Investigating ON and OFF pathway spatial tuning in the upper and lower visual fields using the SSVEP

The ON and OFF retino-cortical processing pathways encode increments and decrements (respectively) in perceived luminance. Recent evidence suggests that the ON and OFF pathways may be differently affected by selective vision loss, encouraging the diagnostic perimetric assessment of ON & OFF pathway function. This calls for an improved understanding of normative ON and OFF pathway function across the visual field. Here, using electroencephalography (EEG), we aim to investigate whether spatial tuning in the ON and OFF pathways differs between the upper and lower visual fields. In 39 healthy observers, we measured ON- and OFF-biased visually evoked potentials (VEPs) using a 128 channel EEG system. Responses from the upper and lower visual fields were simultaneously measured using a spatial array of flicker-frequency tagged hexagonal probes (3.75Hz and 3Hz, respectively). This low-contrast (20%) luminance flicker had a saw-tooth profile, the polarity of which was set to bias responses towards the ON or OFF pathways. Across 8 conditions, we manipulated the polarity and spatial scale (size & number) of the stimuli elements, clamping probe surface area. After a process of dimension reduction, VEP amplitudes were fit with a linear model at the group-level for statistical inference. Reliable upper and lower visual field neural responses for the ON and OFF pathways were found at all spatial scales. As previously reported, OFF pathway responses were larger than ON pathway responses. However, the OFF pathway demonstrated a striking preference for smaller and more numerous probes, while the ON pathway was relatively scale invariant. These preferences were similar in the Upper and Lower visual fields. This difference in scale tuning could be formulated as the ON pathway comprising of sub-units with a broader range of receptive field sizes, perhaps produced by weaker centre-surround inhibition in this pathway. This possibility will be investigated in future work.