Optimum Stimulus Design for fMRI-based pRF Mapping
Regions adjacent to each other in the visual field are represented by adjacent areas in the visual cortex, which is referred to as retinotopy. Functional magnetic resonance imaging (fMRI) has been used to generate population receptive field (pRF) maps that assign areas in the visual field to regions in the visual cortex . The pRF maps generated using fMRI depend on the nature of the stimulus used , which is undesirable for standard mapping applications. Ideally, we would like the pRF maps generated to be robust, which we define as the confluence of good coverage of the entire visual field and superior reliability of the generated maps across subjects and sessions.
While there have been a flurry of recent studies investigating the impact of stimulus choice on pRF maps, the stimuli used in such studies (moving bar, rotating wedge, expanding ring) persist more for historical reasons than their ability to generate robust pRF maps. Yet, a wedge+ring combined stimulus results in higher goodness of fit (better coverage of the visual field) compared to a standard bar stimulus . This was explained by the higher proportion of stimulation time within a run using the wedge+ring stimulus, allowing the pRF model to account for a greater proportion of the variability in the blood-oxygenation-level-dependent (BOLD) signal.
In this study, in addition to the standard bar stimulus we use two novel stimulus patterns involving two parallel/inclined bars sweeping across the visual field. This has the effect of (nearly) doubling the stimulation-related peaks in the BOLD signal, which is hypothesized to enhance the goodness of fit of the pRF model used. We also want to estimate the intersubject and intersession reliability of the pRF maps generated in order to arrive at a holistic assessment of robustness. The goal of this study is to investigate if either of the novel stimuli result in more robust pRF maps than the standard bar stimulus.
A prime use of pRF mapping is for scotoma detection and examination of neural plasticity in the visual cortex in cases of loss in visual input to certain regions. A stimulus design that produces robust pRF maps would allow for standardization of results from different labs, equipment, and methodologies, making it invaluable for further exploration of the visual cortex.References
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 D. Linhardt et al., "Combining stimulus types for improved coverage in population receptive field mapping", NeuroImage, vol. 238, p. 118240, 2021. Available: 10.1016/j.neuroimage.2021.118240 [Accessed 6 May 2022].
 I. Alvarez, B. de Haas, C. Clark, G. Rees and D. Schwarzkopf, "Comparing different stimulus configurations for population receptive field mapping in human fMRI", Frontiers in Human Neuroscience, vol. 9, pp. 96, 2015. Available: 10.3389/fnhum.2015.00096 [Accessed 6 May 2022].