Experience-dependent representation of visual categories in parietal cortex

Categorization is a process by which the brain assigns meaning to sensory stimuli. Through experience, we learn to group stimuli into categories, such as 'chair', 'table' and 'vehicle', which are critical for rapidly and appropriately selecting behavioural responses. Although much is known about the neural representation of simple visual stimulus features (for example, orientation, direction and colour), relatively little is known about how the brain learns and encodes the meaning of stimuli. We trained monkeys to classify 360 degrees of visual motion directions into two discrete categories, and compared neuronal activity in the lateral intraparietal (LIP) and middle temporal (MT) areas, two interconnected brain regions known to be involved in visual motion processing. Here we show that neurons in LIP--an area known to be centrally involved in visuo-spatial attention, motor planning and decision-making-robustly reflect the category of motion direction as a result of learning. The activity of LIP neurons encoded directions of motion according to their category membership, and that encoding shifted after the monkeys were retrained to group the same stimuli into two new categories. In contrast, neurons in area MT were strongly direction selective but carried little, if any, explicit category information. This indicates that LIP might be an important nexus for the transformation of visual direction selectivity to more abstract representations that encode the behavioural relevance, or meaning, of stimuli.     

Dynamic coding of behaviourally relevant stimuli in parietal cortex.

A general function of cerebral cortex is to allow the flexible association of sensory stimuli with specific behaviours. Many neurons in parietal, prefrontal and motor cortical areas are activated both by particular movements and by sensory cues that trigger these movements, suggesting a role in linking sensation to action. For example, neurons in the lateral intraparietal area (LIP) encode both the location of visual stimuli and the direction of saccadic eye movements. LIP is not believed to encode non-spatial stimulus attributes such as colour. Here we investigated whether LIP would encode colour if colour was behaviourally linked to the eye movement. We trained monkeys to make an eye movement in one of two directions based alternately on the colour or location of a visual cue. When cue colour was relevant for directing eye movement, we found a substantial fraction of LIP neurons selective for cue colour. However, when cue location was relevant, colour selectivity was virtually absent in LIP. These results demonstrate that selectivity of cortical neurons can change as a function of the required behaviour.     

用于机器人运动控制的通用小脑认知模块的构建

用具有神经生理学性质的小脑模型实现对机器人系统各种不同感觉协调运动的控制,这是当前机器人控制领域的一个研究热点。但针对一种控制任务就要构建一种对应的小脑模型,这种方式效率很不理想。文中以运动控制为基础,构建了一种包括所有主要细胞类型及其连接的通用小脑认知模块(General Cerebellum CognitiveModule,GCCM)。对给定的背景知识,GCCM能够通过学习生成理想输出,因而,可用于各种不同的机器人感觉运动控制任务。仿真实验证明,通过将脊髓层次上的轨迹误差检测与小脑中的记忆轨迹结合起来,嵌入的GCCM控制系统能对快速手臂延伸运动进行精确的鲁棒控制。     

Direct visuomotor transformations for reaching

The posterior parietal cortex (PPC) is thought to have a function in the sensorimotor transformations that underlie visually guided reaching, as damage to the PPC can result in difficulty reaching to visual targets in the absence of specific visual or motor deficits. This function is supported by findings that PPC neurons in monkeys are modulated by the direction of hand movement, as well as by visual, eye position and limb position signals. The PPC could transform visual target locations from retinal coordinates to hand-centred coordinates by combining sensory signals in a serial manner to yield a body-centred representation of target location, and then subtracting the body-centred location of the hand. We report here that in dorsal area 5 of the PPC, remembered target locations are coded with respect to both the eye and hand. This suggests that the PPC transforms target locations directly between these two reference frames. Data obtained in the adjacent parietal reach region (PRR) indicate that this transformation may be achieved by vectorially subtracting hand location from target location, with both locations represented in eye-centred coordinates.     

Cortical ensemble activity increasingly predicts behaviour outcomes during learning of a motor task

When an animal learns to make movements in response to different stimuli, changes in activity in the motor cortex seem to accompany and underlie this learning. The precise nature of modifications in cortical motor areas during the initial stages of motor learning, however, is largely unknown. Here we address this issue by chronically recording from neuronal ensembles located in the rat motor cortex, throughout the period required for rats to learn a reaction-time task. Motor learning was demonstrated by a decrease in the variance of the rats' reaction times and an increase in the time the animals were able to wait for a trigger stimulus. These behavioural changes were correlated with a significant increase in our ability to predict the correct or incorrect outcome of single trials based on three measures of neuronal ensemble activity: average firing rate, temporal patterns of firing, and correlated firing. This increase in prediction indicates that an association between sensory cues and movement emerged in the motor cortex as the task was learned. Such modifications in cortical ensemble activity may be critical for the initial learning of motor tasks.     

Representation of a perceptual decision in developing oculomotor commands

Behaviour often depends on the ability to make categorical judgements about sensory information acquired over time. Such judgements require a comparison of the evidence favouring the alternatives, but how the brain forms these comparisons is unknown. Here we show that in a visual discrimination task, the accumulating balance of sensory evidence favouring one interpretation over another is evident in the neural circuits that generate the behavioural response. We trained monkeys to make a direction judgement about dynamic random-dot motions and to indicate their judgement with an eye movement to a visual target. We interrupted motion viewing with electrical microstimulation of the frontal eye field and analysed the resulting, evoked eye movements for evidence of ongoing activity associated with the oculomotor response. Evoked eye movements deviated in the direction of the monkey's judgement. The magnitude of the deviation depended on motion strength and viewing time. The oculomotor signals responsible for these deviations reflected the accumulated motion information that informed the monkey's choices on the discrimination task. Thus, for this task, decision formation and motor preparation appear to share a common level of neural organization.     

fMRI-constrained source analysis of visual P300 in Landolt ring task

An fMRI-constrained source analysis was applied to investigate visual P300 in the Landolt ring task. To study the localization and relative activation timing of P300 generators, we implemented simultaneous EEG/fMRI to identify BOLD signal changes and record 64-channel EEG in 10 subjects during a Landolt ring task inside a 1.5-T fMRI scanner using an MR-compatible EEG recording system. MRI artifact subtraction software was applied to obtain continuous EEG data. Then, the simultaneous collecting of EEG and fMRI was validated in preserving relevant ERPs. The fMRI-constrained source analysis resulted in an 8-dipole solution. The bilateral middle frontal and the right inferior parietal dipole waveforms showed a short latency peak corresponding to the early P300 activity, while the four parietal and the anterior cingulate dipole waveforms showed a long latency peak corresponding to the late P300 activity. The longest latency peak of the anterior cingulate dipole agrees with its role in initiation of motor response after successful target recognition. Target detection in the Landolt ring task produces the strongest and most extensive parietal activation （especially superior parietal activation）, which might be due to its particular visual attention switching.     

Modulation of the motion aftereffect by selective attention

The motion aftereffect is a much studied and well documented phenomenon. After viewing a moving visual pattern for a period of time, the same pattern appears to drift in the opposite direction when it is stopped. Psychophysical experiments involving interocular transfer, dichoptic stimulation, and motion aftereffects contingent upon other visual parameters such as colour, orientation and texture, imply that the motion aftereffect is generated at the level of the visual cortex. It has been hypothesized that cortical neurons specialized for the detection of motion along a particular direction become 'fatigued' during the adaptation period so that the resting equilibrium subsequently shifts in the opposite direction to that of the adapting stimulus, giving rise to the sensation of the aftereffect. I have found that if observers are engaged in a separate discrimination task superimposed on a moving textured background, the subsequent motion aftereffect to the background is considerably reduced. It seems that motion aftereffects are susceptible to attentional mechanisms.     

Context-enabled learning in the human visual system

Training was found to improve the performance of humans on a variety of visual perceptual tasks. However, the ability to detect small changes in the contrast of simple visual stimuli could not be improved by repetition. Here we show that the performance of this basic task could be modified after the discrimination of the stimulus contrast was practised in the presence of similar laterally placed stimuli, suggesting a change in the local neuronal circuit involved in the task. On the basis of a combination of hebbian and anti-hebbian synaptic learning rules compatible with our results, we propose a mechanism of plasticity in the visual cortex that is enabled by a change in the context.     

Neuronal correlates of parametric working memory in the prefrontal cortex.

Humans and monkeys have similar abilities to discriminate the difference in frequency between two mechanical vibrations applied sequentially to the fingertips. A key component of this sensory task is that the second stimulus is compared with the trace left by the first (base) stimulus, which must involve working memory. Where and how is this trace held in the brain? This question was investigated by recording from single neurons in the prefrontal cortex of monkeys while they performed the somatosensory discrimination task. Here we describe neurons in the inferior convexity of the prefrontal cortex whose discharge rates varied, during the delay period between the two stimuli, as a monotonic function of the base stimulus frequency. We describe this as 'monotonic stimulus encoding', and we suggest that the result may generalize: monotonic stimulus encoding may be the basic representation of one-dimensional sensory stimulus quantities in working memory. Thus we predict that other behavioural tasks that require ordinal comparisons between scalar analogue stimuli would give rise to monotonic responses similar to those reported here.     

Correlation between neural spike trains increases with firing rate

Populations of neurons in the retina, olfactory system, visual and somatosensory thalamus, and several cortical regions show temporal correlation between the discharge times of their action potentials (spike trains). Correlated firing has been linked to stimulus encoding, attention, stimulus discrimination, and motor behaviour. Nevertheless, the mechanisms underlying correlated spiking are poorly understood, and its coding implications are still debated. It is not clear, for instance, whether correlations between the discharges of two neurons are determined solely by the correlation between their afferent currents, or whether they also depend on the mean and variance of the input. We addressed this question by computing the spike train correlation coefficient of unconnected pairs of in vitro cortical neurons receiving correlated inputs. Notably, even when the input correlation remained fixed, the spike train output correlation increased with the firing rate, but was largely independent of spike train variability. With a combination of analytical techniques and numerical simulations using 'integrate-and-fire' neuron models we show that this relationship between output correlation and firing rate is robust to input heterogeneities. Finally, this overlooked relationship is replicated by a standard threshold-linear model, demonstrating the universality of the result. This connection between the rate and correlation of spiking activity links two fundamental features of the neural code.     

Perceptual consequences of centre-surround antagonism in visual motion processing

Centre-surround receptive field organization is a ubiquitous property in mammalian visual systems, presumably tailored for extracting image features that are differentially distributed over space. In visual motion, this is evident as antagonistic interactions between centre and surround regions of the receptive fields of many direction-selective neurons in visual cortex. In a series of psychophysical experiments we make the counterintuitive observation that increasing the size of a high-contrast moving pattern renders its direction of motion more difficult to perceive and reduces its effectiveness as an adaptation stimulus. We propose that this is a perceptual correlate of centre-surround antagonism, possibly within a population of neurons in the middle temporal visual area. The spatial antagonism of motion signals observed at high contrast gives way to spatial summation as contrast decreases. Evidently, integration of motion signals over space depends crucially on the visibility of those signals, thereby allowing the visual system to register motion information efficiently and adaptively.     

Gaze direction controls response gain in primary visual-cortex neurons

To localize objects in space, the brain needs to combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbits. Interaction between these two types of information occurs in several cortical areas, but the role of the primary visual cortex (area V1) in this process has remained unclear. Here we show that, for half the cells recorded in area V1 of behaving monkeys, the classically described visual responses are strongly modulated by gaze direction. Specifically, we find that selectivity for horizontal retinal disparity-the difference in the position of a stimulus on each retina which relates to relative object distance-and for stimulus orientation may be present at a given gaze direction, but be absent or poorly expressed at another direction. Shifts in preferred disparity also occurred in several neurons. These neural changes were most often present at the beginning of the visual response, suggesting a feedforward gain control by eye position signals. Cortical neural processes for encoding information about the three-dimensional position of a stimulus in space therefore start as early as area V1.     

Motion streaks provide a spatial code for motion direction.

Although many neurons in the primary visual cortex (V1) of primates are direction selective, they provide ambiguous information about the direction of motion of a stimulus. There is evidence that one of the ways in which the visual system resolves this ambiguity is by computing, from the responses of V1 neurons, velocity components in two or more spatial orientations and then combining these velocity components. Here I consider another potential neural mechanism for determining motion direction. When a localized image feature moves fast enough, it should become smeared in space owing to temporal integration in the visual system, creating a spatial signal-a 'motion streak'-oriented in the direction of the motion. The orientation masking and adaptation experiments reported here show that these spatial signals for motion direction exist in the human visual system for feature speeds above about 1 feature width per 100 ms. Computer simulations show that this psychophysical finding is consistent with the known response properties of V1 neurons, and that these spatial signals, when appropriately processed, are sufficient to determine motion direction in natural images.     

Perceptual learning without perception.

The brain is able to adapt rapidly and continually to the surrounding environment, becoming increasingly sensitive to important and frequently encountered stimuli. It is often claimed that this adaptive learning is highly task-specific, that is, we become more sensitive to the critical signals in the tasks we attend to. Here, we show a new type of perceptual learning, which occurs without attention, without awareness and without any task relevance. Subjects were repeatedly presented with a background motion signal so weak that its direction was not visible; the invisible motion was an irrelevant background to the central task that engaged the subject's attention. Despite being below the threshold of visibility and being irrelevant to the central task, the repetitive exposure improved performance specifically for the direction of the exposed motion when tested in a subsequent suprathreshold test. These results suggest that a frequently presented feature sensitizes the visual system merely owing to its frequency, not its relevance or salience.     

Influence of the thalamus on spatial visual processing in frontal cortex

Each of our movements activates our own sensory receptors, and therefore keeping track of self-movement is a necessary part of analysing sensory input. One way in which the brain keeps track of self-movement is by monitoring an internal copy, or corollary discharge, of motor commands. This concept could explain why we perceive a stable visual world despite our frequent quick, or saccadic, eye movements: corollary discharge about each saccade would permit the visual system to ignore saccade-induced visual changes. The critical missing link has been the connection between corollary discharge and visual processing. Here we show that such a link is formed by a corollary discharge from the thalamus that targets the frontal cortex. In the thalamus, neurons in the mediodorsal nucleus relay a corollary discharge of saccades from the midbrain superior colliculus to the cortical frontal eye field. In the frontal eye field, neurons use corollary discharge to shift their visual receptive fields spatially before saccades. We tested the hypothesis that these two components-a pathway for corollary discharge and neurons with shifting receptive fields-form a circuit in which the corollary discharge drives the shift. First we showed that the known spatial and temporal properties of the corollary discharge predict the dynamic changes in spatial visual processing of cortical neurons when saccades are made. Then we moved from this correlation to causation by isolating single cortical neurons and showing that their spatial visual processing is impaired when corollary discharge from the thalamus is interrupted. Thus the visual processing of frontal neurons is spatiotemporally matched with, and functionally dependent on, corollary discharge input from the thalamus. These experiments establish the first link between corollary discharge and visual processing, delineate a brain circuit that is well suited for mediating visual stability, and provide a framework for studying corollary discharge in other sensory systems.     

Columns for visual features of objects in monkey inferotemporal cortex.

At early stages of the mammalian visual cortex, neurons with similar stimulus selectivities are vertically arrayed through the thickness of the cortical sheet and clustered in patches or bands across the surface. This organization, referred to as a 'column', has been found with respect to one-dimensional stimulus parameters such as orientation of stimulus contours, eye dominance of visual inputs, and direction of stimulus motion. It is unclear, however, whether information with extremely high dimensions, such as visual shape, is organized in a similar columnar fashion or in a different manner in the brain. Here we report that the anterior inferotemporal area of the monkey cortex, the final station of the visual cortical stream crucial for object recognition, consists of columns, each containing cells responsive to similar visual features of objects.     

Experience with moving visual stimuli drives the early development of cortical direction selectivity

The onset of vision occurs when neural circuits in the visual cortex are immature, lacking both the full complement of connections and the response selectivity that defines functional maturity. Direction-selective responses are particularly vulnerable to the effects of early visual deprivation, but it remains unclear how stimulus-driven neural activity guides the emergence of cortical direction selectivity. Here we report observations from a motion training protocol that allowed us to monitor the impact of experience on the development of direction-selective responses in visually naive ferrets. Using intrinsic signal imaging techniques, we found that training with a single axis of motion induced the rapid emergence of direction columns that were confined to cortical regions preferentially activated by the training stimulus. Using two-photon calcium imaging techniques, we found that single neurons in visually naive animals exhibited weak directional biases and lacked the strong local coherence in the spatial organization of direction preference that was evident in mature animals. Training with a moving stimulus, but not with a flashed stimulus, strengthened the direction-selective responses of individual neurons and preferentially reversed the direction biases of neurons that deviated from their neighbours. Both effects contributed to an increase in local coherence. We conclude that early experience with moving visual stimuli drives the rapid emergence of direction-selective responses in the visual cortex.     

Perceptual stimulus――A Bayesian-based integration of multi-visual-cue approach and its application

With the view that visual cue could be taken as a kind of stimulus, the study of the mechanism in the visual perception process by using visual cues in their probabilistic representation eventually leads to a class of statistical Integration of multiple visual cues （IMVC） methods which have been applied widely in perceptual grouping, video analysis, and other basic problems in computer vision. In this paper, a survey on the basic ideas and recent advances of IMVC methods is presented, and much focus is on the models and algorithms of IMVC for video analysis within the framework of Bayesian estimation. Furthermore, two typical problems in video analysis, robust visual tracking and “switching problem” in muIti-targèt tracking （MTT） are taken as test beds to verify a series of Bayesian-based IMVC methods proposed by the authors. Furthermore, the relations between the statistical IMVC and the visual perception process, as well as potential future research work for IMVC, are discussed.     

Gamma-band synchronization in visual cortex predicts speed of change detection

Our capacity to process and respond behaviourally to multiple incoming stimuli is very limited. To optimize the use of this limited capacity, attentional mechanisms give priority to behaviourally relevant stimuli at the expense of irrelevant distractors. In visual areas, attended stimuli induce enhanced responses and an improved synchronization of rhythmic neuronal activity in the gamma frequency band (40-70 Hz). Both effects probably improve the neuronal signalling of attended stimuli within and among brain areas. Attention also results in improved behavioural performance and shortened reaction times. However, it is not known how reaction times are related to either response strength or gamma-band synchronization in visual areas. Here we show that behavioural response times to a stimulus change can be predicted specifically by the degree of gamma-band synchronization among those neurons in monkey visual area V4 that are activated by the behaviourally relevant stimulus. When there are two visual stimuli and monkeys have to detect a change in one stimulus while ignoring the other, their reactions are fastest when the relevant stimulus induces strong gamma-band synchronization before and after the change in stimulus. This enhanced gamma-band synchronization is also followed by shorter neuronal response latencies on the fast trials. Conversely, the monkeys' reactions are slowest when gamma-band synchronization is high in response to the irrelevant distractor. Thus, enhanced neuronal gamma-band synchronization and shortened neuronal response latencies to an attended stimulus seem to have direct effects on visually triggered behaviour, reflecting an early neuronal correlate of efficient visuo-motor integration.