Cortical Contributions to Saccadic Suppression

The stability of visual perception is partly maintained by saccadic suppression: the selective reduction of visual sensitivity that accompanies rapid eye movements. The neural mechanisms responsible for this reduced perisaccadic visibility remain unknown, but the Lateral Geniculate Nucleus (LGN) has been proposed as a likely site. Our data show, however, that the saccadic suppression of a target flashed in the right visual hemifield increased with an increase in background luminance in the left visual hemifield. Because each LGN only receives retinal input from a single hemifield, this hemifield interaction cannot be explained solely on the basis of neural mechanisms operating in the LGN. Instead, this suggests that saccadic suppression must involve processing in higher level cortical areas that have access to a considerable part of the ipsilateral hemifield.     

Visuomotor origins of covert spatial attention

Covert spatial attention produces biases in perceptual performance and neural processing of behaviorally relevant stimuli in the absence of overt orienting movements. The neural mechanism that gives rise to these effects is poorly understood. This paper surveys past evidence of a relationship between oculomotor control and visual spatial attention and more recent evidence of a causal link between the control of saccadic eye movements by frontal cortex and covert visual selection. Both suggest that the mechanism of covert spatial attention emerges as a consequence of the reciprocal interactions between neural circuits primarily involved in specifying the visual properties of potential targets and those involved in specifying the movements needed to fixate them.     

The Y cell visual pathway implements a demodulating nonlinearity

Neural encoding of sensory signals involves both linear and nonlinear processes. Determining which nonlinear operations are implemented by neural systems is crucial to understanding sensory processing. Here, we ask if demodulation, the process used to decode AM radio signals, describes how Y cells in the cat LGN nonlinearly encode the visual scene. In response to visual AM signals across?a wide range of carrier frequencies, Y cells were found to transmit a demodulated signal, with the firing rate of single-units fluctuating at the envelope frequency but not the carrier frequency. A comparison of temporal frequency tuning properties between LGN Y cells and neurons in two primary cortical areas suggests that Y cells initiate a distinct pathway that carries a demodulated representation of the visual scene to cortex. The nonlinear signal processing carried out by the Y cell pathway simplifies the neural representation of complex visual features and allows high spatiotemporal frequencies to drive cortical responses.     

Visual Receptive Field Properties of Neurons in the Mouse Lateral Geniculate Nucleus

The lateral geniculate nucleus (LGN) is increasingly regarded as a “smart-gating” operator for processing visual information. Therefore, characterizing the response properties of LGN neurons will enable us to better understand how neurons encode and transfer visual signals. Efforts have been devoted to study its anatomical and functional features, and recent advances have highlighted the existence in rodents of complex features such as direction/orientation selectivity. However, unlike well-researched higher-order mammals such as primates, the full array of response characteristics vis-à-vis its morphological features have remained relatively unexplored in the mouse LGN. To address the issue, we recorded from mouse LGN neurons using multisite-electrode-arrays (MEAs) and analysed their discharge patterns in relation to their location under a series of visual stimulation paradigms. Several response properties paralleled results from earlier studies in the field and these include centre-surround organization, size of receptive field, spontaneous firing rate and linearity of spatial summation. However, our results also revealed “high-pass” and “low-pass” features in the temporal frequency tuning of some cells, and greater average contrast gain than reported by earlier studies. In addition, a small proportion of cells had direction/orientation selectivity. Both “high-pass” and “low-pass” cells, as well as direction and orientation selective cells, were found only in small numbers, supporting the notion that these properties emerge in the cortex. ON- and OFF-cells showed distinct contrast sensitivity and temporal frequency tuning properties, suggesting parallel projections from the retina. Incorporating a novel histological technique, we created a 3-D LGN volume model explicitly capturing the morphological features of mouse LGN and localising individual cells into anterior/middle/posterior LGN. Based on this categorization, we show that the ON/OFF, DS/OS and linear response properties are not regionally restricted. Our study confirms earlier findings of spatial pattern selectivity in the LGN, and builds on it to demonstrate that relatively elaborate features are computed early in the visual pathway.     

Eye-Head Coordination for Visual Cognitive Processing

We investigated coordinated movements between the eyes and head (“eye-head coordination”) in relation to vision for action. Several studies have measured eye and head movements during a single gaze shift, focusing on the mechanisms of motor control during eye-head coordination. However, in everyday life, gaze shifts occur sequentially and are accompanied by movements of the head and body. Under such conditions, visual cognitive processing influences eye movements and might also influence eye-head coordination because sequential gaze shifts include cycles of visual processing (fixation) and data acquisition (gaze shifts). In the present study, we examined how the eyes and head move in coordination during visual search in a large visual field. Subjects moved their eyes, head, and body without restriction inside a 360° visual display system. We found patterns of eye-head coordination that differed those observed in single gaze-shift studies. First, we frequently observed multiple saccades during one continuous head movement, and the contribution of head movement to gaze shifts increased as the number of saccades increased. This relationship between head movements and sequential gaze shifts suggests eye-head coordination over several saccade-fixation sequences; this could be related to cognitive processing because saccade-fixation cycles are the result of visual cognitive processing. Second, distribution bias of eye position during gaze fixation was highly correlated with head orientation. The distribution peak of eye position was biased in the same direction as head orientation. This influence of head orientation suggests that eye-head coordination is involved in gaze fixation, when the visual system processes retinal information. This further supports the role of eye-head coordination in visual cognitive processing.     

Corticothalamic interactions in the transfer of visual information

Thalamic function does not stand apart, as a discrete processing step, from the cortical circuitry. The thalamus receives extensive feedback from the cortex and this influences the firing pattern, synchronization and sensory response mode of relay cells. A crucial question concerns the extent to which the feedback simply controls the state and transmission mode of relay cells and the extent to which the feedback participates in the specific processing of sensory information. Using examples from experiments examining the influence of feedback from the visual cortex to the lateral geniculate nucleus (LGN), we argue that thalamic mechanisms are selectively focused by visually driven feedback to optimize the thalamic contribution to segmentation and global integration. This involves effects on both the temporal and spatial parameters characterizing the responses of LGN cells and includes, for example, motion-driven feedback effects from MT (middle temporal visual area) relayed via layer 6 of V1 (primary visual cortex).     

Predictive Feedback Can Account for Biphasic Responses in the Lateral Geniculate Nucleus

Biphasic neural response properties, where the optimal stimulus for driving a neural response changes from one stimulus pattern to the opposite stimulus pattern over short periods of time, have been described in several visual areas, including lateral geniculate nucleus (LGN), primary visual cortex (V1), and middle temporal area (MT). We describe a hierarchical model of predictive coding and simulations that capture these temporal variations in neuronal response properties. We focus on the LGN-V1 circuit and find that after training on natural images the model exhibits the brain's LGN-V1 connectivity structure, in which the structure of V1 receptive fields is linked to the spatial alignment and properties of center-surround cells in the LGN. In addition, the spatio-temporal response profile of LGN model neurons is biphasic in structure, resembling the biphasic response structure of neurons in cat LGN. Moreover, the model displays a specific pattern of influence of feedback, where LGN receptive fields that are aligned over a simple cell receptive field zone of the same polarity decrease their responses while neurons of opposite polarity increase their responses with feedback. This phase-reversed pattern of influence was recently observed in neurophysiology. These results corroborate the idea that predictive feedback is a general coding strategy in the brain.     

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.     

Integrate-and-fire vs Poisson models of LGN input to V1 cortex: noisier inputs reduce orientation selectivity

One of the reasons the visual cortex has attracted the interest of computational neuroscience is that it has well-defined inputs. The lateral geniculate nucleus (LGN) of the thalamus is the source of visual signals to the primary visual cortex (V1). Most large-scale cortical network models approximate the spike trains of LGN neurons as simple Poisson point processes. However, many studies have shown that neurons in the early visual pathway are capable of spiking with high temporal precision and their discharges are not Poisson-like. To gain an understanding of how response variability in the LGN influences the behavior of V1, we study response properties of model V1 neurons that receive purely feedforward inputs from LGN cells modeled either as noisy leaky integrate-and-fire (NLIF) neurons or as inhomogeneous Poisson processes. We first demonstrate that the NLIF model is capable of reproducing many experimentally observed statistical properties of LGN neurons. Then we show that a V1 model in which the LGN input to a V1 neuron is modeled as a group of NLIF neurons produces higher orientation selectivity than the one with Poisson LGN input. The second result implies that statistical characteristics of LGN spike trains are important for V1’s function. We conclude that physiologically motivated models of V1 need to include more realistic LGN spike trains that are less noisy than inhomogeneous Poisson processes.     

Temporal encoding of spatial information during active visual fixation

Humans and other species continually perform microscopic eye movements, even when attending to a single point. These movements, which include drifts and microsaccades, are under oculomotor control, elicit strong neural responses, and have been thought to serve important functions. The influence of these fixational eye movements on the acquisition and neural processing of visual information remains unclear. Here, we show that during viewing of natural scenes, microscopic eye movements carry out a crucial information-processing step: they remove predictable correlations in natural scenes by equalizing the spatial power of the retinal image within the frequency range of ganglion cells' peak sensitivity. This transformation, which had been attributed to center-surround receptive field organization, occurs prior to any neural processing and reveals a form of matching between the statistics of natural images and those of normal eye movements. We further show that the combined effect of microscopic eye movements and retinal receptive field organization is to convert spatial luminance discontinuities into synchronous firing events, beginning the process of edge detection. Thus, microscopic eye movements are fundamental to two goals of early visual processing: redundancy reduction and feature extraction.     

The Pupillary Light Response Reveals the Focus of Covert Visual Attention

The pupillary light response is often assumed to be a reflex that is not susceptible to cognitive influences. In line with recent converging evidence, we show that this reflexive view is incomplete, and that the pupillary light response is modulated by covert visual attention: Covertly attending to a bright area causes a pupillary constriction, relative to attending to a dark area under identical visual input. This attention-related modulation of the pupillary light response predicts cuing effects in behavior, and can be used as an index of how strongly participants attend to a particular location. Therefore, we suggest that pupil size may offer a new way to continuously track the focus of covert visual attention, without requiring a manual response from the participant. The theoretical implication of this finding is that the pupillary light response is neither fully reflexive, nor under complete voluntary control, but is instead best characterized as a stereotyped response to a voluntarily selected target. In this sense, the pupillary light response is similar to saccadic and smooth pursuit eye movements. Together, eye movements and the pupillary light response maximize visual acuity, stabilize visual input, and selectively filter visual information as it enters the eye.     

Oculomotor control and spatial processing.

In the past year research in the oculomotor system has concentrated on some hitherto neglected areas, and also caused a re-evaluation of several long-standing concepts. Careful studies of the translational (otolith) vestibulo-ocular reflex and the torsional system have demonstrated their importance. A re-evaluation of the role of the superior colliculus in the generation of saccades has provided evidence for its participation in the feedback process. New studies of the interaction of eye movements and visual processing have shown that the brain can compensate for the visual effects of eye movements and maintain a retinotopic representation of visual space for the saccadic system.     

A linear model for symmetric receptive fields: implications for classification tests with flashed and moving images

The purpose of this study was to explore the effects of spatial and temporal properties on the expected responses of visual neurons that have linear receptive fields (RFs), particularly those having a mirror symmetric distribution of spatial subregions. Receptive fields that are symmetric in at least one spatial dimension occur in neurons of the retina, the lateral geniculate nucleus (LGN), and the visual cortex of mammals. Responses to flashing bars, moving bars, and moving edges were studied for different configurations of an analog RF model in which spatial and temporal aspects were varied independently. Responses of the model at intermediate stimulus speeds were found to agree with responses in the literature for X and Y units of the LGN and often for simple units of the visual cortex. In particular, having separated regions of response to light and dark edges, an identifying property of simple cells, was found to be a linear consequence of RF regions responding inversely to stimuli of opposite polarity. Model differences from responses of cortical complex units show that a linear model cannot mimic their responses, and imply that complex units employ major nonlinearities in coding image polarity (light vs dark), which signifies a nonlinearity in coding intensity. Because sudden flux changes inherent in flashing bars test mainly temporal RF properties, and slowly moving edges test mainly spatial properties, these two tests form a useful minimal set with which to describe and classify RFs. The usefulness of this set derives both from its sensitivity to spatial and temporal variables, and from the correlation between the linearity of a cell's processing of stimulus intensity and its RF classification.     

研究: 外膝状体神经元的亮度反应与感受野ON-OFF反应的关系

外膝体是视觉信息进入新皮层的主要通路,其编码亮度信息的神经机制还不清楚.我们采用随机呈现的连续快速变化(50 Hz)的均匀亮度刺激,显著地提高了猫外膝体神经元对均匀亮度的反应强度,通过反相关算法抽提出神经元的亮度反应函数.约81%的神经元的亮度反应函数为单调性上升或下降,有19%的神经元亮度反应函数为V型.通过分析这些神经元对亮度上升和下降的反应强度与感受野ON和OFF反应强度的关系,表明83%的神经元对亮度的反应模式是由其感受野ON-OFF反应的相对强度决定的,其余17%则与其感受野ON-OFF区的兴奋和抑制的变化相关.这些结果揭示了外膝体神经元编码亮度变化的机制.     

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.     

Sensory and behavioral properties of neurons in posterior parietal cortex of the awake, trained monkey.

Neurons in posterior parietal cortex of the awake, trained monkey respond to passive visual and/or somatosensory stimuli. In general, the receptive fields of these cells are large and nonspecific. When these neurons are studied during visually guided hand movements and eye movements, most of their activity can be accounted for by passive sensory stimulation. However, for some visual cells, the response to a stimulus is enhanced when it is to be the target for a saccadic eye movement. This enhancement is selective for eye movements into the visual receptive field since it does not occur with eye movements to other parts of the visual field. Cells that discharge in association with a visual fixation task have foveal receptive fields and respond to the spots of light used as fixation targets. Cells discharging selectively in association with different directions of tracking eye movements have directionally selective responses to moving visual stimuli. Every cell in our sample discharging in association with movement could be driven by passive sensory stimuli. We conclude that the activity of neurons in posterior parietal cortex is dependent on and indicative of external stimuli but not predictive of movement.     

Cortical regions activated after rapid eye movements during REM sleep

The present study investigated the cortical regions activated during rapid eye movement (REM) sleep by identifying the sources of electric currents of brain potentials related to rapid eye movements using low-resolution brain electromagnetic tomography (LORETA). The brain potentials measured were the lambda response (P1 and P2) during wakefulness and the lambda-like response (P1r and P2r) during REM sleep. Fifteen healthy university students participated in this study. During wakefulness, the sources of the electric current of the lambda response (P1 and P2) were estimated to be in the primary and secondary visual cortices (BA 17, 18). During REM sleep, the P1r has a source in a higher order visual area (precuneus; BA 7, 31) and P2r comes from the primary and secondary visual cortices (BA 17, 18). In addition, the density of electric current in the premotor and fronto-central regions including anterior cingulate gyrus was higher after rapid eye movements, which was a discriminative feature of REM sleep. The results of this study suggest that these activities that occur after rapid eye movements might underlie the generation of vivid visual images of dreaming.

     

Eye movements of stumptailed monkeys during discriminative performance: An overview

There are a number of statements that can be made about eye movements of monkeys during the learning of simple and complex discriminative problems that are probably applicable to a wide variety of visual tasks. There are systematic changes in eye movements as a function of practice. Some of these changes occur long after grosser measures of performance, such as frequency of correct choices, have reached an asymptote. Hence, short-term studies of visual information processing may be misleading. Duration of visual fixations and frequency of visual fixations are independent measures, reflecting different cognitive processes. Studies which measure only total looking time confound these two measures and, thus, may miss important information. Eye movements appear to be an important, if not essential, component of the chain of events constituting the cognitive processing underlying performance on visual tasks.     

Goal-related activity in V4 during free viewing visual search. Evidence for a ventral stream visual salience map

Natural exploration of complex visual scenes depends on saccadic eye movements toward important locations. Saccade targeting is thought to be mediated by a retinotopic map that represents the locations of salient features. In this report, we demonstrate that extrastriate ventral area V4 contains a retinotopic salience map that guides exploratory eye movements during a naturalistic free viewing visual search task. In more than half of recorded cells, visually driven activity is enhanced prior to saccades that move the fovea toward the location previously occupied by a neuron's spatial receptive field. This correlation suggests that bottom-up processing in V4 influences the oculomotor planning process. Half of the neurons also exhibit top-down modulation of visual responses that depends on search target identity but not visual stimulation. Convergence of bottom-up and top-down processing streams in area V4 results in an adaptive, dynamic map of salience that guides oculomotor planning during natural vision.     

Fixational Eye Movements in the Earliest Stage of Metazoan Evolution

All known photoreceptor cells adapt to constant light stimuli, fading the retinal image when exposed to an immobile visual scene. Counter strategies are therefore necessary to prevent blindness, and in mammals this is accomplished by fixational eye movements. Cubomedusae occupy a key position for understanding the evolution of complex visual systems and their eyes are assumedly subject to the same adaptive problems as the vertebrate eye, but lack motor control of their visual system. The morphology of the visual system of cubomedusae ensures a constant orientation of the eyes and a clear division of the visual field, but thereby also a constant retinal image when exposed to stationary visual scenes. Here we show that bell contractions used for swimming in the medusae refresh the retinal image in the upper lens eye of Tripedalia cystophora. This strongly suggests that strategies comparable to fixational eye movements have evolved at the earliest metazoan stage to compensate for the intrinsic property of the photoreceptors. Since the timing and amplitude of the rhopalial movements concur with the spatial and temporal resolution of the eye it circumvents the need for post processing in the central nervous system to remove image blur.