Random Dot Kinematograms Research Articles

Time course of early visual cortex dynamics during top-down modulated attention shifts within or between feature dimensions

Shifting attention from one color to another color or from color to another feature dimension such as orientation is imperative when searching for a certain object in a cluttered scene. Most attention models that emphasize feature-based selection implicitly assume that within- and cross-dimensional shifts take equally long. In contrast, the dimensional weighting account (DWA) predicts that cross-dimensional shifts take longer than shifts within a dimension because attentional weights need to be shifted not only between features but also from one dimension to another. We investigated whether shifting costs between feature dimensions is a general mechanism beyond singleton feature search in a non-search task. To this end, we recorded time courses of behavioral data and steady state visual evoked potentials (SSVEPs) in human EEG that provide an objective electrophysiological measure of neural dynamics in early visual cortex. SSVEPs were elicited by four random dot kinematograms (RDKs) that flickered at different frequencies. Each RDK comprised dashes that uniquely combined two features from dimensions color (red or blue) and orientation (slash or backslash). Subjects were cued to either shift attention within a feature dimension (e.g. from color to color) or cross-dimensionally (e.g. from color to orientation) and to detect and respond to short coherent motion events of a subset of dots at the to-be-attended dimension while ignoring all other events. For behavioral as well as electrophysiological data we found that shifts between feature dimensions took longer than shifts within dimensions. Interestingly, shifts towards color were took less time than shifts towards orientation, regardless of the initially to-be-attended feature. In conclusion: While the first result is in line with the DWA the second might reflect – in line with previous studies – that color and orientation are preferentially processed at different stages of the visual cortical hierarchy that receive attentional top-down modulations with different latencies. Meeting abstract presented at VSS 2015

Limited attention facilitates coherent motion processing.

It has been suggested that sensitivities for visual motion are typically impaired by poor attention. Here, we show that limited attention paradoxically improves performance on a global motion detection task. Psychophysical experiments revealed that deliberately attending to an irrelevant stimulus enhanced sensitivity for detecting coherent motion in random-dot kinematograms but did not affect contrast and velocity sensitivity for local luminance motion. Subsequent experiments further demonstrated that the dual task reduced sensitivity for detecting spatial modulations in local motion direction and induced illusory motion assimilation. Additional measurements confirmed that the secondary task had no effect when attentional load was extremely high or when motion stimuli were presented peripherally. These results may be explained by the idea that limited attention dynamically expands the spatial extent of motion integration by reducing center-surround interactions at high-level motion processing.

Construction and evaluation of an integrated dynamical model of visual motion perception

Although numerous models describe the individual neural mechanisms that may be involved in the perception of visual motion, few of them have been constructed to take arbitrary stimuli and map them to a motion percept. Here, we propose an integrated dynamical motion model (IDM), which is sufficiently general to handle diverse moving stimuli, yet sufficiently precise to account for a wide-ranging set of empirical observations made on a family of random dot kinematograms. In particular, we constructed models of the cortical areas involved in motion detection, motion integration and perceptual decision. We analyzed their parameters through dynamical simulations and numerical continuation to constrain their proper ranges. Then, empirical data from a family of random dot kinematograms experiments with systematically varying direction distribution, presentation duration and stimulus size, were used to evaluate our model and estimate corresponding model parameters. The resulting model provides an excellent account of a demanding set of parametrically varied behavioral effects on motion perception, providing both quantitative and qualitative elements of evaluation.

Attentional bias to affective faces and complex IAPS images in early visual cortex follows emotional cue extraction

Emotionally arousing stimuli are known to rapidly draw the brain's processing resources, even when they are task-irrelevant. The steady-state visual evoked potential (SSVEP) response, a neural response to a flickering stimulus which effectively allows measurement of the processing resources devoted to that stimulus, has been used to examine this process of attentional shifting. Previous studies have used a task in which participants detected periods of coherent motion in flickering random dot kinematograms (RDKs) which generate an SSVEP, and found that task-irrelevant emotional stimuli withdraw more attentional resources from the task-relevant RDKs than task-irrelevant neutral stimuli. However, it is not clear whether the emotion-related differences in the SSVEP response are conditional on higher-level extraction of emotional cues as indexed by well-known event-related potential (ERPs) components (N170, early posterior negativity, EPN), or if affective bias in competition for visual attention resources is a consequence of a time-invariant shifting process. In the present study, we used two different types of emotional distractors – IAPS pictures and facial expressions – for which emotional cue extraction occurs at different speeds, being typically earlier for faces (at ~170ms, as indexed by the N170) than for IAPS images (~220–280ms, EPN). We found that emotional modulation of attentional resources as measured by the SSVEP occurred earlier for faces (around 180ms) than for IAPS pictures (around 550ms), after the extraction of emotional cues as indexed by visual ERP components. This is consistent with emotion related re-allocation of attentional resources occurring after emotional cue extraction rather than being linked to a time-fixed shifting process.

The decision, not the decision task, causes repulsive biases away from the decision boundary

The perceived direction of a random-dot kinematogram in a fine discrimination task is systematically biased away from the decision boundary. Stimulus uncertainty combined with a neural read-out that is optimized for that specific discrimination task was thought to cause these biases (Jazayeri and Movshon, 2007). Based on this explanation, we would expect the biases to disappear if subjects did not have to perform the discrimination task. We conducted two experiments to test this prediction. Experiment 1 (control) was aimed to reproduce the effects of the original study, using stimuli that consisted of an array of line segments (length of 0.5 degs). The orientation of each line segment was drawn from a Gaussian around the array's mean orientation (stimulus orientation). Stimulus orientation was uniformly sampled from a range +/−21 degrees relative to a reference orientation. Black marks indicated the randomly chosen reference orientation. Gray wedges around the reference mark indicated the total range of stimulus orientations (prior). Subjects first had to indicate whether the stimulus orientation was clockwise (CW) or counter-clockwise (CCW) relative to reference orientation. Subsequently they had to estimate the stimulus orientation. Experiment 2 was identical to Experiment 1 except that subjects were given the stimulus' category (CW/CCW) before stimulus presentation. The results show very similar repulsive biases in both experiments. Hence, we conclude that the biases are not the result of a specific, task-dependent neural read-out as previously suggested, but instead are consistent with a “self-consistent“ Bayesian observer model (Stocker and Simoncelli, 2008).

Motion-defined surface segregation in human visual cortex.

Surface segregation provides an efficient way to parse the visual scene for perceptual analysis. Here, we investigated the segregation of a bivectorial motion display into transparent surfaces through a psychophysical task and fMRI. We found that perceptual transparency correlated with neural activity in the early areas of the visual cortex, suggesting these areas may be involved in the segregation of motion-defined surfaces. Two oppositely rotating, uniquely colored random dot kinematograms (RDKs) were presented either sequentially or in a spatially interleaved manner, displayed at varying alternation frequencies. Participants reported the color and rotation direction pairing of the RDKs in the psychophysical task. The spatially interleaved display generated the percept of motion transparency across the range of frequencies tested, yielding ceiling task performance. At high alternation frequencies, performance on the sequential display also approached ceiling, indicative of perceived transparency. However, transparency broke down in lower alternation frequency sequential displays, producing performance close to chance. A corresponding pattern mirroring the psychophysical data was also evident in univariate and multivariate analyses of the fMRI BOLD activity in visual cortical areas V1, V2, V3, V3AB, hV4, and V5/MT+. Using gray RDKs, we found significant presentation by frequency interactions in most areas; differences in BOLD signal between presentation types were significant only at the lower alternation frequency. Multivariate pattern classification was similarly unable to discriminate between presentation types at the higher frequency. This study provides evidence that early visual cortex may code for motion-defined surface segregation, which in turn may enable perceptual transparency.

The effect of simultaneous flickering light stimulation on global form and motion perception thresholds

The question regarding the exact function of the primary visual cortex (V1) in vision has been around ever since the description of residual vision after damage to this cortical area by Riddoch in 1917. In 2002, Schoenfeld and colleagues proposed that V1 can be saturated by flashes of light, by which the function of V1-bypassing visual pathways can be “unmasked”. The Schoenfeld group found that light flashes applied on stimulus onset led to the elevation of brightness increment detection thresholds, but left motion detection thresholds unaltered. Although the proposed method (i.e. the use of light flashes to induce refractoriness in V1) could be a simple, cheap and elegant way of exploring V1 functions, no study has followed up on this. Therefore it is not known if it works at all with other types of stimuli. For that reason, we decided to revisit the idea in a modified form. Global form and motion perception thresholds were assessed with static Glass pattern stimuli and random dot kinematograms, with and without 12Hz flickering light stimulation. Global motion thresholds were almost unaltered by flickering stimulation, while a significant threshold elevation was caused in the global form perception task. The strongest conclusion allowed by our data is that simultaneous flickering photostimulation elevates global form perception thresholds but not global motion perception thresholds. This is in some way related to the refractoriness generated in an unsatisfactorily defined part of V1. We suggest that this does not necessarily reflect the activity of V1-bypassing pathways, and propose that the application of light flashes is a method that deserves more attention in the exploration of the V1-dependent and independent elements of visual consciousness in human subjects.

Direction-contingent duration compression is primarily retinotopic.

Previous research has shown that prior adaptation to a spatially circumscribed, oscillating grating results in the duration of a subsequent stimulus briefly presented within the adapted region being underestimated. There is an on-going debate about where in the motion processing pathway the adaptation underlying this distortion of sub-second duration perception occurs. One position is that the LGN and, perhaps, early cortical processing areas are likely sites for the adaptation; an alternative suggestion is that visual area MT+ contains the neural mechanisms for sub-second timing; and a third position proposes that the effect is driven by adaptation at multiple levels of the motion processing pathway. A related issue is in what frame of reference - retinotopic or spatiotopic - does adaptation induced duration distortion occur. We addressed these questions by having participants adapt to a unidirectional random dot kinematogram (RDK), and then measuring perceived duration of a 600 ms test RDK positioned in either the same retinotopic or the same spatiotopic location as the adaptor. We found that, when it did occur, duration distortion of the test stimulus was direction contingent; that is it occurred when the adaptor and test stimuli drifted in the same direction, but not when they drifted in opposite directions. Furthermore the duration compression was evident primarily under retinotopic viewing conditions, with little evidence of duration distortion under spatiotopic viewing conditions. Our results support previous research implicating cortical mechanisms in the duration encoding of sub-second visual events, and reveal that these mechanisms encode duration within a retinotopic frame of reference.

Global versus local: double dissociation between MT+ and V3A in motion processing revealed using continuous theta burst transcranial magnetic stimulation.

The functional properties of motion selective areas in human visual cortex, including V3A, MT+, and intraparietal sulcus (IPS) are not fully understood. To examine the functional specialization of these areas for global and local motion processing, we used off-line, neuronavigated, continuous theta burst (cTBS) transcranial magnetic stimulation to temporarily alter neural activity within unilateral V3A, MT+, and IPS. A within-subjects design was employed and stimulation sessions were separated by at least 24 h. In each session, subjects were asked to discriminate the global motion directions of successively presented random dot kinematograms (RDKs) before and after cTBS. RDKs were presented at either 100 or 40 % coherence in either the left or right visual field. We found that V3A stimulation selectively impaired discrimination of 100 % coherent motion, while MT+ stimulation selectively impaired discrimination of 40 % coherent motion. IPS stimulation impaired discrimination of both motion stimuli. All cTBS effects were specific to stimuli presented contralaterally to the stimulation site and vertex stimulation had no effect. The double dissociation between the cTBS effects on MT+ and V3A indicates distinct roles for these two regions in motion processing. Judging the direction of 100 % coherent motion can rely on local motion processing because every dot moves in the same direction. However, judging the global direction of 40 % coherent motion requires global processing. Thus, our results suggest separate, parallel processing of local and global motion in V3A and MT+, respectively, with the outputs of these two areas being combined within the IPS.

Dynamic integration of information about salience and value for smooth pursuit eye movements

Eye movement behavior can be determined by bottom-up factors like visual salience and by top-down factors like expected value. These different types of signals have to be combined for the control of eye movements. In this study we investigated how smooth pursuit eye movements integrate salience and value information. Observers were asked to track a random-dot kinematogram containing two coherent motion directions. To manipulate salience, the coherence or the density of one of the motion signals was varied. To manipulate value, observers won or lost money in a separate experiment if they were tracking one or the other motion direction. Our results show that pursuit direction was initially determined only by salience. 300–400ms after target motion onset, pursuit steered towards the rewarded direction and the salience effects disappeared. The time course of this effect depended crucially on the difficulty to segment the two signal directions. These results indicate that salience determines early pursuit responses in the same way as saccades with short latencies. Value information is processed slower and dominates pursuit after several 100ms.

Modeling motion perception and perceptual learning in random dot kinematograms

Modeling motion perception and perceptual learning in random dot kinematograms

Attention to multiple locations is limited by spatial working memory capacity.

What limits the ability to attend several locations simultaneously? There are two possibilities: Either attention cannot be divided without incurring a cost, or spatial memory is limited and observers forget which locations to monitor. We compared motion discrimination when attention was directed to one or multiple locations by briefly presented central cues. The cues were matched for the amount of spatial information they provided. Several random dot kinematograms (RDKs) followed the spatial cues; one of them contained task-relevant, coherent motion. When four RDKs were presented, discrimination accuracy was identical when one and two locations were indicated by equally informative cues. However, when six RDKs were presented, discrimination accuracy was higher following one rather than multiple location cues. We examined whether memory of the cued locations was diminished under these conditions. Recall of the cued locations was tested when participants attended the cued locations and when they did not attend the cued locations. Recall was inaccurate only when the cued locations were attended. Finally, visually marking the cued locations, following one and multiple location cues, equalized discrimination performance, suggesting that participants could attend multiple locations when they did not have to remember which ones to attend. We conclude that endogenously dividing attention between multiple locations is limited by inaccurate recall of the attended locations and that attention poses separate demands on the same central processes used to remember spatial information, even when the locations attended and those held in memory are the same.

The ups and downs of global motion perception: a paradoxical advantage for smaller stimuli in the aging visual system.

Recent evidence suggests that normal aging is typically accompanied by impairment in the ability to perceive the global (overall) motion of visual objects in the world. The purpose of this study was to examine the interplay between age-related changes in the ability to perceive translational global motion (up vs. down) and important factors such as the spatial extent (size) over which movement occurs and how cluttered the moving elements are (density). We used random dot kinematograms (RDKs) and measured motion coherence thresholds (% signal elements required to reliably discriminate global direction) for young and older adults. We did so as a function of the number and density of local signal elements, and the aperture area in which they were displayed. We found that older adults’ performance was relatively unaffected by changes in aperture size, the number and density of local elements in the display. In young adults, performance was also insensitive to element number and density but was modulated markedly by display size, such that motion coherence thresholds decreased as aperture area increased (participants required fewer local elements to move coherently to determine the overall image direction). With the smallest apertures tested, young participants’ motion coherence thresholds were considerably higher (~1.5 times worse) than those of their older counterparts. Therefore, when RDK size is relatively small, older participants were actually better than young participants at processing global motion. These findings suggest that the normal (disease-free) aging process does not lead to a general decline in perceptual ability and in some cases may be visually advantageous. The results have important implications for the understanding of the consequences of aging on visual function and a number of potential explanations are explored. These include age-related changes in spatial summation, reduced cortical inhibition, neural blur and attentional resource allocation.

Do older individuals have difficulty processing motion or excluding noise? Implications for safe driving

This study aimed to determine if difficulties extracting signal from noise explained poorer coherent motion thresholds in older individuals, particularly women. In four experimental conditions the contrast of the signal and noise dots used in a random dot kinematogram was manipulated. Coherence thresholds were highest when the signal dots were of a lower contrast than the noise dots and lowest when the signal dots were of a higher contrast than the noise dots. In all conditions the older group had higher coherence thresholds than the younger group, and women had higher thresholds than men. Significant correlations were found between coherence thresholds and self-reported driving difficulties in conditions in which the signal dots had to be extracted from noise only. The results indicate that older individuals have difficulties extracting signal from noise in cluttered visual environments. The implications for safe driving are discussed.

Determinants of motion response anisotropies in human early visual cortex: The role of configuration and eccentricity

Anisotropies in the cortical representation of various stimulus parameters can reveal the fundamental mechanisms by which sensory properties are analysed and coded by the brain. One example is the preference for motion radial to the point of fixation (i.e. centripetal or centrifugal) exhibited in mammalian visual cortex. In two experiments, this study used functional magnetic resonance imaging (fMRI) to explore the determinants of these radial biases for motion in functionally-defined areas of human early visual cortex, and in particular their dependence upon eccentricity which has been indicated in recent reports. In one experiment, the cortical response to wide-field random dot kinematograms forming 16 different complex motion patterns (including centrifugal, centripetal, rotational and spiral motion) was measured. The response was analysed according to preferred eccentricity within four different eccentricity ranges. Response anisotropies were characterised by enhanced activity for centripetal or centrifugal patterns that changed systematically with eccentricity in visual areas V1–V3 and hV4 (but not V3A/B or V5/MT+). Responses evolved from a preference for centrifugal over centripetal patterns close to the fovea, to a preference for centripetal over centrifugal at the most peripheral region stimulated, in agreement with previous work. These effects were strongest in V2 and V3. In a second experiment, the stimuli were restricted to within narrow annuli either close to the fovea (0.75–1.88°) or further in the periphery (4.82–6.28°), in a way that preserved the local motion information available in the first experiment. In this configuration a preference for radial motion (centripetal or centrifugal) persisted but the dependence upon eccentricity disappeared. Again this was clearest in V2 and V3. A novel interpretation of the dependence upon eccentricity of motion anisotropies in early visual cortex is offered that takes into account the spatiotemporal “predictability” of the moving pattern. Such stimulus predictability, and its relationship to models of predictive coding, has found considerable support in recent years in accounting for a number of other perceptual and neural phenomena.

Snake fearfulness is associated with sustained competitive biases to visual snake features: Hypervigilance without avoidance

The extent and time course of competition between a specific fear cue and task-related stimuli in early human visual cortex was investigated using electrophysiology. Steady-state visual evoked potentials (ssVEPs) were evoked using random-dot kinematograms that consisted of rapidly flickering (8.57Hz) dots moving randomly, superimposed upon emotional or neutral distractor pictures. Participants were asked to detect intervals of coherently moving dots, ignoring the distractor pictures that varied in hedonic content. Women reporting high or low levels of snake fear were recruited from a large sample of healthy college students, and snake pictures served as fear-relevant distractors. The time-varying amplitude of the ssVEP evoked by the motion detection task showed significant reduction when viewing emotionally arousing, compared to neutral, distractors, replicating previous studies. For high-fear participants, snake distractors elicited a sustained attenuation of task evoked ssVEP amplitude, greater than the attenuation prompted by other unpleasant arousing content. These findings support a hypothesis that fear cues prompt sustained hypervigilance rather than perceptual avoidance.

Attention is Required for Maintenance of Feature Binding in Visual Working Memory

Working memory and attention are intimately connected. However, understanding the relationship between the two is challenging. Currently, there is an important controversy about whether objects in working memory are maintained automatically or require resources that are also deployed for visual or auditory attention. Here we investigated the effects of loading attention resources on precision of visual working memory, specifically on correct maintenance of feature-bound objects, using a dual-task paradigm. Participants were presented with a memory array and were asked to remember either direction of motion of random dot kinematograms of different colour, or orientation of coloured bars. During the maintenance period, they performed a secondary visual or auditory task, with varying levels of load. Following a retention period, they adjusted a coloured probe to match either the motion direction or orientation of stimuli with the same colour in the memory array. This allowed us to examine the effects of an attention-demanding task performed during maintenance on precision of recall on the concurrent working memory task. Systematic increase in attention load during maintenance resulted in a significant decrease in overall working memory performance. Changes in overall performance were specifically accompanied by an increase in feature misbinding errors: erroneous reporting of nontarget motion or orientation. Thus in trials where attention resources were taxed, participants were more likely to respond with nontarget values rather than simply making random responses. Our findings suggest that resources used during attention-demanding visual or auditory tasks also contribute to maintaining feature-bound representations in visual working memory—but not necessarily other aspects of working memory.

An investigation of the white matter microstructure in motion detection using diffusion MRI

One of the most widely investigated functions of the brain is vision. Whereas special attention is often paid to motion detection and its modulation by attention, comparatively still little is known about the structural background of this function. We therefore, examined the white matter microstructural background of coherent motion detection. A random-dot kinematogram paradigm was used to measure the sensitivity of healthy individuals׳ to movement coherence. The potential correlation was investigated between the motion detection threshold and the white matter microstructure as measured by high angular resolution diffusion MRI. The Track Based Spatial Statistics method was used to address this correlation and probabilistic tractography to reveal the connection between identified regions. A significant positive correlation was found between the behavioural data and the local fractional anisotropy in the posterior part of the right superior frontal gyrus, the right juxta-cortical superior parietal lobule, the left parietal white matter, the left superior temporal gyrus and the left optic radiation. Probabilistic tractography identified pathways that are highly similar to the segregated attention networks, which have a crucial role in the paradigm. This study draws attention to the structural determinant of a behavioural function.

Dissociation between the impact of evidence on eye movement target choice and confidence judgements

It has been suggested that the evidence used to support a decision to move our eyes and the confidence we have in that decision are derived from a common source. Alternatively, confidence may be based on further post-decisional processes. In three experiments, we examined this. In Experiment 1, participants chose between two targets on the basis of varying levels of evidence (i.e., the direction of motion coherence in a random dot kinematogram). They indicated this choice by making a saccade to one of two targets and then indicated their confidence. Saccade trajectory deviation was taken as a measure of the inhibition of the non-selected target. We found that as evidence increased so did confidence and deviations of saccade trajectory away from the non-selected target. However, a correlational analysis suggested they were not related. In Experiment 2, an option to opt-out of the choice was offered on some trials if choice proved too difficult. In this way, we isolated trials on which confidence in target selection was high (i.e., when the option to opt-out was available but not taken). Again saccade trajectory deviations were found not to differ in relation to confidence. In Experiment 3, we directly manipulated confidence, such that participants had high or low task confidence. They showed no differences in saccade trajectory deviations. These results support post-decisional accounts of confidence: evidence supporting the decision to move the eyes is reflected in saccade control, but the confidence that we have in that choice is subject to further post-decisional processes.

Binocular Training Reduces Amblyopic Visual Acuity Impairment

Amblyopia is the most common cause of monocular visual impairment. Patching, which is modestly effective, is the current treatment of amblyopia in children. There is no clinically approved treatment for adults. The present study is a clinical trial (non-sham controlled and non-randomized) that assessed the efficacy of binocular training for improvement of the visual acuity in children and adults with amblyopia. Twenty-two amblyopic subjects ranging in age from 5 to 73 (mean: 36.2) years for whom patching and/or surgical treatments did not correct their visual impairment completed an average of 14.5 sessions of binocular training over a period of 4 to 6 weeks. Random dot kinematograms were presented dichoptically to the two eyes and the participants’ task was to identify the direction of motion of the targets. Mean visual acuity improvement was 0.34 LogMAR (range: 0.1–0.58 LogMAR) and was shown to persist 6 months following the cessation of binocular training. Our study provides results in a large number of patients that confirm the clinical effectiveness of binocular training as a treatment for amblyopia in improving visual acuity in both children and adults. Moreover, this study is the first to demonstrate that the improvements in visual function were maintained for 6 months in the absence of any additional treatment.