Bloch strikes back: a reassessment of the intensity-duration reciprocity.

Perceiving a brief visual stimulus depends on both its intensity and its exposure duration.These two parameters are in a reciprocal relationship, such that doubling the duration andhalving the intensity of a stimulus leads to equivalent perceptual performance. Thisrelationship is known as Bloch’s law. Despite numerous demonstrations of the validity ofBloch’s law, recent evidence shows violations for exposures ranging from few microsecondsto tens of milliseconds. The main goal of this work was to reassess the law’s validity on theperception of simple geometric shapes. The latter were presented for durations between0.15 and 100 ms thanks to a modern LCD-based tachistoscope. We successfully replicatedthe reciprocal relationship between intensity and duration across five different tasksinvolving detection, shape discrimination and subjective visibility judgements. In accordancewith past literature, we report reciprocity only for stimuli shorter than a maximum criticalduration (between 15 and 36 ms, depending on the task). We discuss these results incomparison with the aforementioned violations of Bloch’s law, and identify potential causesof the latter. This work also served as a proof-of-principle that our apparatus is suitable forthe study of the intensity-duration reciprocity with a diverse range of stimuli.