Uniform Stimuli Research Articles

Slow and steady: Validating the rhythmic visual response task as a marker for attentional states.

A principal goal of attention research is to develop tasks with clear behavioral signatures of attentional fluctuations. Measures that index attentional states oftenfall under two broad umbrellas: decision tasks, in which participants make responses based on the changing requirements of each trial, and rhythm tasks, in which participants respond rhythmically to a uniform stimulus (e.g., a metronome tone). In the former, response speeding typically precedes errors (indicative of attention failures). In the latter, increased response variability precedes subjective reports of off-task states. We developed and validated the rhythmic visual response task (RVRT); a rhythm task incorporating trial-unique scene stimuli. The RVRT incorporates two important advances from both task categories: (1) it is free from the influence that differential decision-making has on fluctuations in attentional states, and (2) trial-unique stimuli enable later cognitive judgments to be mapped to specific moments in the task. These features allow a relatively unobtrusive measure of mind wandering that facilitates the downstream assessment of its consequences. Participants completed 900 trials of the RVRT, interrupted periodically by thought probes that assessed their attentional state. We found that both response time variance and speed predicted depth of mind wandering. Encouraged by these findings, we used the same analysis approach on archival data to demonstrate that the combination of variance and speed best predicted attentional states in several rhythm and decision task datasets. We discuss the implications of these findings and suggest future research that uses the RVRT to investigate the impact of spontaneous mind wandering on memory, decision-making, and perception.

Функция временного отклика — новый метод исследования нейрофизиологических механизмов восприятия речи в экологически валидных условиях

<p style="text-align: justify;">The temporal response function is a new method that allows to investigate the brain mechanisms of perception of natural, naturalistic speech stimuli. In contrast to other methods for studying brain activity (e.g., evoked potentials), the temporal response function does not require the presentation of a large number of uniform stimuli to produce a robust brain response - recordings of narrative speech lasting 10 minutes or more can be used in experimental paradigms, increasing their ecological validity. The temporal response function can be used to study brain mechanisms of online processing of different components of natural speech: acoustic (physical properties of the audio signal such as envelope and spectrogram), phonological (individual phonemes and their combinations), lexical (contextual characteristics of individual words) and semantic (semantic meaning of words), as well as the interaction between these components processing mechanisms. The article presents the history of the method, its advantages in comparison with other methods and limitations, mathematical basis, features of natural speech components extraction, and a brief review of the main studies using this method.</p>

Effect of surrounding objects in the adapting scene on chromatic adaptation.

Most of the existing chromatic adaptation transforms (CATs) were developed for flat uniform stimuli presented in a uniform background, which substantially simplifies the complexity of the real scene by excluding surrounding objects from the viewing field. The impact of the background complexity, in terms of the spatial properties of the objects surrounding the stimulus, on chromatic adaptation is ignored in most CATs. This study systematically investigated how the background complexity and color distribution affect the adaptation state. Achromatic matching experiments were conducted in an immersive lighting booth, with the illumination varying in chromaticity and the adapting scene varying in surrounding objects. Results show that compared to the uniform adapting field, increasing the scene complexity can significantly improve the degree of adaptation for the Planckian illuminations with low CCT levels. In addition, the achromatic matching points are substantially biased by the color of the surrounding object, implying the interactive effect of the illumination color and the dominant scene color on the adapting white point.

Exploiting colour space geometry for visual stimulus design across animals.

Colour vision represents a vital aspect of perception that ultimately enables a wide variety of species to thrive in the natural world. However, unified methods for constructing chromatic visual stimuli in a laboratory setting are lacking. Here, we present stimulus design methods and an accompanying programming package to efficiently probe the colour space of any species in which the photoreceptor spectral sensitivities are known. Our hardware-agnostic approach incorporates photoreceptor models within the framework of the principle of univariance. This enables experimenters to identify the most effective way to combine multiple light sources to create desired distributions of light, and thus easily construct relevant stimuli for mapping the colour space of an organism. We include methodology to handle uncertainty of photoreceptor spectral sensitivity as well as to optimally reconstruct hyperspectral images given recent hardware advances. Our methods support broad applications in colour vision science and provide a framework for uniform stimulus designs across experimental systems.This article is part of the theme issue ‘Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods’.

Invited Session V: GABAergic function and dysfunction in visual perception: Strong evidence against a common center-surround mechanism in visual processing

Events and objects in the world generally do not occur in isolation. This surrounding spatial and temporal information has major effects on visual processing, effects that are typically described as contextual modulations. Given qualitative similarities among various contextual surround effects (e.g., the effects tend to exaggerate differences and suppress uniform stimuli), it might seem intuitive that these processes are related. It is unknown, however, whether contextual modulation processes across visual sub-modalities are independent, or at least in part share a common underlying mechanism. This is especially relevant for clinical work where these contextual effects are often linked to inhibitory function. We addressed this question using an individual differences approach in three different populations (total N = 129): neurotypical adults, older adults and patients with schizophrenia. The context battery included six tasks that assessed surround modulations in luminance, contrast, orientation, motion (2X) and size domains. The results revealed robust contextual effects across all tasks. However, despite large sample sizes, we found no significant correlations among different contextual tasks. Across groups, we did find significant differences: (a) weaker surround contrast effect in both schizophrenia and older adults, (b) stronger repulsive surround tilt and motion effects in older adults and (c) weaker surround suppression for motion in both schizophrenia and older adults. We find that (1) strength of contextual modulation does not correlate across visual sub-modalities and (2) deficits in contextual modulation that go in different directions. Together, this indicates an absence of a common underlying mechanism for contextual modulation in vision.

Assessing functional reorganization in visual cortex with simulated retinal lesions

Macular degeneration (MD) causes central vision loss, removing input to corresponding representations in the primary visual cortex. There is disagreement concerning whether the cortical regions deprived of input can remain responsive, and the source of reported cortical responses is still debated. To simulate MD in controls, normally sighted participants viewed a bright central disk to adapt the retina, creating a transient ‘retinal lesion’ during a functional MRI experiment. Participants viewed blocks of faces, scrambled faces and uniform grey stimuli, either passively or whilst performing a one-back task. To assess the impact of the simulated lesion, participants repeated the paradigm using a more conventional mean luminance simulated scotoma without adaptation. Our results suggest our attempt to create a more realistic simulation of a lesion did not impact on responses in the representation of the simulated lesion. While most participants showed no evidence of stimulus-driven activation within the lesion representation, a few individuals (22%) exhibited responses similar to a participant with juvenile MD who completed the same paradigm (without adaptation). Reliability analysis showed that responses in the representation of the lesion were generally consistent irrespective of whether positive or negative. We provide some evidence that peripheral visual stimulation can also produce responses in central representations in controls while performing a task. This suggests that the ‘signature of reorganization of visual processing’, is not found solely in patients with retinal lesions, consistent with the idea that activity may be driven by unmasked top–down feedback.

An image-computable model of human visual shape similarity.

Shape is a defining feature of objects, and human observers can effortlessly compare shapes to determine how similar they are. Yet, to date, no image-computable model can predict how visually similar or different shapes appear. Such a model would be an invaluable tool for neuroscientists and could provide insights into computations underlying human shape perception. To address this need, we developed a model ('ShapeComp'), based on over 100 shape features (e.g., area, compactness, Fourier descriptors). When trained to capture the variance in a database of >25,000 animal silhouettes, ShapeComp accurately predicts human shape similarity judgments between pairs of shapes without fitting any parameters to human data. To test the model, we created carefully selected arrays of complex novel shapes using a Generative Adversarial Network trained on the animal silhouettes, which we presented to observers in a wide range of tasks. Our findings show that incorporating multiple ShapeComp dimensions facilitates the prediction of human shape similarity across a small number of shapes, and also captures much of the variance in the multiple arrangements of many shapes. ShapeComp outperforms both conventional pixel-based metrics and state-of-the-art convolutional neural networks, and can also be used to generate perceptually uniform stimulus sets, making it a powerful tool for investigating shape and object representations in the human brain.

The Reproducibility of Cerebrovascular Reactivity Across MRI Scanners.

Cerebrovascular reactivity (CVR) is defined as the ratio of the cerebral blood flow (CBF) response to an increase in a vasoactive stimulus. We used changes in blood oxygenation level-dependent (BOLD) MRI as surrogates for changes of CBF, and standardized quantitative changes in arterial partial pressure of carbon dioxide as the stimulus. Despite uniform stimulus and test conditions, differences in voxel-wise BOLD changes between testing sites may remain, attributable to physiologic and machine variability. We generated a reference atlas of normal CVR metrics (voxel-wise mean and SD) for each of two sites. We hypothesized that there would be no significant differences in CVR between the two atlases enabling each atlas to be used at any site. A total of 69 healthy subjects were tested to create site-specific atlases, with 20 of those individuals tested at both sites. 38 subjects were scanned at Site 1 (17F, 37.5 ± 16.8 y) and 51 subjects were tested at Site 2 (22F, 40.9 ± 17.4 y). MRI platforms were: Site 1, 3T Magnetom Skyra Siemens scanner with 20-channel head and neck coil; and Site 2, 3T HDx Signa GE scanner with 8-channel head coil. To construct the atlases, test results of individual subjects were co-registered into a standard space and voxel-wise mean and SD CVR metrics were calculated. Map comparisons of z scores found no significant differences between white matter or gray matter in the 20 subjects scanned at both sites when analyzed with either atlas. We conclude that individual CVR testing, and atlas generation are compatible across sites provided that standardized respiratory stimuli and BOLD MRI scan parameters are used. This enables the use of a single atlas to score the normality of CVR metrics across multiple sites.

The impact of transition type on chromatic adaptation under dual lighting conditions

Over the years, many CATs (chromatic adaptation transform), typically based on the von Kries coefficient rule, have been developed to predict the corresponding colors under different illuminants. However, these CATs were derived for uniform stimuli surrounded by a uniform adapting field. To investigate the adaptation state under spatially complex illumination, an achromatic matching experiment was conducted under dual lighting conditions with three color pairs and two transition types. It has been found that the transition type has an impact on both the equivalent chromaticity and degree of adaptation. These results can help build a comprehensive von Kries based CAT model, with considering the spatial complexity of illumination.

Promotion of Poststroke Motor-Function Recovery with Repetitive Transcranial Magnetic Stimulation by Regulating the Interhemispheric Imbalance.

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain-stimulation technique that transiently modulates cerebral cortex excitability, achieving overall positive results in poststroke motor-function recovery. Excessive inhibition of the ipsilesional-affected hemisphere by the contralesional-unaffected hemisphere has seriously hindered poststroke motor-function recovery. Hence, intracortical disinhibition can be used as an approach to managing poststroke brain injury. This technique promotes neural plasticity for faster motor-function recovery. rTMS relieves unilateral inhibition of the brain function by regulatinga interhemispheric-imbalanced inhibition. This paper summarized 12 studies from 2016 to date, focusing on rTMS on motor function after acute and chronic stroke by regulating the interhemispheric imbalance of inhibitory inputs. Although rTMS studies have shown promising outcomes on recovery of motor functions in stroke patients, different intervention methods may lead to discrepancies in results. A uniform optimal stimulus model cannot routinely be used, mainly due to the stimulus schemes, stroke types and outcome-measuring differences among studies. Thus, the effect of rTMS on poststroke motor-function recovery should be investigated further to standardize the rTMS program for optimal poststroke motor-function recovery. More randomized, placebo-controlled clinical trials with standardized rTMS protocols are needed to ensure the effectiveness of the treatment.

Recoverable elastic barrier for robust hydrogel patterning with uniform flow profile for organ-on-a-chip applications

This paper presents a novel recoverable elastic barrier design that can realize robust hydrogel patterning without leakage or bursting from the central tissue chamber into the side microfluidic channels via the method of a temporary or virtual barrier. Compared with previous gel patterning methods using physical barriers like obstacle array or phaseguide microstructure, this method enables more uniform stimuli to the cultured cell/tissue and barrier-free imaging window. Besides the straight one-lane patterning, it also allows the irregular structural or multiple parallel cell-laden hydrogel compartments inside tissue chamber used for different applications. The effectiveness of this design is validated by its incorporation into a microfluidic platform for generating 3D vascularized organ-on-a-chip through different vascularization mechanisms of vasculogenesis and angiogenesis, respectively. We expect that this proof-of-concept will become a new and attractive method for gel patterning inside different microfluidic devices used for versatile biological applications such as organ-on-a-chip.

Chromatic and luminance sensitivity for skin and skinlike textures.

Despite the importance of the appearance of human skin for theoretical and practical purposes, little is known about visual sensitivity to subtle skin-tone changes, and whether the human visual system is indeed optimized to discern skin-color changes that confer some evolutionary advantage. Here, we report discrimination thresholds in a three-dimensional chromatic-luminance color space for natural skin and skinlike textures, and compare these to thresholds for uniform stimuli of the same mean color. We find no evidence that discrimination performance is superior along evolutionarily relevant color directions. Instead, discriminability is primarily determined by the prevailing illumination, and discrimination ellipses are aligned with the daylight locus. More specifically, the area and orientation of discrimination ellipses are governed by the chromatic distance between the stimulus and the illumination. Since this is true for both uniform and textured stimuli, it is likely to be driven by adaptation to mean stimulus color. Natural skin texture itself does not confer any advantage for discrimination performance. Furthermore, we find that discrimination boundaries for skin, skinlike, and scrambled skin stimuli are consistently larger than those for uniform stimuli, suggesting a possible adaptation to higher order color statistics of skin. This is in line with findings by Hansen, Giesel, and Gegenfurtner (2008) for other natural stimuli (fruit and vegetables). Human observers are also more sensitive to skin-color changes under simulated daylight as opposed to fluorescent light. The reduced sensitivity is driven by a decline in sensitivity along the luminance axis, which is qualitatively consistent with predictions from a Von Kries adaptation model.

High-Throughput Analysis of Behavior Under the Control of Optogenetics in Caenorhabditis elegans.

In this unit, we describe an inexpensive and versatile method for optogenetic stimulation of a large population of genetically engineered Caenorhabditis elegans worms while quantitatively analyzing behavior. A custom light-emitting diode light source is used to deliver blue-light stimuli, causing direct depolarization of neurons expressing the light-gated cation channel Channelrhodopsin-2, which in turn evokes behavioral responses. The behavioral responses are recorded by a high-throughput machine vision-based tracking system, the Multi-Worm Tracker, for detailed analysis. This approach allows researchers to bypass technical obstacles to simultaneously deliver uniform stimuli to a large number of freely behaving animals and investigate the neural underpinnings of behavior. © 2018 by John Wiley & Sons, Inc.

The Honeycomb illusion: Uniform textures not perceived as such

We present a series of patterns, in which texture is perceived differently at fixation in comparison to the periphery, such that a physically uniform stimulus yields a nonuniform percept. We call this the Honeycomb illusion, and we discuss it in relation to the similar Extinction illusion (Ninio & Stevens, 2000). The effect remains strong despite multiple fixations, dynamic changes, and manipulations of the size of texture elements. We discuss the phenomenon in relation to how vision achieves a detailed and stable representation of the environment despite changes in retinal spatial resolution and dramatic changes across saccades. The Honeycomb illusion complements previous related observations in suggesting that this representation is not necessarily based on multiple fixations (i.e., memory) or on extrapolation from information available to central vision.

Binocular rivalry from invisible patterns

Binocular rivalry arises when incompatible images are presented to the two eyes. If the two eyes' conflicting features are invisible, leading to identical perceptual interpretations, does rivalry competition still occur? Here we investigated whether binocular rivalry can be induced from conflicting but invisible spatial patterns. A chromatic grating counterphase flickering at 30 Hz appeared uniform, but produced significant tilt aftereffect and orientation-selective adaptation. The invisible pattern also generated significant BOLD activities in the early visual cortex, with minimal response in the parietal and frontal cortical areas. Compared with perceptually matched uniform stimuli, a monocularly presented invisible chromatic grating enhanced the rivalry competition with a low-contrast visible grating presented to the other eye. Furthermore, switching from a uniform field to a perceptually matched invisible chromatic grating produced interocular suppression at approximately 200 ms after onset of the invisible grating. Experiments using briefly presented monocular probes revealed evidence for sustained rivalry competition between two invisible gratings during continuous dichoptic presentations. These findings indicate that even without visible interocular conflict, and with minimal engagement of frontoparietal cortex and consciousness related top-down feedback, perceptually identical patterns with invisible conflict features produce rivalry competition in the early visual cortex.

Adaptation-induced modification of motion selectivity tuning in visual tectal neurons of adult zebrafish.

In the developing brain, training-induced emergence of direction selectivity and plasticity of orientation tuning appear to be widespread phenomena. These are found in the visual pathway across different classes of vertebrates. Moreover, short-term plasticity of orientation tuning in the adult brain has been demonstrated in several species of mammals. However, it is unclear whether neuronal orientation and direction selectivity in nonmammalian species remains modifiable through short-term plasticity in the fully developed brain. To address this question, we analyzed motion tuning of neurons in the optic tectum of adult zebrafish by calcium imaging. In total, orientation and direction selectivity was enhanced by adaptation, responses of previously orientation-selective neurons were sharpened, and even adaptation-induced emergence of selectivity in previously nonselective neurons was observed in some cases. The different observed effects are mainly based on the relative distance between the previously preferred and the adaptation direction. In those neurons in which a shift of the preferred orientation or direction was induced by adaptation, repulsive shifts (i.e., away from the adapter) were more prevalent than attractive shifts. A further novel finding for visually induced adaptation that emerged from our study was that repulsive and attractive shifts can occur within one brain area, even with uniform stimuli. The type of shift being induced also depends on the difference between the adapting and the initially preferred stimulus direction. Our data indicate that, even within the fully developed optic tectum, short-term plasticity might have an important role in adjusting neuronal tuning functions to current stimulus conditions.

A temporo-spatial analysis of the neural correlates of extrinsic perceptual grouping in vision

Principles of perceptual grouping can be divided into intrinsic grouping cues, which are based on built-in properties of the grouped elements (e.g., their shape, position, colour, etc.) like most of the classical Gestalt laws, and extrinsic grouping principles, based on relations between the discrete elements and other external stimuli that induce them to group (e.g., common region, connectedness). Several studies have explored the neural correlates of intrinsic grouping factors but, to our knowledge, no previous study has studied the neural correlates of extrinsic principles. The present study aimed to shed light on this issue by exploiting the high temporal resolution of event-related potentials (ERPs) and recent advances in source localization. Specifically, grouping by common region was compared with two comparison conditions, an intrinsic grouping (luminance similarity) and a uniform stimulus condition, in a perceptual discrimination task. We reported three main neural effects associated with grouping by common region. First, a posterior N210 component with a neural origin in the left extrastriate cortex was related to perceptual analysis of extrinsic elements inducing grouping and the formation of a visual group. Second, an enhanced posterior P280, which presumably reflects higher confidence decisions during response selection. Finally, a P550 originated in the right superior parietal cortex that seems to be associated with top-down suppression activity connected with the termination of the processing of the current trial. Overall, our results suggest that common region cues belong to the category of long latency grouping principles that mainly involve activity in extrastriate cortices.

Noise decomposition of intracellular biochemical signaling networks using nonequivalent reporters.

Experimental measurements of biochemical noise have primarily focused on sources of noise at the gene expression level due to limitations of existing noise decomposition techniques. Here, we introduce a mathematical framework that extends classical extrinsic-intrinsic noise analysis and enables mapping of noise within upstream signaling networks free of such restrictions. The framework applies to systems for which the responses of interest are linearly correlated on average, although the framework can be easily generalized to the nonlinear case. Interestingly, despite the high degree of complexity and nonlinearity of most mammalian signaling networks, three distinct tumor necrosis factor (TNF) signaling network branches displayed linearly correlated responses, in both wild-type and perturbed versions of the network, across multiple orders of magnitude of ligand concentration. Using the noise mapping analysis, we find that the c-Jun N-terminal kinase (JNK) pathway generates higher noise than the NF-κB pathway, whereas the activation of c-Jun adds a greater amount of noise than the activation of ATF-2. In addition, we find that the A20 protein can suppress noise in the activation of ATF-2 by separately inhibiting the TNF receptor complex and JNK pathway through a negative feedback mechanism. These results, easily scalable to larger and more complex networks, pave the way toward assessing how noise propagates through cellular signaling pathways and create a foundation on which we can further investigate the relationship between signaling system architecture and biological noise.

Bidirectional molecular transport shapes cell polarization in a two-dimensional model of eukaryotic chemotaxis

Chemotacting eukaryotic cells can sense shallow gradients of chemoattractants and respond by assuming an asymmetric shape with well-defined front and back regions. Such a striking polarization phenomenon is produced largely through the interconversions and interactions between several cellular components, including Rac GTPase (Rac), phosphoinositide 3-kinase (PI3K), tensin homology protein (PTEN), phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) and phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2). Here, we developed a mathematical model of cell polarization by exploring bidirectional molecular transport that arose from phosphoinositides (PIs) and Rac-mediated feedback loops. We assumed a static gradient of activated Rac derived from an external signal field as the internal trigger signal. The evolution of PI(3,4,5)P3 and PI(4,5)P2 along with PI3K and PTEN that act as activator and inhibitor, respectively, were described by a pair of coupled transient reaction-diffusion equations. The entire system was solved using a Lattice–Boltzmann method with an embedded Monte-Carlo method to track the stochastic translocation behaviors of discrete PI3K/PTEN molecules. We first showed that, upon a graded external stimulus, the Rac to PI(3,4,5)P3 cascade exhibited a short range positive-feedback loop, while the PTEN to PI(4,5)P2 cascade contributed another long range negative-feedback loop, which dominated the “forward” and “backward” molecular transport, respectively. Second, polarization was governed by the ratio of [PI3K] to [PTEN], and manifested a switch-like behavior. Third, with a uniform stimulus, spontaneous polarization could occur in PTEN-deficient cells.

Discomfort glare perception of non-uniform light sources in an office setting

For indoor lighting the glare assessment by Unified Glare Rating (UGR) is widely adopted. However, disagreement exists on how to evaluate glare of non-uniform sources. This recently gained attention with the introduction of small and bright LEDs in general lighting. Earlier studies in a laboratory set-up showed that with equivalent average luminance, discomfort glare from a non-uniform stimulus seems to be greater than that of a uniform stimulus. We investigate the relation between discomfort glare perception of office employees and a set of parameters that is typical for LED luminaires, in a representative office environment. The offered light settings varied in luminance pattern, beam shape and illuminance on the work plane. In agreement with earlier work it can be concluded that point array LED luminaires provoke more discomfort glare in open plan offices than uniform sources and that the currently used UGR is not a good predictor in these cases. The luminance characteristics in the exit window (peak luminance, luminance contrasts, and spatial luminance distribution) play an important role in the design of comfortable (LED) office luminaires. A better understanding of these characteristics and their effects on glare perception, as well as a redefinition of a glare index are essential for a reliable prediction of glare perception.