Abstracts
Below you can find abstracts of the Webinar talks.
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1. Serial dependence in visual working memory: cognitive and neuronal mechanisms
Cora Fischer
Visual perception operates in an object-based manner, by integrating associated features via attention. Working memory allows a flexible access to a limited number of currently relevant objects, even when they are physically no longer present. Recently, it has been shown that we compensate for small changes in an object’s appearance over memory episodes to ensure its perceived continuity. This phenomenon was termed 'serial dependence’ and has mostly been studied in situations that comprised only a single relevant object. I will show that in situations with multiple objects, context features like color, temporal or spatial position are used as anchors to selectively integrate corresponding objects over memory episodes. Moreover, I will present preliminary results based on the high temporal resolution of the MEG signal suggesting that serial dependence operates during access to object information in working memory rather than during the object’s perception or maintenance.
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2. Neural mechanisms of serial dependence in the prefrontal cortex
Albert Compte
Working memory plays a central role in cognition, and yet its fundamental neural mechanisms remain a matter of debate. Persistent activity has been long considered, but recently subthreshold mechanisms have been proposed as alternative. Here, we show how the interaction of both mechanisms determines serial biases of monkeys and humans in spatial working memory. Stimulus information disappears from electrophysiological signals between trials, but remains present in the synchrony of prefrontal neurons as predicted by a computational model integrating attractor dynamics and short-term plasticity. Just prior to the new stimulus, this subthreshold trace in some trials is reignited into activity that recapitulates the previous stimulus representation. We show that the reactivation strength correlates with the strength of serial biases in both monkeys and humans. Single-pulse TMS applied to the prefrontal cortex prior to trial start enhanced serial biases, demonstrating the causal role of prefrontal reactivations in determining serial dependence in working memory.
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3. Perceptual history propagates down to early levels of sensory analysis
Guido Marco Cicchini
One function of perceptual systems is to construct and maintain a reliable representation of the environment. This strategy is to cope with noisy information, but can lead to stimuli being biased towards perceptual history, a phenomenon known as serial dependence. It is still unclear whether serial dependence biases sensory encoding, or only perceptual decisions. We leveraged on the “surround tilt illusion” – where tilted flanking stimuli strongly bias perceived orientation – to measure its influence on the pattern of serial dependence. When a neutral stimulus preceded by an illusory one maximal serial dependence occurred when the perceived orientations matched, suggesting that the sensory history incorporates contextual biases. However, when an illusory stimulus was preceded by a neutral stimulus, maximal serial dependence occurred when their physical orientations matched, suggesting that sensory history interacts with incoming sensory signals before they are biased by flanking stimuli. The evidence suggests that priors are high-level constructs incorporating contextual information, which interact directly with early sensory signals, not with highly processed perceptual representations. It also suggests that serial dependences arise at a network rather than local level.
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4. The role of context in the deployment of visual attention: The Priority Accumulation Framework
Dominique Lamy
Where we shift our attention next is thought to be the location with the strongest combined representation of salience and relevance on the “priority map” at any given moment. But are we at the mercy of the constant changes occurring in our environment, and automatically move our attention to the ever-changing location with the highest priority? Or do we wait for clues that the appropriate moment has arrived to deploy our attention? Here, we address this hitherto neglected issue using a spatial-cueing paradigm. We examined whether attention is deployed as soon as a salient change occurs (the cue), or only when the context signaling that attention should be deployed appears (the search display). The findings from three experiments relying on manual responses and eye movements support the latter possibility. These findings challenge traditional views on how attentional deployment is triggered. They are discussed within the Priority Accumulation Framework (PAF).
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5. Temporal dynamics of Feature Distribution Learning (FDL): A new method for studying visual ensembles
Omer Daglar Tanrikulu
Priming of attention shifts have recently been used as an effective method to uncover internal representations of visual ensembles. This method, Feature Distribution Learning (FDL), takes advantage of attentional priming when role reversals between target and distractors occur in visual search. Results obtained with the FDL method have shown that the visual system can encode feature probability distributions in detail and shows sensitivity to temporal perturbations in the statistical properties of visual scenes. In this talk, I will introduce the FDL method and review studies utilizing it. I will then present our latest study investigating the temporal dynamics of the FDL method, in which we investigated how observers temporally integrate two different orientation distributions from sequentially presented visual search trials. The robust recency effects obtained in this study indicated that the visual system prefers to utilize sensory history when the statistical properties of the environment are relatively stable.
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6. Expectation in Attention
Heleen A. Slagter
Perception is more than meet’s the eye; how we experience the world is critically shaped by attention (what is relevant) and as a rapidly growing body of work indicates, by predictions grounded in past experience and regularities in the outside world (what is likely). Overturning the classical notion of perception as a largely bottom-up process, the idea that our brain is a prediction machine, continually trying to predict what is ‘out there’ based on the probabilistic structure of the world, is quickly growing in scientific stature and influence. Yet, little is still known about how predictions, independently or in interaction with attention, may shape perception and performance. In my talk, I will present findings from several behavioral and EEG studies that examined how predictions are neurally implemented and guide behavior, and how this may depend on the goal-relevance (target, distractor), modality (visual, auditory), and/or dimension (spatial, nonspatial, temporal) of the predicted information. Collectively, these findings reveal how the brain rapidly picks up on the probabilistic structure of the world, guiding what we attend to and what we ignore.
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7. Manipulation of stimulus contrast reveals its complex role in perceptual aftereffects
Nikos Gekas
Visual perception is a perpetually adapting process where the current percept is affected by what has been previously observed. It has been shown that stimuli seen at different times in the past can have opposite effects on the perceived orientation of the current stimulus. Recent history has a repulsive effect on perceived orientation, while more distant history appears to have an attractive effect. Here, we test how the short-term repulsion and long-term attraction are modulated by the saliency of the stimuli presented in the past history. The contrast of both past and current stimuli were simultaneously manipulated in such a way as to decrease the repulsive effect and increase the attractive effect, or vice versa. We found that the manipulation of contrast had a strong effect on the perceptual aftereffects in our orientation discrimination task, which suggests a complex relation between contrast, orientation, and adaptation.
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8. Attractive and repulsive history effects in visual perception
Jan W. Brascamp
In what way is perception of current stimuli altered by what was seen a short while ago? A first distinction is between attractive and repulsive effects. Attractive effects are when the present stimulus appears more similar to a recent stimulus, or when a present stimulus that physically resembles a recent stimulus is more easily detectable. Repulsive effects are the opposite: the present stimulus looks more dissimilar to a previous stimulus than it actually is, or is harder to detect if it physically resembles the previous stimulus. Although both types of history effects are well known, it is surprisingly unclear what factors determine which of these two diametrically opposite effects arises in any given situation. I will discuss my work on bistable perception, where both kinds of effects are observed: in some cases observers tend to perceive an ambiguous stimulus in a way that matches prior experience; in other cases they tend to perceive the opposite interpretation. I will also review a set of results from outside that field. Based on data patterns observed across fields I will tentatively suggest as one important factor in this context a balance between processing at early stages versus advanced stages of the visual hierarchy. In this suggested scheme prior stimuli that are associated with a stronger bottom-up drive and a pronounced representation at early processing stages tend to leave a repulsive trace, whereas relatively weak stimuli whose processing becomes more pronounced at later hierarchical stages (for instance due to perceptual dominance in bistable perception) tend to lead to attraction. Aside from offering this scheme, I will also speculate about the functional significance of these two opposite types of history effects.
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9. On the nature of serial dependence in perceptual decisions
David Pascucci
Human perception and perceptual decisions are highly influenced by their temporal context: There is almost no elementary percept that can be isolated from its preceding history. In psychophysical tasks, this leads to sequential effects in perceptual decisions, in which the judgment of a given stimulus feature is affected by the history of preceding trials. Such effects can be attractive (assimilative) when features of stimuli are erroneously reported as similar to previous trials, or repulsive (contrastive) when the difference between the present and the past stimulus is exaggerated. These opposite biases can be observed under a variety of conditions, complicating a unified understanding. In this talk, I will review some of my recent work on the assimilative effect known as serial dependence (SD), in an attempt to elucidate its nature. I will present evidence that, in perceptual tasks, SD occurs beyond basic visual features and perceptual objects and I will describe cases in which contrasting effects dominate. I will propose that SD resides at an abstract and task-dependent level of representation, where the selectivity to features and objects is lost. The perspective I will outline is not a conclusive framework, but a starting point for debate and discussion towards a cohesive view.
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Gizay Ceylan
