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Jessica M. Thomas, Paola Binda, Ione Fine, Geoffrey M. Boynton; Filling-in of an Induced Foveal Scotoma in Human Visual Cortex. Journal of Vision 2014;14(10):1414. doi: 10.1167/14.10.1414.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: 'Filling-in' occurs when an attribute such as brightness or motion is induced in a blank region of the visual field by the surrounding stimulus. Measuring the neural correlates of filling in with fMRI is complicated by spatio-temporal blurring and nonlinearities associated with the BOLD signal. Here we avoid these complexities by visualizing the effects of filling-in using a novel neural image method based on population receptive fields (pRF) 1. Methods: We estimated the pRFs that best predicted each voxel's time course to a multifocal (spatiotemporally random) stimulus in areas V1-V3 in three normally sighted individuals. We then measured fMRI responses to a drifting bar within a 16° aperture either with or without a central 2° blank 'scotoma'. For both stimuli, 'neural image' time-courses were generated by summing each voxel's Gaussian pRF in visual space scaled by its fMRI response at that time-point. To account for spatio-temporal blurring and nonlinearities in the BOLD signal, we compared these 'real neural images' to a 'model neural image' generated by convolving the stimulus time-course with each subject's estimated HDR and pRFs, with the inclusion of a model of BOLD spatio-temporal nonlinearities. Results: For the stimulus without a scotoma, model and real neural images are remarkably similar and resemble the drifting bar stimulus, delayed in time by BOLD hemodynamics. With the scotoma, the model neural image shows the expected drop in foveal response. In contrast, the scotoma had almost no effect on the real neural image. Differences between the model and real 'neural image' can be attributed to neural 'filling in'. Conclusion: We describe here a novel neural image method that estimates neural responses independently of the effects of spatiotemporal blurring and nonlinearities, and show that it can demonstrate the effects of neural filling in for a drifting bar stimulus in foveal V1-V3.
Meeting abstract presented at VSS 2014
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