Decoding seen and attended motion directions from activity in the human visual cortex

Functional neuroimaging has successfully identified brain areas that show greater responses to visual motion and adapted responses to repeated motion directions. However, such methods have been thought to lack the sensitivity and spatial resolution to isolate direction-selective responses to individual motion stimuli. Here, we used functional magnetic resonance imaging (fMRI) and pattern classification methods to show that ensemble activity patterns in human visual cortex contain robust direction-selective information, from which it is possible to decode seen and attended motion directions. Ensemble activity in areas V1-V4 and MT+/V5 allowed us to decode which of eight possible motion directions the subject was viewing on individual stimulus blocks. Moreover, ensemble activity evoked by single motion directions could effectively predict which of two overlapping motion directions was the focus of the subject's attention and presumably dominant in perception. Our results indicate that feature-based attention can bias direction-selective population activity in multiple visual areas, including MT+/V5 and early visual areas (V1-V4), consistent with gain-modulation models of feature-based attention and theories of early attentional selection. Our approach for measuring ensemble direction selectivity may provide new opportunities to investigate relationships between attentional selection, conscious perception, and direction-selective responses in the human brain.     

Spatio-temporal brain mapping of motion-onset VEPs combined with fMRI and retinotopic maps

Neuroimaging studies have identified several motion-sensitive visual areas in the human brain, but the time course of their activation cannot be measured with these techniques. In the present study, we combined electrophysiological and neuroimaging methods (including retinotopic brain mapping) to determine the spatio-temporal profile of motion-onset visual evoked potentials for slow and fast motion stimuli and to localize its neural generators. We found that cortical activity initiates in the primary visual area (V1) for slow stimuli, peaking 100 ms after the onset of motion. Subsequently, activity in the mid-temporal motion-sensitive areas, MT+, peaked at 120 ms, followed by peaks in activity in the more dorsal area, V3A, at 160 ms and the lateral occipital complex at 180 ms. Approximately 250 ms after stimulus onset, activity fast motion stimuli was predominant in area V6 along the parieto-occipital sulcus. Finally, at 350 ms (100 ms after the motion offset) brain activity was visible again in area V1. For fast motion stimuli, the spatio-temporal brain pattern was similar, except that the first activity was detected at 70 ms in area MT+. Comparing functional magnetic resonance data for slow vs. fast motion, we found signs of slow-fast motion stimulus topography along the posterior brain in at least three cortical regions (MT+, V3A and LOR).     

Tactile motion and pattern processing assessed with high-field FMRI

Processing of motion and pattern has been extensively studied in the visual domain, but much less in the somatosensory system. Here, we used ultra-high-field functional magnetic resonance imaging (fMRI) at 7 Tesla to investigate the neuronal correlates of tactile motion and pattern processing in humans under tightly controlled stimulation conditions. Different types of dynamic stimuli created the sensation of moving or stationary bar patterns during passive touch. Activity in somatosensory cortex was increased during both motion and pattern processing and modulated by motion directionality in primary and secondary somatosensory cortices (SI and SII) as well as by pattern orientation in the anterior intraparietal sulcus. Furthermore, tactile motion and pattern processing induced activity in the middle temporal cortex (hMT+/V5) and in the inferior parietal cortex (IPC), involving parts of the supramarginal und angular gyri. These responses covaried with subjects' individual perceptual performance, suggesting that hMT+/V5 and IPC contribute to conscious perception of specific tactile stimulus features. In addition, an analysis of effective connectivity using psychophysiological interactions (PPI) revealed increased functional coupling between SI and hMT+/V5 during motion processing, as well as between SI and IPC during pattern processing. This connectivity pattern provides evidence for the direct engagement of these specialized cortical areas in tactile processing during somesthesis.     

Stereopsis activates V3A and caudal intraparietal areas in macaques and humans

Stereopsis, the perception of depth from small differences between the images in the two eyes, provides a rich model for investigating the cortical construction of surfaces and space. Although disparity-tuned cells have been found in a large number of areas in macaque visual cortex, stereoscopic processing in these areas has never been systematically compared using the same stimuli and analysis methods. In order to examine the global architecture of stereoscopic processing in primate visual cortex, we studied fMRI activity in alert, fixating human and macaque subjects. In macaques, we found strongest activation to near/far compared to zero disparity in areas V3, V3A, and CIPS. In humans, we found strongest activation to the same stimuli in areas V3A, V7, the V4d topolog (V4d-topo), and a caudal parietal disparity region (CPDR). Thus, in both primate species a small cluster of areas at the parieto-occipital junction appears to be specialized for stereopsis.     

Integration of Motion Responses Underlying Directional Motion Anisotropy in Human Early Visual Cortical Areas

Recent imaging studies have reported directional motion biases in human visual cortex when perceiving moving random dot patterns. It has been hypothesized that these biases occur as a result of the integration of motion detector activation along the path of motion in visual cortex. In this study we investigate the nature of such motion integration with functional MRI (fMRI) using different motion stimuli. Three types of moving random dot stimuli were presented, showing either coherent motion, motion with spatial decorrelations or motion with temporal decorrelations. The results from the coherent motion stimulus reproduced the centripetal and centrifugal directional motion biases in V1, V2 and V3 as previously reported. The temporally decorrelated motion stimulus resulted in both centripetal and centrifugal biases similar to coherent motion. In contrast, the spatially decorrelated motion stimulus resulted in small directional motion biases that were only present in parts of visual cortex coding for higher eccentricities of the visual field. In combination with previous results, these findings indicate that biased motion responses in early visual cortical areas most likely depend on the spatial integration of a simultaneously activated motion detector chain.     

Changes in Choline Acetyltransferase Activity and High-Affinity Choline Uptake,but Not in Acetylcholinesterase Activity and Muscarinic Cholinergic Receptors,in Rat Somatosensory Cortex after Sciatic Nerve Injury

Selected cholinergic markers (choline acetyltransferase, acetylcholinesterase, muscarinic acetylcholine receptor, high-affinity choline uptake) were studied in the hindlimb representation areas of the rat somatosensory cortex and within the visual cortex 1 to 63 days after unilateral transection of the sciatic nerve. In the contralateral somatosensory cortex, peripheral deafferentation resulted in a significant reduction of choline acetyltransferase activity (by 15%) 3 days after sciatic nerve injury, and in a significant reduction of high-affinity choline uptake (by 30%) 1 day after nerve transection, in comparison to untreated control rats. Investigations in individual cortical layers revealed that the decrease of both choline acetyltransferase activity and high-affinity choline uptake sites was mainly due to reductions in cortical layer V. Acetylcholinesterase activity and [³H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors were not affected by unilateral transection of the sciatic nerve. In the ipsilateral somatosensory cortex, as well as in the visual cortex at both cortical hemispheres, no significant changes in the cholinergic parameters studied could be detected. The data indicate that peripheral deafferentation of the somatosensory cortex results in a transient change of presynaptic cholinergic parameters within the affected somatosensory area as early as 1 to 3 days after the lesion; thus, they emphasize the involvement of cholinergic mechanisms in cortical reorganizational events.     

Primary visual cortex activity along the apparent-motion trace reflects illusory perception

The illusion of apparent motion can be induced when visual stimuli are successively presented at different locations. It has been shown in previous studies that motion-sensitive regions in extrastriate cortex are relevant for the processing of apparent motion, but it is unclear whether primary visual cortex (V1) is also involved in the representation of the illusory motion path. We investigated, in human subjects, apparent-motion-related activity in patches of V1 representing locations along the path of illusory stimulus motion using functional magnetic resonance imaging. Here we show that apparent motion caused a blood-oxygenation-level-dependent response along the V1 representations of the apparent-motion path, including regions that were not directly activated by the apparent-motion-inducing stimuli. This response was unaltered when participants had to perform an attention-demanding task that diverted their attention away from the stimulus. With a bistable motion quartet, we confirmed that the activity was related to the conscious perception of movement. Our data suggest that V1 is part of the network that represents the illusory path of apparent motion. The activation in V1 can be explained either by lateral interactions within V1 or by feedback mechanisms from higher visual areas, especially the motion-sensitive human MT/V5 complex.     

Gamma oscillations in human primary somatosensory cortex reflect pain perception

Successful behavior requires selection and preferred processing of relevant sensory information. The cortical representation of relevant sensory information has been related to neuronal oscillations in the gamma frequency band. Pain is of invariably high behavioral relevance and, thus, nociceptive stimuli receive preferred processing. Here, by using magnetoencephalography, we show that selective nociceptive stimuli induce gamma oscillations between 60 and 95 Hz in primary somatosensory cortex. Amplitudes of pain-induced gamma oscillations vary with objective stimulus intensity and subjective pain intensity. However, around pain threshold, perceived stimuli yielded stronger gamma oscillations than unperceived stimuli of equal stimulus intensity. These results show that pain induces gamma oscillations in primary somatosensory cortex that are particularly related to the subjective perception of pain. Our findings support the hypothesis that gamma oscillations are related to the internal representation of behaviorally relevant stimuli that should receive preferred processing.     

纹外皮层PMLS区反馈投射对初级视皮层神经元反应特性的影响

神经系统中存在大量下行投射,与上行输入一起形成复杂的前馈与反馈回路,调控神经信号的传导和处理,但目前对皮层内反馈投射的功能作用认识还比较薄弱.通过微量注射抑制性神经递质γ-氨基丁酸(γ-aminobutyric acid,GABA),使猫纹外皮层后内侧外上雪氏区(area posteromedial lateral suprasylvian,PMLS)局部可逆性失活,使用胞外记录方法,研究初级视皮层17区神经元反应特性的变化.实验结果显示,PMLS区失活后,17区细胞对运动刺激的反应总体减弱,反应的相对稳定性基本不变,最高发放率/自发之比有所下降.与此同时,细胞的方向选择性指数减小,朝向选择性无显著变化.除少数"双向"反应细胞外,绝大部分细胞的最优方向基本不变.进一步分析发现,细胞对各个方向刺激的反应普遍下降,最优方向上的下降程度最大,是导致方向选择性减弱的主要原因.这些结果表明,PMLS区反馈投射可增强初级视皮层的方向选择性,而对朝向选择性影响有限.这一作用特点体现了PMLS区在皮层中偏重处理运动视觉信息的功能.     

Retinotopic mapping of categorical and coordinate spatial relation processing in early visual cortex

Spatial relations are commonly divided in two global classes. Categorical relations concern abstract relations which define areas of spatial equivalence, whereas coordinate relations are metric and concern exact distances. Categorical and coordinate relation processing are thought to rely on at least partially separate neurocognitive mechanisms, as reflected by differential lateralization patterns, in particular in the parietal cortex. In this study we address this textbook principle from a new angle. We studied retinotopic activation in early visual cortex, as a reflection of attentional distribution, in a spatial working memory task with either a categorical or a coordinate instruction. Participants were asked to memorize a dot position, with regard to a central cross, and to indicate whether a subsequent dot position matched the first dot position, either categorically (opposite quadrant of the cross) or coordinately (same distance to the centre of the cross). BOLD responses across the retinotopic maps of V1, V2, and V3 indicate that the spatial distribution of cortical activity was different for categorical and coordinate instructions throughout the retention interval; a more local focus was found during categorical processing, whereas focus was more global for coordinate processing. This effect was strongest for V3, approached significance in V2 and was absent in V1. Furthermore, during stimulus presentation the two instructions led to different levels of activation in V3 during stimulus encoding; a stronger increase in activity was found for categorical processing. Together this is the first demonstration that instructions for specific types of spatial relations may yield distinct attentional patterns which are already reflected in activity early in the visual cortex.     

Direct evidence for encoding of motion streaks in human visual cortex

Temporal integration in the visual system causes fast-moving objects to generate static, oriented traces (‘motion streaks’), which could be used to help judge direction of motion. While human psychophysics and single-unit studies in non-human primates are consistent with this hypothesis, direct neural evidence from the human cortex is still lacking. First, we provide psychophysical evidence that faster and slower motions are processed by distinct neural mechanisms: faster motion raised human perceptual thresholds for static orientations parallel to the direction of motion, whereas slower motion raised thresholds for orthogonal orientations. We then used functional magnetic resonance imaging to measure brain activity while human observers viewed either fast (‘streaky’) or slow random dot stimuli moving in different directions, or corresponding static-oriented stimuli. We found that local spatial patterns of brain activity in early retinotopic visual cortex reliably distinguished between static orientations. Critically, a multivariate pattern classifier trained on brain activity evoked by these static stimuli could then successfully distinguish the direction of fast (‘streaky’) but not slow motion. Thus, signals encoding static-oriented streak information are present in human early visual cortex when viewing fast motion. These experiments show that motion streaks are present in the human visual system for faster motion.     

Separating Fusion from Rivalry

Visual fusion is the process in which differing but compatible binocular information is transformed into a unified percept. Even though this is at the basis of binocular vision, the underlying neural processes are, as yet, poorly understood. In our study we therefore aimed to investigate neural correlates of visual fusion. To this end, we presented binocularly compatible, fusible (BF), and incompatible, rivaling (BR) stimuli, as well as an intermediate stimulus type containing both binocularly fusible and monocular, incompatible elements (BFR). Comparing BFR stimuli with BF and BR stimuli, respectively, we were able to disentangle brain responses associated with either visual fusion or rivalry. By means of functional magnetic resonance imaging, we measured brain responses to these stimulus classes in the visual cortex, and investigated them in detail at various retinal eccentricities. Compared with BF stimuli, the response to BFR stimuli was elevated in visual cortical areas V1 and V2, but not in V3 and V4 – implying that the response to monocular stimulus features decreased from V1 to V4. Compared to BR stimuli, the response to BFR stimuli decreased with increasing eccentricity, specifically within V3 and V4. Taken together, it seems that although the processing of exclusively monocular information decreases from V1 to V4, the processing of binocularly fused information increases from earlier to later visual areas. Our findings suggest the presence of an inhibitory neural mechanism which, depending on the presence of fusion, acts differently on the processing of monocular information.     

Orientation-selective neurons in the squirrel visual cortex

The organization of receptive fields of neurons sensitive to orientation of visual stimuli was investigated in the squirrel visual cortex. Neurons with mutually inhibitory on- and off-areas of the receptive field, with partially and completely overlapping excitatory and inhibitory mechanisms, were distinguished. Neurons of the second group are most typical. They exhibit orientation selectivity within the excitatory area of the receptive field because, if the stimulus widens in the zero direction, perpendicular to the preferred direction, lateral inhibition is much stronger than if it widens in the preferred direction. Additional inhibitory areas (outside the excitatory area) potentiate this inhibition and increase selectivity. It is suggested that there is no strict separation of simple (with separate excitatory and inhibitory mechanisms in the receptive field) and complex (with overlapping of these mechanisms) neurons in the squirrel visual cortex.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 6, pp. 540–549, November–December, 1979.     

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.     

Parkinson-Related Changes of Activation in Visuomotor Brain Regions during Perceived Forward Self-Motion

Radial expanding optic flow is a visual consequence of forward locomotion. Presented on screen, it generates illusionary forward self-motion, pointing at a close vision-gait interrelation. As particularly parkinsonian gait is vulnerable to external stimuli, effects of optic flow on motor-related cerebral circuitry were explored with functional magnetic resonance imaging in healthy controls (HC) and patients with Parkinson’s disease (PD). Fifteen HC and 22 PD patients, of which 7 experienced freezing of gait (FOG), watched wide-field flow, interruptions by narrowing or deceleration and equivalent control conditions with static dots. Statistical parametric mapping revealed that wide-field flow interruption evoked activation of the (pre-)supplementary motor area (SMA) in HC, which was decreased in PD. During wide-field flow, dorsal occipito-parietal activations were reduced in PD relative to HC, with stronger functional connectivity between right visual motion area V5, pre-SMA and cerebellum (in PD without FOG). Non-specific ‘changes’ in stimulus patterns activated dorsolateral fronto-parietal regions and the fusiform gyrus. This attention-associated network was stronger activated in HC than in PD. PD patients thus appeared compromised in recruiting medial frontal regions facilitating internally generated virtual locomotion when visual motion support falls away. Reduced dorsal visual and parietal activations during wide-field optic flow in PD were explained by impaired feedforward visual and visuomotor processing within a magnocellular (visual motion) functional chain. Compensation of impaired feedforward processing by distant fronto-cerebellar circuitry in PD is consistent with motor responses to visual motion stimuli being either too strong or too weak. The ‘change’-related activations pointed at covert (stimulus-driven) attention.     

Implications on visual apperception: Energy, duration, structure and synchronization

Although primary visual cortex (V1 or striate) activity per se is not sufficient for visual apperception (normal conscious visual experiences and conscious functions such as detection, discrimination, and recognition), the same is also true for extrastriate visual areas (such as V2, V3, V4/V8/VO, V5/M5/MST, IT, and GF). In the lack of V1 area, visual signals can still reach several extrastriate parts but appear incapable of generating normal conscious visual experiences. It is scarcely emphasized in the scientific literature that conscious perceptions and representations must have also essential energetic conditions. These energetic conditions are achieved by spatiotemporal networks of dynamic mitochondrial distributions inside neurons. However, the highest density of neurons in neocortex (number of neurons per degree of visual angle) devoted to representing the visual field is found in retinotopic V1. It means that the highest mitochondrial (energetic) activity can be achieved in mitochondrial cytochrome oxidase-rich V1 areas. Thus, V1 bear the highest energy allocation for visual representation.In addition, the conscious perceptions also demand structural conditions, presence of adequate duration of information representation, and synchronized neural processes and/or ‘interactive hierarchical structuralism.’ For visual apperception, various visual areas are involved depending on context such as stimulus characteristics such as color, form/shape, motion, and other features. Here, we focus primarily on V1 where specific mitochondrial-rich retinotopic structures are found; we will concisely discuss V2 where smaller riches of these structures are found. We also point out that residual brain states are not fully reflected in active neural patterns after visual perception. Namely, after visual perception, subliminal residual states are not being reflected in passive neural recording techniques, but require active stimulation to be revealed.     

Bistable percepts in the brain: FMRI contrasts monocular pattern rivalry and binocular rivalry

The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1.     

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.     

Predicting the stream of consciousness from activity in human visual cortex

Can the rapid stream of conscious experience be predicted from brain activity alone? Recently, spatial patterns of activity in visual cortex have been successfully used to predict feature-specific stimulus representations for both visible and invisible stimuli. However, because these studies examined only the prediction of static and unchanging perceptual states during extended periods of stimulation, it remains unclear whether activity in early visual cortex can also predict the rapidly and spontaneously changing stream of consciousness. Here, we used binocular rivalry to induce frequent spontaneous and stochastic changes in conscious experience without any corresponding changes in sensory stimulation, while measuring brain activity with fMRI. Using information that was present in the multivariate pattern of responses to stimulus features, we could accurately predict, and therefore track, participants' conscious experience from the fMRI signal alone while it underwent many spontaneous changes. Prediction in primary visual cortex primarily reflected eye-based signals, whereas prediction in higher areas reflected the color of the percept. Furthermore, accurate prediction during binocular rivalry could be established with signals recorded during stable monocular viewing, showing that prediction generalized across viewing conditions and did not require or rely on motor responses. It is therefore possible to predict the dynamically changing time course of subjective experience with only brain activity.     

Stronger Neural Modulation by Visual Motion Intensity in Autism Spectrum Disorders

Theories of autism spectrum disorders (ASD) have focused on altered perceptual integration of sensory features as a possible core deficit. Yet, there is little understanding of the neuronal processing of elementary sensory features in ASD. For typically developed individuals, we previously established a direct link between frequency-specific neural activity and the intensity of a specific sensory feature: Gamma-band activity in the visual cortex increased approximately linearly with the strength of visual motion. Using magnetoencephalography (MEG), we investigated whether in individuals with ASD neural activity reflect the coherence, and thus intensity, of visual motion in a similar fashion. Thirteen adult participants with ASD and 14 control participants performed a motion direction discrimination task with increasing levels of motion coherence. A polynomial regression analysis revealed that gamma-band power increased significantly stronger with motion coherence in ASD compared to controls, suggesting excessive visual activation with increasing stimulus intensity originating from motion-responsive visual areas V3, V6 and hMT/V5. Enhanced neural responses with increasing stimulus intensity suggest an enhanced response gain in ASD. Response gain is controlled by excitatory-inhibitory interactions, which also drive high-frequency oscillations in the gamma-band. Thus, our data suggest that a disturbed excitatory-inhibitory balance underlies enhanced neural responses to coherent motion in ASD.