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Using Psychophysical And Neuroimaging Tools To Identify The Mechanism Underlying Perceptual Dominancy In Vision

Periodic Reporting for period 1 - Perceptual Dominancy (Using Psychophysical And Neuroimaging Tools To Identify The Mechanism Underlying Perceptual Dominancy In Vision)

Reporting period: 2016-07-01 to 2018-06-30

When we open our eyes, a massive amount of unstructured information falls on the retina. The construction of the bits and pieces of this information into a coherent percept of the environment is aided by perceptual organization: Grouping principles are utilized in order to determine which visual elements are ‘linked together’ and are segregated from other elements. The traditional theories of perceptual organization assumed that grouping is a single, low-level, pre-attentive process that provides preliminary units for more elaborate attentional processing. Today, perceptual organization is thought of as multiplicity of multi-stage, iterative processes, with different attentional demands, time courses, developmental progression, and neural correlates. Still, there is a lack of knowledge about the mechanism by which the competition between alternative organizations is resolved. The current study was designed to fill this gap using behavioral measures (i.e. accuracy and reaction times) as well as neuroimaging tools (i.e. EEG).

In simple terms, the current study concerned the following questions: Why do we perceive one visual interpretation and not another one, although several alternatives are possible? What happens to the interpretations that we do not consciously experience?
By exploring the relationships of conflicting visual stimuli we reveal the processes used in the brain in order to create this unified experience, that ultimately allows us to manage in our environment without stumbling upon and bumping into the objects around us.
Advancing our knowledge of how the visual system works allows for advances in technology, such as the development of apparatus and software that can be used in medical applications, for example, providing sergeants better tools to detect tumors in a cluttered x-ray or MRI scans, or in rehabilitation of brain injuries and loss of vision.

The overall objectives were:
1. Identifying the factors that contribute to the dynamics of the time course of the competition and the relations between them
2. Evaluating the impact of iterations (i.e. extent of visual encoding) within the time course of the competition
3. Identifying the neural correlates of the competition between organizations
The work in this project was pursued in two tracks: observing behavior and measuring brain activation. The behavioral part was conducted through the first and second year of the project, in which participants were performing a computerized task and their reaction time and accuracy to the task were measured.
Paradigm – we used primed-matching paradigm, in which the participant was asked to respond to a pair of test figures following a prime stimulus. The participants’ task was to indicate whether the two test figures are identical, or were they different from one another. The figures in the test-pair could be similar or dissimilar to the prime in the organization they depicted. Thus, although the task did not involve the prime, if a representation of the organization in the prime was constructed, then it should have facilitated responses to a test-pair that was similar in organization to the prime, compared with when it was dissimilar. To measure dominancy of representations, dissimilar test-pairs of one organization in the prime stimulus were also similar to the other organization. For example, if the prime presented columns by one grouping principle (e.g. proximity) and rows by another (e.g. brightness similarity), then a test-pair of columns was dissimilar to the latter organization but also similar to the former. Priming effects were calculated as the difference in facilitation between ‘same’-responses to similar and dissimilar test pairs according to one of the organizations in the competition. Thus, positive priming effects indicate that one organization was better represented than the other, negative priming effects indicate that the latter organization was better represented than the first, and absence of priming effects indicates that the two were represented to the same degree (i.e. competition between representations).
We tested the effect of several factors on the resolution of a conflict in the visual stimulus. In particular, we were interested in the progression of the processes, i.e. the time course of the competition between alternative perceptions. Hence, in the first set of experiments our main manipulation was of prime duration. In order to examine the dynamics between alternatives in the time-course of the competition we manipulated the strength of each alternative organization in the stimulus. We were also interested in examining the role of re-sampling of the visual input in the resolution of a visual conflict, thus, in another set of experiments the prime was flashed briefly and the duration between prime offset and the test onset varied.
The results from this part indicate that several mechanisms are involved in this competition, and that it is resolved in a dynamic, rather than a rigid manner. Initial results of this part of the project were presented in an international conference in 2017.
The second part of the project has been conducted starting the second year. In this part, participants performed a computerized task, similar to the one used in the behavioral part, only they were connected to an electroencephalogram (EEG), which records brain waves from electrodes placed on the scalp. The Global Field Power (GFP), representing the standard deviation across all electrodes, was extracted for each time point. The GFP measure has the advantage to be reference independent. Using the GFP we defined Event-Related (ERP) components indicating stimulus related activity. These components were then tested for differences in amplitudes in correlation with the experimental manipulation. Preliminary results of these analyses suggest one component to be specifically involved in the competition process. In addition, inverse solutions for source localization was computed using the Local Auto-Regressive Average (LAURA) algorithm. The method is known as Distributed Electrical Source Imaging and estimates the underlying current densities. The data is also being analysed with frequency analysis methods to reveal whether there is a change in frequency power in the time-course of the competition.
This project made substantial contributions to state-of-the-art research in the field: first, the project significantly expands our understanding of the cognitive mechanism responsible for the selection between alternative organizations, by identifying the factors affecting perceptual dominancy, and the role of iterations in the time course of the competition. Second, it inquired into the neural mechanism associated with perceptual dominancy, while providing a systematic examination of the time course of the competition and its associated neural correlates. In addition, it disproved the traditional and somewhat ambiguous treatment of similarity-based grouping operations and provided an account for the mechanism(s) underlying this type of grouping. Thus, the project provided a novel interdisciplinary approach of cognitive and neural aspects in the research on perceptual dominancy.