A new neural framework for visuospatial processing

The division of cortical visual processing into distinct dorsal and ventral streams is a key framework that has guided visual neuroscience. The characterization of the ventral stream as a 'What' pathway is relatively uncontroversial, but the nature of dorsal stream processing is less clear. Originally proposed as mediating spatial perception ('Where'), more recent accounts suggest it primarily serves non-conscious visually guided action ('How'). Here, we identify three pathways emerging from the dorsal stream that consist of projections to the prefrontal and premotor cortices, and a major projection to the medial temporal lobe that courses both directly and indirectly through the posterior cingulate and retrosplenial cortices. These three pathways support both conscious and non-conscious visuospatial processing, including spatial working memory, visually guided action and navigation, respectively.     

Stereoscopic Vision in the Absence of the Lateral Occipital Cortex

Both dorsal and ventral cortical visual streams contain neurons sensitive to binocular disparities, but the two streams may underlie different aspects of stereoscopic vision. Here we investigate stereopsis in the neurological patient D.F., whose ventral stream, specifically lateral occipital cortex, has been damaged bilaterally, causing profound visual form agnosia. Despite her severe damage to cortical visual areas, we report that DF''s stereo vision is strikingly unimpaired. She is better than many control observers at using binocular disparity to judge whether an isolated object appears near or far, and to resolve ambiguous structure-from-motion. DF is, however, poor at using relative disparity between features at different locations across the visual field. This may stem from a difficulty in identifying the surface boundaries where relative disparity is available. We suggest that the ventral processing stream may play a critical role in enabling healthy observers to extract fine depth information from relative disparities within one surface or between surfaces located in different parts of the visual field.     

Evolution and Optimality of Similar Neural Mechanisms for Perception and Action during Search

A prevailing theory proposes that the brain''s two visual pathways, the ventral and dorsal, lead to differing visual processing and world representations for conscious perception than those for action. Others have claimed that perception and action share much of their visual processing. But which of these two neural architectures is favored by evolution? Successful visual search is life-critical and here we investigate the evolution and optimality of neural mechanisms mediating perception and eye movement actions for visual search in natural images. We implement an approximation to the ideal Bayesian searcher with two separate processing streams, one controlling the eye movements and the other stream determining the perceptual search decisions. We virtually evolved the neural mechanisms of the searchers'' two separate pathways built from linear combinations of primary visual cortex receptive fields (V1) by making the simulated individuals'' probability of survival depend on the perceptual accuracy finding targets in cluttered backgrounds. We find that for a variety of targets, backgrounds, and dependence of target detectability on retinal eccentricity, the mechanisms of the searchers'' two processing streams converge to similar representations showing that mismatches in the mechanisms for perception and eye movements lead to suboptimal search. Three exceptions which resulted in partial or no convergence were a case of an organism for which the targets are equally detectable across the retina, an organism with sufficient time to foveate all possible target locations, and a strict two-pathway model with no interconnections and differential pre-filtering based on parvocellular and magnocellular lateral geniculate cell properties. Thus, similar neural mechanisms for perception and eye movement actions during search are optimal and should be expected from the effects of natural selection on an organism with limited time to search for food that is not equi-detectable across its retina and interconnected perception and action neural pathways.     

The processing of audio-visual speech: empirical and neural bases

In this selective review, I outline a number of ways in which seeing the talker affects auditory perception of speech, including, but not confined to, the McGurk effect. To date, studies suggest that all linguistic levels are susceptible to visual influence, and that two main modes of processing can be described: a complementary mode, whereby vision provides information more efficiently than hearing for some under-specified parts of the speech stream, and a correlated mode, whereby vision partially duplicates information about dynamic articulatory patterning.Cortical correlates of seen speech suggest that at the neurological as well as the perceptual level, auditory processing of speech is affected by vision, so that 'auditory speech regions' are activated by seen speech. The processing of natural speech, whether it is heard, seen or heard and seen, activates the perisylvian language regions (left>right). It is highly probable that activation occurs in a specific order. First, superior temporal, then inferior parietal and finally inferior frontal regions (left>right) are activated. There is some differentiation of the visual input stream to the core perisylvian language system, suggesting that complementary seen speech information makes special use of the visual ventral processing stream, while for correlated visual speech, the dorsal processing stream, which is sensitive to visual movement, may be relatively more involved.     

Top-Down Control in Contour Grouping

Human observers tend to group oriented line segments into full contours if they follow the Gestalt rule of ''good continuation''. It is commonly assumed that contour grouping emerges automatically in early visual cortex. In contrast, recent work in animal models suggests that contour grouping requires learning and thus involves top-down control from higher brain structures. Here we explore mechanisms of top-down control in perceptual grouping by investigating synchronicity within EEG oscillations. Human participants saw two micro-Gabor arrays in a random order, with the task to indicate whether the first (S1) or the second stimulus (S2) contained a contour of collinearly aligned elements. Contour compared to non-contour S1 produced a larger posterior post-stimulus beta power (15–21 Hz). Contour S2 was associated with a pre-stimulus decrease in posterior alpha power (11–12 Hz) and in fronto-posterior theta (4–5 Hz) phase couplings, but not with a post-stimulus increase in beta power. The results indicate that subjects used prior knowledge from S1 processing for S2 contour grouping. Expanding previous work on theta oscillations, we propose that long-range theta synchrony shapes neural responses to perceptual groupings regulating lateral inhibition in early visual cortex.     

Neuroimaging of visual awareness in patients and normal subjects

The immediacy and directness of our visual experience belies the complexity of the underlying neural mechanisms, which remain incompletely understood. Recent neuroimaging studies suggest that activity in ventral visual cortex is necessary but not sufficient for visual awareness. Experiments in both patients and normal subjects indicate that parietal and frontal areas make an important contribution to visual awareness, suggesting that reciprocal interactions between dorsal frontoparietal areas and ventral visual cortex may provide a fundamental neural substrate for conscious visual experience.     

Posterior Parietal Cortex Drives Inferotemporal Activations During Three-Dimensional Object Vision

The primate visual system consists of a ventral stream, specialized for object recognition, and a dorsal visual stream, which is crucial for spatial vision and actions. However, little is known about the interactions and information flow between these two streams. We investigated these interactions within the network processing three-dimensional (3D) object information, comprising both the dorsal and ventral stream. Reversible inactivation of the macaque caudal intraparietal area (CIP) during functional magnetic resonance imaging (fMRI) reduced fMRI activations in posterior parietal cortex in the dorsal stream and, surprisingly, also in the inferotemporal cortex (ITC) in the ventral visual stream. Moreover, CIP inactivation caused a perceptual deficit in a depth-structure categorization task. CIP-microstimulation during fMRI further suggests that CIP projects via posterior parietal areas to the ITC in the ventral stream. To our knowledge, these results provide the first causal evidence for the flow of visual 3D information from the dorsal stream to the ventral stream, and identify CIP as a key area for depth-structure processing. Thus, combining reversible inactivation and electrical microstimulation during fMRI provides a detailed view of the functional interactions between the two visual processing streams.     

Two Speed Factors of Visual Recognition Independently Correlated with Fluid Intelligence

Growing evidence indicates a moderate but significant relationship between processing speed in visuo-cognitive tasks and general intelligence. On the other hand, findings from neuroscience proposed that the primate visual system consists of two major pathways, the ventral pathway for objects recognition and the dorsal pathway for spatial processing and attentive analysis. Previous studies seeking for visuo-cognitive factors of human intelligence indicated a significant correlation between fluid intelligence and the inspection time (IT), an index for a speed of object recognition performed in the ventral pathway. We thus presently examined a possibility that neural processing speed in the dorsal pathway also represented a factor of intelligence. Specifically, we used the mental rotation (MR) task, a popular psychometric measure for mental speed of spatial processing in the dorsal pathway. We found that the speed of MR was significantly correlated with intelligence scores, while it had no correlation with one’s IT (recognition speed of visual objects). Our results support the new possibility that intelligence could be explained by two types of mental speed, one related to object recognition (IT) and another for manipulation of mental images (MR).     

A stimulus-driven approach to object identity and location processing in the human brain

The primate visual system is considered to be segregated into ventral and dorsal streams specialized for processing object identity and location, respectively. We reexamined the dorsal/ventral model using a stimulus-driven approach to object identity and location processing. While looking at repeated presentations of a standard object at a standard location, subjects monitored for any infrequent "oddball" changes in object identity, location, or identity and location (conjunction). While the identity and location oddballs preferentially activated ventral and dorsal brain regions respectively, each oddball type activated both pathways. Furthermore, all oddball types recruited the lateral temporal cortex and the temporo-parietal junction. These findings suggest that a strict dorsal/ventral dual-stream model does not fully account for the perception of novel objects in space.     

Lifting without Seeing: The Role of Vision in Perceiving and Acting upon the Size Weight Illusion

Background

Our expectations of an object''s heaviness not only drive our fingertip forces, but also our perception of heaviness. This effect is highlighted by the classic size-weight illusion (SWI), where different-sized objects of identical mass feel different weights. Here, we examined whether these expectations are sufficient to induce the SWI in a single wooden cube when lifted without visual feedback, by varying the size of the object seen prior to the lift.

Methodology/Principal Findings

Participants, who believed that they were lifting the same object that they had just seen, reported that the weight of the single, standard-sized cube that they lifted on every trial varied as a function of the size of object they had just seen. Seeing the small object before the lift made the cube feel heavier than it did after seeing the large object. These expectations also affected the fingertip forces that were used to lift the object when vision was not permitted. The expectation-driven errors made in early trials were not corrected with repeated lifting, and participants failed to adapt their grip and load forces from the expected weight to the object''s actual mass in the same way that they could when lifting with vision.

Conclusions/Significance

Vision appears to be crucial for the detection, and subsequent correction, of the ostensibly non-visual grip and load force errors that are a common feature of this type of object interaction. Expectations of heaviness are not only powerful enough to alter the perception of a single object''s weight, but also continually drive the forces we use to lift the object when vision is unavailable.     

Photoreceptor projection and termination pattern in the lamina of gonodactyloid stomatopods (mantis shrimp)

The apposition compound eyes of gonodactyloid stomatopods are divided into a ventral and a dorsal hemisphere by six equatorial rows of enlarged ommatidia, the mid-band (MB). Whereas the hemispheres are specialized for spatial vision, the MB consists of four dorsal rows of ommatidia specialized for colour vision and two ventral rows specialized for polarization vision. The eight retinula cell axons (RCAs) from each ommatidium project retinotopically onto one corresponding lamina cartridge, so that the three retinal data streams (spatial, colour and polarization) remain anatomically separated. This study investigates whether the retinal specializations are reflected in differences in the RCA arrangement within the corresponding lamina cartridges. We have found that, in all three eye regions, the seven short visual fibres (svfs) formed by retinula cells 1–7 (R1–R7) terminate at two distinct lamina levels, geometrically separating the terminals of photoreceptors sensitive to either orthogonal e-vector directions or different wavelengths of light. This arrangement is required for the establishment of spectral and polarization opponency mechanisms. The long visual fibres (lvfs) of the eighth retinula cells (R8) pass through the lamina and project retinotopically to the distal medulla externa. Differences between the three eye regions exist in the packing of svf terminals and in the branching patterns of the lvfs within the lamina. We hypothesize that the R8 cells of MB rows 1–4 are incorporated into the colour vision system formed by R1–R7, whereas the R8 cells of MB rows 5 and 6 form a separate neural channel from R1 to R7 for polarization processing.This research was supported by the Swiss National Science Foundation (PBSKB-104268/1), the Australian Research Council (LP0214956) and the American Air Force (AOARD/AFOSR) (F62562-03-P-0227).     

形状和空间位置知觉两条通路的功能磁共振研究

利用功能磁共振成像（ｆＭＲＩ） 技术,研究在处理形状知觉、位置知觉和特定形状图形的空间位置知觉的情况下,人类视皮层背侧（Ｄｏｒｓａｌ ｓｔｒｅａｍ） 和腹侧（Ｖｅｎｔｒａｌ ｓｔｒｅａｍ） 两条通路是怎样反应的。结果发现：形状知觉仅引起腹侧通路的兴奋;空间位置的知觉引起背侧通路的兴奋;特定形状的空间位置知觉引起腹侧通路和背侧通路的共同兴奋。这一结果丰富了对人类视觉皮层的两条通路在功能上定位的认识。     

spalt-induced specification of distinct dorsal and ventral domains is required for Drosophila tracheal patterning

Morphogenesis of the Drosophila tracheal system relies on different signalling pathways that have distinct roles in specifying both the migration of the tracheal cells and the particular morphological features of the primary branches. The current view is that the tracheal cells are initially specified as an equivalent group of cells whose diversification depends on signals from the surrounding cells. In this work, we show that the tracheal primordia are already specified as distinct dorsal and ventral cell populations. This subdivision depends on the activity of the spalt (sal) gene and occurs prior to the activity of the signalling pathways that dictate the development of the primary branches. Finally, we show that the specification of these two distinct cell populations, which are not defined by cell lineage, are critical for proper tracheal patterning. These results indicate that tracheal patterning depends not only on signalling from surrounding cells but also in the different response of the tracheal cells depending on their allocation to the dorsal or ventral domains.     

The role of prefrontal cortex in working memory: examining the contents of consciousness.

Working memory enables us to hold in our ''mind''s eye'' the contents of our conscious awareness, even in the absence of sensory input, by maintaining an active representation of information for a brief period of time. In this review we consider the functional organization of the prefrontal cortex and its role in this cognitive process. First, we present evidence from brain-imaging studies that prefrontal cortex shows sustained activity during the delay period of visual working memory tasks, indicating that this cortex maintains on-line representations of stimuli after they are removed from view. We then present evidence for domain specificity within frontal cortex based on the type of information, with object working memory mediated by more ventral frontal regions and spatial working memory mediated by more dorsal frontal regions. We also propose that a second dimension for domain specificity within prefrontal cortex might exist for object working memory on the basis of the type of representation, with analytic representations maintained preferentially in the left hemisphere and image-based representations maintained preferentially in the right hemisphere. Furthermore, we discuss the possibility that there are prefrontal areas brought into play during the monitoring and manipulation of information in working memory in addition to those engaged during the maintenance of this information. Finally, we consider the relationship of prefrontal areas important for working memory, both to posterior visual processing areas and to prefrontal areas associated with long-term memory.     

The role of temporally coarse form processing during binocular rivalry

Presenting the eyes with spatially mismatched images causes a phenomenon known as binocular rivalry-a fluctuation of awareness whereby each eye's image alternately determines perception. Binocular rivalry is used to study interocular conflict resolution and the formation of conscious awareness from retinal images. Although the spatial determinants of rivalry have been well-characterized, the temporal determinants are still largely unstudied. We confirm a previous observation that conflicting images do not need to be presented continuously or simultaneously to elicit binocular rivalry. This process has a temporal limit of about 350 ms, which is an order of magnitude larger than the visual system's temporal resolution. We characterize this temporal limit of binocular rivalry by showing that it is independent of low-level information such as interocular timing differences, contrast-reversals, stimulus energy, and eye-of-origin information. This suggests the temporal factors maintaining rivalry relate more to higher-level form information, than to low-level visual information. Systematically comparing the role of form and motion-the processing of which may be assigned to ventral and dorsal visual pathways, respectively-reveals that this temporal limit is determined by form conflict rather than motion conflict. Together, our findings demonstrate that binocular conflict resolution depends on temporally coarse form-based processing, possibly originating in the ventral visual pathway.     

The effect of stimulus duration and motor response in hemispatial neglect during a visual search task

Patients with hemispatial neglect exhibit a myriad of profound deficits. A hallmark of this syndrome is the patients' absence of awareness of items located in their contralesional space. Many studies, however, have demonstrated that neglect patients exhibit some level of processing of these neglected items. It has been suggested that unconscious processing of neglected information may manifest as a fast denial. This theory of fast denial proposes that neglected stimuli are detected in the same way as non-neglected stimuli, but without overt awareness. We evaluated the fast denial theory by conducting two separate visual search task experiments, each differing by the duration of stimulus presentation. Specifically, in Experiment 1 each stimulus remained in the participants' visual field until a response was made. In Experiment 2 each stimulus was presented for only a brief duration. We further evaluated the fast denial theory by comparing verbal to motor task responses in each experiment. Overall, our results from both experiments and tasks showed no evidence for the presence of implicit knowledge of neglected stimuli. Instead, patients with neglect responded the same when they neglected stimuli as when they correctly reported stimulus absence. These findings thus cast doubt on the concept of the fast denial theory and its consequent implications for non-conscious processing. Importantly, our study demonstrated that the only behavior affected was during conscious detection of ipsilesional stimuli. Specifically, patients were slower to detect stimuli in Experiment 1 compared to Experiment 2, suggesting a duration effect occurred during conscious processing of information. Additionally, reaction time and accuracy were similar when reporting verbally versus motorically. These results provide new insights into the perceptual deficits associated with neglect and further support other work that falsifies the fast denial account of non-conscious processing in hemispatial visual neglect.     

Occipital Alpha Activity during Stimulus Processing Gates the Information Flow to Object-Selective Cortex

Given the limited processing capabilities of the sensory system, it is essential that attended information is gated to downstream areas, whereas unattended information is blocked. While it has been proposed that alpha band (8–13 Hz) activity serves to route information to downstream regions by inhibiting neuronal processing in task-irrelevant regions, this hypothesis remains untested. Here we investigate how neuronal oscillations detected by electroencephalography in visual areas during working memory encoding serve to gate information reflected in the simultaneously recorded blood-oxygenation-level-dependent (BOLD) signals recorded by functional magnetic resonance imaging in downstream ventral regions. We used a paradigm in which 16 participants were presented with faces and landscapes in the right and left hemifields; one hemifield was attended and the other unattended. We observed that decreased alpha power contralateral to the attended object predicted the BOLD signal representing the attended object in ventral object-selective regions. Furthermore, increased alpha power ipsilateral to the attended object predicted a decrease in the BOLD signal representing the unattended object. We also found that the BOLD signal in the dorsal attention network inversely correlated with visual alpha power. This is the first demonstration, to our knowledge, that oscillations in the alpha band are implicated in the gating of information from the visual cortex to the ventral stream, as reflected in the representationally specific BOLD signal. This link of sensory alpha to downstream activity provides a neurophysiological substrate for the mechanism of selective attention during stimulus processing, which not only boosts the attended information but also suppresses distraction. Although previous studies have shown a relation between the BOLD signal from the dorsal attention network and the alpha band at rest, we demonstrate such a relation during a visuospatial task, indicating that the dorsal attention network exercises top-down control of visual alpha activity.     

Modelling Visual Neglect: Computational Insights into Conscious Perception

Background

Visual neglect is an attentional deficit typically resulting from parietal cortex lesion and sometimes frontal lesion. Patients fail to attend to objects and events in the visual hemifield contralateral to their lesion during visual search.

Methodology/Principal Finding

The aim of this work was to examine the effects of parietal and frontal lesion in an existing computational model of visual attention and search and simulate visual search behaviour under lesion conditions. We find that unilateral parietal lesion in this model leads to symptoms of visual neglect in simulated search scan paths, including an inhibition of return (IOR) deficit, while frontal lesion leads to milder neglect and to more severe deficits in IOR and perseveration in the scan path. During simulations of search under unilateral parietal lesion, the model''s extrastriate ventral stream area exhibits lower activity for stimuli in the neglected hemifield compared to that for stimuli in the normally perceived hemifield. This could represent a computational correlate of differences observed in neuroimaging for unconscious versus conscious perception following parietal lesion.

Conclusions/Significance

Our results lead to the prediction, supported by effective connectivity evidence, that connections between the dorsal and ventral visual streams may be an important factor in the explanation of perceptual deficits in parietal lesion patients and of conscious perception in general.     

Gdf6a is required for the initiation of dorsal–ventral retinal patterning and lens development

Dorsal–ventral patterning of the vertebrate retina is essential for accurate topographic mapping of retinal ganglion cell (RGC) axons to visual processing centers. Bone morphogenetic protein (Bmp) growth factors regulate dorsal retinal identity in vertebrate models, but the developmental timing of this signaling and the relative roles of individual Bmps remain unclear. In this study, we investigate the functions of two zebrafish Bmps, Gdf6a and Bmp4, during initiation of dorsal retinal identity, and subsequently during lens differentiation. Knockdown of zebrafish Gdf6a blocks initiation of retinal Smad phosphorylation and dorsal marker expression, while knockdown of Bmp4 produces no discernable retinal phenotype. These data, combined with analyses of embryos ectopically expressing Bmps, demonstrate that Gdf6a is necessary and sufficient for initiation of dorsal retinal identity. We note a profound expansion of ventral retinal identity in gdf6a morphants, demonstrating that dorsal BMP signaling antagonizes ventral marker expression. Finally, we demonstrate a role for Gdf6a in non-neural ocular tissues. Knockdown of Gdf6a leads to defects in lens-specific gene expression, and when combined with Bmp signaling inhibitors, disrupts lens fiber cell differentiation. Taken together, these data indicate that Gdf6a initiates dorsal retinal patterning independent of Bmp4, and regulates lens differentiation.     

Rhodopsin and Porphyropsin Fields In the Adult Bullfrog Retina

Though it had been supposed earlier that the bullfrog undergoes a virtually complete metamorphosis of visual systems from vitamin A₂ and porphyropsin in the tadpole to vitamin A₁ and rhodopsin in the adult, the present observations show that the retina of the adult frog may contain as much as 30–40% porphyropsin, all of it segregated in the dorsal zone. The most dorsal quarter of the adult retina may contain 81–89% porphyropsin mixed with a minor amount of rhodopsin; the ventral half contains only rhodopsin. Further, the dorsal zone contains a two to three times higher concentration of visual pigments than the ventral retina. The pigment epithelium underlying the retina contains a corresponding distribution of vitamins A₁ and A₂, predominantly vitamin A₂ in the dorsal pigment epithelium, exclusively vitamin A₁ in the ventral zone. The retina accepts whatever vitamin A the pigment epithelium provides it with, and turns it into the corresponding visual pigment. Thus, a piece of light-adapted dorsal retina laid back on ventral pigment epithelium regenerates rhodopsin, whereas a piece of light-adapted ventral retina laid back on dorsal pigment epithelium regenerates predominantly porphyropsin. Vitamin A₂ must be made from vitamin A₁, by dehydrogenation at the 3,4-bond in the ring. This conversion must occur in the pigment epithelium, presumably through the action of a vitamin A-3,4-dehydrogenase. The essential change at metamorphosis is to make much less of this dehydrogenase, and to sequester it in the dorsal pigment epithelium. Some adult bullfrogs, perhaps characteristically taken in the summer, contain very little porphyropsin—only perhaps 5%—still sequestered in the dorsal retina. The gradient of light over the retinal surface has little if any effect on this distribution. The greater density of visual pigments in the dorsal retina, and perhaps also—although this is less clear—the presence of porphyropsin in this zone, has some ecological importance in increasing the retinal sensitivity to the dimmer and, on occasion, redder light received from below.