Biomedical subjects

Rufin VanRullen

Publications and source records attributed to Rufin VanRullen.

6 recordsLinked to original sources

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↗