Region Of Visual Space Research Articles

Topographically Localized Modulation of Tectal Cell Spatial Tuning by Complex Natural Scenes.

The tuning properties of neurons in the visual system can be contextually modulated by the statistics of the area surrounding their receptive field (RF), particularly when the surround contains natural features. However, stimuli presented in specific egocentric locations may have greater behavioral relevance, raising the possibility that the extent of contextual modulation may vary with position in visual space. To explore this possibility, we utilized the small size and optical transparency of the larval zebrafish to describe the form and spatial arrangement of contextually modulated cells throughout an entire tectal hemisphere. We found that the spatial tuning of tectal neurons to a prey-like stimulus sharpens when the stimulus is presented against a background with the statistics of complex natural scenes, relative to a featureless background. These neurons are confined to a spatially restricted region of the tectum and have receptive fields centered within a region of visual space in which the presence of prey preferentially triggers hunting behavior. Our results suggest that contextual modulation of tectal neurons by complex backgrounds may facilitate prey-localization in cluttered visual environments.

A Unifying Motif for Spatial and Directional Surround Suppression.

In the visual system, the response to a stimulus in a neuron's receptive field can be modulated by stimulus context, and the strength of these contextual influences vary with stimulus intensity. Recent work has shown how a theoretical model, the stabilized supralinear network (SSN), can account for such modulatory influences, using a small set of computational mechanisms. Although the predictions of the SSN have been confirmed in primary visual cortex (V1), its computational principles apply with equal validity to any cortical structure. We have therefore tested the generality of the SSN by examining modulatory influences in the middle temporal area (MT) of the macaque visual cortex, using electrophysiological recordings and pharmacological manipulations. We developed a novel stimulus that can be adjusted parametrically to be larger or smaller in the space of all possible motion directions. We found, as predicted by the SSN, that MT neurons integrate across motion directions for low-contrast stimuli, but that they exhibit suppression by the same stimuli when they are high in contrast. These results are analogous to those found in visual cortex when stimulus size is varied in the space domain. We further tested the mechanisms of inhibition using pharmacological manipulations of inhibitory efficacy. As predicted by the SSN, local manipulation of inhibitory strength altered firing rates, but did not change the strength of surround suppression. These results are consistent with the idea that the SSN can account for modulatory influences along different stimulus dimensions and in different cortical areas.SIGNIFICANCE STATEMENT Visual neurons are selective for specific stimulus features in a region of visual space known as the receptive field, but can be modulated by stimuli outside of the receptive field. The SSN model has been proposed to account for these and other modulatory influences, and tested in V1. As this model is not specific to any particular stimulus feature or brain region, we wondered whether similar modulatory influences might be observed for other stimulus dimensions and other regions. We tested for specific patterns of modulatory influences in the domain of motion direction, using electrophysiological recordings from MT. Our data confirm the predictions of the SSN in MT, suggesting that the SSN computations might be a generic feature of sensory cortex.

Immunity to attentional capture at ignored locations.

Certain stimuli have the power to rapidly and involuntarily capture spatial attention against our will. The present study investigated whether such stimuli capture spatial attention even when they appear in ignored regions of visual space. In other words, which force is more powerful: attentional capture or spatial filtering? Participants performed a spatial cuing task, searching for a letter target defined by color (e.g., green) and then reporting that letter's identity. Two of the four search locations were always irrelevant. Unlike many previous experiments, participants were forced to ignore these locations because they always contained a target-colored distractor letter. Experiment 1 assessed capture by a salient-but-irrelevant abrupt onset cue appearing 150 ms before the search display. One might expect onset cues to capture attention even at ignored locations given that the main function of capture, presumably, is to rapidly alert observers to unexpected yet potentially important stimuli. However, they did not. Experiment 2 replicated this result with a different neutral baseline condition. Experiment 3 replicated the absence of capture effects at ignored locations with an even more potent stimulus: a relevant cue possessing the target color. We propose that people are effectively immune to attentional capture by objects in ignored locations - spatial filtering dominates attentional capture.

Homeostatic plasticity in human extrastriate cortex following a simulated peripheral scotoma

Neuroimaging and patient work over the past decade have indicated that, following retinal deafferentation, the human visual cortex undergoes a large-scale and enduring reorganization of its topography such that the classical retinotopic organization of deafferented visual cortex remaps to represent non-classical regions of visual space. Such long-term visual reorganization is proposed to occur through changes in the functional balance of deafferented visual circuits that engage more lasting changes through activity-dependent neuroplasticity. Here, we investigated the short-term changes in functional balance (short-term plasticity; homeostatic plasticity) that occur within deafferented human visual cortices. We recorded electroencephalogram (EEG) while observers were conditioned for 6s with a simulated retinal scotoma (artificial scotoma) positioned 8.0° in the periphery. Visual evoked potentials (VEPs) evoked by the onset of sinusoidal visual probes that varied in their tilt were used to examine changes in cortical excitability within and around cortical representations of the simulated scotoma. Psychophysical orientation functions obtained from discrimination of visual probe tilt were used to examine alterations in the stimulus selectivity within the scotoma representations. Consistent with a mechanism of homeostatic disinhibition, an early extrastriate component of the VEP (the early phase P1) exhibited increased amplitude following the condition with a simulated scotoma relative to a stimulus-matched control condition. This increased visual cortical response was associated with a reduction in the slope of the psychophysical orientation function, suggesting a broader tuning of neural populations within scotoma representations. Together, these findings support a mechanism of disinhibition in promoting visual plasticity and topographical reorganization.

The spatially global control of attentional target selection in visual search

ABSTRACTGlyn Humphreys and his co-workers have made numerous important theoretical and empirical contributions to research on visual search. They have introduced the concept of attentional target templates and investigated the nature of these templates and how they are involved in the control of search performance. In the experiments reported here, we investigated whether feature-specific search templates for particular colours can guide target selection independently for different regions of visual space. We employed behavioural and electrophysiological markers of attentional selection in tasks with targets defined by specific colour/location combinations. In Experiment 1, participants searched for pairs of colour targets in a particular spatial configuration (e.g., red in the upper and blue in the lower visual field). In Experiment 2, they searched for single colour-defined targets at specific locations (e.g., red on the left or blue on the right). Target displays were preceded by non-informative cues containing target-colour items at task-set matching or mismatching locations. Contingent attentional capture was observed only for matching cues. However, both matching and mismatching cues elicited identical N2pc components, indicating equivalent attentional capture. This shows that the rapid deployment of attention towards target features is spatially non-selective, and that selection of colour/location combinations occurs at later post-perceptual stages. This was further corroborated in search displays where targets were accompanied by target-colour distractors at nonmatching locations. Here, spatial biases towards the target emerged late and were strongly attenuated relative to displays without such distractors. These results demonstrate that attentional templates for target-defining features operate in a spatially-global fashion. Feature-based guidance of visual search cannot be restricted to particular locations even when this is required by the demands of an attentional selection task.

Impairing the useful field of view in natural scenes: Tunnel vision versus general interference.

A fundamental issue in visual attention is the relationship between the useful field of view (UFOV), the region of visual space where information is encoded within a single fixation, and eccentricity. A common assumption is that impairing attentional resources reduces the size of the UFOV (i.e., tunnel vision). However, most research has not accounted for eccentricity-dependent changes in spatial resolution, potentially conflating fixed visual properties with flexible changes in visual attention. Williams (1988, 1989) argued that foveal loads are necessary to reduce the size of the UFOV, producing tunnel vision. Without a foveal load, it is argued that the attentional decrement is constant across the visual field (i.e., general interference). However, other research asserts that auditory working memory (WM) loads produce tunnel vision. To date, foveal versus auditory WM loads have not been compared to determine if they differentially change the size of the UFOV. In two experiments, we tested the effects of a foveal (rotated L vs. T discrimination) task and an auditory WM (N-back) task on an extrafoveal (Gabor) discrimination task. Gabor patches were scaled for size and processing time to produce equal performance across the visual field under single-task conditions, thus removing the confound of eccentricity-dependent differences in visual sensitivity. The results showed that although both foveal and auditory loads reduced Gabor orientation sensitivity, only the foveal load interacted with retinal eccentricity to produce tunnel vision, clearly demonstrating task-specific changes to the form of the UFOV. This has theoretical implications for understanding the UFOV.

ON and OFF subfield organization of layer 2/3 neurons in tree shrew visual cortex.

Abstract In vivo 2-photon imaging of calcium sensors in visual cortex has provided a host of new insights into the fine scale columnar mapping of response properties like orientation, direction, and visual space. Extracellular recording in carnivores have shown a columnar segregation of ON- and OFF-center geniculate inputs in layer 4, which could provide the basis for the generation of orientation selectivity. However, the spatial arrangement of ON and OFF in layer 2/3 has not been addressed. In this study we used 2-photon imaging of GCamP6s calcium fluorescent signals to map the receptive fields of thousands of neurons in layer 2/3 of tree shrew visual cortex with reverse correlation using sparse noise. We found a diverse array of receptive field properties in layer 2/3 including neurons with classic simple, complex and single sign receptive fields (either ON or OFF). The ON and OFF subfields in layer 2/3 were found to exhibit topologically distinct relationships with the maps of visual space and orientation preference. In most cases, the centers of OFF subfields for neurons in a given region of cortex were confined to a compact region of visual space and displayed a smooth retinotopic progression, while the centers of the ON subfields were distributed over a wider region of visual space and displayed less retinotopic precision. Consistent with the arrangement of ON and OFF subfields of simple cells in other species, the angle of displacement in visual space of the ON and OFF subfields for individual neurons could be used to predict the organization of the orientation map. Taken together, these results suggest that the differential arrangement of ON and OFF subfield centers by cortical circuits meets the conjoint constraints of mapping both visuotopy and orientation in a single population of neurons and in a fashion that preserves continuity for both stimulus features. Meeting abstract presented at VSS 2015.

Topography and areal organization of mouse visual cortex.

To guide future experiments aimed at understanding the mouse visual system, it is essential that we have a solid handle on the global topography of visual cortical areas. Ideally, the method used to measure cortical topography is objective, robust, and simple enough to guide subsequent targeting of visual areas in each subject. We developed an automated method that uses retinotopic maps of mouse visual cortex obtained with intrinsic signal imaging (Schuett et al., 2002; Kalatsky and Stryker, 2003; Marshel et al., 2011) and applies an algorithm to automatically identify cortical regions that satisfy a set of quantifiable criteria for what constitutes a visual area. This approach facilitated detailed parcellation of mouse visual cortex, delineating nine known areas (primary visual cortex, lateromedial area, anterolateral area, rostrolateral area, anteromedial area, posteromedial area, laterointermediate area, posterior area, and postrhinal area), and revealing two additional areas that have not been previously described as visuotopically mapped in mice (laterolateral anterior area and medial area). Using the topographic maps and defined area boundaries from each animal, we characterized several features of map organization, including variability in area position, area size, visual field coverage, and cortical magnification. We demonstrate that higher areas in mice often have representations that are incomplete or biased toward particular regions of visual space, suggestive of specializations for processing specific types of information about the environment. This work provides a comprehensive description of mouse visuotopic organization and describes essential tools for accurate functional localization of visual areas.

Asynchronous Broadband Signals Are the Principal Source of the BOLD Response in Human Visual Cortex

Activity in the living human brain can be studied using multiple methods, spanning a wide range of spatial and temporal resolutions. We investigated the relationship between electric field potentials measured with electrocorticography (ECoG) and the blood oxygen level-dependent (BOLD) response measured with functional magnetic resonance imaging (fMRI). We set out to explain the full set of measurements by modeling the underlying neural circuits. ECoG responses in visual cortex can be separated into two visually driven components. One component is a specific temporal response that follows each stimulus contrast reversal ("stimulus locked"); the other component is an increase in the response variance ("asynchronous"). For electrodes in visual cortex (V1, V2, V3), the two measures respond to stimuli in the same region of visual space, but they have different spatial summation properties. The stimulus-locked ECoG component sums contrast approximately linearly across space; spatial summation in the asynchronous ECoG component is subadditive. Spatial summation measured using BOLD closely matches the asynchronous component. We created a neural simulation that accurately captures the main features of the ECoG time series; in the simulation, the stimulus-locked and asynchronous components arise from different neural circuits. These observations suggest that the two ECoG components arise from different neural sources within the same cortical region. The spatial summation measurements and simulations suggest that the BOLD response arises primarily from neural sources that generate the asynchronous broadband ECoG component.

Functional Characterization of the Extraclassical Receptive Field in Macaque V1: Contrast, Orientation, and Temporal Dynamics

Neurons in primary visual cortex, V1, very often have extraclassical receptive fields (eCRFs). The eCRF is defined as the region of visual space where stimuli cannot elicit a spiking response but can modulate the response of a stimulus in the classical receptive field (CRF). We investigated the dependence of the eCRF on stimulus contrast and orientation in macaque V1 cells for which the laminar location was determined. The eCRF was more sensitive to contrast than the CRF across the whole population of V1 cells with the greatest contrast differential in layer 2/3. We confirmed that many V1 cells experience stronger suppression for collinear than orthogonal stimuli in the eCRF. Laminar analysis revealed that the predominant bias for collinear suppression was found in layers 2/3 and 4b. The laminar pattern of contrast and orientation dependence suggests that eCRF suppression may derive from different neural circuits in different layers, and may be comprised of two distinct components: orientation-tuned and untuned suppression. On average tuned suppression was delayed by ∼25 ms compared with the onset of untuned suppression. Therefore, response modulation by the eCRF develops dynamically and rapidly in time.

Adaptation to implied tilt: extensive spatial extrapolation of orientation gradients

To extract the global structure of an image, the visual system must integrate local orientation estimates across space. Progress is being made toward understanding this integration process, but very little is known about whether the presence of structure exerts a reciprocal influence on local orientation coding. We have previously shown that adaptation to patterns containing circular or radial structure induces tilt-aftereffects (TAEs), even in locations where the adapting pattern was occluded. These spatially “remote” TAEs have novel tuning properties and behave in a manner consistent with adaptation to the local orientation implied by the circular structure (but not physically present) at a given test location. Here, by manipulating the spatial distribution of local elements in noisy circular textures, we demonstrate that remote TAEs are driven by the extrapolation of orientation structure over remarkably large regions of visual space (more than 20°). We further show that these effects are not specific to adapting stimuli with polar orientation structure, but require a gradient of orientation change across space. Our results suggest that mechanisms of visual adaptation exploit orientation gradients to predict the local pattern content of unfilled regions of space.

Dynamics of Normalization Underlying Masking in Human Visual Cortex

Stimulus visibility can be reduced by other stimuli that overlap the same region of visual space, a process known as masking. Here we studied the neural mechanisms of masking in humans using source-imaged steady state visual evoked potentials and frequency-domain analysis over a wide range of relative stimulus strengths of test and mask stimuli. Test and mask stimuli were tagged with distinct temporal frequencies and we quantified spectral response components associated with the individual stimuli (self terms) and responses due to interaction between stimuli (intermodulation terms). In early visual cortex, masking alters the self terms in a manner consistent with a reduction of input contrast. We also identify a novel signature of masking: a robust intermodulation term that peaks when the test and mask stimuli have equal contrast and disappears when they are widely different. We fit all of our data simultaneously with family of a divisive gain control models that differed only in their dynamics. Models with either very short or very long temporal integration constants for the gain pool performed worse than a model with an integration time of ∼30 ms. Finally, the absolute magnitudes of the response were controlled by the ratio of the stimulus contrasts, not their absolute values. This contrast-contrast invariance suggests that many neurons in early visual cortex code relative rather than absolute contrast. Together, these results provide a more complete description of masking within the normalization framework of contrast gain control and suggest that contrast normalization accomplishes multiple functional goals.

Attentional Capture? Synchronized Feedback Signals from the Isthmi Boost Retinal Signals to Higher Visual Areas

When a salient object in the visual field captures attention, the neural representation of that object is enhanced at the expense of competing stimuli. How neural activity evoked by a salient stimulus evolves to take precedence over the neural activity evoked by other stimuli is a matter of intensive investigation. Here, we describe in pigeons (Columba livia) how retinal inputs to the optic tectum (TeO, superior colliculus in mammals), triggered by moving stimuli, are selectively relayed on to the rotundus (Rt, caudal pulvinar) in the thalamus, and to its pallial target, the entopallium (E, extrastriate cortex). We show that two satellite nuclei of the TeO, the nucleus isthmi parvocelullaris (Ipc) and isthmi semilunaris (SLu), send synchronized feedback signals across tectal layers. Preventing the feedback from Ipc but not from SLu to a tectal location suppresses visual responses to moving stimuli from the corresponding region of visual space in all Rt subdivisions. In addition, the bursting feedback from the Ipc imprints a bursting rhythm on the visual signals, such that the visual responses of the Rt and the E acquire a bursting modulation significantly synchronized to the feedback from Ipc. As the Ipc feedback signals are selected by competitive interactions, the visual responses within the receptive fields in the Rt tend to synchronize with the tectal location receiving the "winning" feedback from Ipc. We propose that this selective transmission of afferent activity combined with the cross-regional synchronization of the areas involved represents a bottom-up mechanism by which salient stimuli capture attention.

Unmasking the Perky Effect: Spatial Extent of Image Interference on Visual Acuity

We have previously argued that visual mental images are not substitutable for visual percepts, because the interfering effects of visual stimuli such as line maskers on visual targets differ markedly in their properties from the interfering effects of visual images (the “Perky effect”). Imagery interference occurs over a much wider temporal and spatial extent than masking, and unlike masking, image interference is insensitive to relative orientation. The lack of substitutability is theoretically interesting because the Perky effect can be compared meaningfully to real line masking in that both types of interference are visual, not due to optical factors (accommodative blur or poor fixation) or to high-level factors (attentional distraction, demand characteristics, or effects of uncertainty). In this report, however, we question our earlier position that spatial extents of interference are markedly different: when images and real lines are matched in contrast, which was not done previously, their interference effects have very similar spatial extents. These data add weight to the view that spatial properties of images and percepts are similar in respect to extent. Along with the wider temporal extent and the insensitivity to orientation, the new results remain compatible with our older hypothesis that to create a clear mental image in a region of visual space, incoming signals from the eye must be suppressed (Craver-Lemley and Reeves, 1992). We have pursued this idea in this report using “unmasking,” in which adding elements to the visual image in the region beyond the zone of suppression reduces the Perky effect.

Synaptic Adaptation and Sustained Generation of Waves in a Model of Turtle Visual Cortex

Both single and repeated visual stimuli produce waves of activity in the visual cortex of freshwater turtles. Large-scale, biophysically realistic models of the visual cortex capture the basic features of the waves produced by single stimuli. However, these models do not respond to repetitive stimuli due to the presence of a long-lasting hyperpolarization that follows the initial wave. This paper modifies the large-scale model so that it responds to repetitive stimuli by incorporating Hebbian and anti-Hebbian learning rules in synapses in the model. The resulting adaptive model responds to repetitive stimuli with repetitive waves. However, repeated presentation of a stimulus to a restricted region of visual space produces a habituation in the model in the same way it does in the real cortex.

A Cholinergic Gating Mechanism Controlled by Competitive Interactions in the Optic Tectum of the Pigeon

We describe the operation of a midbrain neural circuit in pigeons that may participate in selecting and attending to one visual stimulus from the myriad displayed in their visual environment. This mechanism is based on a topographically organized cholinergic signal reentering the optic tectum (TeO). We have shown previously that, whenever a visual stimulus activates neurons in a given tectal location, this location receives a strong bursting feedback from cholinergic neurons of the nucleus isthmi pars parvocellularis (Ipc), situated underneath the tectum. Here we show that, if a second visual stimulus is presented, even far from the first, the feedback signal to the first tectal location is diminished or suppressed, and feedback to the second tectal location is initiated. We found that this long-range suppressive interaction is mostly mediated by the nucleus isthmi pars magnocellularis, which sends a wide-field GABAergic projection to Ipc and TeO. In addition, two sets of findings indicate that the feedback from the Ipc modulates the ascending output from the TeO. First, visually evoked extracellular responses recorded in the dorsal anterior subdivision of the thalamic nucleus rotundus (RtDa), receiving the ascending tectal output, are closely synchronized to this feedback signal. Second, local inactivation of the Ipc prevents visual responses in RtDa to visual targets moving in the corresponding region of visual space. These results suggest that the ascending transmission of visual activity through the tectofugal pathway is gated by this cholinergic re-entrant signal, whose location within the tectal visual map is dynamically defined by competitive interactions.

Microstimulation of V1 delays visually guided saccades: a parametric evaluation of delay fields

Electrical microstimulation of macaque striate cortex (area V1) delays the execution of saccadic eye movements made to a visual target placed in the receptive field of the stimulated neurons. The region of visual space within which saccades are delayed is called a delay field. We examined the effects of changing the parameters of stimulation and target size on the size of a delay field. Rhesus monkeys were required to generate a saccadic eye movement to a punctate and white visual target presented within or outside the receptive field of the neurons under study. On 50% of trials, a train of stimulation consisting of 0.2-ms anode-first pulses was delivered to the neurons before the onset of the visual target. Stimulations were performed in the operculum at 0.9-2.0 mm below the cortical surface. It was found that increases in current (50-100 microA), pulse frequency (100-300 Hz), or train duration (75-300 ms) increased the size of a delay field and increases in target size (0.1 degrees -0.2 degrees of visual angle) decreased the size of a delay field. Delay fields varied in size between 0.1 and 0.6 degrees of visual angle. These results are related to the properties of phosphenes induced by electrical stimulation of V1 in humans and compared to the interference effects observed following transcranial magnetic stimulation of human V1.

The spatial filtering properties of local edge detectors and brisk–sustained retinal ganglion cells

We compared image computation in the rabbit retina by two different cell types: the so-called 'local edge detecting' ganglion cells and the well-known brisk-sustained ganglion cells. From both anatomical and physiological evidence, these cells are present in nearly equal numbers and thus overlap to sample the same regions of visual space. We recorded simultaneously from overlapping cells on a dense microelectrode array. The results were analysed using an anatomically realistic simulation of the retina's processing levels. The 'local edge detecting' cell was found to be tuned to higher spatial frequencies and to have a narrower spatial frequency bandpass than the brisk-sustained cells. Simulation revealed that this is due primarily to the 'zero-crossing' detector implied by the definition of the local edge detector. The outputs of the simulations in response to complex images were analysed quantitatively. The results showed the population of local edge detectors to transmit a sparser code than the brisk-sustained cells.

A morphological basis for orientation tuning in primary visual cortex.

Feedforward connections are thought to be important in the generation of orientation-selective responses in visual cortex by establishing a bias in the sampling of information from regions of visual space that lie along a neuron's axis of preferred orientation. It remains unclear, however, which structural elements-dendrites or axons-are ultimately responsible for conveying this sampling bias. To explore this question, we have examined the spatial arrangement of feedforward axonal connections that link non-oriented neurons in layer 4 and orientation-selective neurons in layer 2/3 of visual cortex in the tree shrew. Target sites of labeled boutons in layer 2/3 resulting from focal injections of biocytin in layer 4 show an orientation-specific axial bias that is sufficient to confer orientation tuning to layer 2/3 neurons. We conclude that the anisotropic arrangement of axon terminals is the principal source of the orientation bias contributed by feedforward connections.

Are neural correlates of visual consciousness retinotopic?

Some visual neurons code what we see, their defining characteristic being a response profile which mirrors conscious percepts rather than veridical sensory attributes. One issue yet to be resolved is whether, within a given cortical area, conscious visual perception relates to diffuse activity across the entire population of such cells or focal activity within the sub-population mapping the location of the perceived stimulus. Here we investigate the issue in the human brain with fMRI, using a threshold stimulation technique to dissociate perceptual from non-perceptual activity. Our results point to a retinotopic organisation of perceptual activity in early visual areas, with independent perceptual activations for different regions of visual space.