Visibility reflects dynamic changes of effective connectivity between V1 and fusiform cortex

Identifying the neural basis of visibility is central to understanding conscious visual perception. Visibility of basic features such as brightness is often thought to reflect activity in just early visual cortex. But here we show under metacontrast masking that fMRI activity in stimulus-driven areas of early visual cortex did not reflect parametric changes in the visibility of a brightness stimulus. The psychometric visibility function was instead correlated with activity in later visual regions plus parieto-frontal areas, and surprisingly, in representations of the unstimulated stimulus surround for primary visual cortex. Critically, decreased stimulus visibility was associated with a regionally-specific decoupling between early visual cortex and higher visual areas. This provides evidence that dynamic changes in effective connectivity can closely reflect visual perception.     

Delayed striate cortical activation during spatial attention

Recordings of event-related potentials (ERPs) and event-related magnetic fields (ERMFs) were combined with functional magnetic resonance imaging (fMRI) to study visual cortical activity in humans during spatial attention. While subjects attended selectively to stimulus arrays in one visual field, fMRI revealed stimulus-related activations in the contralateral primary visual cortex and in multiple extrastriate areas. ERP and ERMF recordings showed that attention did not affect the initial evoked response at 60-90 ms poststimulus that was localized to primary cortex, but a similarly localized late response at 140-250 ms was enhanced to attended stimuli. These findings provide evidence that the primary visual cortex participates in the selective processing of attended stimuli by means of delayed feedback from higher visual-cortical areas.     

宽周边视视网膜皮层映射的核磁共振研究

目的:人类视觉皮层的组织方式是视网膜皮层映射组织,先前研究已经证实视觉皮层在中心视采用这种组织方式,本文主要研究宽周边视的视觉皮层组织方式.方法:本文采用一种可以在核磁共振室中使用的光纤设备,设计了30度、40度、50度、60度的类圆环block刺激,使用1.5T的功能性核磁共振仪器,T1高分辨率图像分辨率为1*1*5.5mm,T2加权图像分辨率为4*4*5.5mm,TR反应时间为60,矩阵大小为64*64.核磁共振数据分析使用了SPM2和Brain voyager软件.结果:通过对试验者的数据处理分析,周边视的刺激的反应区域在枕叶上,主要分布在枕叶的前部,刺激反应区域随着偏心率的增大而沿着距状沟从距状沟的后部向前部移动.结论:周边视的视网膜皮层映射组织特性和中心视的特性非常相似.     

A network that uses few active neurones to code visual input predicts the diverse shapes of cortical receptive fields

Computational models of primary visual cortex have demonstrated that principles of efficient coding and neuronal sparseness can explain the emergence of neurones with localised oriented receptive fields. Yet, existing models have failed to predict the diverse shapes of receptive fields that occur in nature. The existing models used a particular "soft" form of sparseness that limits average neuronal activity. Here we study models of efficient coding in a broader context by comparing soft and "bard" forms of neuronal sparseness. As a result of our analyses, we propose a novel network model for visual cortex. The model forms efficient visual representations in which the number of active neurones, rather than mean neuronal activity, is limited. This form of hard sparseness also economises cortical resources like synaptic memory and metabolic energy. Furthermore, our model accurately predicts the distribution of receptive field shapes found in the primary visual cortex of cat and monkey.     

Sensorimotor mismatch signals in primary visual cortex of the behaving mouse

Studies in anesthetized animals have suggested that activity in early visual cortex is mainly driven by visual input and is well described by a feedforward processing hierarchy. However, evidence from experiments on awake animals has shown that both eye movements and behavioral state can strongly modulate responses of neurons in visual cortex; the functional significance of this modulation, however, remains elusive. Using visual-flow feedback manipulations during locomotion in a virtual reality environment, we found that responses in layer 2/3 of mouse primary visual cortex are strongly driven by locomotion and by mismatch between actual and expected visual feedback. These data suggest that processing in visual cortex may be based on predictive coding strategies that use motor-related and visual input to detect mismatches between predicted and actual visual feedback.     

Sound-driven synaptic inhibition in primary visual cortex

Multimodal objects and events activate many sensory cortical areas simultaneously. This is possibly reflected in reciprocal modulations of neuronal activity, even at the level of primary cortical areas. However, the synaptic character of these interareal interactions, and their impact on synaptic and behavioral sensory responses are unclear. Here, we found that activation of auditory cortex by a noise burst drove local GABAergic inhibition on supragranular pyramids of the mouse primary visual cortex, via cortico-cortical connections. This inhibition was generated by sound-driven excitation of a limited number of cells in infragranular visual cortical neurons. Consequently, visually driven synaptic and spike responses were reduced upon bimodal stimulation. Also, acoustic stimulation suppressed conditioned behavioral responses to a dim flash, an effect that was prevented by acute blockade of GABAergic transmission in visual cortex. Thus, auditory cortex activation by salient stimuli degrades potentially distracting sensory processing in visual cortex by recruiting local, translaminar, inhibitory circuits.     

The representation of erroneously perceived stimuli in the primary visual cortex

In order to attain a correct interpretation of an ambiguous visual stimulus, the brain may have to elaborate on the sensory evidence. Are the neurons that carry the sensory evidence also involved in generating an interpretation? To address this question, we studied the activity of neurons in the primary visual cortex of macaque monkeys involved in a task in which they have to trace a curve mentally, without moving their eyes. On a percentage of trials, the monkeys made errors and traced the wrong curve. Here, we show that these errors are predicted by activity in area V1. Thus, neurons in the primary visual cortex do not only represent sensory events, but also the way in which they are interpreted by the monkey.     

Spontaneous retinal activity mediates development of ocular dominance columns and binocular receptive fields in v1

The mechanisms that give rise to ocular dominance columns (ODCs) during development are controversial. Early experiments indicated a key role for retinal activity in ODC formation. However, later studies showed that in those early experiments, the retinal activity perturbation was initiated after ODCs had already formed. Moreover, recent studies concluded that early eye removals do not impact ODC segregation. Here we blocked spontaneous retinal activity during the very early stages of ODC development. This permanently disrupted the anatomical organization of ODCs and led to a dramatic increase in receptive field size for binocular cells in primary visual cortex. Our data suggest that early spontaneous retinal activity conveys crucial information about whether thalamocortical axons represent one or the other eye and that this activity mediates binocular competition important for shaping receptive fields in primary visual cortex.     

Total activation change of visual and motor area due to various disturbances

Brain activation is affected by gradient acoustic noise and various disturbances as well as by other primary tasks. Therefore, we have studied the effects of various disturbances of two different levels of difficulty, that is, weak and strong difficulty levels for primary visual and motor tasks. In the case of visual task with motor and mental disturbances, we found it decreased as motor and mental disturbance difficulty-level increased, compared with the case of without motor and mental disturbances. To the contrary, in the case of motor activity, the total activation of motor cortex with weak and with strong mental disturbance increased as mental disturbance difficulty-levels increased. Therefore, one can conclude that when mental disturbance is added, the visual cortex and motor cortex have an opposite result and when the difficulty-level of the disturbance is increased, the primary tasks are affected more significantly. Although the current observation is preliminary and requires more careful experimental study, it appears that various disturbance effects on brain functions (such as motor and visual cortical responses) produce significant differences in data observations.     

Effects of cholinergic enhancement on visual stimulation, spatial attention, and spatial working memory

We compared behavioral and neural effects of cholinergic enhancement between spatial attention, spatial working memory (WM), and visual control tasks, using fMRI and the anticholinesterase physostigmine. Physostigmine speeded responses nonselectively but increased accuracy selectively for attention. Physostigmine also decreased activations to visual stimulation across all tasks within primary visual cortex, increased extrastriate occipital cortex activation selectively during maintained attention and WM encoding, and decreased parietal activation selectively during maintained attention. Finally, lateralization of occipital activation as a function of the visual hemifield toward which attention or memory was directed was decreased under physostigmine. In the case of attention, this effect correlated strongly with a decrease in a behavioral measure of selective spatial processing. Our results suggest that, while cholinergic enhancement facilitates visual attention by increasing activity in extrastriate cortex generally, it accomplishes this in a manner that reduces expectation-driven selective biasing of extrastriate cortex.     

Knowing with which eye we see: utrocular discrimination and eye-specific signals in human visual cortex

Neurophysiological and behavioral reports converge to suggest that monocular neurons in the primary visual cortex are biased toward low spatial frequencies, while binocular neurons favor high spatial frequencies. Here we tested this hypothesis with functional magnetic resonance imaging (fMRI). Human participants viewed flickering gratings at one of two spatial frequencies presented to either the left or the right eye, and judged which of the two eyes was being stimulated (utrocular discrimination). Using multivoxel pattern analysis we found that local spatial patterns of signals in primary visual cortex (V1) allowed successful decoding of the eye-of-origin. Decoding was above chance for low but not high spatial frequencies, confirming the presence of a bias reported by animal studies in human visual cortex. Behaviorally, we found that reliable judgment of the eye-of-origin did not depend on spatial frequency. We further analyzed the mean response in visual cortex to our stimuli and revealed a weak difference between left and right eye stimulation. Our results are thus consistent with the interpretation that participants use overall levels of neural activity in visual cortex, perhaps arising due to local luminance differences, to judge the eye-of-origin. Taken together, we show that it is possible to decode eye-specific voxel pattern information in visual cortex but, at least in healthy participants with normal binocular vision, these patterns are unrelated to awareness of which eye is being stimulated.     

猫视皮层星形胶质细胞的老年性变化

电生理研究结果显示,在衰老过程中猫的视皮层神经元对视觉刺激的反应性出现显著的功能衰退,是否这种功能性衰退伴随胶质细胞活动的改变尚无直接的实验证据。以前期电生理实验猫为材料,用免疫形态学方法比较青年猫和老年猫初级视皮层内星形胶质细胞的活动状况。利用Nissl染色显示猫初级视皮层组织结构,用免疫组织化学方法（SABC法）显示GFAP免疫阳性（GFAP-IR）星形胶质细胞。光镜下观察、拍照,对GFAP-IR细胞计数并换算成密度,测量GFAP-IR直径取平均值。老年猫初级视皮层灰质各层及白质内的GFAP-IR细胞密度比青年猫的显著升高（p〈0.001）。与青年猫相比,老年猫视皮层灰质和白质中GFAP-IR细胞的平均直径均比青年猫的显著增大（p〈0.0001）,且老年猫视皮层内GFAP阳性免疫反应较青年猫的明显增强。老年猫初级视皮层神经元功能衰退伴随着星形胶质细胞活动的增强,胶质细胞活动增强有助于神经元之间的信息交流,因而可能对衰老过程中神经元的功能衰退起补偿作用。     

Visual physiology: perceived size looms large

Visual illusions tell us that size perception depends heavily upon complex contextual cues, often thought to be extracted by brain areas high in the visual hierarchy. Now, a new study shows that perceived size is reflected in activity as early as the primary visual cortex.     

Auditory Evoked Bursts in Mouse Visual Cortex during Isoflurane Anesthesia

General anesthesia is not a uniform state of the brain. Ongoing activity differs between light and deep anesthesia and cortical response properties are modulated in dependence of anesthetic dosage. We investigated how anesthesia level affects cross-modal interactions in primary sensory cortex. To examine this, we continuously measured the effects of visual and auditory stimulation during increasing and decreasing isoflurane level in the mouse visual cortex and the subiculum (from baseline at 0.7 to 2.5 vol % and reverse). Auditory evoked burst activity occurred in visual cortex after a transition during increase of anesthesia level. At the same time, auditory and visual evoked bursts occurred in the subiculum, even though the subiculum was unresponsive to both stimuli previous to the transition. This altered sensory excitability was linked to the presence of burst suppression activity in cortex, and to a regular slow burst suppression rhythm (∼0.2 Hz) in the subiculum. The effect disappeared during return to light anesthesia. The results show that pseudo-heteromodal sensory burst responses can appear in brain structures as an effect of an anesthesia induced state change.     

Peripheral and central inputs shape network dynamics in the developing visual cortex in vivo

Spontaneous network activity constitutes a central theme during the development of neuronal circuitry [1, 2]. Before the onset of vision, retinal neurons generate waves of spontaneous activity that are relayed along the ascending visual pathway [3, 4] and shape activity patterns in these regions [5, 6]. The spatiotemporal nature of retinal waves is required to establish precise functional maps in higher visual areas, and their disruption results in enlarged axonal projection areas (e.g., [7-10]). However, how retinal inputs shape network dynamics in the visual cortex on the cellular level is unknown. Using in vivo two-photon calcium imaging, we identified two independently occurring patterns of network activity in the mouse primary visual cortex (V1) before and at the onset of vision. Acute manipulations of spontaneous retinal activity revealed that one type of network activity largely originated in the retina and was characterized by low synchronicity (L-) events. In addition, we identified a type of high synchronicity (H-) events that required gap junction signaling but were independent of retinal input. Moreover, the patterns differed in wave progression and developmental profile. Our data suggest that different activity patterns have complementary functions during the formation of synaptic circuits in the developing visual cortex.     

The role of activity in development of the visual system

Neuronal activity is important for both the initial formation and the subsequent refinement of anatomical and physiological features of the mammalian visual system. Here we examine recent evidence concerning the role that spontaneous activity plays in axonal segregation, both of retinogeniculate afferents into eye-specific layers and of geniculocortical afferents into ocular dominance bands. We also assess the role of activity in the generation and plasticity of orientation selectivity in the primary visual cortex. Finally, we review recent challenges to textbook views on how inputs representing the two eyes interact during the critical period of visual cortical plasticity.     

Traveling waves in visual cortex

Electrode recordings and imaging studies have revealed that localized visual stimuli elicit waves of activity that travel across primary visual cortex. Traveling waves are present also during spontaneous activity, but they can be greatly reduced by widespread and intensive visual stimulation. In this Review, we summarize the evidence in favor of these traveling waves. We suggest that their substrate may lie in long-range horizontal connections and that their functional role may involve the integration of information over large regions of space.     

Pupillary reactions during transcranial magnetic stimulation

Pupillary reactions have been studied in healthy volunteers before, during, and after transcranial magnetic stimulation (TMS) of the primary visual cortex. During TMS in the projection of the primary visual cortex, a significant increase in pupil size was observed. Three minutes after the end of the TMS, a significant decrease in pupil size was recorded. These data point to a role of the primary visual cortex in the mechanisms of correcting pupillary reactions in humans.     

Innate visual learning through spontaneous activity patterns

Patterns of spontaneous activity in the developing retina, LGN, and cortex are necessary for the proper development of visual cortex. With these patterns intact, the primary visual cortices of many newborn animals develop properties similar to those of the adult cortex but without the training benefit of visual experience. Previous models have demonstrated how V1 responses can be initialized through mechanisms specific to development and prior to visual experience, such as using axonal guidance cues or relying on simple, pairwise correlations on spontaneous activity with additional developmental constraints. We argue that these spontaneous patterns may be better understood as part of an "innate learning" strategy, which learns similarly on activity both before and during visual experience. With an abstraction of spontaneous activity models, we show how the visual system may be able to bootstrap an efficient code for its natural environment prior to external visual experience, and we continue the same refinement strategy upon natural experience. The patterns are generated through simple, local interactions and contain the same relevant statistical properties of retinal waves and hypothesized waves in the LGN and V1. An efficient encoding of these patterns resembles a sparse coding of natural images by producing neurons with localized, oriented, bandpass structure-the same code found in early visual cortical cells. We address the relevance of higher-order statistical properties of spontaneous activity, how this relates to a system that may adapt similarly on activity prior to and during natural experience, and how these concepts ultimately relate to an efficient coding of our natural world.     

Exploring BOLD Changes during Spatial Attention in Non-Stimulated Visual Cortex

Blood oxygen level-dependent (BOLD) responses were measured in parts of primary visual cortex that represented unstimulated visual field regions at different distances from a stimulated central target location. The composition of the visual scene varied by the presence or absence of additional peripheral distracter stimuli. Bottom-up effects were assessed by comparing peripheral activity during central stimulation vs. no stimulation. Top-down effects were assessed by comparing active vs. passive conditions. In passive conditions subjects simply watched the central letter stimuli and in active conditions they had to report occurrence of pre-defined targets in a rapid serial letter stream. Onset of the central letter stream enhanced activity in V1 representations of the stimulated region. Within representations of the periphery activation decreased and finally turned into deactivation with increasing distance from the stimulated location. This pattern was most pronounced in the active conditions and during the presence of peripheral stimuli. Active search for a target did not lead to additional enhancement at areas representing the attentional focus but to a stronger deactivation in the vicinity. Suppressed neuronal activity was also found in the non distracter condition suggesting a top-down attention driven effect. Our observations suggest that BOLD signal decreases in primary visual cortex are modulated by bottom-up sensory-driven factors such as the presence of distracters in the visual field as well as by top-down attentional processes.