Effects of perceptual load on population receptive fields
24.25, Saturday, May 11, 2:30 - 4:15 pm, Royal Ballroom 4-5
Benjamin de Haas1, 2, D. Samuel Schwarzkopf1,2, Elaine J. Anderson1,2,3, Geraint Rees1,2; 1Institute of Cognitive Neuroscience, University College London, 2Wellcome Trust Centre for Neuroimaging, University College London, 3Institute of Ophthalmology, University College London
Perceptual load at fixation reduces detection performance for peripheral stimuli and the neural responses they evoke. Here we used functional MRI and population receptive field (pRF) mapping (Dumoulin, 2008, NeuroImage, 39(2), 647-60) to test whether foveal load changes the spatial tuning functions of voxels in human V1-3. Participants (n=28) performed a fixation task at the centre of the screen (Schwartz, 2005, Cereb Cortex, 15(6), 770-786) while dynamic, high-contrast bar stimuli traversed the visual field. Each participant completed 4-8 mapping scan runs. High and low load fixation tasks alternated between runs (n=14) or once per run (n=14). Additionally, each participant completed two scan runs to estimate their individual hemodynamic response function (HRF) under either condition. We used the resulting HRF estimates for high and low perceptual load to generate predictions for the time series of respective mapping runs. Predictions were based on the assumption of two-dimensional Gaussian pRFs. We estimated pRF centre position and size according to the best fit between predictions and actual data. High perceptual load at fixation increased pRF sizes in para-foveal V1-3 (~3-6 degrees eccentricity). Additionally, the eccentricity of foveal pRFs in V1-3 (up to ~1 degree eccentricity) was pulled outwards. Thus, high attentional demand at central fixation yielded a blurring effect on representations outside the central visual field and a centrifugal effect for the representation of mapping stimuli very close to the task stimuli. These effects may reflect the competition between the representation of central target and task-irrelevant mapping stimuli. They indicate that attention changes the spatial tuning properties of pRFs. This could be a physiological mechanism for early attentional selection under high perceptual load, as suggested by Lavies load theory (Lavie, 2005, TiCS, 9(2), 75-82).