Biomedical subjects

Rufin VanRullen

Publications and source records attributed to Rufin VanRullen.

16 recordsLinked to original sources

PubMed2006

The continuous Wagon Wheel Illusion is object-based.

The occurrence of perceived reversed motion while observers view a periodic, continuously moving stimulus (the "continuous Wagon Wheel Illusion") has been taken as evidence that some aspects of motion perception rely on discrete sampling of visual information. The spatial extent of this sampling is currently under debate. When two separate motion stimuli are viewed simultaneously, the illusion of reversed motion rarely occurs for both objects together: this rules out global sampling of the visual field. The same result holds when the objects are superimposed by transparency: this argues against location-based sampling. Here we show that the sampling is in fact object-based: we use a rotating ring stimulus split in two halves. When the two halves move in opposite directions, appearing to belong to separate objects, perceptual reversals occur in either half at a time, but rarely in both. When the two halves physically move in compatible directions, they generally appear to reverse simultaneously: the illusion keeps the perceptual object united. Rather than the local low-level properties of the motion stimulus (which are comparable in both cases), it is thus the high-level organization of the scene that determines the extent of perceived motion reversals. These results imply that the continuous Wagon Wheel Illusion, and any discrete perceptual sampling that may cause it, is restricted to the object of our attention.

Adaptation, Physiological↗

PubMed2006

The continuous wagon wheel illusion is associated with changes in electroencephalogram power at approximately 13 Hz.

Continuously moving objects sometimes appear to spontaneously reverse their motion direction. The mechanisms underlying this bistable phenomenon (the "continuous wagon wheel illusion") are heavily debated, but one interpretation suggests that motion information is perceived in discrete episodes at a rate between 10 and 15 Hz. Here, we asked observers to report the perceived direction of a continuously rotating wheel while 32-channel electroencephalogram (EEG) was recorded. We then separated periods of perceived true from illusory (reversed) motion and compared the EEG power spectrum under these two perceptually distinct yet physically identical conditions. The only reliable difference was observed approximately 13 Hz over centroparietal electrodes, independent of the temporal frequency of the wheel. Thus, it is likely to reflect internal processes rather than purely stimulus-driven activity. EEG power (13 Hz) decreased before the onset of illusory motion and increased before transitions back to real motion. Using this relationship, it was possible to predict above chance, on a trial-by-trial basis, the direction of the upcoming perceptual transition. These data are compatible with the idea that motion perception occurs in snapshots <100 ms in duration.

Adult↗

PubMed2005

On second glance: still no high-level pop-out effect for faces.

A recent article in this journal (Hershler, O., & Hochstein, S. (2005). At first sight: A high-level pop out effect for faces. Vision Research, 45(13), 1707-1724) reported, in contradiction to several earlier studies, that photographs of human faces can be searched for efficiently (i.e., "pop out") among photographs of other objects (as long as these objects are not "too similar" to faces). An apparent search asymmetry between faces and other categories (houses, cars) pointed to the existence of a specialized "face map". Findings of impaired performance for scrambled images were presented as evidence that this face pop out is a high-level, "holistic" effect. While the main pop-out effect cannot be disputed, several choices made in that study in terms of experiment design, analysis and interpretation are questionable. After discussing these issues, I report novel experiments which show that (i) the face pop-out effect can be replicated, but under controlled conditions there is no asymmetry between faces and other objects (cars); (ii) inverting pictures and hence disrupting holistic face processing has only a minor effect on search performance; (iii) finally, search becomes inefficient when Fourier amplitude information (which carries global low-level statistical properties of images) is made irrelevant, and only phase information (carrying contour localization) can be used to detect faces. These results imply, contrary to the target article, that the face pop-out effect is mostly based on low-level factors.

Adult↗

PubMed2005

Binding is a local problem for natural objects and scenes.

Current theories hold that attention is necessary for binding the features of a visual object into a coherent representation, implying that interference should be observed when two objects must be recognized simultaneously: this is the well-known binding problem. Recent studies have suggested, however, that discriminating isolated natural scenes, objects or faces might be possible in the near absence of attention. It is still unclear what mechanisms underlie this remarkable ability. Here, we investigate whether the binding problem affects natural objects in the same way as other stimuli: is interference observed when two natural objects or scenes must be simultaneously processed? We show that in the presence of competing objects, performance in the near absence of attention depends on the relative distance between stimuli: discrimination is good for stimuli far enough apart, and poor for close enough stimuli. In contrast, seemingly simpler but unfamiliar synthetic objects could not be bound in the near absence of attention, independent of the distance between them. Thus, natural objects are special in that they suffer from the binding problem, but only locally. We surmise that this particular type of local binding for natural objects and scenes could be "hardwired" by dedicated neuronal populations.

Adult↗

PubMed2005

Attention-driven discrete sampling of motion perception.

In movies or on TV, a wheel can seem to rotate backwards, due to the temporal subsampling inherent in the recording process (the wagon wheel illusion). Surprisingly, this effect has also been reported under continuous light, suggesting that our visual system, too, might sample motion in discrete "snapshots." Recently, these results and their interpretation have been challenged. Here, we investigate the continuous wagon wheel illusion as a form of bistable percept. We observe a strong temporal frequency dependence: the illusion is maximal at alternation rates around 10 Hz but shows no spatial frequency dependence. We introduce an objective method, based on unbalanced counterphase gratings, for measuring this phenomenon and demonstrate that the effect critically depends on attention: the continuous wagon wheel illusion was almost abolished in the absence of focused attention. A motion-energy model, coupled with attention-dependent temporal subsampling of the perceptual stream at rates between 10 and 20 Hz, can quantitatively account for the observed data.

Attention↗

PubMed2005

Spike times make sense.

Many behavioral responses are completed too quickly for the underlying sensory processes to rely on estimation of neural firing rates over extended time windows. Theoretically, first-spike times could underlie such rapid responses, but direct evidence has been lacking. Such evidence has now been uncovered in the human somatosensory system. We discuss these findings and their potential generalization to other sensory modalities, and we consider some future challenges for the neuroscientific community.

Action Potentials↗

PubMed2005

Neurons tune to the earliest spikes through STDP.

Spike timing-dependent plasticity (STDP) is a learning rule that modifies the strength of a neuron's synapses as a function of the precise temporal relations between input and output spikes. In many brains areas, temporal aspects of spike trains have been found to be highly reproducible. How will STDP affect a neuron's behavior when it is repeatedly presented with the same input spike pattern? We show in this theoretical study that repeated inputs systematically lead to a shaping of the neuron's selectivity, emphasizing its very first input spikes, while steadily decreasing the postsynaptic response latency. This was obtained under various conditions of background noise, and even under conditions where spiking latencies and firing rates, or synchrony, provided conflicting informations. The key role of first spikes demonstrated here provides further support for models using a single wave of spikes to implement rapid neural processing.

Action Potentials↗

PubMed2004

A simple translation in cortical log-coordinates may account for the pattern of saccadic localization errors.

During saccadic eye movements, the visual world shifts rapidly across the retina. Perceptual continuity is thought to be maintained by active neural mechanisms that compensate for this displacement, bringing the presaccadic scene into a postsaccadic reference frame. Because of this active mechanism, objects appearing briefly around the time of the saccade are perceived at erroneous locations, a phenomenon called perisaccadic mislocalization. The position and direction of localization errors can inform us about the different reference frames involved. It has been found, for example, that errors are not simply made in the direction of the saccade but directed toward the saccade target, indicating that the compensatory mechanism involves spatial compression rather than translation. A recent study confirmed that localization errors also occur in the direction orthogonal to saccade direction, but only for eccentricities far from the fovea, beyond the saccade target. This spatially specific pattern of distortion cannot be explained by a simple compression of space around the saccade target. Here I show that a change of reference frames (i.e., translation) in cortical (logarithmic) coordinates, taking into account the cortical magnification factor, can accurately predict these spatial patterns of mislocalization. The flashed object projects onto the cortex in presaccadic (fovea-centered) coordinates but is perceived in postsaccadic (target-centered) coordinates.

Animals↗

PubMed2003

Visual selective behavior can be triggered by a feed-forward process.

The ventral visual pathway implements object recognition and categorization in a hierarchy of processing areas with neuronal selectivities of increasing complexity. The presence of massive feedback connections within this hierarchy raises the possibility that normal visual processing relies on the use of computational loops. It is not known, however, whether object recognition can be performed at all without such loops (i.e., in a purely feed-forward mode). By analyzing the time course of reaction times in a masked natural scene categorization paradigm, we show that the human visual system can generate selective motor responses based on a single feed-forward pass. We confirm these results using a more constrained letter discrimination task, in which the rapid succession of a target and mask is actually perceived as a distractor. We show that a masked stimulus presented for only 26 msec - and often not consciously perceived - can fully determine the earliest selective motor responses: The neural representations of the stimulus and mask are thus kept separated during a short period corresponding to the feed-forward "sweep." Therefore, feedback loops do not appear to be "mandatory" for visual processing. Rather, we found that such loops allow the masked stimulus to reverberate in the visual system and affect behavior for nearly 150 msec after the feed-forward sweep.

Attention↗

PubMed2003

Attention and scintillation.

The scintillating grid is a recently discovered visual illusion, created by superimposing white discs on the intersections of orthogonal gray bars on a black background. Even though the entire display is stationary, observers perceive dark spots appearing and disappearing rapidly within some of the white discs. This scintillation effect is correlated with eye position and eye movements. Here we investigate whether covert shifts of attention, as revealed by cueing and visual search paradigms, can also affect the illusion. We find that the chance of a particular intersection scintillating is directly correlated with distance from the attentional focus, regardless of the location of the fixation point. This suggests that the dynamics of this illusion might reflect the distribution of attention in space and time.

Analysis of Variance↗

PubMed2003

Is perception discrete or continuous?

How does conscious perception evolve following stimulus presentation? The idea that perception relies on discrete processing epochs has been often considered, but never widely accepted. The alternative, a continuous translation of the external world into explicit perception, although more intuitive and subjectively appealing, cannot satisfactorily account for a large body of psychophysical data. Cortical and thalamocortical oscillations in different frequency bands could provide a neuronal basis for such discrete processes, but are rarely analyzed in this context. This article reconciles the unduly abandoned topic of discrete perception with current views and advances in neuroscience.

Journal Article↗

PubMed2003

Competition and selection during visual processing of natural scenes and objects.

When a visual scene, containing many discrete objects, is presented to our retinae, only a subset of these objects will be explicitly represented in visual awareness. The number of objects accessing short-term visual memory might be even smaller. Finally, it is not known to what extent "ignored" objects (those that do not enter visual awareness) will be processed--or recognized. By combining free recall, forced-choice recognition and visual priming paradigms for the same natural visual scenes and subjects, we were able to estimate these numbers, and provide insights as to the fate of objects that are not explicitly recognized in a single fixation. When presented for 250 ms with a scene containing 10 distinct objects, human observers can remember up to 4 objects with full confidence, and between 2 and 3 more when forced to guess. Importantly, the objects that the subjects consistently failed to report elicited a significant negative priming effect when presented in a subsequent task, suggesting that their identity was represented in high-level cortical areas of the visual system, before the corresponding neural activity was suppressed during attentional selection. These results shed light on neural mechanisms of attentional competition, and representational capacity at different levels of the human visual system.

Attention↗

PubMed2002

Rapid natural scene categorization in the near absence of attention.

What can we see when we do not pay attention? It is well known that we can be "blind" even to major aspects of natural scenes when we attend elsewhere. The only tasks that do not need attention appear to be carried out in the early stages of the visual system. Contrary to this common belief, we report that subjects can rapidly detect animals or vehicles in briefly presented novel natural scenes while simultaneously performing another attentionally demanding task. By comparison, they are unable to discriminate large T's from L's, or bisected two-color disks from their mirror images under the same conditions. We conclude that some visual tasks associated with "high-level" cortical areas may proceed in the near absence of attention.

Adult↗

PubMed2002

Surfing a spike wave down the ventral stream.

Numerous theories of neural processing, often motivated by experimental observations, have explored the computational properties of neural codes based on the absolute or relative timing of spikes in spike trains. Spiking neuron models and theories however, as well as their experimental counterparts, have generally been limited to the simulation or observation of isolated neurons, isolated spike trains, or reduced neural populations. Such theories would therefore seem inappropriate to capture the properties of a neural code relying on temporal spike patterns distributed across large neuronal populations. Here we report a range of computer simulations and theoretical considerations that were designed to explore the possibilities of one such code and its relevance for visual processing. In a unified framework where the relation between stimulus saliency and spike relative timing plays the central role, we describe how the ventral stream of the visual system could process natural input scenes and extract meaningful information, both rapidly and reliably. The first wave of spikes generated in the retina in response to a visual stimulation carries information explicitly in its spatio-temporal structure: the most salient information is represented by the first spikes over the population. This spike wave, propagating through a hierarchy of visual areas, is regenerated at each processing stage, where its temporal structure can be modified by (i). the selectivity of the cortical neurons, (ii). lateral interactions and (iii). top-down attentional influences from higher order cortical areas. The resulting model could account for the remarkable efficiency and rapidity of processing observed in the primate visual system.

Action Potentials↗

PubMedDate not supplied

Visual saliency and spike timing in the ventral visual pathway.

Visual saliency is a fundamental yet hard to define property of objects or locations in the visual world. In a context where objects and their representations compete to dominate our perception, saliency can be thought of as the "juice" that makes objects win the race. It is often assumed that saliency is extracted and represented in an explicit saliency map, which serves to determine the location of spatial attention at any given time. It is then by drawing attention to a salient object that it can be recognized or categorized. I argue against this classical view that visual "bottom-up" saliency automatically recruits the attentional system prior to object recognition. A number of visual processing tasks are clearly performed too fast for such a costly strategy to be employed. Rather, visual attention could simply act by biasing a saliency-based object recognition system. Under natural conditions of stimulation, saliency can be represented implicitly throughout the ventral visual pathway, independent of any explicit saliency map. At any given level, the most activated cells of the neural population simply represent the most salient locations. The notion of saliency itself grows increasingly complex throughout the system, mostly based on luminance contrast until information reaches visual cortex, gradually incorporating information about features such as orientation or color in primary visual cortex and early extrastriate areas, and finally the identity and behavioral relevance of objects in temporal cortex and beyond. Under these conditions the object that dominates perception, i.e. the object yielding the strongest (or the first) selective neural response, is by definition the one whose features are most "salient"--without the need for any external saliency map. In addition, I suggest that such an implicit representation of saliency can be best encoded in the relative times of the first spikes fired in a given neuronal population. In accordance with our subjective experience that saliency and attention do not modify the appearance of objects, the feed-forward propagation of this first spike wave could serve to trigger saliency-based object recognition outside the realm of awareness, while conscious perceptions could be mediated by the remaining discharges of longer neuronal spike trains.

Action Potentials↗

PubMedDate not supplied

Visual search and dual tasks reveal two distinct attentional resources.

Most theories of visual processing assume that a target will "pop out" from an array of distractors ("parallel" visual search, e.g., color or orientation discrimination) if targets and distractors can be discriminated without attention. When the discrimination requires attention (e.g., rotated L vs. T or red-green vs. green-red bisected disks), "serial" examination is needed in visual search. Attentional requirements are also frequently assessed by measuring interference from a concurrently performed attentionally demanding task. It is commonly believed that attention acts equivalently in dual-task and visual search paradigms, based on the implicit assumption that visual attentional requirements can be defined along a single dimension. Here we show that there is no such equivalence: We report on targets that do not trigger pop-out, even though they can be discriminated from distractors with attention occupied elsewhere (natural scenes, color-orientation conjunctions); conversely, we show that certain targets that pop out among distractors need undivided attention to be effectively discriminated from distractors when presented in isolation (rotated L vs. +, depth-rotated cubes). In other words, visual search and dual-task performance reveal attentional resources along two independent dimensions. We suggest an interpretation of these results in terms of neuronal selectivities and receptive field size effects.

Attention↗