Neuronal mechanisms of perceptual learning: changes in human brain activity with training in orientation discrimination

Modeling mechanisms of perceptual learning with augmented Hebbian re-weighting

Using the external noise plus training paradigm, we have consistently found that two independent mechanisms, stimulus enhancement and external noise exclusion, support perceptual learning in a range of tasks. Here, we show that re-weighting of stable early sensory representations through Hebbian learning (Petrov et al., 2005, 2006) can generate performance patterns that parallel a large range of empirical data: (1) perceptual learning reduced contrast thresholds at all levels of external noise in peripheral orientation identification (Dosher & Lu, 1998, 1999), (2) training with low noise exemplars transferred to performance in high noise, while training with exemplars embedded in high external noise transferred little to performance in low noise (Dosher & Lu, 2005), and (3) pre-training in high external noise only reduced subsequent learning in high external noise, whereas pre-training in zero external noise left very little additional learning in all the external noise conditions (Lu et al., 2006). In the augmented Hebbian re-weighting model (AHRM), perceptual learning strengthens or maintains the connections between the most closely tuned visual channels and a learned categorization structure, while it prunes or reduces inputs from task-irrelevant channels. Reducing the weights on irrelevant channels reduces the contributions of external noise and additive internal noise. Manifestation of stimulus enhancement or external noise exclusion depends on the initial state of internal noise and connection weights in the beginning of a learning task. Both mechanisms reflect re-weighting of stable early sensory representations.

Electrophysiological correlates of learning-induced modulation of visual motion processing in humans

Training on a visual task leads to increased perceptual and neural responses to visual features that were attended during training as well as decreased responses to neglected distractor features. However, the time course of these attention-based modulations of neural sensitivity for visual features has not been investigated before. Here we measured event related potentials (ERP) in response to motion stimuli with different coherence levels before and after training on a speed discrimination task requiring object-based attentional selection of one of the two competing motion stimuli. We found that two peaks on the ERP waveform were modulated by the strength of the coherent motion signal; the response amplitude associated with motion directions that were neglected during training was smaller than the response amplitude associated with motion directions that were attended during training. The first peak of motion coherence-dependent modulation of the ERP responses was at 300 ms after stimulus onset and it was most pronounced over the occipitotemporal cortex. The second peak was around 500 ms and was focused over the parietal cortex. A control experiment suggests that the earlier motion coherence-related response modulation reflects the extraction of the coherent motion signal whereas the later peak might index accumulation and readout of motion signals by parietal decision mechanisms. These findings suggest that attention-based learning affects neural responses both at the sensory and decision processing stages.

ERP evidence for distinct mechanisms of fast and slow visual perceptual learning

Perceptual learning (PL) occurs not only within the first training session but also between sessions. Once acquired, the learning effects can last for a long time. By examining the time course of learning-associated ERP changes, this study explores whether fast and slow visual PL contribute to long-term preservation. Subjects first participated in a visual task for three training sessions, and were then given one test session six months later. ERP results showed that fast learning effects, as reflected by the decrement of posterior N1 and increment of posterior P2 within session 1, were preserved in session 3 but not in the test session. However, slow learning effects, as reflected by the increment of posterior N1 and decrement of frontal P170 between sessions 1 and 3, were retained completely in the test session. This study indicates that PL induces different changes in the human adult brain during and after active training, and only the delayed changes of brain activity can be preserved for a long period of six months.

The effect of short-term training on cardinal and oblique orientation discrimination: an ERP study

The adult brain shows remarkable plasticity, as demonstrated by the improvement in most visual discrimination tasks after intensive practice. However, previous studies have demonstrated that practice improved the discrimination only around oblique orientations, while performance around cardinal orientations (vertical or horizontal orientations) remained stable despite extensive training. The two experiments described here used event-related potentials (ERPs) to investigate the neural substrates underlying different training effects in the two kinds of orientation. Event-related potentials were recorded from subjects when they were trained with a grating orientation discrimination task. Psychophysical threshold measurements were performed before and after the training. For oblique gratings, psychophysical thresholds decreased significantly across training sessions. ERPs showed larger P2 and P3 amplitudes and smaller N1 amplitudes over the parietal/occipital areas with more practice. In line with the psychophysical thresholds, the training effect on the P2 and P3 was specific to stimulus orientation. However, the N1 effect was generalized over differently oriented gratings stimuli. For cardinally oriented gratings, no significant changes were found in the psychophysical thresholds during the training. ERPs still showed similar generalized N1 effect as the oblique gratings. However, the amplitudes of P2 and P3 were unchanged during the whole training. Compared with cardinal orientations, more visual processing stages and later ERP components were involved in the training of oblique orientation discrimination. These results contribute to understanding the neural basis of the asymmetry between cardinal and oblique orientation training effects.

Visual learning for perceptual and categorical decisions in the human brain

Successful actions and interactions in the complex environments we inhabit entail making fast and optimal perceptual decisions. Extracting the key features from our sensory experiences and deciding how to interpret them is a computationally challenging task that is far from understood. Accumulating evidence suggests that the brain may solve this challenge by combining sensory information and previous knowledge about the environment acquired through evolution, development, and everyday experience. Here, we review the role of visual learning and experience-dependent plasticity in shaping decisions. We propose that learning plays an important role in translating sensory experiences to decisions and actions by shaping neural representations across cortical circuits in a task-dependent manner.

Shared mechanisms of perceptual learning and decision making

Perceptual decisions require the brain to weigh noisy evidence from sensory neurons to form categorical judgments that guide behavior. Here we review behavioral and neurophysiological findings suggesting that at least some forms of perceptual learning do not appear to affect the response properties of neurons that represent the sensory evidence. Instead, improved perceptual performance results from changes in how the sensory evidence is selected and weighed to form the decision. We discuss the implications of this idea for possible sites and mechanisms of training-induced improvements in perceptual processing in the brain.

Practice-related improvement in working memory is modulated by changes in processing external interference

Working memory (WM) performance is impaired by the presence of external interference. Accordingly, more efficient processing of intervening stimuli with practice may lead to enhanced WM performance. To explore the role of practice on the impact that interference has on WM performance, we studied young adults with electroencephalographic (EEG) recordings as they performed three motion-direction, delayed-recognition tasks. One task was presented without interference, whereas two tasks introduced different types of interference during the interval of memory maintenance: distractors and interruptors. Distractors were to be ignored, whereas interruptors demanded attention based on task instructions for a perceptual discrimination. We show that WM performance was disrupted by both types of interference, but interference-induced disruption abated across a single experimental session through rapid learning. WM accuracy and response time improved in a manner that was correlated with changes in early neural measures of interference processing in visual cortex (i.e., P1 suppression and N1 enhancement). These results suggest practice-related changes in processing interference exert a positive influence on WM performance, highlighting the importance of filtering irrelevant information and the dynamic interactions that exist between neural processes of perception, attention, and WM during learning.

Hebbian Reweighting on Stable Representations in Perceptual Learning

Perceptual learning is the improvement in perceptual task performance with practice or training. The observation of specificity in perceptual learning has been widely associated with plasticity in early visual cortex representations. Here, we review the evidence supporting the plastic reweighting of readout from stable sensory representations, originally proposed by Dosher & Lu (1998), as an alternative explanation of perceptual learning. A task-analysis that identifies circumstances in which specificity supports representation enhancement and those in which it implies reweighting provides a framework for evaluating the literature; reweighting is broadly consistent with the behavioral results and almost all of the physiological reports. We also consider the evidence that the primary mode of perceptual learning is through augmented Hebbian learning of the reweighted associations, which has implications for the role and importance of feedback. Feedback is not necessary for perceptual learning, but can improve it in some circumstances, and in some cases block feedback is also helpful - all effects that are generally compatible with an augmented Hebbian model (Petrov, Dosher, & Lu, 2005). The two principles of perceptual learning through reweighting evidence from stable sensory representations and of augmented Hebbian learning provide a theoretical structure for the consideration of issues such as task difficulty, task roving, and cuing in perceptual learning.

MECHANISMS OF PERCEPTUAL LEARNING

What is learned in perceptual learning? How does perceptual learning change the perceptual system? We investigate these questions using a systems analysis of the perceptual system during the course of perceptual learning using psychophysical methods and models of the observer. Effects of perceptual learning on an observer's performance are characterized by external noise tests within the framework of noisy observer models. We find evidence that two independent mechanisms, external noise exclusion and stimulus enhancement support perceptual learning across a range of tasks. We suggest that both mechanisms may reflect re-weighting of stable early sensory representations.

Neural signatures of phonetic learning in adulthood: a magnetoencephalography study

The present study used magnetoencephalography (MEG) to examine perceptual learning of American English /r/ and /l/ categories by Japanese adults who had limited English exposure. A training software program was developed based on the principles of infant phonetic learning, featuring systematic acoustic exaggeration, multi-talker variability, visible articulation, and adaptive listening. The program was designed to help Japanese listeners utilize an acoustic dimension relevant for phonemic categorization of /r-l/ in English. Although training did not produce native-like phonetic boundary along the /r-l/ synthetic continuum in the second language learners, success was seen in highly significant identification improvement over twelve training sessions and transfer of learning to novel stimuli. Consistent with behavioral results, pre-post MEG measures showed not only enhanced neural sensitivity to the /r-l/ distinction in the left-hemisphere mismatch field (MMF) response but also bilateral decreases in equivalent current dipole (ECD) cluster and duration measures for stimulus coding in the inferior parietal region. The learning-induced increases in neural sensitivity and efficiency were also found in distributed source analysis using Minimum Current Estimates (MCE). Furthermore, the pre-post changes exhibited significant brain-behavior correlations between speech discrimination scores and MMF amplitudes as well as between the behavioral scores and ECD measures of neural efficiency. Together, the data provide corroborating evidence that substantial neural plasticity for second-language learning in adulthood can be induced with adaptive and enriched linguistic exposure. Like the MMF, the ECD cluster and duration measures are sensitive neural markers of phonetic learning.

A review of the generalization of auditory learning

The ability to detect and discriminate attributes of sounds improves with practice. Determining how such auditory learning generalizes to stimuli and tasks that are not encountered during training can guide the development of training regimens used to improve hearing abilities in particular populations as well as provide insight into the neural mechanisms mediating auditory performance. Here we review the newly emerging literature on the generalization of auditory learning, focusing on behavioural investigations of generalization on basic auditory tasks in human listeners. The review reveals a variety of generalization patterns across different trained tasks that can not be summarized with a simple rule, and a diversity of views about the definition, evaluation and interpretation of generalization.

Item-specific Training Reduces Prefrontal Cortical Involvement in Perceptual Awareness

Previous studies on the neural correlates of perceptual awareness implicate sensory-specific regions and higher cortical regions such as the prefrontal cortex (PFC) in this process. The specific role of PFC regions is, however, unknown. PFC activity could be bottom-up driven, integrating signals from sensory regions. Alternatively, PFC regions could serve more active top-down processes that help to define the content of consciousness. To compare these alternative views of PFC function, we used functional magnetic resonance imaging and measured brain activity specifically related to conscious perception of items that varied in ease of identification (by being presented 0, 12, or 60 times previously). A bottom-up account predicts that PFC activity would be largely insensitive to stimulus difficulty, whereas a top-down account predicts reduced PFC activity as identification becomes easier. The results supported the latter prediction by showing reduced activity for previously presented compared to novel items in the PFC and several other regions. This was further confirmed by a functional connectivity analysis showing that the interaction between frontal and visual sensory regions declined as a function of ease of identification. Given the attribution of top-down processing to PFC regions in combination with the marked decline in PFC activity for easy items, these findings challenge the prevailing notion that the PFC is necessary for consciousness.

Different dynamics of performance and brain activation in the time course of perceptual learning

Perceptual learning is regarded as a manifestation of experience-dependent plasticity in the sensory systems, yet the underlying neural mechanisms remain unclear. We measured the dynamics of performance on a visual task and brain activation in the human primary visual cortex (V1) across the time course of perceptual learning. Within the first few weeks of training, brain activation in a V1 subregion corresponding to the trained visual field quadrant and task performance both increased. However, while performance levels then saturated and were maintained at a constant level, brain activation in the corresponding areas decreased to the level observed before training. These findings indicate that there are distinct temporal phases in the time course of perceptual learning, related to differential dynamics of BOLD activity in visual cortex.

Perceptual learning and dynamic changes in primary visual cortex

Perceptual learning is the improved performance that follows practice in a perceptual task. In this issue of Neuron, Yotsumoto et al. use fMRI to show that stimuli presented at the location used in training initially evoke greater activation in primary visual cortex than stimuli presented elsewhere, but this difference disappears once learning asymptotes.

The neural substrates of visual perceptual learning of words: implications for the visual word form area hypothesis

Abstract It remains under debate whether the fusiform visual word form area (VWFA) is specific to visual word form and whether visual expertise increases its sensitivity (Xue et al., 2006; Cohen et al., 2002). The present study examined three related issues: (1) whether the VWFA is also involved in processing foreign writing that significantly differs from the native one, (2) the effect of visual word form training on VWFA activation after controlling the task difficulty, and (3) the transfer of visual word form learning. Eleven native English speakers were trained, during five sessions, to judge whether two subsequently flashed (100-msec duration with 200-msec interval) foreign characters (i.e., Korean Hangul) were identical or not. Visual noise was added to the stimuli to manipulate task difficulty. In functional magnetic resonance imaging scans before and after training, subjects performed the task once with the same noise level (i.e., parameter-matched scan) and once with noise level changed to match performance from pretraining to posttraining (i.e., performance-matched scan). Results indicated that training increased the accuracy in parameter-matched condition but remained constant in performance-matched condition (because of increasing task difficulty). Pretraining scans revealed stronger activation for English words than for Korean characters in the left inferior temporal gyrus and the left inferior frontal cortex, but not in the VWFA. Visual word form training significantly decreased the activation in the bilateral middle and left posterior fusiform when either parameters or performance were matched and for both trained and new items. These results confirm our conjecture that the VWFA is not dedicated to words, and visual expertise acquired with training reduces rather than increases its activity.

Activations in visual and attention-related areas predict and correlate with the degree of perceptual learning

Repeated experience with a visual stimulus can result in improved perception of the stimulus, i.e., perceptual learning. To understand the underlying neural mechanisms of this process, we used functional magnetic resonance imaging to track brain activations during the course of training on a contrast discrimination task. Based on their ability to improve on the task within a single scan session, subjects were separated into two groups: "learners" and "nonlearners." As learning progressed, learners showed progressively reduced activation in both visual cortex, including Brodmann's areas 18 and 19 and the fusiform gyrus, and several cortical regions associated with the attentional network, namely, the intraparietal sulcus (IPS), frontal eye field (FEF), and supplementary eye field. Among learners, the decrease in brain activations in these regions was highly correlated with the magnitude of performance improvement. Unlike learners, nonlearners showed no changes in brain activations during training. Learners showed stronger activation than nonlearners during the initial period of training in all these brain regions, indicating that one could predict from the initial activation level who would learn and who would not. In addition, over the course of training, the functional connectivity between IPS and FEF in the right hemisphere with early visual areas was stronger for learners than nonlearners. We speculate that sharpened tuning of neuronal representations may cause reduced activation in visual cortex during perceptual learning and that attention may facilitate this process through an interaction of attention-related and visual cortical regions.

Neural systems for perceptual skill learning

Recent studies have begun to use functional neuroimaging techniques to examine the changes in brain activity that occur as humans learn new skills. This review outlines results from a number of imaging studies examining visual perceptual skill learning. Although the regions engaged during skill learning differ across tasks, a common finding has been increasing activation in the inferior temporal and fusiform gyri as skill is acquired and activation of the caudate nucleus in association with learning. Neuroimaging has great promise for the understanding of learning at the level of large neural populations, but further work is necessary to understand the specificity of learning-related changes and their relation to underlying neurophysiological plasticity.

Brain activity associated with stimulation therapy of the visual borderzone in hemianopic stroke patients

BACKGROUND AND OBJECTIVE

Visual restoration therapy is a home-based treatment program intended to expand visual fields of hemianopic patients through repetitive stimulation of the borderzone adjacent to the blind field. We hypothesized that the training itself would induce visual field location-specific changes in the brain's response to stimuli, a phenomenon demonstrated in animal experiments but never in humans with brain injury.

METHODS

Six chronic right hemianopic patients underwent functional magnetic resonance imaging (fMRI)--responding to stimuli in the trained visual borderzone versus the nontrained seeing field before and after 1 month of visual restoration therapy. Spatially normalized fMRI time-series data were analyzed in a fixed-effects group analysis comparing blood oxygen level dependent (BOLD) activity in the borderzone versus seeing location at baseline and at 1 month. Percent BOLD change was measured to determine each condition's contribution to the time-by-condition interaction.

RESULTS

There was a significant time by condition interaction manifested as increased BOLD activity for borderzone detection relative to seeing detection after the first month of therapy, which correlated with a relative improvement in response times in the borderzone location out-of-scanner. The right inferior and lateral temporal, right dorsolateral frontal, bilateral anterior cingulate, and bilateral basal ganglia showed the greatest response.

CONCLUSION

Visual restoration therapy appears to induce an alteration in brain activity associated with a shift of attention from the nontrained seeing field to the trained borderzone. The effect appears to be mediated by the anterior cingulate and dorsolateral frontal cortex in conjunction with other higher order visual areas in the occipitotemporal and middle temporal regions. Demonstration of a visual field-specific training effect on brain activity provides an important starting point for understanding the potential for visual therapy in hemianopia.

An event-related potential study on perceptual learning in grating orientation discrimination

This study aims to investigate the neural mechanism of visual perceptual learning in grating orientation discrimination. We recorded event-related potentials of human adults when they were trained with a grating orientation discrimination task. Although psychophysical thresholds decreased significantly across training sessions, event-related potentials showed larger P2 and P3 amplitudes and smaller N1 amplitudes over the parietal/occipital areas with more practice. In line with the psychophysical thresholds, the training effect on the P2 and P3 were specific to stimulus orientation. The N1 effect, however, was generalized over differently oriented gratings. Our findings show that several stages of visual processing are involved in perceptual learning and provide an illustration of the temporal relationship between specific and generalized perceptual learning in human adults.

Discrimination training alters object representations in human extrastriate cortex

Visual object recognition relies critically on learning. However, little is known about the effect of object learning in human visual cortex, and in particular how the spatial distribution of training effects relates to the distribution of object and face selectivity across the cortex before training. We scanned human subjects with high-resolution functional magnetic resonance imaging (fMRI) while they viewed novel object classes, both before and after extensive training to discriminate between exemplars within one of these object classes. Training increased the strength of the response in visual cortex to trained objects compared with untrained objects. However, training did not simply induce a uniform increase in the response to trained objects: the magnitude of this training effect varied substantially across subregions of extrastriate cortex, with some showing a twofold increase in response to trained objects and others (including the right fusiform face area) showing no significant effect of training. Furthermore, the spatial distribution of training effects could not be predicted from the spatial distribution of either pretrained responses or face selectivity. Instead, training changed the spatial distribution of activity across the cortex. These findings support a dynamic view of the ventral visual pathway in which the cortical representation of an object category is continuously modulated by experience.

Language experience shapes fusiform activation when processing a logographic artificial language: An fMRI training study

The significant role of the left midfusiform cortex in reading found in recent neuroimaging studies has led to the visual word form area (VWFA) hypothesis. This hypothesis suggests that years of experience reading native language change the visual expertise of this region to be especially sensitive to the visual form of native language. The present study aimed at testing this hypothesis by exploring the role of language experience in shaping the fusiform activation. We designed a logographic artificial language (LAL) using the visual form and pronunciation of Korean Hangul characters (but their correspondence was shuffled) and assigning arbitrary meanings to these characters. Twelve native Chinese Mandarin speakers (6 male and 6 female, 18 to 21 years old) with no prior knowledge of Korean language were trained in the visual form of these characters for 2 weeks, followed by 2 weeks each of phonological and semantic training. Behavioral data indicated that training was effective in increasing the efficiency of visual form processing and establishing the connections among visual form, sounds, and meanings. Imaging data indicated that at the pre-training stage, subjects showed stronger activation in the fusiform regions for LAL than for Chinese across both one-back visual matching task and the passive viewing task. Visual form training significantly decreased the activation of bilateral fusiform cortex and the left inferior occipital cortex, whereas phonological training increased activation in these regions, and the right fusiform remained more active after semantic training. Increased activations after phonological and semantic training were also evident in other regions involved in language processing. These findings thus do not seem to be consistent with the visual-expertise-induced-sensitivity hypothesis about fusiform regions. Instead, our results suggest that visual familiarity, phonological processing, and semantic processing all make significant but different contributions to shaping the fusiform activation.

Learning and neural plasticity in visual object recognition

The capability of the adult primate visual system for rapid and accurate recognition of targets in cluttered, natural scenes far surpasses the abilities of state-of-the-art artificial vision systems. Understanding this capability remains a fundamental challenge in visual neuroscience. Recent experimental evidence suggests that adaptive coding strategies facilitated by underlying neural plasticity enable the adult brain to learn from visual experience and shape its ability to integrate and recognize coherent visual objects.

Shape-specific perceptual learning in a figure-ground segregation task

What does perceptual experience contribute to figure-ground segregation? To study this question, we trained observers to search for symmetric dot patterns embedded in random dot backgrounds. Training improved shape segmentation, but learning did not completely transfer either to untrained locations or to untrained shapes. Such partial specificity persisted for a month after training. Interestingly, training on shapes in empty backgrounds did not help segmentation of the trained shapes in noisy backgrounds. Our results suggest that perceptual training increases the involvement of early sensory neurons in the segmentation of trained shapes, and that successful segmentation requires perceptual skills beyond shape recognition alone.

The dynamics of perceptual learning: an incremental reweighting model

The mechanisms of perceptual learning are analyzed theoretically, probed in an orientation-discrimination experiment involving a novel nonstationary context manipulation, and instantiated in a detailed computational model. Two hypotheses are examined: modification of early cortical representations versus task-specific selective reweighting. Representation modification seems neither functionally necessary nor implied by the available psychophysical and physiological evidence. Computer simulations and mathematical analyses demonstrate the functional and empirical adequacy of selective reweighting as a perceptual learning mechanism. The stimulus images are processed by standard orientation- and frequency-tuned representational units, divisively normalized. Learning occurs only in the "read-out" connections to a decision unit; the stimulus representations never change. An incremental Hebbian rule tracks the task-dependent predictive value of each unit, thereby improving the signal-to-noise ratio of their weighted combination. Each abrupt change in the environmental statistics induces a switch cost in the learning curves as the system temporarily works with suboptimal weights.

fMRI Reveals a Common Neural Substrate of Illusory and Real Contours in V1 after Perceptual Learning

Perceptual learning involves the specific and relatively permanent modification of perception following a sensory experience. In psychophysical experiments, the specificity of the learning effects to the trained stimulus attributes (e.g., visual field position or stimulus orientation) is often attributed to assumed neural modifications at an early cortical site within the visual processing hierarchy. We directly investigated a neural correlate of perceptual learning in the primary visual cortex using fMRI. Twenty volunteers practiced a curvature discrimination on Kanizsa-type illusory contours in the MR scanner. Practice-induced changes in the BOLD response to illusory contours were compared between the pretraining and the posttraining block in those areas of the primary visual cortex (V1) that, in the same session, had been identified to represent real contours at corresponding visual field locations. A retinotopically specific BOLD signal increase to illusory contours was observed as a consequence of the training, possibly signaling the formation of a contour representation, which is necessary for performing the curvature discrimination. The effects of perceptual training were maintained over a period of about 10 months, and they were specific to the trained visual field position. The behavioral specificity of the learning effects supports an involvement of V1 in perceptual learning, and not in unspecific attentional effects.

Neural changes in the ventral and dorsal visual streams during pattern recognition learning

The learning process related to pattern and object recognition is difficult to study because the human brain has a remarkable capacity to recognise complex visual forms from early infancy. In the present study, we investigated on-going neural changes underlying the learning process of visual pattern recognition by means of a device substituting audition for vision. Functional MRI evidenced the gradual pattern recognition-induced recruitment of the ventral visual stream, bilaterally, from learning session 1 to session 3, and a slight decrease in these activation foci from session 3 to session 4. The initial increase in activation is thought to reflect the gradually enhanced visualisation of patterns in the subjects' mind across sessions. By contrast the subsequent decrease reported at the end of the training period is interpreted as the progressive optimisation of neuronal responses elicited by the task. Our results, in accordance with previous observations, suggest that the succession of activation increase and decrease in sensori-motor areas could be a general rule in sensory and sensori-motor learning.

Top-down reorganization of activity in the visual pathway after learning a shape identification task

Learning in shape identification led to global changes in activation across the entire visual pathway, as revealed with whole-brain fMRI. Following extensive training in a shape identification task, brain activity associated with trained shapes relative to the untrained shapes showed: (1) an increased level of activity in retinotopic cortex (RC), (2) a decrease in activation of the lateral occipital cortex (LO), and (3) a decrease in the dorsal attentional network. In addition, RC activations became more correlated (and LO activation, less correlated) with performance. When comparing target-present and target-absent trials within the trained condition, we observed a similar decrease in the dorsal attentional network but not in the visual cortices. These findings indicate a large-scale reorganization of activity in the visual pathway as a result of learning, with the RC becoming more involved (and the LO, less involved) and that these changes are triggered in a top-down manner depending on the perceptual task performed.

Distributed neural plasticity for shape learning in the human visual cortex

Expertise in recognizing objects in cluttered scenes is a critical skill for our interactions in complex environments and is thought to develop with learning. However, the neural implementation of object learning across stages of visual analysis in the human brain remains largely unknown. Using combined psychophysics and functional magnetic resonance imaging (fMRI), we show a link between shape-specific learning in cluttered scenes and distributed neuronal plasticity in the human visual cortex. We report stronger fMRI responses for trained than untrained shapes across early and higher visual areas when observers learned to detect low-salience shapes in noisy backgrounds. However, training with high-salience pop-out targets resulted in lower fMRI responses for trained than untrained shapes in higher occipitotemporal areas. These findings suggest that learning of camouflaged shapes is mediated by increasing neural sensitivity across visual areas to bolster target segmentation and feature integration. In contrast, learning of prominent pop-out shapes is mediated by associations at higher occipitotemporal areas that support sparser coding of the critical features for target recognition. We propose that the human brain learns novel objects in complex scenes by reorganizing shape processing across visual areas, while taking advantage of natural image correlations that determine the distinctiveness of target shapes.

Learning to recognize objects involves distinct neural changes in several visual cortical areas.

Learning to see biological motion: brain activity parallels behavior

Individuals improve with practice on a variety of perceptual tasks, presumably reflecting plasticity in underlying neural mechanisms. We trained observers to discriminate biological motion from scrambled (nonbiological) motion and examined whether the resulting improvement in perceptual performance was accompanied by changes in activation within the posterior superior temporal sulcus and the fusiform ''face area,'' brain areas involved in perception of biological events. With daily practice, initially naive observers became more proficient at discriminating biological from scrambled animations embedded in an array of dynamic ''noise'' dots, with the extent of improvement varying among observers. Learning generalized to animations never seen before, indicating that observers had not simply memorized specific exemplars. In the same observers, neural activity prior to and following training was measured using functional magnetic resonance imaging. Neural activity within the posterior superior temporal sulcus and the fusiform ''face area'' reflected the participants' learning: BOLD signals were significantly larger after training in response both to animations experienced during training and to novel animations. The degree of learning was positively correlated with the amplitude changes in BOLD signals.

The nature of learned categorical perception effects: a psychophysical approach

Categorical perception is often cited as a striking example of cognitive influences on perception. However, some evidence suggests the term is a misnomer, with effects based on cognitive not perceptual processing. Here, using a psychophysical approach, we provide evidence consistent with a learned categorical perception effect that is dependent on analysis within the visual processing stream. An improvement in participants' discrimination between grating patterns that they had learned to place in different categories was 'tuned' around the orientation of the patterns experienced during category learning. Thus, here, categorical perception may result from attentionally modulated perceptual learning about diagnostic category features, based upon orientation-selective stages of analysis. This argues strongly that category learning can alter our perception of the world.

Neural substrates of visual perceptual learning of simple and complex stimuli

OBJECTIVE

The two experiments described here used event-related potentials (ERPs) to investigate whether perceptual learning of different complexities of stimuli involves different levels of visual cortical processing in human adults.

METHODS

Reaction times and ERPs were recorded during 3 consecutive training sessions in which subjects discriminated between simple stimuli made of line segments or complex stimuli made of compound shapes.

RESULTS

Reaction times in both experiments were shortened across training sessions. For simple stimuli, training resulted in a decreased N1 (125-155ms) and an increased P2 (180-240ms) over the occipital area. For complex stimuli, however, training resulted in a decreased N1 (125-155ms) and N2 (290-340ms) and an increased P3 (350-550ms) over the central/parietal areas.

CONCLUSIONS

These findings suggest that perceptual learning modifies the response at different levels of visual cortical processing related to the complexity of the stimulus.

SIGNIFICANCE

The neuronal mechanisms involved in perceptual learning may depend on the nature (e.g. the complexity) of the stimuli used in the discrimination task.

Human Functional Neuroimaging of Brain Changes Associated with Practice

The discovery that experience-driven changes in the human brain can occur from a neural to a cortical level throughout the lifespan has stimulated a proliferation of research into how neural function changes in response to experience, enabled by neuroimaging methods such as positron emission tomography and functional magnetic resonance imaging. Studies attempt to characterize these changes by examining how practice on a task affects the functional anatomy underlying performance. Results are incongruous, including patterns of increases, decreases and functional reorganization of regional activations. Following an extensive review of the practice-effects literature, we distinguish a number of factors affecting the pattern of practice effects observed, including the effects of task domain, changes at the level of behavioural and cognitive processes, the time-window of imaging and practice, and of a number of other influences and miscellaneous confounding factors. We make a novel distinction between patterns of reorganization and redistribution as effects of task practice on brain activation, and emphasize the need for careful attention to practice-related changes occurring on the behavioural, cognitive and neural levels of analysis. Finally, we suggest that functional and effective connectivity analyses may make important contributions to our understanding of changes in functional anatomy occurring as a result of practice on tasks.

Perceptual learning of line orientation modifies the effects of transcranial magnetic stimulation of visual cortex

Perceptual learning may be accompanied by physiological changes in early visual cortex. We used transcranial magnetic stimulation (TMS) to test the postulate that perceptual learning of a visual task initially performed at 60-65% accuracy strengthens visual processing in early visual cortex. Single pulse TMS was delivered to human occipital cortex at time delays of 70-154 ms after the onset of an odd-element, line orientation discrimination task. When TMS was delivered at a delay of 84 ms or later the accuracy of visual discrimination was transiently degraded in ten subjects. As visual performance in control trials without TMS improved with training, the absolute magnitude of TMS suppression of performance decreased in parallel. This result occurred both when TMS was delivered to broad areas of occipital cortex and when TMS was optimally delivered to early occipital cortex. No change in TMS suppression was observed when three new subjects were given feedback during an odd-element task that did not require substantial perceptual learning. Thus, perceptual learning improved visual performance and reduced TMS suppression of early visual cortex in parallel.

Learning strengthens the response of primary visual cortex to simple patterns

Training can significantly improve performance on even the most basic visual tasks, such as detecting a faint patch of light or determining the orientation of a bar (for reviews, see ). The neural mechanisms of visual learning, however, remain controversial. One simple way to improve behavior is to increase the overall neural response to the trained stimulus by increasing the number or gain of responsive neurons. Learning of this type has been observed in other sensory modalities, where training increases the number of receptive fields that cover the trained stimulus. Here, we show that visual learning can selectively increase the overall response to trained stimuli in primary visual cortex (V1). We used functional magnetic resonance imaging (fMRI) to measure neural signals before and after one month of practice at detecting very low-contrast oriented patterns. Training increased V1 response for practiced orientations relative to control orientations by an average of 39%, and the magnitude of the change in V1 correlated moderately well with the magnitude of changes in detection performance. The elevation of V1 activity by training likely results from an increase in the number of neurons responding to the trained stimulus or an increase in response gain.

Neuronal Bases of Perceptual Learning Revealed by a Synaptic Balance Scheme

Our ability to perceive external sensory stimuli improves as we experience the same stimulus repeatedly. This perceptual enhancement, called perceptual learning, has been demonstrated for various sensory systems, such as vision, audition, and somatosensation. I investigated the contribution of lateral excitatory and inhibitory synaptic balance to perceptual learning. I constructed a simple associative neural network model in which sensory features were expressed by the activities of specific cell assemblies. Each neuron is sensitive to a specific sensory feature, and the neurons belonging to the same cell assembly are sensitive to the same feature. During perceptual learning processes, the network was presented repeatedly with a stimulus that was composed of a sensory feature and noise, and the lateral excitatory and inhibitory synaptic connection strengths between neurons were modified according to a pulse-timing-based Hebbian rule. Perceptual learning enhanced the cognitive performance of the network, increasing the signal-to-noise ratio of neuronal activity. I suggest here that the alteration of the synaptic balance may be essential for perceptual learning, especially when the brain tries to adopt the most suitable strategy—signal enhancement, noise reduction, or both—for a given perceptual task.

Perceptual learning improves efficiency by re-tuning the decision 'template' for position discrimination

Visual position discrimination improves with practice; however, the mechanism(s) underlying this improvement are not yet known. We used positional noise to explore the underlying neural mechanisms and found that position discrimination improved with practice over a range of noise levels. This improvement can be largely explained by an increasing efficiency with which observers used positional information in the stimulus. In a second experiment, we tested the hypothesis that the improved efficiency reflects a re-tuning of the observers' perceptual 'template'--the weightings of inputs from basic visual mechanisms--to more closely match the ideal template required to perform the perceptual task. Using a new technique to measure which parts of the stimulus influenced the observer's performance, we were able to record the re-tuning of the decision template across training sessions; we found a robust and steady increase in template efficiency during learning.

Cerebral regions processing first- and higher-order motion in an opposed-direction discrimination task

Using PET, we studied the processing of different types of motion in an opposed-direction discrimination task. We used first-order motion and two types of higher-order motion (presented as moving gratings with stripes defined by flickering texture and kinetic boundaries, respectively). In these experiments, we found that all types of motion activate a common set of cortical regions when comparing a direction discrimination task to a detection of the dimming of the fixation point. This set includes left hV3A, bilateral hMT/V5+ and regions in the middle occipital gyrus, bilateral activations in the posterior and anterior parts of the intraparietal sulcus, bilateral precentral gyrus, medial frontal cortex and regions in the cerebellum. No significant differences were observed between different types of motion, even at low statistical thresholds. From this we conclude that, under our experimental conditions, the same cerebral regions are involved in the processing of first-order and higher-order motion in an opposed-direction discrimination task.

Specificity and generalization of visual perceptual learning in humans: an event-related potential study

To investigate the neural correlates of specificity and generalization of visual perceptual learning, we recorded event-related potentials from human adults when they were trained with a simple visual discrimination task. While reaction times decreased significantly across training sessions, event-related potentials showed larger P2 amplitudes ( approximately 210 ms) over the left occipital/parietal areas and smaller N1 amplitudes ( approximately 140 ms) at the left parietal site with more practice. Similar to reaction times, the training effect on the P2 amplitudes was specific to stimulus orientation. However, the N1 effect was generalized over differently oriented stimuli. These results indicated the complexity of the neural substance underlying perceptual learning, relative to behavioral level.

Neural correlates of perceptual learning: a functional MRI study of visual texture discrimination

Visual texture discrimination has been shown to induce long-lasting behavioral improvement restricted to the trained eye and trained location in visual field [Karni, A. & Sagi, D. (1991) Proc. Natl. Acad. Sci. USA 88, 4966-4970]. We tested the hypothesis that such learning involves durable neural modifications at the earliest cortical stages of the visual system, where eye specificity, orientation, and location information are mapped with highest resolution. Using functional magnetic resonance imaging in humans, we measured neural activity 24 h after a single session of intensive monocular training on visual texture discrimination, performed in one visual quadrant. Within-subject comparisons between trained and untrained eye for targets presented within the same quadrant revealed higher activity in a corresponding retinotopic area of visual cortex. Functional connectivity analysis showed that these learning-dependent changes were not associated with an increased engagement of other brain areas remote from early visual cortex. We suggest that these new data are consistent with recent proposals that the cellular mechanisms underlying this type of perceptual learning may involve changes in local connections within primary visual cortex. Our findings provide a direct demonstration of learning-dependent reorganization at early processing stages in the visual cortex of adult humans.

An event-related potential study on visual perceptual learning under short-term and long-term training conditions

The present study investigated learning-induced changes of event-related potentials in a visual discrimination task, focusing on making a comparison between short-term and long-term learning perceptual processes. Event-related potentials were recorded from two groups of human adults. One group (short-term training group) was given 1.5 h of training in a single day, and another group (long-term training group) was given the same training procedure (1.5 hour of training) on each of 3 consecutive days. The results demonstrated that for short-term training group, along with the reduction of reaction times, the amplitudes of N1 and N2 negativities over the central/parietal areas decreased during the training. For long-term training group, however, after long-term training, the N2 effect disappeared and the N1 effect occurred over the posterior areas. In addition, the amplitudes of N2 for long-term training group were less than those for short-term training group. Our results suggest that neural activity depends not only on perceptual mechanisms and on the parameters of the physical stimuli but also on the extent of the observer's previous learning.