Irem Akdogan^1,2 (), Simay Uner^1,2, Hulusi Kafaligonul^1,2,3; ¹Department of Neuroscience, Aysel Sabuncu Brain Research Center, Bilkent University, Ankara, Turkiye, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkiye, ³Neuroscience and Neurotechnology Center of Excellence (NÖROM), Department of Anatomy, Faculty of Medicine, Gazi University, Ankara, Turkiye

Peripheral visual processing is inherently non-uniform, characterized by robust asymmetries in polar angle. The Horizontal-Vertical Anisotropy (HVA) reflects superior performance along the horizontal meridian, while the Vertical Meridian Asymmetry (VMA) often favors the lower over the upper visual field. While the anatomical and functional bases of these inhomogeneities are well-documented, the degree to which they can be modulated under targeted neuromodulation protocols remains unclear. This study systematically investigated whether tRNS alters the contrast sensitivity function (CSF) across the visual field and whether such effects depend on the electrical stimulation frequency. Participants performed an orientation discrimination task at four isoeccentric visual field locations (left, right, upper, and lower) under either high-frequency (hf-tRNS) or low-frequency (lf-tRNS) stimulation conditions (1.5 mA), as well as baseline controls. CSF data confirmed the expected asymmetry pattern via maximum contrast sensitivities, indicating the presence of HVA and VMA, both at baseline and tRNS conditions. Moreover, tRNS applications revealed modulation patterns that were strictly specific to the electrical stimulation frequency. Hf-tRNS significantly enhanced contrast sensitivity across the spatial frequencies in the lower visual field. However, this was location-specific, as it attenuated sensitivity in both horizontal locations—where anatomical and functional processing is naturally strongest—independent of spatial frequency. The upper visual field showed no reliable modulation. Conversely, lf-tRNS failed to induce significant changes in any visual field location. These results provide the first comparative mapping of peripheral CSF under different tRNS types. A fixed stimulation intensity elicits divergent effects—facilitation in the lower field versus reduction in the horizontal meridian—illustrating that the potential for plasticity and neural gain varies significantly across the periphery, while asymmetries were preserved. Together, these findings underscore intrinsic biological constraints of polar angle asymmetries and suggest that effective neuromodulation of peripheral vision may require frequency- and location-specific tuning of intensity rather than a uniform stimulation approach.

Acknowledgements: This work was supported by The Scientific and Technological Research Council of Turkiye (ARDEB 424K278, BIDEB 2211 Program) and the BAGEP Award of the Science Academy-Turkiye.