镜像等效或守恒及其打破：从行为到认知神经机制的研究证据

镜像等效或守恒是动物与人类个体对两侧对称自然物体的一种进化自适应加工。但是, 这种知觉特性会妨碍包含镜像字符的文字阅读学习。阅读者有必要学会利用镜像泛化抑制的“去学习”机制, 打破镜像等效或守恒, 以获得识别镜像字符的能力。这一过程中, 左侧梭状回皮层通过与早期视觉皮层、顶叶皮层和口语脑网络的交互作用, 逐渐发展出一个可以识别镜像字符的视觉词形区(visual word form area, VWFA)。今后的研究需要关注两半球及其连合纤维在镜像等效或守恒加工中的作用、镜像泛化与抑制的详尽加工机制及其对镜像书写的影响、正常汉语儿童的汉字镜像泛化加工等问题。     

Readout from iconic memory and selective spatial attention involve similar neural processes

Iconic memory and spatial attention are often considered separately, but they may have functional similarities. Here we provide functional magnetic resonance imaging evidence for some common underlying neural effects. Subjects judged three visual stimuli in one hemifield of a bilateral array comprising six stimuli. The relevant hemifield for partial report was indicated by an auditory cue, administered either before the visual array (precue, spatial attention) or shortly after the array (postcue, iconic memory). Pre- and postcues led to similar activity modulations in lateral occipital cortex contralateral to the cued side. This finding indicates that readout from iconic memory can have some neural effects similar to those of spatial attention. We also found common bilateral activation of a fronto-parietal network for postcue and precue trials. These neuroimaging data suggest that some common neural mechanisms underlie selective spatial attention and readout from iconic memory. Some differences were also found; compared with precues, postcues led to higher activity in the right middle frontal gyrus.     

Capacity limits of information processing in the brain

Despite the impressive complexity and processing power of the human brain, it is severely capacity limited. Behavioral research has highlighted three major bottlenecks of information processing that can cripple our ability to consciously perceive, hold in mind, and act upon the visual world, illustrated by the attentional blink (AB), visual short-term memory (VSTM), and psychological refractory period (PRP) phenomena, respectively. A review of the neurobiological literature suggests that the capacity limit of VSTM storage is primarily localized to the posterior parietal and occipital cortex, whereas the AB and PRP are associated with partly overlapping fronto-parietal networks. The convergence of these two networks in the lateral frontal cortex points to this brain region as a putative neural locus of a common processing bottleneck for perception and action.     

Tracing trajectories of audio‐visual learning in the infant brain

Although infants begin learning about their environment before they are born, little is known about how the infant brain changes during learning. Here, we take the initial steps in documenting how the neural responses in the brain change as infants learn to associate audio and visual stimuli. Using functional near‐infrared spectroscopy (fNRIS) to record hemodynamic responses in the infant cortex (temporal, occipital, and frontal cortex), we find that across the infant brain, learning is characterized by an increase in activation followed by a decrease. We take this U‐shaped response as evidence of repetition enhancement during early stages of learning and repetition suppression during later stages, a result that mirrors the Hunter and Ames model of infant visual preference. Furthermore, we find that the neural response to violations of the learned associations can be predicted by the shape of the learning curve in temporal and occipital cortex. These data provide the first look at the shape of the neural response during audio‐visual associative learning in infancy establishing that diverse regions of the infant brain exhibit systematic changes across the time‐course of learning.     

Cortical integration of audio–visual speech and non-speech stimuli

Using fMRI we investigated the neural basis of audio–visual processing of speech and non-speech stimuli using physically similar auditory stimuli (speech and sinusoidal tones) and visual stimuli (animated circles and ellipses). Relative to uni-modal stimuli, the different multi-modal stimuli showed increased activation in largely non-overlapping areas. Ellipse-Speech, which most resembles naturalistic audio–visual speech, showed higher activation in the right inferior frontal gyrus, fusiform gyri, left posterior superior temporal sulcus, and lateral occipital cortex. Circle-Tone, an arbitrary audio–visual pairing with no speech association, activated middle temporal gyri and lateral occipital cortex. Circle-Speech showed activation in lateral occipital cortex, and Ellipse-Tone did not show increased activation relative to uni-modal stimuli. Further analysis revealed that middle temporal regions, although identified as multi-modal only in the Circle-Tone condition, were more strongly active to Ellipse-Speech or Circle-Speech, but regions that were identified as multi-modal for Ellipse-Speech were always strongest for Ellipse-Speech. Our results suggest that combinations of auditory and visual stimuli may together be processed by different cortical networks, depending on the extent to which multi-modal speech or non-speech percepts are evoked.     

The role of the human extrastriate visual cortex in mirror symmetry discrimination: a TMS-adaptation study

The human visual system is able to efficiently extract symmetry information from the visual environment. Prior neuroimaging evidence has revealed symmetry-preferring neuronal representations in the dorsolateral extrastriate visual cortex; the objective of the present study was to investigate the necessity of these representations in symmetry discrimination. This was accomplished by the use of state-dependent transcranial magnetic stimulation, which combines the fine resolution of adaptation paradigms with the assessment of causality. Subjects were presented with adapters and targets consisting of dot configurations that could be symmetric along either the vertical or horizontal axis (or they could be non-symmetric), and they were asked to perform a symmetry discrimination task on the targets while fixating the center of the screen. TMS was applied during the delay between the adapter and the test stimulus over one of four different sites: Left or Right V1/V2, or left or right dorsolateral extrastriate cortex (DLO). TMS over both Left and Right DLO reduced the adaptation effect in detecting vertical and horizontal symmetry, although the Left DLO effect on horizontal symmetry and the Right DLO effect on both vertical and horizontal symmetry were present only when considering subjects who showed a behavioral adaptation effect in the baseline No-TMS condition. Application of TMS over the Left or Right V1/V2 did not modulate the adaptation effect. Overall, these data suggest that both the Left and Right DLO contain neuronal representations tuned to mirror symmetry which play a causal role in symmetry discrimination.     

Do object-category selective regions in the ventral visual stream represent perceived distance information?

It is well established that scenes and objects elicit a highly selective response in specific brain regions in the ventral visual cortex. An inherent difference between these categories that has not been explored yet is their perceived distance from the observer (i.e. scenes are distal whereas objects are proximal). The current study aimed to test the extent to which scene and object selective areas are sensitive to perceived distance information independently from their category-selectivity and retinotopic location. We conducted two studies that used a distance illusion (i.e., the Ponzo lines) and showed that scene regions (the parahippocampal place area, PPA, and transverse occipital sulcus, TOS) are biased toward perceived distal stimuli, whereas the lateral occipital (LO) object region is biased toward perceived proximal stimuli. These results suggest that the ventral visual cortex plays a role in representing distance information, extending recent findings on the sensitivity of these regions to location information. More broadly, our findings imply that distance information is inherent to object recognition.     

Frontal Lobe Function and Pain in the Elderly

We review the literature on pain and aging and conclude that evidence supports a hypothesis that right frontal cortex contributes to the mediation of the chronic pain experience in elderly persons with chronic pain syndromes. Evidence for the right frontal pain hypothesis comes from clinical, neurocognitive, and neuroimaging studies, which implicate right inferior and orbitofrontal cortex in (1) the persistent pain experience, (2) negative emotional states, (3) retrieval of negative emotional and autobiographical memories, (4) regulation of autonomic arousal, and (5) regulation of attentional and pain functions of the anterior cingulate region. Right frontal dysfunction is also implicated in the effects of cognitive aging. If right frontal neurocognitive systems are affected in cognitively impaired elderly, and if (by hypothesis) the right frontal cortex also plays a major role in the experience of chronic pain, then cognitively impaired elderly with right frontal dysfunction should be protected to some extent from persistent pain syndromes. Available evidence supports this proposition.     

The cortical representation of centrally presented words: A magnetic stimulation study

The right and left visual fields each project to the contralateral cerebral hemispheres. The current study aimed to investigate the extent of the functional overlap of the two hemifields along the vertical meridian. We applied repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex to investigate whether the foveal representation of words is bilaterally represented or is split between the two hemispheres. Employing a lateralized lexical decision task, we first showed a double dissociation between the stimulated cortical site and performance; right visual field (RVF) but not left visual field (LVF) performance was impaired when the left visual cortex was stimulated, and LVF but not RVF performance was impairred when the right visual cortex was stimulated. Unilateral stimulation also significantly impaired lexical decision latencies to centrally presented words. These findings support the suggestion that foveal representation of words is split. We discuss future strategies for the use of TMS in further tests of the split representation account.     

Inter-individual variability in metacognitive ability for visuomotor performance and underlying brain structures

Metacognition refers to the ability to discriminate between one’s own correct and incorrect decisions. The neurobiological underpinnings of metacognition have mainly been studied in perceptual decision-making. Here we investigated whether differences in brain structure predict individual variability in metacognitive sensitivity for visuomotor performance. Participants had to draw straight trajectories toward visual targets, which could unpredictably deviate around detection threshold, report such deviations when detected, and rate their confidence level for such reports. Structural brain MRI analyses revealed that larger gray-matter volume (GMV) in the left middle occipital gyrus, left medial parietal cortex, and right postcentral gyrus predicted higher deviation detection sensitivity. By contrast, larger GMV in the right prefrontal cortex but also right anterior insula and right fusiform gyrus predicted higher metacognitive sensitivity. These results extend past research by linking metacognitive sensitivity for visuomotor behavior to brain areas involved in action agency (insula), executive control (prefrontal cortex) and vision (fusiform).     

Characteristics and models of human symmetry detection

Symmetry is everywhere - in natural objects, from crystals to living organisms, in manufactured articles of many kinds, and in art works from all cultures. Symmetry is a salient visual property that is detected efficiently and rapidly by the human visual system. In this paper, several decades of experimental research on human symmetry detection are reviewed. By examining the effects of several factors on symmetry detection, this research has revealed some important characteristics of how humans perceive symmetry. These characteristics constrain the general principles of putative underlying mechanisms and models of human symmetry detection. For example, the orientation of the axis of symmetry and its location in the visual field have effects that suggest that the bilateral symmetry of the visual system at cortical levels of the brain might partly determine the salience of vertical mirror symmetry. At the same time, there is a surprisingly high degree of flexibility and robustness that remains to be explained. Thus, symmetry provides a major challenge to model human flexibility and efficiency within the constraints of the biology of the visual system.     

Why is “blindsight” blind? A new perspective on primary visual cortex,recurrent activity and visual awareness

The neuropsychological phenomenon of blindsight has been taken to suggest that the primary visual cortex (V1) plays a unique role in visual awareness, and that extrastriate activation needs to be fed back to V1 in order for the content of that activation to be consciously perceived. The aim of this review is to evaluate this theoretical framework and to revisit its key tenets. Firstly, is blindsight truly a dissociation of awareness and visual detection? Secondly, is there sufficient evidence to rule out the possibility that the loss of awareness resulting from a V1 lesion simply reflects reduced extrastriate responsiveness, rather than a unique role of V1 in conscious experience? Evaluation of these arguments and the empirical evidence leads to the conclusion that the loss of phenomenal awareness in blindsight may not be due to feedback activity in V1 being the hallmark awareness. On the basis of existing literature, an alternative explanation of blindsight is proposed. In this view, visual awareness is a “global” cognitive function as its hallmark is the availability of information to a large number of perceptual and cognitive systems; this requires inter-areal long-range synchronous oscillatory activity. For these oscillations to arise, a specific temporal profile of neuronal activity is required, which is established through recurrent feedback activity involving V1 and the extrastriate cortex. When V1 is lesioned, the loss of recurrent activity prevents inter-areal networks on the basis of oscillatory activity. However, as limited amount of input can reach extrastriate cortex and some extrastriate neuronal selectivity is preserved, computations involving comparison of neural firing rates within a cortical area remain possible. This enables “local” read-out from specific brain regions, allowing for the detection and discrimination of basic visual attributes. Thus blindsight is blind due to lack of “global” long-range synchrony, and it functions via “local” neural readout from extrastriate areas.     

Effects of surface pre-presentation on symmetry detection on a 3-D bumpy surface

Abstract:  The aim of this study was to investigate the visual detection mechanism of mirror symmetrical dot patterns drawn on a 3-D bumpy surface (i.e., 3-D noisy curved surface). The focus of this investigation was whether symmetry detection on the bumpy surface is associated with structure recovery of the surface. In Experiment 1, we confirmed that the human visual system can distinguish the bumpy surface from a distorted dot pattern on a transparent bumpy surface. In Experiment 2, we required participants to discriminate between a symmetrical pattern on a transparent (non-visible) bumpy surface and an opaque (visible) bumpy surface for the same stimuli as in Experiment 1. Finally, in Experiment 3, we examined the effect of pre-presentation of the opaque bumpy surface on pattern discrimination. The results showed that pre-presentation of the opaque surface facilitated the detection of a diagonal symmetry, but not in the case of the detection of a cardinal symmetry. These results suggested that diagonal symmetry involves the process of surface recovery, whereas cardinal symmetry does not.     

TMS to the “occipital face area” affects recognition but not categorization of faces

The human cortical system for face perception is comprised of a network of connected regions including the middle fusiform gyrus (“fusiform face area” or FFA), the inferior occipital cortex (“occipital face area” or OFA), and the superior temporal sulcus. The traditional hierarchical feedforward model of visual processing suggests information flows from early visual cortex to the OFA for initial face feature analysis to higher order regions including the FFA for identity recognition. However, patient data suggest an alternative model. Patients with acquired prosopagnosia, an inability to visually recognize faces, have been documented with lesions to the OFA but who nevertheless show face-selective activation in the FFA. Moreover, their ability to categorize faces remains intact. This suggests that the FFA is not solely responsible for face recognition and the network is not strictly hierarchical, but may be organized in a reverse hierarchical fashion. We used transcranial magnetic stimulation (TMS) to temporarily disrupt processing in the OFA in neurologically-intact individuals and found participants’ ability to categorize intact versus scrambled faces was unaffected, however face identity discrimination was significantly impaired. This suggests that face categorization but not recognition can occur without the “earlier” OFA being online and indicates that “lower level” face category processing may be assumed by other intact face network regions such as the FFA. These results are consistent with the patient data and support a non-hierarchical, global-to-local model with re-entrant connections between the OFA and other face processing areas.     

Abnormal retinotopic representations in human visual cortex revealed by fMRI

The representation of the visual field in early visual areas is retinotopic. The point-to-point relationship on the retina is therefore maintained on the convoluted cortical surface. Functional magnetic resonance imaging (fMRI) has been able to demonstrate the retinotopic representation of the visual field in occipital cortex of normal subjects. Furthermore, visual areas that are retinotopic can be identified on computationally flattened cortical maps on the basis of positions of the vertical and horizontal meridians. Here, we investigate abnormal retinotopic representations in human visual cortex with fMRI. We present three case studies in which patients with visual disorders are investigated. We have tested a subject who only possesses operating rod photoreceptors. We find in this case that the cortex undergoes a remapping whereby regions that would normally represent central field locations now map more peripheral positions in the visual field: In a human albino we also find abnormal visual cortical activity. Monocular stimulation of each hemifield resulted in activations in the hemisphere contralateral to the stimulated eye. This is consistent with abnormal decussation at the optic chiasm in albinism. Finally, we report a case where a lesion to white matter has resulted in a lack of measurable activity in occipital cortex. The activity was absent for a small region of the visual field, which was found to correspond to the subject's field defect. The cases selected have been chosen to demonstrate the power of fMRI in identifying abnormalities in the cortical representations of the visual field in patients with visual dysfunction. Furthermore, the experiments are able to show how the cortex is capable of modifying the visual field representation in response to abnormal input.     

Grouping reduces visual extinction: Neuropsychological evidence for weight-linkage in visual selection

Abstract

We tested two subjects following damage to right parietal cortex to see if their failure to detect a left visual stimulus in the presence of a simultaneous right stimulus (visual extinction) could be modulated by perceptual grouping between the left and right stimuli. Subjects performed a simple detection task for brief displays in which items could appear in the left or right visual field, both fields, or neither field. On trials in which items appeared in both fields, we found that left omissions (extinction errors) were dramatically reduced when the two items formed a good perceptual group, either on the basis of Gestalt factors such as similarity and symmetry (Experiment 1). or by forming a familiar configuration (Experiment 2). We suggest that extinction may be a spatially specific exaggeration of a normal attention limitation, in which the contralesional item is disadvantaged in the competition for selection. However, this obstacle to selection can be overcome if, as a result of grouping, ipsilesional and contralesional items become allies rather than competitors for selection.     

Conditioned interhemispheric transfer by cerebral tetanization

Electrodes were implanted over the motor and occipital cortex in both cerebral hemispheres of five dogs and also over the right ectosylvian cortex in one dog. Stimulation of these areas resulted mainly in contralateral movements opposite to the stimulated cortex. At appropriate voltages, stimulation of the occipital and ectosylvian cortex resulted in no movement at all. A non-motivated conditioned reflex (CR) was elaborated by coupling electrical stimulation of the occipital (CS) and the ipsilateral motor cortex (US). After long-term consolidation of this CR, electrical stimulations of the opposite hemisphere were performed again and a complete interhemispheric transfer was observed in all five dogs, i.e., stimulation of either right or left occipital cortex as well as stimulation of the right or left motor cortex constantly resulted in the same movement. The dynamics of the interhemispheric transfer were different in each individual dog. The transfer was limited to the symmetrical motor and occipital cortex. Stimulation of the ectosylvian cortex never resulted in any movement. This interhemispheric transfer is discussed as a function of the previous experience of the animal.     

Posterior parietal cortex activity predicts individual differences in visual short-term memory capacity

Humans show a severe capacity limit in the number of objects they can store in visual short-term memory (VSTM). We recently demonstrated with functional magnetic resonance imaging that VSTM storage capacity estimated in averaged group data correlated strongly with posterior parietal/superior occipital cortex activity (Todd & Marois, 2004). However, individuals varied widely in their VSTM capacity. Here, we examined the neural basis of these individual differences. A voxelwise, individualdifferences analysis revealed a significant correlation between posterior parietal cortex (PPC) activity and individuals’ VSTM storage capacity. In addition, a region-of-interest analysis indicated that other brain regions, particularly visual occipital cortex, may contribute to individual differences in VSTM capacity. Thus, although not ruling out contributions from other brain regions, the individual-differences approach supports a key role for the PPC in VSTM by demonstrating that its activity level predicts individual differences in VSTM storage capacity.     

TMS to the “occipital face area” affects recognition but not categorization of faces

The human cortical system for face perception is comprised of a network of connected regions including the middle fusiform gyrus (“fusiform face area” or FFA), the inferior occipital cortex (“occipital face area” or OFA), and the superior temporal sulcus. The traditional hierarchical feedforward model of visual processing suggests information flows from early visual cortex to the OFA for initial face feature analysis to higher order regions including the FFA for identity recognition. However, patient data suggest an alternative model. Patients with acquired prosopagnosia, an inability to visually recognize faces, have been documented with lesions to the OFA but who nevertheless show face-selective activation in the FFA. Moreover, their ability to categorize faces remains intact. This suggests that the FFA is not solely responsible for face recognition and the network is not strictly hierarchical, but may be organized in a reverse hierarchical fashion. We used transcranial magnetic stimulation (TMS) to temporarily disrupt processing in the OFA in neurologically-intact individuals and found participants’ ability to categorize intact versus scrambled faces was unaffected, however face identity discrimination was significantly impaired. This suggests that face categorization but not recognition can occur without the “earlier” OFA being online and indicates that “lower level” face category processing may be assumed by other intact face network regions such as the FFA. These results are consistent with the patient data and support a non-hierarchical, global-to-local model with re-entrant connections between the OFA and other face processing areas.