Crossmodal spatial influences of touch on extrastriate visual areas take current gaze direction into account

Recent results indicate that crossmodal interactions can affect activity in cortical regions traditionally regarded as "unimodal." Previously we found that combining touch on one hand with visual stimulation in the anatomically corresponding hemifield could boost responses in contralateral visual cortex. Here we manipulated which visual hemifield corresponded to the location of the stimulated hand, by changing gaze direction such that right-hand touch could now arise in either the left or right visual field. Crossmodal effects on visual cortex switched from one hemisphere to the other, depending on gaze direction, regardless of whether the hand was seen. This indicates that crossmodal influences of touch upon visual cortex depend on spatial alignment for the multimodal stimuli, with gaze posture taken into account.     

Prefrontal cortex and neural mechanisms of executive function

Executive function is a product of the coordinated operation of multiple neural systems and an essential prerequisite for a variety of cognitive functions. The prefrontal cortex is known to be a key structure for the performance of executive functions. To accomplish the coordinated operations of multiple neural systems, the prefrontal cortex must monitor the activities in other cortical and subcortical structures and control and supervise their operations by sending command signals, which is called top-down signaling. Although neurophysiological and neuroimaging studies have provided evidence that the prefrontal cortex sends top-down signals to the posterior cortices to control information processing, the neural correlate of these top-down signals is not yet known. Through use of the paired association task, it has been demonstrated that top-down signals are used to retrieve specific information stored in long-term memory. Therefore, we used a paired association task to examine the neural correlates of top-down signals in the prefrontal cortex. The preliminary results indicate that 32% of visual neurons exhibit pair-selectivity, which is similar to the characteristics of pair-coding activities in temporal neurons. The latency of visual responses in prefrontal neurons was longer than bottom-up signals but faster than top-down signals in inferior temporal neurons. These results suggest that pair-selective visual responses may be top-down signals that the prefrontal cortex provides to the temporal cortex, although further studies are needed to elucidate the neural correlates of top-down signals and their characteristics to understand the neural mechanism of executive control by the prefrontal cortex.     

Functional visual streams.

The idea that visual signals relayed by the parvocellular and magnocellular subdivisions of the lateral geniculate nucleus remain segregated in the cerebral cortex has attracted considerable attention. It has been proposed that parvocellular contributions dominate in the temporal visual cortex, and that magnocellular contributions dominate in the parietal cortex. Recent experiments have shown that the organization of primate visual pathways is not this simple.     

Image phase or amplitude? Rapid scene categorization is an amplitude-based process

Models of the visual cortex are based on image decomposition according to the Fourier spectrum (amplitude and phase). On one hand, it is commonly believed that phase information is necessary to identify a scene. On the other hand, it is known that complex cells of the visual cortex, the most numerous ones, code only the amplitude spectrum. This raises the question of knowing if these cells carry sufficient information to allow visual scene categorization. In this work, using the same experiments in computer simulation and in psychophysics, we provide arguments to show that the amplitude spectrum alone is sufficient for categorization task.     

Spatial updating in human parietal cortex

Single neurons in monkey parietal cortex update visual information in conjunction with eye movements. This remapping of stimulus representations is thought to contribute to spatial constancy. We hypothesized that a similar process occurs in human parietal cortex and that we could visualize it with functional MRI. We scanned subjects during a task that involved remapping of visual signals across hemifields. We observed an initial response in the hemisphere contralateral to the visual stimulus, followed by a remapped response in the hemisphere ipsilateral to the stimulus. We ruled out the possibility that this remapped response resulted from either eye movements or visual stimuli alone. Our results demonstrate that updating of visual information occurs in human parietal cortex.     

Neuronal mechanisms of sensory integration in the visual analyzer system

It has been established in experiments on immobilized cats that somatic and interoceptive signals produce complex reorganizations of the spontaneous and evoked activity of visual cortex units. Either long diffuse changes of spike frequency or phasic reactions have been observed. The dynamics of sensory integration in the visual cortex is determined by unconditioned and conditioned mechanisms both of an intra- and interanalyser nature. Functioning of the microsystem of learning elements in the visual cortex is based on units capable of fixing the elaborated changes of evoked activity and constituting 18.6 percent of the total number of cellular elements in the visual projection cortex, responding to direct cortical stimulation. Microinophoresis of synaptically active agents has shown that complexely organized choline- and serotoninergic structures involved in the processes of unconditioned and conditioned interaction of heteromodal excitations are located in the visual cortex.     

用脑光学成像精确测定猫初级视皮层视野拓扑投射关系

利用基于脑内源信号的光学成像和二维互相关分析的方法,对猫初级视皮层17区的视野拓扑离心度(即视网膜-皮层拓扑关系)进行了精确测量。当采用在同一屏幕内处于上下视野的、方位互相垂直的两个相邻光栅刺激时,皮层中一部分区域的绝大部分细胞因同时兴奋而导致方位功能图模糊不清。将这种方位功能图和用单一方位(水平或垂直)全屏光栅刺激所得到的功能图进行比较,通过计算每一像素的互相关系数,从而获得皮层的精确视野拓扑离心度。同时用电生理的方法测量了同一视皮层内的单细胞的感受野位置,证明这种方法得到的视野离心度和光学记录方法得到的相同。因此,本研究为大面积地确定视皮层细胞感受野在视野中的位置提供了一种快速和较准确的方法。     

Vision: attention makes the cup flow over

Scalp potentials are surprisingly informative about visual attention: a recent study that used them to record neural responses to up to four superimposed visual patterns simultaneously has now revealed the flow of attentional signals back to visual cortex.     

Behavioral detection of electrical microstimulation in different cortical visual areas

The extent to which areas in the visual cerebral cortex differ in their ability to support perceptions has been the subject of considerable speculation. Experiments examining the activity of individual neurons have suggested that activity in later stages of the visual cortex is more closely linked to perception than that in earlier stages [1-9]. In contrast, results from functional imaging, transcranial magnetic stimulation, and lesion studies have been interpreted as showing that earlier stages are more closely coupled to perception [10-15]. We examined whether neuronal activity in early and later stages differs in its ability to support detectable signals by measuring behavioral thresholds for detecting electrical microstimulation in different cortical areas in two monkeys. By training the animals to perform a two-alternative temporal forced-choice task, we obtained criterion-free thresholds from five visual areas--V1, V2, V3A, MT, and the inferotemporal cortex. Every site tested yielded a reliable threshold. Thresholds varied little within and between visual areas, rising gradually from early to later stages. We similarly found no systematic differences in the slopes of the psychometric detection functions from different areas. These results suggest that neuronal signals of similar magnitude evoked in any part of visual cortex can generate percepts.     

Increased activity in human visual cortex during directed attention in the absence of visual stimulation

When subjects direct attention to a particular location in a visual scene, responses in the visual cortex to stimuli presented at that location are enhanced, and the suppressive influences of nearby distractors are reduced. What is the top-down signal that modulates the response to an attended versus an unattended stimulus? Here, we demonstrate increased activity related to attention in the absence of visual stimulation in extrastriate cortex when subjects covertly directed attention to a peripheral location expecting the onset of visual stimuli. Frontal and parietal areas showed a stronger signal increase during this expectation than did visual areas. The increased activity in visual cortex in the absence of visual stimulation may reflect a top-down bias of neural signals in favor of the attended location, which derives from a fronto-parietal network.     

Correlation between activities of neurons of sensorimotor and visual cortices after forming a hidden focus of excitation (defensive dominanta) in cortical representation of a forelimb in rabbits

The interaction between neurons of sensorimotor and visual cortices was investigated by cross-correlation analysis. In this interaction, we examined the role of sensorimotor neurons responding to light. In rabbits with a hidden focus of excitation, neurons of the sensorimotor cortex responding to light significantly more often formed correlation joints with cells of the visual cortex than neurons not responding to light. On the other hand, neurons of the visual cortex significantly more often formed correlation joints with neurons of the sensorimotor cortex not responding to light.     

A neural model of predictive recognition in form pathway of visual cortex

We present a functional model of form pathway in visual cortex based on predictive coding scheme, in which the prediction is compared with feedforward signals filtered by two kinds of spatial resolution maps, broad and fine resolution map. We propose here the functional role of the prediction and of the two kinds of resolution maps in perception of object form in visual system. The prediction is represented based on memory of dynamical attractors in temporal cortex, categorized by an elemental figure in posterior temporal cortex. The prediction is generated by the feedforward signals of main neurons in broad resolution maps of V(1) and V(4), and then is compared with the feedforward signals of main neurons in fine resolution map of V(1) and V(4).     

Binocular depth perception and the cerebral cortex

Our ability to coordinate the use of our left and right eyes and to make use of subtle differences between the images received by each eye allows us to perceive stereoscopic depth, which is important for the visual perception of three-dimensional space. Binocular neurons in the visual cortex combine signals from the left and right eyes. Probing the roles of binocular neurons in different perceptual tasks has advanced our understanding of the stages within the visual cortex that lead to binocular depth perception.     

Prior expectations evoke stimulus-specific activity in the deep layers of the primary visual cortex

The way we perceive the world is strongly influenced by our expectations. In line with this, much recent research has revealed that prior expectations strongly modulate sensory processing. However, the neural circuitry through which the brain integrates external sensory inputs with internal expectation signals remains unknown. In order to understand the computational architecture of the cortex, we need to investigate the way these signals flow through the cortical layers. This is crucial because the different cortical layers have distinct intra- and interregional connectivity patterns, and therefore determining which layers are involved in a cortical computation can inform us on the sources and targets of these signals. Here, we used ultra-high field (7T) functional magnetic resonance imaging (fMRI) to reveal that prior expectations evoke stimulus-specific activity selectively in the deep layers of the primary visual cortex (V1). These findings are in line with predictive processing theories proposing that neurons in the deep cortical layers represent perceptual hypotheses and thereby shed light on the computational architecture of cortex.

The way we perceive the world is strongly influenced by our expectations, but the neural circuitry through which the brain achieves this remains unknown. A study using ultra-high field fMRI reveals that prior expectations evoke stimulus-specific signals in the deep layers of the primary visual cortex.     

Features of corticofugal effects of the motor area of the cortex on processing of heterogeneous afferentation in the parafascicular complex of the thalamus in the cat

Evoked potentials to somatic and visual stimuli were recorded in the parafascicular complex (parafascicular nuclei--centrum medianum--Pf--CM) of the thalamus of cats anaesthetized by nembutal. Cooling of the motor cortex was also used. The influence of the motor cortex on processing of the visual and somatic afferent signals at the thalamic level was found to be direct but different by its character. The motor cortex exerted unidirectional facilitatory modulatory action of a tonic type on the processing of visual afferentation and general facilitatory influence against the background of which particular excitatory and inhibitory effects were seen which it exerted on the processing of somatic signals. Episodically the motor cortex completely controlled the afferent inputs activated by somatic impulses. The motor cortical area non-equally influenced afferentation of the same modality forming the different components of the evoked potentials in Pf--CM. On the base of our present and earlier obtained data an idea has been formed of existence of a general principle of differentiated influences of polysensory areas on heterogeneous afferentation on nonspecific and association thalamic nuclei, and of realization of these influences through separation of functionally isolated subsystems in descending pathways. Each of the subsystems by closing separate thalamo-cortical circuits might transmit signals of a single modality.     

Spatial attention changes excitability of human visual cortex to direct stimulation

Conscious perception depends not only on sensory input, but also on attention [1, 2]. Recent studies in monkeys [3-6] and humans [7-12] suggest that influences of spatial attention on visual awareness may reflect top-down influences on excitability of visual cortex. Here we tested this specifically, by providing direct input into human visual cortex via cortical transcranial magnetic stimulation (TMS) to produce illusory visual percepts, called phosphenes. We found that a lower TMS intensity was needed to elicit a conscious phosphene when its apparent spatial location was attended, rather than unattended. Our results indicate that spatial attention can enhance visual-cortex excitability, and visual awareness, even when sensory signals from the eye via the thalamic pathway are bypassed.     

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

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

Proprioceptive alignment of visual and somatosensory maps in the posterior parietal cortex

A touch on one hand can enhance the response to a visual stimulus delivered at a nearby location [1, 2], improving our interactions with the external world. In order to keep such visual-tactile spatial interactions effective, the brain updates the continuous postural changes, like those typically accompanying hand actions, through proprioception, thus maintaining the somatosensory and visual maps in spatial register [2, 3]. The posterior parietal cortex (PPC) might be critical for such a spatial remapping [4]; nevertheless, a direct causal demonstration of its involvement is lacking. Here, we found that unattended touches to one hand enhanced visual sensitivity for phosphenes induced by occipital trancranial magnetic stimulation (TMS) [5] when the touched hand was spatially coincident to the reported location of the phosphenes in external space. Notably, this spatially specific crossmodal facilitation was maintained after hand crossing, suggesting an efficient visual-tactile remapping. Critically, after 1 Hz repetitive TMS interference [6] over the PPC, but not over the primary somatosensory cortex, phosphene detection was still enhanced by spatially coincident touches with uncrossed hands, but it was enhanced by spatially noncoincident touches after hand crossing. This is the first causal evidence in humans that the PPC constantly updates the representation of the body in space in order to facilitate crossmodal interactions.     

人工视觉辅助系统：现状与展望

通过视觉获取图像信息是人类学习和生活的重要功能,失明则会显著降低其生活质量. 因视网膜色素变性、青光眼和黄斑变性等疾病而造成后天失明者,以及由意外事故、战争等造成眼部创伤者,有可能通过人工视觉辅助系统的帮助恢复部分视觉,或者完成复杂的生活任务. 一些盲症患者视觉通路的神经传导剩余部分依然有功能,因此可以借助电极阵列刺激视神经向大脑传递视觉信息,也可在大脑视觉皮层贴敷电极阵列的方法输入视觉信息. 此外,还能借助体外装置,如通过人工智能将视觉转换成语音指令、触觉阵列编码等,帮助盲症患者获得环境信息. 本文综述各类人工视觉辅助系统的现状,展望其发展趋势,并提出了新的植入器件与随身体外装置的新设想.