The detection and discrimination of spatial offsets

By detection of spatial offsets is meant the ability to indicate whether or not a given (vernier) stimulus has a spatial offset. Discrimination, on the other hand, implies that the direction of offset has been correctly identified. We compared vernier thresholds for these two tasks and found a consistent difference by a factor of around 2 in favour of discrimination. This is to say that observers are able to correctly indicate the direction of offsets that are too small to be reliably detected by the same observers. This apparent paradox can be explained on the assumptions that one single, bipolar mechanism is involved in both tasks, and that observers use a direction selective cue such as the orientation difference of the implicit lines through the vernier stimuli. The implications for estimates of the sensitivity of hyperacuity are discussed.