Thresholds vary between spatial and temporal forced-choice paradigms: the case of lateral interactions in peripheral vision.

Psychophysicists use spatial or temporal two-alternative forced-choice (2AFC) paradigms interchangeably. Thus, experiments with the same general goal are carried out using one or the other paradigm by distinct or even the same research groups. For example, 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 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 results showed that spatial 2AFC renders significantly lower sensitivity than temporal 2AFC when the target is presented along with suprathreshold flankers, but about the same sensitivity as temporal 2AFC when the target is presented alone. In the end, this resulted in statistically significant facilitation in peripheral vision only when measured with temporal 2AFC. Separate experiments at each of several peripheral locations revealed that the presence and magnitude of this effect varies not only with psychophysical paradigm but also with retinal locus.