Spatial or temporal 2AFC may give different results depending on context

Experiments with the same general goal have usually been carried out using spatial or temporal two-alternative forced-choice (2AFC) paradigms interchangeably by distinct or even the same research groups. This situation has occurred both in studies on visual sensitivity in dyslexia and in studies on lateral interactions in peripheral vision. Conflicting results in either field (e.g., whether or not dyslexics have a visual sensitivity deficit and whether or not peripheral detection is facilitated by the presence of flankers) appear to be resolved on the surmise that spatial and temporal 2AFC paradigms indeed produce different results. We designed experiments in which peripheral detection thresholds for Gabor patches (in the presence or absence of suprathreshold flankers) could be measured using completely equivalent spatial and temporal 2AFC paradigms so that any resultant difference can be unequivocally attributed to the effect of the paradigms themselves. The main results showed that spatial 2AFC renders significantly lower sensitivity than temporal 2AFC when the target is surrounded by flankers, but it renders about the same sensitivity as temporal 2AFC when the target is presented alone. In the end, this resulted in statistically significant facilitation only when measured with temporal 2AFC. These results suggest that splitting resources across two spatial locations appears to elevate target detection thresholds only when a peripheral discrimination task is involved. Separate experiments involving several peripheral locations further revealed that the magnitude of this differential effect of paradigm varies with retinal locus. The proven effect of these two factors (i.e., variations across paradigms and variations across retinal loci) along with large individual differences (i.e., some subjects consistently show impairment instead of facilitation) indicates that previous conclusions about lateral interactions in the periphery may need to be revisited.