature on unfamiliar face matching and perceptual training. Given the reliability of the performance enhancement and its generalization to diverse image sets, we suggest that feedback training may be useful for face matching in occupational settings.

Can training enhance face cognition abilities in middle-aged adults?

Face cognition is a crucial skill for social interaction and shows large individual differences in healthy adults, suggesting a possibility for improvement in some. We developed and tested specific training procedures for the accuracy of face memory and the speed of face cognition. Two groups each of 20 healthy middle-aged trainees practiced for 29 daily sessions of 15 minutes duration with different computerized home-based training procedures. In addition, 20 matched and 59 non-matched controls were included. Face cognition speed training enhanced performance during the training and transferred to the latent factor level as measured in a pre-post comparison. Persistence of the training effect was evidenced at the manifest level after three months. However, the training procedure influenced the speed of processing object stimuli to the same extent as face stimuli and therefore seems to have affected a more general ability of processing complex visual stimuli and not only faces. No effects of training on the accuracy of face memory were found. This study demonstrates that face-specific abilities may be hard to improve but also shows the plasticity of the speed of processing complex visual stimuli – for the first time in middle-aged, normal adults.

Characterization of field loss based on microperimetry is predictive of face recognition difficulties

PURPOSE

To determine how visual field loss as assessed by microperimetry is correlated with deficits in face recognition.

METHODS

Twelve patients (age range, 26-70 years) with impaired visual sensitivity in the central visual field caused by a variety of pathologies and 12 normally sighted controls (control subject [CS] group; age range, 20-68 years) performed a face recognition task for blurred and unblurred faces. For patients, we assessed central visual field loss using microperimetry, fixation stability, Pelli-Robson contrast sensitivity, and letter acuity.

RESULTS

Patients were divided into two groups by microperimetry: a low vision (LV) group (n = 8) had impaired sensitivity at the anatomical fovea and/or poor fixation stability, whereas a low vision that excluded the fovea (LV:F) group (n = 4) was characterized by at least some residual foveal sensitivity but insensitivity in other retinal regions. The LV group performed worse than the other groups at all blur levels, whereas the performance of the LV:F group was not credibly different from that of the CS group. The performance of the CS and LV:F groups deteriorated as blur increased, whereas the LV group showed consistently poor performance regardless of blur. Visual acuity and fixation stability were correlated with face recognition performance.

CONCLUSIONS

Persons diagnosed as having disease affecting the central visual field can recognize faces as well as persons with no visual disease provided that they have residual sensitivity in the anatomical fovea and show stable fixation patterns. Performance in this task is limited by the upper resolution of nonfoveal vision or image blur, whichever is worse.

Implicit learning of geometric eigenfaces

The human visual system can implicitly extract a prototype of encountered visual objects (Posner & Keele, 1968). While learning a prototype provides an efficient way of encoding objects at the category level, discrimination among individual objects requires encoding of variations among them as well. Here we show that in addition to the prototype, human adults also implicitly learn the feature correlations that capture the most significant geometric variations among faces. After studying a group of synthetic faces, observers mistook as seen previously unseen faces representing the first two principal components (eigenfaces, Turk & Pentland, 1991) of the studied faces at significantly higher rates than the correct recognition of the faces actually studied. Implicit learning of the most significant eigenfaces provides an optimal way for encoding variations among faces. The data thus extend the types of summary statistics that can be implicitly extracted by the visual system to include several principal components.

The effect of spatial frequency on perceptual learning of inverted faces

We investigated the efficacy of training adults to recognize full spectrum inverted faces presented with different viewpoints. To examine the role of different spatial frequencies in any learning, we also used high-pass filtered faces that preserved featural information and low-pass filtered faces that severely reduced that featural information. Although all groups got faster over the 2 days of training, there was more improvement in accuracy for the group exposed to full spectrum faces than in the two groups exposed to filtered faces, both of which improved more modestly and only when the same faces were shown on the 2 days of training. For the group exposed to the full spectrum range and, to a lesser extent, for those exposed to high frequency faces, training generalized to a new set of full spectrum faces of a different size in a different task, but did not lead to evidence of holistic processing or improved sensitivity to feature shape or spacing in inverted faces. Overall these results demonstrate that only 2h of practice in recognizing full-spectrum inverted faces presented from multiple points of view is sufficient to improve recognition of the trained faces and to generalize to novel instances. Perceptual learning also occurred for low and high frequency faces but to a smaller extent.

Neural correlates of face gender discrimination learning

Using combined psychophysics and event-related potentials (ERPs), we investigated the effect of perceptual learning on face gender discrimination and probe the neural correlates of the learning effect. Human subjects were trained to perform a gender discrimination task with male or female faces. Before and after training, they were tested with the trained faces and other faces with the same and opposite genders. ERPs responding to these faces were recorded. Psychophysical results showed that training significantly improved subjects' discrimination performance and the improvement was specific to the trained gender, as well as to the trained identities. The training effect indicates that learning occurs at two levels-the category level (gender) and the exemplar level (identity). ERP analyses showed that the gender and identity learning was associated with the N170 latency reduction at the left occipital-temporal area and the N170 amplitude reduction at the right occipital-temporal area, respectively. These findings provide evidence for the facilitation model and the sharpening model on neuronal plasticity from visual experience, suggesting a faster processing speed and a sparser representation of face induced by perceptual learning.

The transfer of object learning across exemplars and their orientation is related to perceptual similarity

Recognition of objects improves after training. The exact characteristics of this visual learning process remain unclear. We examined to which extent object learning depends on the exact exemplar and orientation used during training. Participants were trained to name object pictures at as short a picture presentation time as possible. The required presentation time diminished over training. After training participants were tested with a completely new set of objects as well as with two variants of the trained object set, namely an orientation change and a change of the exact exemplar shown. Both manipulations led to a decrease in performance compared to the original picture set. Nevertheless, performance with the manipulated versions of the trained stimuli was better than performance with the completely new set, at least when only one manipulation was performed. Amount of transfer to new images of an object was related to perceptual similarity, but not to pixel overlap or to measurements of similarity in the different layers of a popular hierarchical object recognition model (HMAX). Thus, object learning generalizes only partially over changes in exemplars and orientation, which is consistent with the tuning properties of neurons in object-selective cortical regions and the role of perceptual similarity in these representations.

The rapid emergence of stimulus specific perceptual learning

Is stimulus specific perceptual learning the result of extended practice or does it emerge early in the time course of learning? We examined this issue by manipulating the amount of practice given on a face identification task on Day 1, and altering the familiarity of stimuli on Day 2. We found that a small number of trials was sufficient to produce stimulus specific perceptual learning of faces: on Day 2, response accuracy decreased by the same amount for novel stimuli regardless of whether observers practiced 105 or 840 trials on Day 1. Current models of learning assume early procedural improvements followed by late stimulus specific gains. Our results show that stimulus specific and procedural improvements are distributed throughout the time course of learning.

Superior identification of familiar visual patterns a year after learning

Practice improves visual performance on simple tasks in which stimuli vary along one dimension. Such learning frequently is stimulus-specific and enduring, and has been associated with plasticity in striate cortex. It is unclear if similar lasting effects occur for naturalistic patterns that vary on multiple dimensions. We measured perceptual learning in identification tasks that used faces and textures, stimuli that engage multiple stages in visual processing. Performance improved significantly across 2 consecutive days of practice. More important, the effects of practice were remarkably stable across time: Improvements were maintained approximately 1 year later, and both the relative difficulty of identifying individual stimuli and individual differences in performance were essentially constant across sessions. Finally, the effects of practice were largely stimulus-specific. Our results suggest that the characteristics of perceptual learning are similar across a spectrum of stimulus complexities.

Learning to Discriminate Face Views

Although perceptual learning of simple visual features has been studied extensively and intensively for many years, we still know little about the mechanisms of perceptual learning of complex object recognition. In a series of seven experiments, human perceptual learning in discrimination of in-depth orientation of face view was studied using psychophysical methods. We trained subjects to discriminate face orientations around a face view (i.e., 30°) over eight daily sessions, which resulted in a significant improvement in sensitivity to the face view orientation. This improved sensitivity was highly specific to the trained orientation and persisted up to 6 mo. Different from perceptual learning of simple visual features, this orientation-specific learning effect could completely transfer across changes in face size, visual field, and face identity. A complete transfer also occurred between two partial face images that were mutually exclusive but constituted a complete face. However, the transfer of the learning effect between upright and inverted faces and between a face and a paperclip object was very weak. These results shed light on the mechanisms of the perceptual learning of face view discrimination. They suggest that the visual system had learned how to compute face orientation from face configural information more accurately and that a large amount of plastic changes took place at a level of higher visual processing where size-, location-, and identity-invariant face views are represented.

Effects of aging on biological motion discrimination

Previous studies have shown that older subjects have difficulties discriminating the walking direction of point-light walkers. In two experiments, we investigated the underlying cause in further detail. In Experiment 1, subjects had to discriminate the walking direction of upright and inverted point-light walkers in a cloud of randomly moving dots. In general, older subjects performed less accurately and showed an increased inversion effect. Nevertheless, they were as accurate as young subjects for upright walkers during training, in which no noise was added to the display. These results indicate that older subjects are less able to extract relevant information from noisy displays. In Experiment 2, subjects discriminated the walking direction of scrambled walkers that primarily contained local motion information, random-position walkers that primarily contained global form information, and normal point-light walkers that contained both kinds of information. Both age groups performed at chance when no global form information was present in the display but were equally accurate for walkers that only contained global form information. However, when both local motion and global form information were present in the display, older subjects were less accurate then younger subjects. Older subjects again exhibited an increased inversion effect. These results indicate that both older and younger subjects rely more on global form than local motion to discriminate the direction of point-light walkers. Also, older subjects seem to have difficulties integrating global form and local motion information as efficiently as younger subjects.

How much practice is needed to produce perceptual learning?

We examined the amount of practice needed to improve performance on 10-AFC face- and texture identification tasks. On Day 1, subjects were grouped by amount of practice: a control group had 0 trials of practice, and several experimental groups had practice that ranged from 1 to 40 trials per condition. On Day 2, all groups performed 40 trials per condition of the trained task. The effect of practice was estimated by comparing performance across groups on Day 2. In both tasks, increasing practice was associated with greater learning, but surprisingly small amounts of practice were required to improve performance. In the face identification task, for example, only one trial per condition on Day 1 was required to increase performance relative to the control group at the start of testing on Day 2. In the texture identification task, five trials per condition on Day 1 were required to increase performance relative to the control group. In both tasks, the advantage associated with small amounts of practice declined during the Day 2 session due to larger within-session learning in the control group. Sleep had little to no effect on learning; performance depended primarily on the amount of preceding practice.