Selectivity to Translational Egomotion in Human Brain Motion Areas

The optic flow generated when a person moves through the environment can be locally decomposed into several basic components, including radial, circular, translational and spiral motion. Since their analysis plays an important part in the visual perception and control of locomotion and posture it is likely that some brain regions in the primate dorsal visual pathway are specialized to distinguish among them. The aim of this study is to explore the sensitivity to different types of egomotion-compatible visual stimulations in the human motion-sensitive regions of the brain. Event-related fMRI experiments, 3D motion and wide-field stimulation, functional localizers and brain mapping methods were used to study the sensitivity of six distinct motion areas (V6, MT, MST+, V3A, CSv and an Intra-Parietal Sulcus motion [IPSmot] region) to different types of optic flow stimuli. Results show that only areas V6, MST+ and IPSmot are specialized in distinguishing among the various types of flow patterns, with a high response for the translational flow which was maximum in V6 and IPSmot and less marked in MST+. Given that during egomotion the translational optic flow conveys differential information about the near and far external objects, areas V6 and IPSmot likely process visual egomotion signals to extract information about the relative distance of objects with respect to the observer. Since area V6 is also involved in distinguishing object-motion from self-motion, it could provide information about location in space of moving and static objects during self-motion, particularly in a dynamically unstable environment.     

The representation of egomotion in the human brain

An essential function of visual processing is to establish the position of the body in space and, in concert with the other sense systems, to monitor movement of the whole body, or "egomotion." A key cue to egomotion is optic flow. For example, forward motion through the environment generates an expanding pattern of flow on the retina, and (with eyes fixed centrally) the direction of heading corresponds to the center of expansion [1]. In macaques, visual cortical area MST is sensitive to optic-flow structure [2, 3], and it has been suggested that MST has a central role in the computation of heading [4]. However, here we identify two areas of the human brain that represent visual cues to egomotion more directly than does MST. These areas respond strongly to a single optic-flow stimulus but become relatively unresponsive when the stimulus is surrounded with further flow patches and thereby made inconsistent with egomotion. One is putative area VIP in the anterior portion of the intraparietal sulcus. The other is a new visual area, which we refer to as cingulate sulcus visual area (CSv). Areas V1-V4 and MT respond about equally to both types of flow stimulus. MST has intermediate properties, responding well to multiple patches but with a modest preference for a single, egomotion-compatible patch. We suggest that MST is merely an intermediate processing stage for visual cues to egomotion and that such cues are more comprehensively encoded by VIP and CSv.     

Form and motion coherence activate independent, but not dorsal/ventral segregated, networks in the human brain

There is much evidence in primates' visual processing for distinct mechanisms involved in object recognition and encoding object position and motion, which have been identified with 'ventral' and 'dorsal' streams, respectively, of the extra-striate visual areas [1] [2] [3]. This distinction may yield insights into normal human perception, its development and pathology. Motion coherence sensitivity has been taken as a test of global processing in the dorsal stream [4] [5]. We have proposed an analogous 'form coherence' measure of global processing in the ventral stream [6]. In a functional magnetic resonance imaging (fMRI) experiment, we found that the cortical regions activated by form coherence did not overlap with those activated by motion coherence in the same individuals. Areas differentially activated by form coherence included regions in the middle occipital gyrus, the ventral occipital surface, the intraparietal sulcus, and the temporal lobe. Motion coherence activated areas consistent with those previously identified as V5 and V3a, the ventral occipital surface, the intraparietal sulcus, and temporal structures. Neither form nor motion coherence activated area V1 differentially. Form and motion foci in occipital, parietal, and temporal areas were nearby but showed almost no overlap. These results support the idea that form and motion coherence test distinct functional brain systems, but that these do not necessarily correspond to a gross anatomical separation of dorsal and ventral processing streams.     

Sensitive period for a multimodal response in human visual motion area MT/MST

The middle temporal complex (MT/MST) is a brain region specialized for the perception of motion in the visual modality. However, this specialization is modified by visual experience: after long-standing blindness, MT/MST responds to sound. Recent evidence also suggests that the auditory response of MT/MST is selective for motion. The developmental time course of this plasticity is not known. To test for a sensitive period in MT/MST development, we used fMRI to compare MT/MST function in congenitally blind, late-blind, and sighted adults. MT/MST responded to sound in congenitally blind adults, but not in late-blind or sighted adults, and not in an individual who lost his vision between ages of 2 and 3 years. All blind adults had reduced functional connectivity between MT/MST and other visual regions. Functional connectivity was increased between MT/MST and lateral prefrontal areas in congenitally blind relative to sighted and late-blind adults. These data suggest that early blindness affects the function of feedback projections from prefrontal cortex to MT/MST. We conclude that there is a sensitive period for visual specialization in MT/MST. During typical development, early visual experience either maintains or creates a vision-dominated response. Once established, this response profile is not altered by long-standing blindness.     

Practicing coarse orientation discrimination improves orientation signals in macaque cortical area v4

Practice improves the performance in visual tasks, but mechanisms underlying this adult brain plasticity are unclear. Single-cell studies reported no [1], weak [2], or moderate [3, 4] perceptual learning-related changes in macaque visual areas V1 and V4, whereas none were found in middle temporal (MT) [5]. These conflicting results and modeling of human (e.g., [6, 7]) and monkey data [8] suggested that changes in the readout of visual cortical signals underlie perceptual learning, rather than changes in these signals. In the V4 learning studies, monkeys discriminated small differences in orientation, whereas in the MT study, the animals discriminated opponent motion directions. Analogous to the latter study, we trained monkeys to discriminate static orthogonal orientations masked by noise. V4 neurons showed robust increases in their capacity to discriminate the trained orientations during the course of the training. This effect was observed during discrimination and passive fixation but specifically for the trained orientations. The improvement in neural discrimination was due to decreased response variability and an increase of the difference between the mean responses for the two trained orientations. These findings demonstrate that perceptual learning in a coarse discrimination task indeed can change the response properties of a cortical sensory area.     

Neural population representation hypothesis of visual flow and its illusory after effect in the brain: psychophysics,neurophysiology and computational approaches

The neural representation of motion aftereffects induced by various visual flows (translational, rotational, motion-in-depth, and translational transparent flows) was studied under the hypothesis that the imbalances in discharge activities would occur in favor in the direction opposite to the adapting stimulation in the monkey MST cells (cells in the medial superior temporal area) which can discriminate the mode (i.e., translational, rotational, or motion-in-depth) of the given flow. In single-unit recording experiments conducted on anaesthetized monkeys, we found that the rate of spontaneous discharge and the sensitivity to a test stimulus moving in the preferred direction decreased after receiving an adapting stimulation moving in the preferred direction, whereas they increased after receiving an adapting stimulation moving in the null direction. To consistently explain the bidirectional perception of a transparent visual flow and its unidirectional motion aftereffect by the same hypothesis, we need to assume the existence of two subtypes of MST D cells which show directionally selective responses to a translational flow: component cells and integration cells. Our physiological investigation revealed that the MST D cells could be divided into two types: one responded to a transparent flow by two peaks at the instances when the direction of one of the component flow matched the preferred direction of the cell, and the other responded by a single peak at the instance when the direction of the integrated motion matched the preferred direction. In psychophysical experiments on human subjects, we found evidence for the existence of component and integration representations in the human brain. To explain the different motion perceptions, i.e., two transparent flows during presentation of the flows and a single flow in the opposite direction to the integrated flows after stopping the flow stimuli, we suggest that the pattern-discrimination system can select the motion representation that is consistent with the perception of the pattern from two motion representations. We discuss the computational aspects related to the integration of component motion fields.     

An fMRI study of the selective activation of human extrastriate form vision areas by radial and concentric gratings

The ventral form vision pathway of the primate brain comprises a sequence of areas that include V1, V2, V4 and the inferior temporal cortex (IT) [1]. Although contour extraction in the V1 area and responses to complex images, such as faces, in the IT have been studied extensively, much less is known about shape extraction at intermediate cortical levels such as V4. Here, we used functional magnetic resonance imaging (fMRI) to demonstrate that the human V4 is more strongly activated by concentric and radial patterns than by conventional sinusoidal gratings. This is consistent with global pooling of local V1 orientations to extract concentric and radial shape information in V4. Furthermore, concentric patterns were found to be effective in activating the fusiform face area. These findings support recent psychophysical [2,3] and physiological [4,5] data indicating that analysis of concentric and radial structure represents an important aspect of processing at intermediate levels of form vision.     

Adaptation to left-right reversed vision rapidly activates ipsilateral visual cortex in humans.

The brain mechanisms of adaptation to visual transposition are of increasing interest, not only for research on sensory-motor coordination, but also for neuropsychological rehabilitation. Sugita [Nature 380 (1996) 523] found that after adaptation to left-right reversed vision for one and a half months, monkey V1 neurons responded to stimuli presented not only in the contralateral visual field, but also in the ipsilateral visual field. To identify the underlying neuronal mechanisms of adaptation to visual transposition, we conducted fMRI and behavioral experiments for which four adult human subjects wore left-right reversing goggles for 35/39 days, and investigated: (1) whether ipsilateral V1 activation can be induced in human adult subjects; (2) if yes, when the ipsilateral activity starts, and what kind of behavioral/psychological changes occur accompanying the ipsilateral activity; (3) whether other visual cortices also show an ipsilateral activity change. The results of behavioral experiments showed that visuomotor coordinative function and internal representation of peripersonal space rapidly adapted to the left-right reversed vision within the first or second week. Accompanying these behavioral changes, we found that both primary (V1) and extrastriate (MT/MST) visual cortex in human adults responded to visual stimuli presented in the ipsilateral visual field. In addition, the ipsilateral activity started much sooner than the one and a half months, which had been expected from the monkey neurophysiological study. The results of the present study serve as physiological evidence of large-scale, cross-hemisphere, cerebral plasticity that exists even in adult human brain.     

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.     

Radial Frequency Analysis of Contour Shapes in the Visual Cortex

Cumulative psychophysical evidence suggests that the shape of closed contours is analysed by means of their radial frequency components (RFC). However, neurophysiological evidence for RFC-based representations is still missing. We investigated the representation of radial frequency in the human visual cortex with functional magnetic resonance imaging. We parametrically varied the radial frequency, amplitude and local curvature of contour shapes. The stimuli evoked clear responses across visual areas in the univariate analysis, but the response magnitude did not depend on radial frequency or local curvature. Searchlight-based, multivariate representational similarity analysis revealed RFC specific response patterns in areas V2d, V3d, V3AB, and IPS0. Interestingly, RFC-specific representations were not found in hV4 or LO, traditionally associated with visual shape analysis. The modulation amplitude of the shapes did not affect the responses in any visual area. Local curvature, SF-spectrum and contrast energy related representations were found across visual areas but without similar specificity for visual area that was found for RFC. The results suggest that the radial frequency of a closed contour is one of the cortical shape analysis dimensions, represented in the early and mid-level visual areas.     

暗示性运动加工的认知神经机制

暗示性运动是指个体观看静止图片时从中知觉到的运动.研究者采用高低认知水平两类暗示性运动刺激材料,借助"冻结帧"、直接观看、运动后效和f MRI适应等任务范式,探讨了注意和意识在暗示性运动加工中的作用及其记忆特点;并借助脑成像等技术,考察了颞中区、颞上皮层区、颞上沟、镜像神经元系统等脑区在暗示性运动加工中的作用.但由于暗示性运动加工涉及"视觉腹侧通路与背侧通路功能的分离与整合"问题,目前对相关研究结果和解释还存在争议,暗示性运动加工的认知神经机制仍有待于进一步研究.     

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).     

Cardinal directions for visual optic flow.

As we move through our environment, the flow of deforming images on the retinae provides a rich source of information about the three-dimensional structure of the external world and how to navigate through it. Recent evidence from psychophysical [1] [2] [3] [4], electrophysiological [5] [6] [7] [8] [9] and imaging [10] [11] studies suggests that there are neurons in the primate visual system - in the medial superior temporal cortex - that are specialised to respond to this type of complex 'optic flow' motion. In principle, optic flow could be encoded by a small number of neural mechanisms tuned to 'cardinal directions', including radial and circular motion [12] [13]. There is little support for this idea at present, however, from either physiological [6] [7] or psychophysical [14] research. We have measured the sensitivity of human subjects for detection of motion and for discrimination of motion direction over a wide and densely sampled range of complex motions. Average sensitivity was higher for inward and outward radial movement and for both directions of rotation, consistent with the existence of detectors tuned to these four types of motion. Principle component analysis revealed two clear components, one for radial stimuli (outward and inward) and the other for circular stimuli (clockwise and counter-clock-wise). The results imply that the mechanisms that analyse optic flow in humans tend to be tuned to the cardinal axes of radial and rotational motion.     

A direct projection from area V1 to area V3A of rhesus monkey visual cortex

Small cortical lesions were made in regions of the primary visual cortex (V1) representing different retinal eccentricities. It was found that, whereas all parts of V1 project to visual areas V2, V3 and the motion area of the superior temporal sulcus, only parts of V1 representing peripheral eccentricities (in excess of 30 degrees) project directly to visual area V3A.     

Human V6: Functional Characterisation and Localisation

Human visual area V6, in the parieto-occipital sulcus, is thought to have an important role in the extraction of optic flow for the monitoring and guidance of self-motion (egomotion) because it responds differentially to egomotion-compatible optic flow when compared to: (a) coherent but egomotion-incompatible flow (Cardin & Smith, 2010), and (b) incoherent motion (Pitzalis et al., 2010). It is not clear, however, whether V6 responds more strongly to egomotion-incompatible global motion than to incoherent motion. This is relevant not only for determining the functional properties of V6, but also in order to choose optimal stimuli for localising V6 accurately with fMRI. Localisation with retinotopic mapping is difficult and there is a need for a simple, reliable method. We conducted an event-related 3T fMRI experiment in which participants viewed a display of dots which either: a) followed a time-varying optic flow trajectory in a single, egomotion-compatible (EC) display; b) formed an egomotion-incompatible (EI) 3×3 array of optic flow patches; or c) moved randomly (RM). Results from V6 show an ordering of response magnitudes: EC > EI > RM. Neighbouring areas V3A and V7 responded more strongly to EC than to RM, but about equally to EC and EI. Our results suggest that although V6 may have a general role in the extraction of global motion, in clear contrast to neighbouring motion areas it is especially concerned with encoding EC stimuli. They suggest two strategies for localising V6: (1) contrasting EC and EI; or (2) contrasting EC and RM, which is more sensitive but carries a risk of including voxels from neighbouring regions that also show a EC > RM preference.     

Interhemispheric connections shape subjective experience of bistable motion

The right and left visual hemifields are represented in different cerebral hemispheres and are bound together by connections through the corpus callosum. Much has been learned on the functions of these connections from split-brain patients [1-4], but little is known about their contribution to conscious visual perception in healthy humans. We used diffusion tensor imaging and functional magnetic resonance imaging to investigate which callosal connections contribute to the subjective experience of a visual motion stimulus that requires interhemispheric integration. The "motion quartet" is an ambiguous version of apparent motion that leads to perceptions of either horizontal or vertical motion [5]. Interestingly, observers are more likely to perceive vertical than horizontal motion when the stimulus is presented centrally in the visual field [6]. This asymmetry has been attributed to the fact that, with central fixation, perception of horizontal motion requires integration across hemispheres whereas perception of vertical motion requires only intrahemispheric processing [7]. We are able to show that the microstructure of individually tracked callosal segments connecting motion-sensitive areas of the human MT/V5 complex (hMT/V5+; [8]) can predict the conscious perception of observers. Neither connections between primary visual cortex (V1) nor other surrounding callosal regions exhibit a similar relationship.     

From 1D to 2D via 3D: dynamics of surface motion segmentation for ocular tracking in primates.

In primates, tracking eye movements help vision by stabilising onto the retinas the images of a moving object of interest. This sensorimotor transformation involves several stages of motion processing, from the local measurement of one-dimensional luminance changes up to the integration of first and higher-order local motion cues into a global two-dimensional motion immune to antagonistic motions arising from the surrounding. The dynamics of this surface motion segmentation is reflected into the various components of the tracking responses and its underlying neural mechanisms can be correlated with behaviour at both single-cell and population levels. I review a series of behavioural studies which demonstrate that the neural representation driving eye movements evolves over time from a fast vector average of the outputs of linear and non-linear spatio-temporal filtering to a progressive and slower accurate solution for global motion. Because of the sensitivity of earliest ocular following to binocular disparity, antagonistic visual motion from surfaces located at different depths are filtered out. Thus, global motion integration is restricted within the depth plane of the object to be tracked. Similar dynamics were found at the level of monkey extra-striate areas MT and MST and I suggest that several parallel pathways along the motion stream are involved albeit with different latencies to build-up this accurate surface motion representation. After 200-300 ms, most of the computational problems of early motion processing (aperture problem, motion integration, motion segmentation) are solved and the eye velocity matches the global object velocity to maintain a clear and steady retinal image.     

Seeing invisible motion: a human FMRI study

A common view about visual consciousness is that it could arise when and where activity reaches some higher level of processing along the cortical hierarchy. Reports showing that activity in striate cortex can be dissociated from awareness , whereas the latter modulates activity in higher areas , point in this direction. In the specific case of visual motion, a central, "perceptual" role has been assigned to area V5: several human and monkey studies have shown V5 activity to correlate with the motion percept. Here we show that activity in this and other higher cortical areas can be also dissociated from perception and follow the physical stimulus instead. The motion information in a peripheral grating modulated fMRI responses, despite being invisible to human volunteers: under crowding conditions , areas V3A, V5, and parietal cortex still showed increased activity when the grating was moving compared to when it was flickering. We conclude that stimulus-specific activation of higher cortical areas does not necessarily result in awareness of the underlying stimulus.     

Attention-dependent representation of a size illusion in human V1

One of the most fundamental properties of human primary visual cortex (V1) is its retinotopic organization, which makes it an ideal candidate for encoding spatial properties, such as size, of objects. However, three-dimensional (3D) contextual information can lead to size illusions that are reflected in the spatial pattern of activity in V1 [1]. A critical question is how complex 3D contextual information can influence spatial activity patterns in V1. Here, we assessed whether changes in the spatial distribution of activity in V1 depend on the focus of attention, which would be suggestive of feedback of 3D contextual information from higher visual areas. We presented two 3D rings at close and far apparent depths in a 3D scene. When subjects fixated its center, the far ring appeared to be larger and occupy a more eccentric portion of the visual field, relative to the close ring. Using functional magnetic resonance imaging, we found that the spatial distribution of V1 activity induced by the far ring was also shifted toward a more eccentric representation of the visual field, whereas that induced by the close ring was shifted toward the foveal representation, consistent with their perceptual appearances. This effect was significantly reduced when the focus of spatial attention was narrowed with a demanding central fixation task. We reason that focusing attention on the fixation task resulted in reduced activity in--and therefore reduced feedback from--higher visual areas that process the 3D depth cues.