How to test blindsight without light scatter artefacts?

light scatter artefacts are a methodological problem in testing residual visual capacities (RVCs), for instance blindsight, in patients with homonymous visual field defects (HVFDs). The term light scatter artefact describes the phenomenon that light from targets directed towards the HVFD can stray into the sighted visual field. This might enable an observer to respond correctly to information directed at her blind field despite the fact that she is unable to process that information in the blind field itself. In this manuscript, we present a review of the relevance of light scatter in visual neuroscience, discuss factors that influence the impact of light scatter and evaluate means to test for light scatter artefacts. Furthermore, we present findings from an empirical study that was aimed at developing tests for RVCs that are free of light scatter artefacts. Previous studies on light scatter only used small sample sizes and equipment that is no longer in use. Hence, their results cannot be generalized to future experiments making it necessary to run laborious light scatter tests for every new study on RVCs. To avoid this, we hereby start a pool of stimuli and paradigms which demonstrably do not elicit light scatter artefacts. To this end, we investigated 21 healthy young participants in three frequently used RVC-paradigms: (1) temporal 2AFC task, (2) movement direction discrimination, and (3) redundant target paradigm. For each paradigm, we applied the blind-spot method. But first, we had to establish that our testing paradigm was sufficiently sensitive to detect light scatter artefacts. For this, we used conditions that are known to produce strong light scatter and a paradigm that is very sensitive to such effects. Specifically, we presented white targets on a black background in a dark room. The stimuli were presented to observers' blind spot. To check for light scatter artefacts, we used a target-detection task in a temporal 2AFC format. We obtained clear light scatter artefacts. Participants produced reliably above-chance detection performance under these conditions. The other two luminance conditions, measured in an illuminated room, did not produce light scatter artefacts. Accuracy in the temporal 2AFC task was at chance level for white targets on a grey background at the blind-spot position. Additionally, black targets on a grey background avoided light scatter artefacts in all three RVC-paradigms. In future, researchers can apply these stimulus and illumination conditions when using one of the three above paradigms in their studies. Using these conditions, they will be able to avoid light scatter artefacts without having to perform their own blind-spot tests.