Independent hemispheric attentional systems mediate visual search in split-brain patients

The primate visual system is adept at identifying objects embedded within complex displays that contain a variety of potentially distracting elements. Theories of visual perception postulate that this ability depends on spatial selective attention, a mechanism analogous to a spotlight or zoom lens, which concentrates high-level processing resources on restricted portions of the visual field. Previous studies in which attention was pre-cued to specific locations in the visual field have shown that the spotlight has a single, unified focus, even in the disconnected hemispheres of patients who have undergone surgical transection of the corpus callosum. Here we demonstrate that an independent focus of attention is deployed by each of the surgically separated hemispheres in a visual search task, such that bilateral stimulus arrays can be scanned at a faster rate by 'split-brain' subjects than by normal control subjects. The attentional system used for visual search therefore seems to be functionally and anatomically distinct from the system that mediates voluntary orienting of attention.     

Early scale effect and hemisphere superiority on the visual spatial attention: From the electrophysiological evidence of ERP

The visual attention mechanism in the brain was studied among 16 young subjects through the precue-target visual search paradigm using the event-related potentials (ERPs) technique, with the attentive ranges cued with different scales of Chinese words. The results showed that the response time was shortened as the cue scale was reduced, while the amplitudes of the P1 and N1 components of the ERPs increased. These results not only provided the electrophysiological evidence supporting the spotlight theory, but also indicated that the spotlight effect occurred during the early period of the selected attention. Two kinds of separation in the P2 effect were observed. One separation was between the P1 effect and P2 effect, which meant that additional computation was needed when the spatial scale of attention was enlarged; the other was between the left and right hemisphere of the P2 effect, which indicates that the attentive processing of the cue range mainly occurred in the left hemisphere.     

Tracking an object through feature space

Visual attention allows an observer to select certain visual information for specialized processing. Selection is readily apparent in 'tracking' tasks where even with the eyes fixed, observers can track a target as it moves among identical distractor items. In such a case, a target is distinguished by its spatial trajectory. Here we show that one can keep track of a stationary item solely on the basis of its changing appearance--specified by its trajectory along colour, orientation, and spatial frequency dimensions--even when a distractor shares the same spatial location. This ability to track through feature space bears directly on competing theories of attention, that is, on whether attention can select locations in space, features such as colour or shape, or particular visual objects composed of constellations of visual features. Our results affirm, consistent with a growing body of psychophysical and neurophysiological evidence, that attention can indeed select specific visual objects. Furthermore, feature-space tracking extends the definition of visual object to include not only items with well defined spatio-temporal trajectories, but also those with well defined featuro-temporal trajectories.     

Feature-based attention influences motion processing gain in macaque visual cortex.

Changes in neural responses based on spatial attention have been demonstrated in many areas of visual cortex, indicating that the neural correlate of attention is an enhanced response to stimuli at an attended location and reduced responses to stimuli elsewhere. Here we demonstrate non-spatial, feature-based attentional modulation of visual motion processing, and show that attention increases the gain of direction-selective neurons in visual cortical area MT without narrowing the direction-tuning curves. These findings place important constraints on the neural mechanisms of attention and we propose to unify the effects of spatial location, direction of motion and other features of the attended stimuli in a 'feature similarity gain model' of attention.     

fMRI evidence for objects as the units of attentional selection.

Contrasting theories of visual attention emphasize selection by spatial location, visual features (such as motion or colour) or whole objects. Here we used functional magnetic resonance imaging (fMRI) to test key predictions of the object-based theory, which proposes that pre-attentive mechanisms segment the visual array into discrete objects, groups, or surfaces, which serve as targets for visual attention. Subjects viewed stimuli consisting of a face transparently superimposed on a house, with one moving and the other stationary. In different conditions, subjects attended to the face, the house or the motion. The magnetic resonance signal from each subject's fusiform face area, parahippocampal place area and area MT/MST provided a measure of the processing of faces, houses and visual motion, respectively. Although all three attributes occupied the same location, attending to one attribute of an object (such as the motion of a moving face) enhanced the neural representation not only of that attribute but also of the other attribute of the same object (for example, the face), compared with attributes of the other object (for example, the house). These results cannot be explained by models in which attention selects locations or features, and provide physiological evidence that whole objects are selected even when only one visual attribute is relevant.     

Miniature eye movements enhance fine spatial detail

Our eyes are constantly in motion. Even during visual fixation, small eye movements continually jitter the location of gaze. It is known that visual percepts tend to fade when retinal image motion is eliminated in the laboratory. However, it has long been debated whether, during natural viewing, fixational eye movements have functions in addition to preventing the visual scene from fading. In this study, we analysed the influence in humans of fixational eye movements on the discrimination of gratings masked by noise that has a power spectrum similar to that of natural images. Using a new method of retinal image stabilization, we selectively eliminated the motion of the retinal image that normally occurs during the intersaccadic intervals of visual fixation. Here we show that fixational eye movements improve discrimination of high spatial frequency stimuli, but not of low spatial frequency stimuli. This improvement originates from the temporal modulations introduced by fixational eye movements in the visual input to the retina, which emphasize the high spatial frequency harmonics of the stimulus. In a natural visual world dominated by low spatial frequencies, fixational eye movements appear to constitute an effective sampling strategy by which the visual system enhances the processing of spatial detail.     

Involuntary orienting to sound improves visual perception

To perceive real-world objects and events, we need to integrate several stimulus features belonging to different sensory modalities. Although the neural mechanisms and behavioural consequences of intersensory integration have been extensively studied, the processes that enable us to pay attention to multimodal objects are still poorly understood. An important question is whether a stimulus in one sensory modality automatically attracts attention to spatially coincident stimuli that appear subsequently in other modalities, thereby enhancing their perceptual salience. The occurrence of an irrelevant sound does facilitate motor responses to a subsequent light appearing nearby. However, because participants in previous studies made speeded responses rather than psychophysical judgements, it remains unclear whether involuntary auditory attention actually affects the perceptibility of visual stimuli as opposed to postperceptual decision and response processes. Here we provide psychophysical evidence that a sudden sound improves the detectability of a subsequent flash appearing at the same location. These data show that the involuntary orienting of attention to sound enhances early perceptual processing of visual stimuli.     

Selective gating of visual signals by microstimulation of frontal cortex

Several decades of psychophysical and neurophysiological studies have established that visual signals are enhanced at the locus of attention. What remains a mystery is the mechanism that initiates biases in the strength of visual representations. Recent evidence argues that, during spatial attention, these biases reflect nascent saccadic eye movement commands. We examined the functional interaction of saccade preparation and visual coding by electrically stimulating sites within the frontal eye fields (FEF) and measuring its effect on the activity of neurons in extrastriate visual cortex. Here we show that visual responses in area V4 could be enhanced after brief stimulation of retinotopically corresponding sites within the FEF using currents below that needed to evoke saccades. The magnitude of the enhancement depended on the effectiveness of receptive field stimuli as well as on the presence of competing stimuli outside the receptive field. Stimulation of non-corresponding FEF representations could suppress V4 responses. The results suggest that the gain of visual signals is modified according to the strength of spatially corresponding eye movement commands.     

Improved auditory spatial tuning in blind humans.

Despite reports of improved auditory discrimination capabilities in blind humans and visually deprived animals, there is no general agreement as to the nature or pervasiveness of such compensatory sensory enhancements. Neuroimaging studies have pointed out differences in cerebral organization between blind and sighted humans, but the relationship between these altered cortical activation patterns and auditory sensory acuity remains unclear. Here we compare behavioural and electrophysiological indices of spatial tuning within central and peripheral auditory space in congenitally blind and normally sighted but blindfolded adults to test the hypothesis (raised by earlier studies of the effects of auditory deprivation on visual processing) that the effects of visual deprivation might be more pronounced for processing peripheral sounds. We find that blind participants displayed localization abilities that were superior to those of sighted controls, but only when attending to sounds in peripheral auditory space. Electrophysiological recordings obtained at the same time revealed sharper tuning of early spatial attention mechanisms in the blind subjects. Differences in the scalp distribution of brain electrical activity between the two groups suggest a compensatory reorganization of brain areas in the blind that may contribute to the improved spatial resolution for peripheral sound sources.     

Selective deficit of visual search in moving displays after extrastriate damage

A visual cue that is often associated with significant stimuli, such as those provided by prey and predators, is movement relative to the observer. An efficient visual system should be able to direct attention to those parts of the visual field that contain such stimuli. What is needed is a system that can filter by movement difference. This could direct attention to a moving item among stationary items, or an item moving in one direction against a background moving in a different direction. Visual search experiments have shown that people are indeed able to filter by movement; that is, they can attend to just the moving items in arrays of moving and stationary stimuli. Single-cell recordings from monkey visual cortex show that the medial temporal cortical area (MT) has some of the properties required to filter by movement. We have now linked these two observations by showing that a patient with bilateral lesions to the presumed human homologue of MT cannot restrict visual attention to the moving items in arrays of both moving and stationary items. This suggests that MT is the site of a movement filter used in normal visual processing.     

Naloxone augments electrophysiological signs of selective attention in man

Previous research on the behavioural functions of endogenous opioid systems in rodents suggested a possible opioid role in the regulation of attention. This proposal was consistent with reports that opiate administration in man impairs the ability to concentrate while opiate antagonists augment behavioural and electrophysiological indices of arousal and attention. We examined the effects of the opiate antagonist naloxone on electrophysiological measures of attention in normal human subjects, using a paradigm which dissociates selective information processing from concurrent processes of general arousal or alertness that may be present. We now report electrophysiological evidence that naloxone improves the selectivity of auditory attention in the presence of competing sources of stimuli. These findings indicate a role for the endogenous opioid systems in the regulation of selective attention in man.     

Cellular networks underlying human spatial navigation

Place cells of the rodent hippocampus constitute one of the most striking examples of a correlation between neuronal activity and complex behaviour in mammals. These cells increase their firing rates when the animal traverses specific regions of its surroundings, providing a context-dependent map of the environment. Neuroimaging studies implicate the hippocampus and the parahippocampal region in human navigation. However, these regions also respond selectively to visual stimuli. It thus remains unclear whether rodent place coding has a homologue in humans or whether human navigation is driven by a different, visually based neural mechanism. We directly recorded from 317 neurons in the human medial temporal and frontal lobes while subjects explored and navigated a virtual town. Here we present evidence for a neural code of human spatial navigation based on cells that respond at specific spatial locations and cells that respond to views of landmarks. The former are present primarily in the hippocampus, and the latter in the parahippocampal region. Cells throughout the frontal and temporal lobes responded to the subjects' navigational goals and to conjunctions of place, goal and view.     

Attentional modulation of motion-induced blindness

When a global moving pattern is superimposed on high-contrast stationary or slowly moving stimuli, the stimuli can be perceived as disappearing and reappearing alternately for periods of several sec- onds.This visual illusory phenomenon was named“motion-induced blindness”(MIB)in recent litera- ture.So far there is no consensus on the mechanism of MIB,especially on the role of attention in this phenomenon.To examine the effect of spatial attention on MIB,the present study manipulated the participants’spatial attention by asking them to respond to two targets simultaneously presented in bilateral visual fields(the divided-attention condition)or only respond to one of them(the fo- cused-attention condition).A central arrow was presented as an endogenous cue to index the target visual field in the focused-attention condition,while a point was presented instead in the di- vided-attention condition.The results show that the percentage of accumulated invisibility period was larger for the targets in the focused-attention condition than for those in the divided-attention condition. This effect of attention is significant in upper visual field(UVF)and left lower visual field(left LVF);that is,this effect shows a hemispheric asymmetry in LVF but not in UVF.Furthermore,the percentage of accumulated invisibility period was larger for targets in left LVF than for those in right LVF in the fo- cused-attention condition,but no hemispheric asymmetry was found in the divided-attention condition. In addition,the increased percentage of accumulated invisibility period in the focused-attention condi- tion originated merely in the enhancement of the mean phase duration of disappearance in LVF,while the disappearance occurred more frequently and lasted longer for each occurrence,which led to an increase in the total invisibility period,in the focused-than divided-attention condition in UVF.These results suggest that the modulation of spatial attention on MIB has different patterns in UVF and LVF.     

Electrophysiological measurement of rapid shifts of attention during visual search.

The perception of natural visual scenes that contain many objects poses computational problems that are absent when objects are perceived in isolation. Vision researchers have captured this attribute of real-world perception in the laboratory by using visual search tasks, in which subjects search for a target object in arrays containing varying numbers of non-target distractor objects. Under many conditions, the amount of time required to detect a visual search target increases as the number of objects in the stimulus array increases, and some investigators have proposed that this reflects the serial application of attention to the individual objects in the array. However, other investigators have argued that this pattern of results may instead be due to limitations in the processing capacity of a parallel processing system that identifies multiple objects concurrently. Here we attempt to address this longstanding controversy by using an electrophysiological marker of the moment-by-moment direction of attention-the N2pc component of the event-related potential waveform--to show that attention shifts rapidly among objects during visual search.     

Dissociable neural mechanisms supporting visual short-term memory for objects

Using visual information to guide behaviour requires storage in a temporary buffer, known as visual short-term memory (VSTM), that sustains attended information across saccades and other visual interruptions. There is growing debate on whether VSTM capacity is limited to a fixed number of objects or whether it is variable. Here we report four experiments using functional magnetic resonance imaging that resolve this controversy by dissociating the representation capacities of the parietal and occipital cortices. Whereas representations in the inferior intra-parietal sulcus (IPS) are fixed to about four objects at different spatial locations regardless of object complexity, those in the superior IPS and the lateral occipital complex are variable, tracking the number of objects held in VSTM, and representing fewer than four objects as their complexity increases. These neural response patterns were observed during both VSTM encoding and maintenance. Thus, multiple systems act together to support VSTM: whereas the inferior IPS maintains spatial attention over a fixed number of objects at different spatial locations, the superior IPS and the lateral occipital complex encode and maintain a variable subset of the attended objects, depending on their complexity. VSTM capacity is therefore determined both by a fixed number of objects and by object complexity.     

Attentional modulation in visual cortex depends on task timing

Paying attention to a stimulus selectively increases the ability to process it. For example, when subjects attend to a specific region of a visual scene, their sensitivity to changes at that location increases. A large number of studies describe the behavioural consequences and neurophysiological correlates of attending to spatial locations. There has, in contrast, been little study of the allocation of attention over time. Because subjects can anticipate predictable events with great temporal precision, it seems probable that they might dynamically shift their attention when performing a familiar perceptual task whose constraints changed over time. We trained monkeys to respond to a stimulus change where the probability of occurrence changed over time. Recording from area V4 of the visual cortex in these animals, we found that the modulation of neuronal responses changed according to the probability of the change occurring at that instant. Thus, we show that the attentional modulation of sensory neurons reflects a subject's anticipation of the timing of behaviourally relevant events.     

视觉神经表征中底-顶和顶-底的双向激活——来自人与猴的实验证据

长期以来,建立在坚实生理学证据之上的底-顶加工说和特征检测理论在视觉研究中占主导地位。对于顶-底加工人们只能靠一般常识,即知识或经验通过激活记忆中的神经表征影响视觉过程。但是近年来,来自人和猴的研究为顶-底的加工提供了实验证据。 首先位于猴腹侧加工系统内的物体和面孔视觉记忆表征,提供了神经编码是怎样创立、组织和再激活的最佳实验证据。联想性编码是通过学习由一些具有特殊功能的神经元建立的,这些神经元具有将时间性关联刺激的表征联系起来的能力。其次,不仅来自视网膜的底-顶信号,而且来自前额叶的顶-底信号都能触发联想性编码的提取,既可以作为有意识回忆的神经基础,又是顶-底加工影响视觉过程的基础。脑损伤病人研究、具有高时间分辨率的人类功能性核磁共振成像(functional magnetic resonance imaging, fMRI)和猴fMRI研究以及猴细胞电生理分析相结合,将进一步加强人们对视觉脑机制的全面理解。     

Top-down gain control of the auditory space map by gaze control circuitry in the barn owl

High-level circuits in the brain that control the direction of gaze are intimately linked with the control of visual spatial attention. Immediately before an animal directs its gaze towards a stimulus, both psychophysical sensitivity to that visual stimulus and the responsiveness of high-order neurons in the cerebral cortex that represent the stimulus increase dramatically. Equivalent effects on behavioural sensitivity and neuronal responsiveness to visual stimuli result from focal electrical microstimulation of gaze control centres in monkeys. Whether the gaze control system modulates neuronal responsiveness in sensory modalities other than vision is unknown. Here we show that electrical microstimulation applied to gaze control circuitry in the forebrain of barn owls regulates the gain of midbrain auditory responses in an attention-like manner. When the forebrain circuit was activated, midbrain responses to auditory stimuli at the location encoded by the forebrain site were enhanced and spatial selectivity was sharpened. The same stimulation suppressed responses to auditory stimuli represented at other locations in the midbrain map. Such space-specific, top-down regulation of auditory responses by gaze control circuitry in the barn owl suggests that the central nervous system uses a common strategy for dynamically regulating sensory gain that applies across modalities, brain areas and classes of vertebrate species. This approach provides a path for discovering mechanisms that underlie top-down gain control in the central nervous system.     

Motion-induced blindness in normal observers.

Cases in which salient visual stimuli do not register consciously are known to occur in special conditions, such as the presentation of dissimilar stimuli to the two eyes or when images are stabilized on the retina. Here, we report a striking phenomenon of 'visual disappearance' observed with normal-sighted observers under natural conditions. When a global moving pattern is superimposed on high-contrast stationary or slowly moving stimuli, the latter disappear and reappear alternately for periods of several seconds. We show that this motion-induced blindness (MIB) phenomenon is unlikely to reflect retinal suppression, sensory masking or adaptation. The phenomenology observed includes perceptual grouping effects, object rivalry and visual field anisotropy. This is very similar to that found in other types of visual disappearance, as well as in clinical cases of attention deficits, in which partial invisibility might occur despite the primary visual areas being intact. Disappearance might reflect a disruption of attentional processing, which shifts the system into a winner-takes-all mode, uncovering the dynamics of competition between object representations within the human visual system.     

Preserved figure-ground segregation and symmetry perception in visual neglect.

A central controversy in current research on visual attention is whether figures are segregated from their background preattentively, or whether attention is first directed to unstructured regions of the image. Here we present neurological evidence for the former view from studies of a brain-injured patient with visual neglect. His attentional impairment arises after normal segmentation of the image into figures and background has taken place. Our results indicate that information which is neglected and unavailable to higher levels of visual processing can nevertheless be processed by earlier stages in the visual system concerned with segmentation.