Biology subjects

Sprague, T.

Publications and source records attributed to Sprague, T..

3 recordsLinked to original sources

bioRxiv2018

Reconciling fMRI and EEG indices of attentional modulations in human visual cortex

Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) are the two most popular non-invasive methods used to study the neural mechanisms underlying human cognition. These approaches are considered complementary: fMRI has higher spatial resolution but sluggish temporal resolution, whereas EEG has millisecond temporal resolution, but only at a broad spatial scale. Beyond the obvious fact that fMRI measures properties of blood and EEG measures changes in electric fields, many foundational studies assume that, aside from differences in spatial and temporal precision, these two methods index the same underlying neural modulations. We tested this assumption by using EEG and fMRI to measure attentional modulations of neural responses to stimuli of different visual contrasts. We found that equivalent experiments performed using fMRI and EEG on the same participants revealed remarkably different patterns of attentional modulations: event-related fMRI responses provided evidence for an additive increase in responses across all contrasts equally, whereas early stimulus-evoked event-related potentials (ERPs) showed larger modulations with increasing stimulus contrast and only a later negative-going ERP and low-frequency oscillatory EEG signals showed effects similar to fMRI. These results demonstrate that there is not a one-to-one correspondence between the physiological mechanisms that give rise to modulations of fMRI responses and the most commonly used ERP markers, and that the typical approach of employing fMRI and EEG to gain complementary information about localization and temporal dynamics is over-simplified. Instead, fMRI and EEG index different physiological modulations and their joint application affords synergistic insights into the neural mechanisms supporting human cognition.

neuroscience↗

bioRxiv2017

Dissociable signatures of visual salience and behavioral relevance across attentional priority maps in human cortex

Computational models posit that visual attention is guided by activity within spatial maps that index the image-computable salience and the behavioral relevance of objects in the scene. However, the simultaneous influence of these factors on putative neural attentional priority maps in human cortex is not well understood. We tested the hypothesis that visual salience and behavioral relevance independently impact the activation profile across retinotopically-organized cortical regions by quantifying attentional priority maps measured in human brains using functional MRI while participants attended one of two differentially-salient stimuli. We find that the topography of activation in priority maps, as reflected in the modulation of region-level patterns of population activity, independently indexed the physical salience and behavioral relevance of each scene element. Moreover, salience strongly impacted activation patterns in early visual areas, whereas later visual areas were dominated by relevance. This suggests that prioritizing spatial locations relies on distributed neural codes containing graded representations of salience and relevance across the visual hierarchy.\n\nSignificance StatementOften, it is necessary to orient towards bright, unique, or sudden events in the environment - that is, salient stimuli. However, we can focus processing resources on less salient visual information if it is relevant to the task at hand. We tested a theory which supposes that we represent different scene elements according to both their salience and their relevance in a series of priority maps by measuring fMRI activation patterns across the human brain and reconstructing spatial maps of the visual scene under different task conditions. We found that different regions indexed either the salience or the relevance of scene items, but not their interaction, suggesting an evolving representation of salience and relevance across different visual areas.

neuroscience↗

bioRxiv2016

Spatial tuning shifts increase the discriminability and fidelity of population codes in visual cortex

AcknowledgementsMany thanks to the lab and particularly to Rosanne Rademaker and Edward Vul for comments on analyses and on the manuscript. This work was supported by National Science Foundation Graduate Research Fellowships to V.A.V. and T.C.S., a grant from the National Eye Institute (R01-EY025872) and a Scholar Award from the James S. McDonnell Foundation to J.T.S.\n\nABSTRACTSelective visual attention enables organisms to enhance the representation of behaviorally relevant stimuli by altering the encoding properties of single receptive fields (RFs). Yet we know little about how the attentional modulations of single RFs contribute to the encoding of an entire visual scene. Addressing this issue requires (1) measuring a group of RFs that tile a continuous portion of visual space, (2) constructing a population-level measurement of spatial representations based on these RFs, and (3) linking how different types of RF attentional modulations change the population-level representation. To accomplish these aims, we used fMRI to characterize the responses of thousands of voxels in retinotopically organized human cortex. First, we found that the response modulations of voxel RFs (vRFs) depend on the spatial relationship between the RF center and the visual location of the attended target. Second, we used two analyses to assess the spatial encoding quality of a population of voxels. We found that attention increased fine spatial discriminability and representational fidelity near the attended target. Third, we linked these findings by manipulating the observed vRF attentional modulations and recomputing our population measures. Surprisingly, we discovered that attentional enhancements of population-level representations largely depend on position shifts of vRFs, rather than changes in size or gain. Our data suggest that position shifts of single RFs are a principal mechanism by which attention enhances population-level representations in visual cortex.

neuroscience↗