Discrimination of short time intervals by the human observer

For time intervals in the 150-1500 ms range, the difference-discrimination thresholds are about 5%. The value of this Weber fraction varies somewhat d epending whether the stimulus modality is vision, bearing or touch. Thresho lds are higher when a time interval signaled in one modality has to be comp ared with one in another, and also when two different modalities are used t o delineate a single time interval, as well as when onset and offset are in the same modality but signaled to opposite cortical hemispheres. There is a prominent practice effect. This effect was used to show that there is com plete transfer of training between the two visual hemispheres. These findin gs imply that the time-discrimination mechanism is not located at an early stage of visual processing. If there is a single central time-discriminatio n apparatus, the observed intermodal differences must relate to the relativ e ease of access to it via different modalities. The mechanism involved nee ds elucidating. Counting of spikes or internal time modules would seem to b e too simplistic a concept; there is still a need for a process in which th e duration of a just concluded presentation and an internally stored interv al duration can be compared.