Adaptation to visual numerosity can affect time perception but not the other way around

The theory of magnitude suggests number and time are linked by a common cortical metric, and their specialisa- tion develops from a single magnitude system. We inves- tigated the presence of a common processing mechanism underlying numerical and temporal perception using uni- modal and cross-modal adaptation. We conducted four ex- periments, each using a rapid adaptation protocol (100 trials; average trial duration: 1.2 sec) and a 2AFC adaptive staircase method. In a two by two adaptation and discrimination task design, participants adapted to either numerosity or dura- tion and subsequently performed a numerosity or duration discrimination task. Each adaptation-task combination was presented in a separate block. Data analysis revealed that adapting to a high numerosity (80 dots) led to underestima- tion of the reference numerosity (40 dots), while adapting to a low numerosity (20 dots) led to overestimation of the ref- erence numerosity presented in the adapted location. Sim- ilarly, adapting to a long duration (600 ms) led to underes- timation of the reference duration (333 ms), whereas adapt- ing to a short duration (67 ms) led to an overestimation of the reference duration. In the cross-modal adaptation condi- tions, numerosity adaptation affected time discrimination in 60% of the subjects, where adaptation to a high and low nu- merosity resulted in an underestimation and overestimation of the reference duration respectively. There was no effect of time adaptation on numerosity discrimination. These re- sults indicate that both numerosity and time are susceptible to rapid adaptation, and provide only limited support for the existence of a common magnitude system in numerical and temporal perception.