Involvement of striate and extrastriate visual cortical areas in spatial attention

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70-75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.     

Working memory for visual objects: complementary roles of inferior temporal, medial temporal, and prefrontal cortex

Humans have an extraordinary ability to maintain and manipulate visual image information in the absence of perceptual stimulation. The neural substrates of visual working memory have been extensively researched, but there have been few attempts to integrate these findings into a model of how different cortical areas interact to form and maintain visual memories. In this paper, I review findings from neurophysiological, neuropsychological, and neuroimaging studies of visual working memory in human and nonhuman primates. These data support a model in which visual working memory operations rely on activation of object representations in inferior temporal cortex, via top-down feedback from neocortical areas in the prefrontal and medial temporal cortex, and also from the hippocampus.     

Retinotopic maps and foveal suppression in the visual cortex of amblyopic adults

Amblyopia is a developmental visual disorder associated with loss of monocular acuity and sensitivity as well as profound alterations in binocular integration. Abnormal connections in visual cortex are known to underlie this loss, but the extent to which these abnormalities are regionally or retinotopically specific has not been fully determined. This functional magnetic resonance imaging (fMRI) study compared the retinotopic maps in visual cortex produced by each individual eye in 19 adults (7 esotropic strabismics, 6 anisometropes and 6 controls). In our standard viewing condition, the non-tested eye viewed a dichoptic homogeneous mid-level grey stimulus, thereby permitting some degree of binocular interaction. Regions-of-interest analysis was performed for extrafoveal V1, extrafoveal V2 and the foveal representation at the occipital pole. In general, the blood oxygenation level-dependent (BOLD) signal was reduced for the amblyopic eye. At the occipital pole, population receptive fields were shifted to represent more parafoveal locations for the amblyopic eye, compared with the fellow eye, in some subjects. Interestingly, occluding the fellow eye caused an expanded foveal representation for the amblyopic eye in one early–onset strabismic subject with binocular suppression, indicating real-time cortical remapping. In addition, a few subjects actually showed increased activity in parietal and temporal cortex when viewing with the amblyopic eye. We conclude that, even in a heterogeneous population, abnormal early visual experience commonly leads to regionally specific cortical adaptations.     

Top-down influence in early visual processing: a Bayesian perspective

Traditional views of visual processing suggest that early visual neurons are static spatiotemporal filters that extract local features by feedforward computation. The extracted information is then fed forward through a chain of modules to successively higher visual areas for further analysis. Recording from early visual neurons in awake behaving monkeys, we revealed there are many levels of complexity in the information processing of the early visual cortex. We found that the early visual neurons not only are sensitive to features within their receptive fields (RFs) but also to the global context of a visual scene, the behavioral relevance of the stimuli and the experience of the animals. These findings suggest that the early visual cortex (V1 and V2) is tightly coupled to and highly interactive with the rest of the visual system. The top-down interaction, mediated by recurrent feedback connections, introduces contextual information to influence the perceptual inference in the early visual cortex.     

Conserved patterns of cortico-cortical connections define areal hierarchy in rat visual cortex

Summary The prevalence of reciprocal connections in the cerebral cortex indicates that they play a fundamental role in the processing of sensory information. We have investigated the laminar termination patterns of such paired connections between different visual cortical areas of the rat, and have found two basic projection types: one which includes layer 4 and a second which includes layer 1 and avoids layer 4. The projections from primary visual cortex (area 17) to extrastriate visual cortical targets in the cytoarchitectonical areas 18a and 18b, and from 18a to a site in 18b, are of the first type. In contrast, the return projections from 18a and 18b to area 17 and from 18b to 18a, are of the second type. Thus each pair of connections has one element of each type, giving every circuit a nearly identical asymmetric structure. These laminar patterns resemble those of forward and feedback connections in primate cortex, indicating that cortico-cortical connectivity patterns are highly conserved through evolution, and that, as in monkeys, these connections define a hierarchical organization of areas in rat visual cortex.     

Retinotopic organization of extra-retinal saccade-related input to the visual cortex in the cat

Summary Single unit activity of 842 cells has been recorded in cat visual cortex and analyzed with respect to vestibular induced, and spontaneous saccadic eye movements performed in the dark. This study has been done in awake, chronically implanted cats, subsequently placed in acute conditions to achieve the precise retinotopic mapping of the cortical areas previously investigated.In areas 17 and 18, respectively, 27% and 24% of the cells tested were influenced by horizontal saccadic eye movements in the dark (E. M. cells). In the Clare-Bishop area, the proportion of E. M. cells was 12%, while only 2% of such cells were found in areas 19 and 21.The distribution of E.M. cells in areas 17 and 18 with respect to retinotopy showed that E.M. cells were more numerous in the cortical zones devoted to the representation of the area centralis (38% in area 17, 27% in area 18) than in the zones subserving the periphery of the visual field (17% and 12%, respectively).Two of the characteristics of E. M. cell activations appear dependant on the retinotopic organization. First, a larger number of E.M. cells presenting an asymmetry in their responses to horizontal saccadic eye movements in opposite directions (directional E.M. cells) were encountered in the cortical representation of the peripheral visual field. 53% of E. M. cells recorded in area 17 and 71% in area 18 were directional in the cortex corresponding to the peripheral visual field. This percentage was of 23% and 25% respectively in the cortex devoted to area centralis. Second, E.M. cells were found to have a latency from the onset of the saccade systematically larger than 100 ms (i.e, they discharged at, or after the end of the eye movement) if they were located in the cortical representation of the area centralis, while E.M. cells related to the peripheral visual field displayed a wider range of latencies (0–240 ms).Results obtained in Clare Bishop area, although limited to the representation of the peripheral visual field, were quantitatively and qualitatively similar to those observed in the homologous retinotopic zones of areas 17 and 18.It is concluded that an extra-retinal input related to oculomotor activity is sent to the cat visual cortex and is organized, at least in areas 17 and 18, with respect to the retinotopic representation of the visual field. These data support the hypothesis of a functional duality between central and peripheral vision and are discussed in the context of visual-oculomotor integration.Supported by INSERM (C.R.L. 79-53336)     

Occipital network for figure/ground organization

To study the cortical mechanism of figure/ground categorization in the human brain, we employed fMRI and the temporal-asynchrony paradigm. This paradigm is able to eliminate any differential activation for local stimulus features, and thus to identify only global perceptual interactions. Strong segmentation of the image into different spatial configurations was generated solely from temporal asynchronies between zones of homogeneous dynamic noise. The figure/ground configuration was a single geometric figure enclosed in a larger surround region. In a control condition, the figure/ground organization was eliminated by segmenting the noise field into many identical temporal-asynchrony stripes. The manipulation of the type of perceptual organization triggered dramatic reorganization in the cortical activation pattern. The figure/ground configuration generated suppression of the ground representation (limited to early retinotopic visual cortex, V1 and V2) and strong activation in the motion complex hMT+/V5+; conversely, both responses were abolished when the figure/ground organization was eliminated. These results suggest that figure/ground processing is mediated by top-down suppression of the ground representation in the earliest visual areas V1/V2 through a signal arising in the motion complex. We propose a model of a recurrent cortical architecture incorporating suppressive feedback that operates in a topographic manner, forming a figure/ground categorization network distinct from that for “pure” scene segmentation and thus underlying the perceptual organization of dynamic scenes into cognitively relevant components. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cognitive mechanisms of transitive inference

Objects in natural scenes are rarely seen in isolation, but are usually overlapping or partially occluding other objects. To recognize individual objects, the visual system must be able to segregate overlapping objects from one another. Evidence from lesions in humans and monkeys suggest that perceptual segregation of occluded or overlapping objects involves extrastriate visual cortex. In monkeys, area V4 has been shown to play an important role in recognizing occluded or poorly salient shapes. In humans, a retinotopic homologue of ventral V4 (V4v) has been described, but it is not known whether this area is also functionally homologous to area V4 in monkeys. In this study, we tried to localize the visual cortical regions involved in perceptual segregation of overlapping shapes using positron emission tomography (PET). Regional cerebral blood flow (rCBF) was measured in seven subjects while they discriminated the relative areas of simultaneously presented rectangular shapes. In the control condition, the shapes were displayed without overlaps; in a second condition, the shapes overlapped each other partially. In a third condition, the shapes did not overlap but had been reduced in salience by adding random noise to the stimuli. Contrasting the overlapping shape condition with the control condition identified a single region in the left posterior lateral occipital cortex. The rCBF in this region also increased, though more weakly, during discrimination of shapes embedded in noise, relative to the control condition. The region activated by segregation of overlapping shapes was located in the posterior occipital cortex close to the anterior border of area V2, near the average location of human V4v as determined by retinotopic mapping studies. The activation of this region of extrastriate visual cortex by a task that involved segregation of overlapping shapes is consistent with monkey V4 and human V4v being functionally homologous. We conclude that discrimination of overlapping shapes involves in particular a region of extrastriate visual cortex located in the left lateral occipital cortex and that this region may correspond to human V4v.     

Enhancement of oblique effect in the cat's primary visual cortex via orientation preference shifting induced by excitatory feedback from higher-order cortical area 21a

It is often suggested that the oblique effect, the well-known phenomenon whereby both humans and animals are visually more sensitive to vertical and horizontal contours than to oblique ones, is due to the overrepresentation of cardinal orientations in the visual cortex. The functional role of feedback projections from higher-order cortical areas to lower-order areas is not fully understood. Combining the two issues in a study using optical imaging here, we report that the neural oblique effect was significantly enhanced (3.7 times higher than the normal) in the cat's primary visual cortex through orientation shifting induced by excitatory feedback from the higher-order cortical area 21a. This suggests that a reciprocal co-excitatory mechanism may underlie the perceptual oblique effect.     

A physiological correlate of the 'spotlight' of visual attention

Here we identify a neural correlate of the ability to precisely direct visual attention to locations other than the center of gaze. Human subjects performed a task requiring shifts of visual attention (but not of gaze) from one location to the next within a dense array of targets and distracters while functional MRI was used to map corresponding displacements of neural activation within visual cortex. The cortical topography of the purely attention-driven activity precisely matched the topography of activity evoked by the cued targets when presented in isolation. Such retinotopic mapping of attention-related activation was found in primary visual cortex, as well as in dorsomedial and ventral occipital visual areas previously implicated in processing the attended target features. These results identify a physiological basis for the effects of spatially directed visual attention.     

The effect of musical experience on hemispheric lateralization in musical feature processing

In the monkey's visual cortex, there are two well-documented information processing streams: the dorsal motion and ventral form/color pathways. Similarly, two corresponding information streams were also found in the cat's visual cortices, and PMLS and area 21a are the gateways for distinct motion and form information processing. It has been shown that the feedback from PMLS solely modulates motion direction, but not orientation response, while the feedback from area 21a modulates form related features, such as spatial frequency dependency and neuronal oblique effect. Here, we postulate that feedback signals from higher cortical areas in the form or the motion information pathway may solely modulate the corresponding properties in neurons in the lower areas of the visual system. To examine the above hypothesis, the impact of feedback from higher area 21a on both orientation and direction maps was investigated in area 17 of the cat using intrinsic signal optical imaging. The results showed that the feedback from area 21a did not affect the amplitude and preference of direction, but did modulate orientation response in area 17, supporting the above hypothesis.     

Differential contribution of early visual areas to the perceptual process of contour processing

We investigated contour processing and figure-ground detection within human retinotopic areas using event-related functional magnetic resonance imaging (fMRI) in 6 healthy and na?ve subjects. A figure (6 degrees side length) was created by a 2nd-order texture contour. An independent and demanding foveal letter-discrimination task prevented subjects from noticing this more peripheral contour stimulus. The contour subdivided our stimulus into a figure and a ground. Using localizers and retinotopic mapping stimuli we were able to subdivide each early visual area into 3 eccentricity regions corresponding to 1) the central figure, 2) the area along the contour, and 3) the background. In these subregions we investigated the hemodynamic responses to our stimuli and compared responses with or without the contour defining the figure. No contour-related blood oxygenation level-dependent modulation in early visual areas V1, V3, VP, and MT+ was found. Significant signal modulation in the contour subregions of V2v, V2d, V3a, and LO occurred. This activation pattern was different from comparable studies, which might be attributable to the letter-discrimination task reducing confounding attentional modulation. In V3a, but not in any other retinotopic area, signal modulation corresponding to the central figure could be detected. Such contextual modulation will be discussed in light of the recurrent processing hypothesis and the role of visual awareness.     

A comparison of magnification functions in area 19 and the lateral suprasylvian visual area in the cat

A retinotopic map can be described by a magnification function that relates magnification factor to visual field eccentricity. Magnification factor for primary visual cortex (VI) in both the cat and the macaque monkey is directly proportional to retinal ganglion cell density. However, among those extrastriate areas for which a magnification function has been described, this is often not the case. Deviations from the pattern established in V1 are of considerable interest because they may provide insight into an extrastriate area's role in visual processing. The present study explored the magnification function for the lateral suprasylvian area (LS) in the cat. Because of its complex retinotopic organization, magnification was calculated indirectly using the known magnification function for area 19. Small tracer injections were made in area 17, and the extent of anterograde label in LS and in area 19 was measured. Using the ratio of cortical area labeled in LS to that in area 19, and the known magnification factor for area 19 at the corresponding retinotopic location, we were able to calculate magnification factor for LS. We found that the magnification function for LS differed substantially from that for area 19: central visual field was expanded, and peripheral field compressed in LS compared with area 19. Additionally, we found that the lower vertical meridian's representation was compressed relative to that of the horizontal meridian. We also examined receptive field size in areas 17, 19, and LS and found that, for all three areas, receptive field size was inversely proportional to magnification factor.     

Directional pursuit deficits following lesions of the foveal representation within the superior temporal sulcus of the macaque monkey

Ibotenic acid lesions of the middle temporal visual area (MT) have previously been shown to impair a monkey's ability to initiate smooth pursuit eye movements to targets moving in the extrafoveal visual field (30). This is a retinotopic deficit: pursuit is impaired in all directions within the affected portion of the contralateral visual field. In the present experiments we analyzed the effects of lesions of the foveal representation of MT on the maintenance of foveal pursuit. Injections of ibotenic acid were directed toward the representation of the fovea within MT but spread into extrafoveal regions of MT and adjacent visual areas within the superior temporal sulcus. Chemical lesions of the foveal representation produced a directional deficit in the maintenance of pursuit: the monkey failed to match eye speed to target speed when pursuing a target that moved toward the side of the brain with the lesion. This deficit was evident regardless of the part of the visual field in which target motion began, and pursuit at higher target speeds was more severely affected. The directional deficit was qualitatively similar to pursuit deficits observed in human patients following large parietal-occipital lesions. Extension of the lesions into extrafoveal regions of the contralateral visual field representation also resulted in retinotopic deficits for pursuit initiation: the monkey was unable to match the speed of its pursuit eye movement to that of a target or to adjust the amplitude of its saccade to compensate for target motion. The errors in pursuit speed and saccade amplitude for initiation of pursuit into the contralateral visual field were linearly related, which supports the hypothesis that both deficits arise from damage to the same underlying visual motion processing mechanism. The selectivity of the retinotopic deficit for motion information was also investigated by reducing retinal motion through the use of a stabilized image. After the lesion, the monkeys continued normal pursuit when a position error was present during stabilization, supporting the view that the deficit was related to loss of motion but not position information.     

Long-term voltage-sensitive dye imaging reveals cortical dynamics in behaving monkeys

A novel method of chronic optical imaging based on new voltage-sensitive dyes (VSDs) was developed to facilitate the explorations of the spatial and temporal patterns underlying higher cognitive functions in the neocortex of behaving monkeys. Using this system, we were able to explore cortical dynamics, with high spatial and temporal resolution, over period of 相似文献   

Neural fate of ignored stimuli: dissociable effects of perceptual and working memory load

Observers commonly experience functional blindness to unattended visual events, and this problem has fuelled an intense debate concerning the fate of unattended visual information in neural processing. Here we used functional magnetic resonance imaging (fMRI) to demonstrate that the type of task that a human subject engages in determines the way in which ignored visual background stimuli are processed in parahippocampal cortex. Increasing the perceptual difficulty of a foveal target task attenuated processing of task-irrelevant background scenes, whereas increasing the number of objects held in working memory did not have this effect. These dissociable effects of perceptual and working memory load clarify how task-irrelevant, unattended stimuli are processed in category-selective areas in human ventral visual cortex.     

Thalamocortical connections of rat posterior parietal cortex.

The neuronal connections of rat posterior parietal cortex (PPC) have been examined using retrograde fluorescent axonal tracers. We have found that PPC receives thalamic input predominantly from the lateral posterior and lateral dorsal nuclei, and not from the ventrobasal nucleus, which projects to the rostrally adjacent hindlimb cortex, or from the dorsal lateral geniculate nucleus, which projects to the caudally adjacent visual association area. PPC has reciprocal corticocortical connections with medial agranular cortex and orbital cortex; together, these three cortical areas may function as a network for directed attention in rats.     

Rat posterior parietal cortex: topography of corticocortical and thalamic connections

Anatomical and functional findings support the contention that there is a distinct posterior parietal cortical area (PPC) in the rat, situated between the rostrally adjacent hindlimb sensorimotor area and the caudally adjacent secondary visual areas. The PPC is distinguished from these areas by receiving thalamic afferents from the lateral dorsal (LD), lateral posterior (LP), and posterior (Po) nuclei, in the absence of input from the ventrobasal complex (VB) or dorsal lateral geniculate (DLG) nuclei. Behavioral studies have demonstrated that PPC is involved in spatial orientation and directed attention. In the present study we used fluorescent retrograde axonal tracers primarily to investigate the cortical connections of PPC, in order to determine the organization of the circuitry by which PPC is likely to participate in these functions, and also to determine how the topography of its thalamic connections differs from that of neighboring cortical areas. The cortical connections of PPC involve the ventrolateral (VLO) and medial (MO) orbital areas, medial agranular cortex (area Fr2), portions of somatic sensory areas Par1 and Par2, secondary visual areas Oc2M and Oc2L, auditory area Tel, and retrosplenial cortex. The secondary visual areas Oc2L and Oc2M have cortical connections which are similar to those of PPC, but are restricted within orbital cortex to area VLO, and within area Fr2 to its caudal portion, and do not involve auditory area Te1. The cortical connections of hindlimb cortex are largely restricted to somatic sensory and motor areas. Retrosplenial cortex, which is medially adjacent to PPC, has cortical connections that are prominent with visual cortex, do not involve somatic sensory or auditory cortex, and include the presubiculum. We conclude that PPC is distinguished by its pattern of cortical connections with the somatic sensory, auditory and visual areas, and with areas Fr2, and VLO/MO, in addition to its exclusive thalamic connectivity with LD, LP and Po. Because recent behavioral studies indicate that PPC, Fr2 and VLO are involved in directed attention and spatial learning, we suggest that the interconnections among these three cortical areas represent a major component of the circuitry for these functions in rats.     

Frequency-Following and Connectivity of Different Visual Areas in Response to Contrast-Reversal Stimulation

The sensitivity of visual areas to different temporal frequencies, as well as the functional connections between these areas, was examined using magnetoencephalography (MEG). Alternating circular sinusoids (0, 3.1, 8.7 and 14 Hz) were presented to foveal and peripheral locations in the visual field to target ventral and dorsal stream structures, respectively. It was hypothesized that higher temporal frequencies would preferentially activate dorsal stream structures. To determine the effect of frequency on the cortical response we analyzed the late time interval (220–770 ms) using a multi-dipole spatio-temporal analysis approach to provide source locations and timecourses for each condition. As an exploratory aspect, we performed cross-correlation analysis on the source timecourses to determine which sources responded similarly within conditions. Contrary to predictions, dorsal stream areas were not activated more frequently during high temporal frequency stimulation. However, across cortical sources the frequency-following response showed a difference, with significantly higher power at the second harmonic for the 3.1 and 8.7 Hz stimulation and at the first and second harmonics for the 14 Hz stimulation with this pattern seen robustly in area V1. Cross-correlations of the source timecourses showed that both low- and high-order visual areas, including dorsal and ventral stream areas, were significantly correlated in the late time interval. The results imply that frequency information is transferred to higher-order visual areas without translation. Despite the less complex waveforms seen in the late interval of time, the cross-correlation results show that visual, temporal and parietal cortical areas are intricately involved in late-interval visual processing.     

Saccade-related spread of activity across superior colliculus may arise from asymmetry of internal connections

The superior colliculus (SC) receives a retinotopic projection of the contralateral visual field in which the representation of the central field is expanded with respect to the peripheral field. The visual projection forms a nonlinear, approximately logarithmic, map on the SC. Models of the SC commonly assume that the function defining the strength of neuronal connections within this map (the kernel) depends only on the distance between two neurons, and is thus isotropic and homogeneous. However, if the connection strength is based on the distance between two stimuli in sensory space, the kernel will be asymmetric because of the nonlinear projection onto the brain map. We show, using a model of the SC, that one consequence of these asymmetric intrinsic connections is that activity initiated at one point spreads across the map. We compare this simulated spread with the spread observed experimentally around the time of saccadic eye movements with respect to direction of spread, differing effects of local and global inhibition, and the consequences of localized inactivation on the SC map. Early studies suggested that the SC spread was caused by feedback of eye displacement during a saccade, but subsequent studies were inconsistent with this feedback hypothesis. In our new model, the spread is autonomous, resulting from intrinsic connections within the SC, and thus does not depend on eye movement feedback. Other sensory maps in the brain (e.g., visual cortex) are also nonlinear and our analysis suggests that the consequences of asymmetric connections in those areas should be considered.