Tatiana Aloi Emmanouil

Amodal completion of unconsciously presented objects

In the visual environment, objects often appear behind occluding surfaces, yet they are automatically and effortlessly perceived as complete. Here, we examined whether visually occluded objects that are presented below the threshold of awareness are amodally completed. We used a priming paradigm in which participants responded to consciously perceived targets that were preceded by unconsciously presented primes. In two experiments, we show that discrimination responses to targets were faster when they were preceded by congruent shapes, regardless of whether these shapes were intact and complete or occluded by a horizontal bar. This priming effect was not produced by a partial match in features, since the occluded primes did not facilitate responses to targets that shared local features (Experiment 1) or contained only the object features that remained visible after occlusion (Experiment 2). These results show that objects presented below the threshold of awareness can be amodally completed and provide compelling evidence that unconscious processing occurs to a greater extent than previously considered.

Saliency affects feedforward more than feedback processing in early visual cortex.

Early visual cortex activity is influenced by both bottom-up and top-down factors. To investigate the influences of bottom-up (saliency) and top-down (task) factors on different stages of visual processing, we used transcranial magnetic stimulation (TMS) of areas V1/V2 to induce visual suppression at varying temporal intervals. Subjects were asked to detect and discriminate the color or the orientation of briefly-presented small lines that varied on color saliency based on color contrast with the surround. Regardless of task, color saliency modulated the magnitude of TMS-induced visual suppression, especially at earlier temporal processing intervals that reflect the feedforward stage of visual processing in V1/V2. In a second experiment we found that our color saliency effects were also influenced by an inherent advantage of the color red relative to other hues and that color discrimination difficulty did not affect visual suppression. These results support the notion that early visual processing is stimulus driven and that feedforward and feedback processing encode different types of information about visual scenes. They further suggest that certain hues can be prioritized over others within our visual systems by being more robustly represented during early temporal processing intervals.

Neural evidence for sequential selection of object features

Behavioral and neural evidence suggests that attention selects entire objects, amplifying all of their features regardless of task relevance. A new magnetoencephalography (MEG) study by Schoenfeld et al. elucidates the time course of this selection, showing that object features are activated sequentially, with attention spreading from task-relevant to task-irrelevant modules.

Working memory for self-initiated and provided spatial configurations

During a typical day, people temporarily memorise information provided to them. However, they memorise as often information they actively choose themselves. Although prevalent in everyday behaviour, this aspect of working memory (WM), we term self-initiated WM, has been largely unexplored. In this study, we used a modified spatial span task in which participants constructed the spatial sequences they maintained in memory. The results of three experiments demonstrated that participants planned and constructed structured spatial sequences by minimising the distances between successive locations and by selecting sequences with fewer path crossings. The sequences were initiated most often on the top left side. Memory accuracy was enhanced when participants memorised self-initiated spatial sequences, even when the self-initiated and provided sequences were matched for structure. When asked to construct spatial sequences for a hypothetical competitor in a memory contest, participants constructed complex sequences with longer paths and more path crossings, suggesting that these sequence parameters were under their control. The tendency to initiate the spatial sequences on the top left side remained. Overall, the results suggest that self-initiated WM can benefit from explicit metacognitive knowledge of the ideal structure of memory representations and also demonstrate that self-initiation benefits memory beyond structure.

Perceptual overloading reveals illusory contour perception without awareness of the inducers

Unconscious perception is frequently examined by restricting visual input (e.g., using short stimulus durations followed by masking) to prevent that information from entering visual awareness. Failures to demonstrate perception without awareness may thus be a consequence of this restricted input rather than of limitations in unconscious perception. Here, we demonstrate a novel method that circumvents these significant drawbacks inherent in other methods. Using this new perceptual overloading technique (POT), in which stimuli are repeatedly presented in alternation with a stream of variable masks, we demonstrate illusory contour perception and modal completion even when subjects are completely unaware of the inducing elements. In addition to demonstrating a powerful new method to study consciousness by effectively gating robust visual input from visual awareness, we show that more complex contextual effects, previously considered to be a privilege only of conscious vision, can occur without awareness.

Unconscious processing of unattended features in human visual cortex

Unconscious processing has been convincingly demonstrated for task-relevant feature dimensions. However, it is possible that the visual system is capable of more complex unconscious operations, extracting visual features even when they are unattended and task irrelevant. In the current study, we addressed this question by measuring unconscious priming using a task in which human participants attended to a target objects shape while ignoring its color. We measured both behavioral priming effects and priming-related fMRI activations from primes that were unconsciously presented using metacontrast masking. The results showed faster RTs and decreases in fMRI activation only when the primes were identical to the targets, indicating that primes were processed both in the attended shape and the unattended color dimensions. Reductions in activation were observed in early visual areas, including primary visual cortex, as well as in feature-responsive areas for shape and color. These results indicate that multiple features can be unconsciously encoded and possibly bound using the same visual networks activated by consciously perceived images.

Ensemble coding remains accurate under object and spatial visual working memory load

A number of studies have provided evidence that the visual system statistically summarizes large amounts of information that would exceed the limitations of attention and working memory (ensemble coding). However the necessity of working memory resources for ensemble coding has not yet been tested directly. In the current study, we used a dual task design to test the effect of object and spatial visual working memory load on size averaging accuracy. In Experiment 1, we tested participants’ accuracy in comparing the mean size of two sets under various levels of object visual working memory load. Although the accuracy of average size judgments depended on the difference in mean size between the two sets, we found no effect of working memory load. In Experiment 2, we tested the same average size judgment while participants were under spatial visual working memory load, again finding no effect of load on averaging accuracy. Overall our results reveal that ensemble coding can proceed unimpeded and highly accurately under both object and spatial visual working memory load, providing further evidence that ensemble coding reflects a basic perceptual process distinct from that of individual object processing.

Estimation in self-initiated working memory for spatial locations

The short-term maintenance of precise location information is fundamental in many daily activities. Often, individuals memorize spatial information provided to them, but in many other occasions memory is self-initiated, meaning that individuals memorize locations they selected themselves. While prevalent in everyday behavior, research on self-initiated working memory (WM) is scarce. The current study employed a modified spatial precision memory task in which participants temporarily memorized single locations they themselves selected within a square area that appeared during encoding. We explored the distribution of locations participants selected, guided by the assumption that participants would select locations that they perceived would maximize accuracy. The results of two experiments demonstrated that, to enhance memory performance, participants most often selected prototypical locations within the square area, namely, at its center and at its corners, which were closest to the center and the corners of the entire screen. When asked to disrupt memory performance, participants mostly avoided these locations. Furthermore, memory accuracy for self-initiated locations was superior to memory for provided locations, even when the distribution of locations in the two conditions was matched. We interpret the results within the framework of models that emphasize the utility of spatial categories in maximizing memory accuracy in spatial-estimation tasks. The results provide the first illustration of self-initiated representations in spatial precision WM tasks, suggesting that participants have access to metacognitive knowledge about the usefulness of spatial categories in location memory. Moreover, the results demonstrate that spatial precision is enhanced when participants reproduce locations they selected themselves.

Statistical processing of partly occluded sets.

The visual environment contains sets of objects with similar properties, such as the pebbles on the beach or the cars on the highway. Even though the objects in these sets exceed the capacity of attention, people can accurately encode their average properties, such as average size or speed. Studies thus far suggest that statistical summaries are computed effortlessly and automatically, however the exact stage at which statistical processing occurs remains unclear. In this study, we investigated whether averaging occurs before or after objects are perceptually completed. Participants viewed sets of circles that appeared either in front of or partly behind horizontal bars and they were asked to judge their average size. Estimates of the mean were similar for occluded and unoccluded sets, suggesting that objects were amodally completed before they were averaged. The results suggest that statistical processing is informed by complex computations that occur at later stages of visual processing. Meeting abstract presented at VSS 2015.