For science

Phenomenology of Transitions in Binocular Rivalry

Ph.D. thesis under the supervision of Natalia Zaretskaya, Ph.D., Visual Neuroscience Lab, University of Graz, AT

Binocular rivalry occurs when two eyes are presented with two different images that cannot be fused into each other as a uniform perception. As a result, the subject has a perception alternating between the dominant image of either of the presentations with a transition phase. The studies on the phenomenology of binocular rivalry showed that several features of stimuli such as contrast, luminance, and content affect the dominance duration and the frequency of transition whereas different perceptual transitions have been reported, including traveling wave, superimposed, piecemeal, and transparency. Some recent studies also showed that the duration of transition during binocular rivalry can be correlated with some clinical conditions such as ADHD and ASD. Despite the evidence for the various features of perceptual transition, there is no systematic investigation of all possible transition types and factors that affect transition appearance. The goal of this study is to define different transition types in binocular rivalry and the factors affecting them.

Inverted Encoding Model of Serial Dependence

Project intern under the supervision of David Pascucci, Ph.D. and Micheal Herzog, Ph.D., Brain Mind Institute, Ecole polytechnique federale de Lausanne (EPFL), CH

Previous studies showed that the prior experience has a positive effect on the current perception such that the prior experience can lead to a bias towards perceiving the current stimulus more similar to the previous stimulus than actually it is. This effect is known as serial dependence. To investigate the effect of serial dependence, 21 participants performed an orientation adjustment task. Behaviorally, an effect of serial dependence was shown in this experiment. I preprocess and analyze the EEG data that was previously collected. We model the orientation channels and serial dependence by using the paradigm of an inverted encoding model. Our next step will be to model the fMRI data for a similar model of serial dependence as we applied to EEG data.

Effect of Visual Stimuli with Fearful Emotional Cue on Population Receptive Field Estimates

M.Sc. thesis under supervision of Hüseyin Boyacı, Assoc. Prof. Dr., Computational and Biological Vision Lab, Bilkent University, TR

It is known that the emotional state can affect visual perception, especially in temporo-occipital cortex. The common approach of emotional studies on the visual system is the subtraction method for brain activity. I investigated the effect of emotional modulation on the low-level features of the visual system by using the population receptive field (pRF) estimation method since it has been shown that its estimations were sensitive to the stimulus content. The advantage of the pRF method was to study on dynamic functionality of the visual system with a model-based approach. I hypothesized that stimulus with fearful content might affect the pRF sizes and locations in the visual cortex from the lowest level, V1, to the higher levels. Six participants performed voluntarily our 3-session fMRI experiment. I presented the simultaneous wedge and ring stimuli rendered with scrambled, neutral, and fearful images in three sessions separately and respectively. Then, I preprocessed data by using FreeSurfer and SPM, followed by the analysis with the SamSrf toolbox on MATLAB. Results showed the fearful content of stimulus might lead to increased pRF sizes as well as a shift in pRF centers toward the eccentric side in visual areas hV4 and V3A, as well as lower retinotopic regions: V1, V2, and V3. Hence, our findings showed that the fearful content of visual stimuli might have a modulatory effect on the visual system. This modulatory effect of emotion can facilitate visual perception by increasing position tolerance due to the increased pRF sizes.

Determination of Motor Cortex Location by using fNIRS

B.Sc. thesis under the supervision of Aykut Eken, Ph.D. and Didem gökçay, ph.D., NeuroLab, Middle East Technical University (METU), TR

The motor cortex is one of the well-studied brain regions. I aimed to determine the motor cortex by using functional near-infrared spectroscopy (fNIRS). My thesis was a part of a Ph.D. project on the pain perception of fibromyalgia patients. We collected brain imaging data from 15 healthy individuals and 15 fibromyalgia patients while the participants were performing finger-tapping tasks (FTT). They were also applied median nerve stimulation (MNS) in a separate experimental condition for the Ph.D. project. I used basic MS Excel and statistics to analyse the data of the finger-tapping task to determine the location of the motor cortex. I defined the motor cortex successfully and I also found some activations on the ipsilateral hemisphere, which indicated the inter-hemispheric interactions.

Further analysis of the data showed the difference in brain activities of healthy individuals and fibromyalgia patients. It is known that fibromyalgia syndrome includes motor control dysfunction besides allodynia. We found that fibromyalgia patients had lower hemodynamic activity on the bilateral superior parietal gyrus, left supramarginal gyrus, and right somatosensory cortex during the finger tapping (FTT) task whereas they had higher activity on the bilateral superior parietal gyrus, right supramarginal gyrus, and right postcentral gyrus in response to the median nerve stimulation (MNS). Moreover, the correlation between FTT and MNS results indicated that the left superior parietal gyrus might play an important role in the association of fine motor loss and allodynia in fibromyalgia.