Browsing by Subject "Functional MRI"

Now showing 1 - 2 of 2

Open Access
BolT: Fused window transformers for fMRI time series analysis
(Elsevier B.V., 2023-05-18) Bedel, Hasan Atakan; Şıvgın, Irmak; Dalmaz, Onat; Ul Hassan Dar, Salman ; Çukur, Tolga
Deep-learning models have enabled performance leaps in analysis of high-dimensional functional MRI (fMRI) data. Yet, many previous methods are suboptimally sensitive for contextual representations across diverse time scales. Here, we present BolT, a blood-oxygen-level-dependent transformer model, for analyzing multi-variate fMRI time series. BolT leverages a cascade of transformer encoders equipped with a novel fused window attention mechanism. Encoding is performed on temporally-overlapped windows within the time series to capture local representations. To integrate information temporally, cross-window attention is computed between base tokens in each window and fringe tokens from neighboring windows. To gradually transition from local to global representations, the extent of window overlap and thereby number of fringe tokens are progressively increased across the cascade. Finally, a novel cross-window regularization is employed to align high-level classification features across the time series. Comprehensive experiments on large-scale public datasets demonstrate the superior performance of BolT against state-of-the-art methods. Furthermore, explanatory analyses to identify landmark time points and regions that contribute most significantly to model decisions corroborate prominent neuroscientific findings in the literature.
Open Access
Category-selective top-down modulation in the fusiform face area of the human brain during visual search
(IEEE, 2017) Dar, Salman Ul Hassan; Çukur, Tolga
Several regions in the ventral-temporal cortex of the human brain are thought to have representations of specific categories of objects. Furthermore, a distributed network of frontal and parietal brain regions is implicated in attentional control. It is assumed that during visual search, attention-control regions send top-down signals to the target category-selective areas to bias the processing in favour of the attended object category. However, little is known about such causal interactions during naturalistic visual search. Here we assess the influence of attention-control brain regions on a well-known face selective area fusiform face area (FFA) during natural visual search using Granger causality analysis. Our results indicate that attending to humans enhances the influence of attention-control regions on the fusiform face area.