Garikoitz Lerma-Usabiaga^1,2 (), Miguel Martinez-Zaldivar¹, Yongning Lei¹; ¹BCBL. Basque Center on Cognition, Brain and Language, San Sebastián, Gipuzkoa, Spain, ²IKERBASQUE. Basque Foundation for Science, Bilbao, Bizkaia, Spain

Population receptive field (pRF) modeling characterizes the aggregate spatial tuning of neuronal populations within a voxel. However, these estimates theoretically depend on the specific neural subpopulations recruited by the texture and features of the stimulus carrier pattern. Prompted by preliminary observations that word stimuli yield more foveal eccentricity estimates than standard checkerboards, this study systematically investigates how differing stimulus carrier patterns, flickering frequency, and pattern size modulate recovered pRF parameters in healthy human visual cortex. We employed a high-precision, dense-sampling fMRI protocol, scanning 10 participants over 10 sessions each. Retinotopic mapping was performed using 2 runs of standard 8-bar apertures filled with one of three carrier patterns: flickering checkerboards, words, or false fonts. To rigorously test the stability of carrier-dependent effects, we varied the internal element size and refresh frequency of the patterns across sessions. We quantified systematic deviations in recovered pRF parameters and assessed the inter-subject replicability of these shifts. Results demonstrate high consistency across sessions, confirming measurement reliability. Recovered pRF parameters were modulated by stimulus properties. Beyond the eccentricity shifts driven by carrier content, variations in element size and refresh frequency altered pRF estimates. The magnitude of these shifts was small in the early visual cortex but substantially larger in more anterior regions such as VO-1 and hV4, reaching more than 1 standard deviations outside of the fovea. Notably, we observed substantial inter-subject variability, with different participants showing distinct patterns of carrier-dependent modulation. We conclude that pRF parameters are not fixed anatomical constants but dynamic measures dependent on the specific neural subpopulations recruited by stimulus features. By quantifying how texture, size, and frequency drive these variations, our findings provide a first step toward resolving discrepancies in prior literature and establish essential constraints for designing future visual mapping experiments.

Acknowledgements: This research is supported by the Basque Government through the BERC 2022-2025 program and Funded by the Spanish State Research Agency through BCBL Severo Ochoa excellence accreditation CEX2020-001010/AEI/10.13039/501100011033 and through project PID2024-161512NB-I00 funded by Spanish Ministry of Science and Innovation