Does practice in orientation discrimination lead to changes in the response properties of macaque inferior temporal neurons?

Perceptual learning

Perceptual learning involves long-term changes to perception due to practice or experience. While perceptual learning has been studied for over a century in philosophy and psychology, research into the cognitive and neural mechanisms underlying perceptual learning remains an area of ongoing development. This review explores what perceptual learning is and how it occurs, with a focus on areas of controversy. It then turns to several current debates. First, it explores the debate as to whether such learning involves genuine perceptual change at all, rather than a change in action, attention, or decision-making. Second, it questions the role that higher-cognitive mechanisms, like attention, might play in perceptual learning. Does perceptual learning require attention, or can it occur through mere exposure in the absence of attention? Third, it examines a debate about what perceptual learning means for the perception-cognition divide. Does it blur the divide or preserve it? This article is categorized under: Philosophy > Psychological Capacities Psychology > Perception and Psychophysics Psychology > Learning.

The effect of face patch microstimulation on perception of faces and objects

What is the range of stimuli encoded by face-selective regions of the brain? We asked how electrical microstimulation of face patches in macaque inferotemporal cortex affects perception of faces and objects. We found that microstimulation strongly distorted face percepts and that this effect depended on precise targeting to the center of face patches. While microstimulation had no effect on the percept of many non-face objects, it did affect the percept of some, including non-face objects whose shape is consistent with a face (for example, apples) as well as somewhat facelike abstract images (for example, cartoon houses). Microstimulation even perturbed the percept of certain objects that did not activate the stimulated face patch at all. Overall, these results indicate that representation of facial identity is localized to face patches, but activity in these patches can also affect perception of face-compatible non-face objects, including objects normally represented in other parts of inferotemporal cortex.

The transition in the ventral stream from feature to real-world entity representations

We propose that the ventral visual pathway of human and non-human primates is organized into three levels: (1) ventral retinotopic cortex including what is known as TEO in the monkey but corresponds to V4A and PITd/v, and the phPIT cluster in humans, (2) area TE in the monkey and its homolog LOC and neighboring fusiform regions, and more speculatively, (3) TGv in the monkey and its possible human equivalent, the temporal pole. We attribute to these levels the visual representations of features, partial real-world entities (RWEs), and known, complete RWEs, respectively. Furthermore, we propose that the middle level, TE and its homolog, is organized into three parallel substreams, lower bank STS, dorsal convexity of TE, and ventral convexity of TE, as are their corresponding human regions. These presumably process shape in depth, 2D shape and material properties, respectively, to construct RWE representations.

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 neural basis of visual object learning

Object vision in human and nonhuman primates is often cited as a primary example of adult plasticity in neural information processing. It has been hypothesized that visual experience leads to single neurons in the monkey brain with strong selectivity for complex objects, and to regions in the human brain with a preference for particular categories of highly familiar objects. This view suggests that adult visual experience causes dramatic local changes in the response properties of high-level visual cortex. Here, we review the current neurophysiological and neuroimaging evidence and find that the available data support a different conclusion: adult visual experience introduces moderate, relatively distributed effects that modulate a pre-existing, rich and flexible set of neural object representations.

Perceptual learning and sensomotor flexibility: cortical plasticity under attentional control?

Recent research reveals long-lasting cortical plasticity of early sensory cortices even in adults. Sensory signals could be modified under top-down control if necessary quite early in order to optimize their signal-to-noise ratio, leading to 'low level' or 'early' perceptual learning (PL). For easy tasks, such elaborate top-down influences are usually not required, and learning is restricted to late selection of the appropriate signals on higher cortical levels, which seems easier and faster to achieve. But to reach the absolute limits of sensory performance, PL seems to optimize the entire chain of sensory processing. Hence, improvement for these extreme perceptual abilities is quite specific for a number of stimulus parameters, such as the position in the visual field and sometimes even the trained eye, reflecting the specificity of receptive fields in early sensory cortices. Early PL may be just one example--even if a very extensive one--of the mechanisms of neuronal plasticity and sensomotor flexibility that are constantly updating our sensomotor representations as a result of experience. As an illustration, this review contains some new experimental results on PL and sensory flexibility in the context of adaptation to multifocal intraocular lenses.

Cortical mechanisms of sensory learning and object recognition

Learning about the world through our senses constrains our ability to recognise our surroundings. Experience shapes perception. What is the neural basis for object recognition and how are learning-induced changes in recognition manifested in neural populations? We consider first the location of neurons that appear to be critical for object recognition, before describing what is known about their function. Two complementary processes of object recognition are considered: discrimination among diagnostic object features and generalization across non-diagnostic features. Neural plasticity appears to underlie the development of discrimination and generalization for a given set of features, though tracking these changes directly over the course of learning has remained an elusive task.

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.

The reverse hierarchy theory of visual perceptual learning

Perceptual learning can be defined as practice-induced improvement in the ability to perform specific perceptual tasks. We previously proposed the Reverse Hierarchy Theory as a unifying concept that links behavioral findings of visual learning with physiological and anatomical data. Essentially, it asserts that learning is a top-down guided process, which begins at high-level areas of the visual system, and when these do not suffice, progresses backwards to the input levels, which have a better signal-to-noise ratio. This simple concept has proved powerful in explaining a broad range of findings, including seemingly contradicting data. We now extend this concept to describe the dynamics of skill acquisition and interpret recent behavioral and electrophysiological findings.

The effect of perceptual learning on neuronal responses in monkey visual area V4

Previous studies have shown that perceptual learning can substantially alter the response properties of neurons in the primary somatosensory and auditory cortices. Although psychophysical studies suggest that perceptual learning induces similar changes in primary visual cortex (V1), studies that have measured the response properties of individual neurons have failed to find effects of the size described for the other sensory systems. We have examined the effect of learning on neuronal response properties in a visual area that lies at a later stage of cortical processing, area V4. Adult macaque monkeys were trained extensively on orientation discrimination at a specific retinal location using a narrow range of orientations. During the course of training, the subjects achieved substantial improvement in orientation discrimination that was primarily restricted to the trained location. After training, neurons in V4 with receptive fields overlapping the trained location had stronger responses and narrower orientation tuning curves than neurons with receptive fields in the opposite, untrained hemifield. The changes were most prominent for neurons that preferred orientations close to the trained range of orientations. These results provide the first demonstration of perceptual learning modifying basic neuronal response properties at an intermediate level of visual cortex and give insights into the distribution of plasticity across adult visual cortex.

Impact of learning on representation of parts and wholes in monkey inferotemporal cortex

Here we investigated the impact of visual discrimination training on neuronal responses to parts of images and to whole images in inferotemporal (IT) cortex. Monkeys were trained to discriminate among 'baton' stimuli consisting of discrete top and bottom parts joined by a vertical stem. With separate features at each end, we were able to manipulate the two parts of each baton independently. After training the monkeys, we used single-cell recording to compare neuronal responses to learned and unlearned batons. Responses to learned batons, though not enhanced in strength, were enhanced in selectivity for both individual parts and for whole batons. Whole-baton selectivity arose from a form of conjunctive encoding whereby two parts together exerted a greater influence on neuronal activity than predicted by the additive influence of each part considered individually. These results indicate a possible neural mechanism for holistic or configural effects in expert versus novice observers.

Physiological correlates of perceptual learning in monkey V1 and V2

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation. During training the monkeys' discrimination thresholds gradually improved to much better than those of naive monkeys or humans. Although this improvement was specific to the trained orientation, it showed little retinotopic specificity. The receptive field properties of single neurons from regions representing the trained location and a location in the opposite visual hemifield were measured in V1 and V2. In most respects the receptive field properties in the representations of the trained and untrained regions were indistinguishable. However, in the regions of V1 and V2 representing the trained location, there were slightly fewer neurons whose optimal orientation was near the trained orientation. This resulted in a small but significant decrease in the V1 population response to the trained orientation at the trained location. Consequently, the observed neuronal populations did not exhibit any orientation-specific biases sufficient to explain the orientation specificity of the behavioral improvement. Pooling models suggest that the behavioral improvement was accomplished with a task-dependent and orientation-selective pooling of unaltered signals from early visual neurons. These data suggest that, even for training with stimuli suited to the selectivities found in early areas of visual cortex, behavioral improvements can occur in the absence of pronounced changes in the physiology of those areas.

Complex sound processing during human REM sleep by recovering information from long-term memory as revealed by the mismatch negativity (MMN)

Perceptual learning is thought to be the result of neural changes that take place over a period of several hours or days, allowing information to be transferred to long-term memory. Evidence suggests that contents of long-term memory may improve attentive and pre-attentive sensory processing. Therefore, it is plausible to hypothesize that learning-induced neural changes that develop during wakefulness could improve automatic information processing during human REM sleep. The MMN, an objective measure of the automatic change detection in auditory cortex, was used to evaluate long-term learning effects on pre-attentive processing during wakefulness and REM sleep. When subjects learned to discriminate two complex auditory patterns in wakefulness, an increase in the MMN was obtained in both wake and REM states. The automatic detection of the infrequent complex auditory pattern may therefore be improved in both brain states by reactivating information from long-term memory. These findings suggest that long-term learning-related neural changes are accessible during REM sleep as well.

Spatial sensitivity of macaque inferior temporal neurons

Recent findings in dorsal visual stream areas and computational work raise the question whether neurons at the end station of the ventral visual stream can code for stimulus position. The authors provide the first detailed, quantitative data on the spatial sensitivity of neurons in the anterior part of the inferior temporal cortex (area TE) in awake, fixating monkeys. They observed a large variation in receptive field (RF) size (ranging from 2.8 degrees to 26 degrees ). TE neurons differed in their optimal position, with a bias toward the foveal position. Moreover, the RF profiles of most TE neurons could be fitted well with a two-dimensional Gaussian function. Most neurons had only one region of high sensitivity and showed a smooth decline in sensitivity toward more distal positions. In addition, the authors investigated some of the possible determinants of such spatial sensitivity. First, testing with low-pass filtered versions of the stimuli revealed that the general preference for the foveal position and the size of the RFs was not due simply to TE neurons receiving input with a lower spatial resolution at more eccentric positions. The foveal position was still preferred after intense low-pass filtering. Second, although an increase in stimulus size consistently broadened spatial sensitivity profiles, it did not change the qualitative features of these profiles. Moreover, size selectivity of TE neurons was generally position invariant. Overall, the results suggest that TE neurons can code for the position of stimuli in the central region of the visual field.

Effects of visual experience on the representation of objects in the prefrontal cortex

The perception and recognition of objects are improved by experience. Here, we show that monkeys' ability to recognize degraded objects was improved by several days of practice with these objects. This improvement was reflected in the activity of neurons in the prefrontal (PF) cortex, a brain region critical for a wide range of visual behaviors. Familiar objects activated fewer neurons than did novel objects, but these neurons were more narrowly tuned, and the object representation was more resistant to the effects of degradation, after experience. These results demonstrate a neural correlate of visual learning in the PF cortex of adult monkeys.

Mirror-image confusion in single neurons of the macaque inferotemporal cortex

Humans and animals confuse lateral mirror images, such as the letters "b" and "d," more often than vertical mirror images, such as the letters "b" and "p." Experiments were performed to find a neural correlate of this phenomenon. Visually responsive pattern-selective neurons in the inferotemporal cortex of macaque monkeys responded more similarly to members of a lateral mirror-image pair than to members of a vertical mirror-image pair. The phenomenon developed within 20 milliseconds of the onset of the visual response and persisted to its end. It occurred during presentation of stimuli both at the fovea and in the periphery.

Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.

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

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

Effects of shape-discrimination training on the selectivity of inferotemporal cells in adult monkeys

Through extensive training, humans can become "visual experts, " able to visually distinguish subtle differences among similar objects with greater ease than those who are untrained. To understand the neural mechanisms behind this acquired discrimination ability, adult monkeys were fully trained to discriminate 28 moderately complex shapes. The training effects on the stimulus selectivity of cells in area TE of the inferotemporal cortex were then examined in anesthetized preparations. Area TE represents a later stage of the ventral visual cortical pathway that is known to mediate visual object discrimination and recognition. The recordings from the trained monkeys and untrained controls showed that the proportion of TE cells responsive to some member of the 28 stimuli was significantly greater in the trained monkeys than that in the control monkeys. Cell responses recorded from the trained monkeys were not sharply tuned to single training stimuli, but rather broadly covered several training stimuli. The distances among the training stimuli in the response space spanned by responses of the recorded TE cells were significantly greater in the trained monkeys than those in the control monkeys. The subset of training stimuli to which individual cells responded differed from cell to cell with only partial overlaps, suggesting that the cells responded to features common to several stimuli. These results are consistent with a model in which visual expertise is acquired through the development of differential responses by inferotemporal cells to the images of relevant objects.

The neuronal machinery involved in successive orientation discrimination

Following our strategy of using simple discrimination tasks to investigate the primate visual system, we trained both human and monkey subjects for two orientation discrimination tasks: an identification and a successive discrimination. Contrasting these two tasks allowed us to isolate the temporal comparison component and to relate this component to activity in right fusiform gyrus using Positron Emission Tomography (PET) and to infero-temporal cortex using a lesion approach in monkeys. Single-cell recordings in infero-temporal cortex demonstrated that neurons in this region can contribute to the three processes underlying temporal comparison: (1) sensorial representation of visual stimuli, (2) maintaining a trace of the preceding stimulus, and (3) comparison of the incoming stimulus with that trace. By the same token, a comparison of these two tasks, which use the same input and the same attribute, demonstrates the task dependency of processing in the human and non-human primate visual system.

Neuromagnetic fields reveal cortical plasticity when learning an auditory discrimination task

Auditory evoked neuromagnetic fields of the primary and association auditory cortices were recorded while subjects learned to discriminate small differences in frequency and intensity between two consecutive tones. When discrimination was no better than chance, evoked field patterns across the scalp manifested no significant differences between correct and incorrect responses. However, when performance was correct on at least 75% of the trials, the spatial pattern of magnetic field differed significantly between correct and incorrect responses during the first 70 ms following the onset of the second tone. In this respect, the magnetic field pattern predicted when the subject would make an incorrect judgment more than 100 ms prior to indicating the judgment by a button press. One subject improved discrimination for much smaller differences between stimuli after 200 h of training. Evidence of cortical plasticity with improved discrimination is provided by an accompanying decrease of the relative magnetic field amplitude of the 100 ms response components in the primary and association auditory cortices.

The primate temporal pole: its putative role in object recognition and memory

In this article, we consider both the ventral temporopolar cortex and the perirhinal cortex (areas 35 and 36) as the anterior ventromedial temporal (aVMT) cortex, and discuss its role based on recent data in monkeys and human subjects. In monkeys, the aVMT cortex receives its primary input from area TE, and only minor input from other cortical areas. Laminar patterns of connections suggest that the aVMT cortex is a hierarchically higher-order area than area TE. Lesions of this cortex produce deficits in the learning and performance of visual memory tasks. Neurons in the aVMT cortex respond selectively to complex stimuli and changes in activity related to visual memory tasks. In humans, damage of this cortex induces deficits in the recognition of familiar objects and faces. The aVMT cortex is activated during recognition of familiar faces. In addition, the aVMT cortex is one of the most vulnerable areas in Alzheimer's disease. All these data indicate that the aVMT cortex is a higher-order visual cortical area that is related to object recognition and memory. The anterior area TE has been implicated in both functions. We propose here that these areas and the anterior entorhinal cortex are designated as the temporal pole, a brain region which is specialized for both object recognition and memory.