Attention to object files defined by transparent motion

Two interspersed and differently colored sets of dots were rotated in oppos ite directions and were perceived as superimposed transparent surfaces. Pro bes consisting of brief changes in dot motion direction were reported. Two probes affecting the same surface were discriminated accurately. The 2nd pr obe was discriminated poorly if it affected a surface different from the Is t and if the time between probes was less than 600 ms. This reflects a diff iculty in switching attention rapidly between surfaces. Spatial proximity i ncreased the interference. Controls were incompatible with traditional spat ial mechanisms (2- or 3-dimensional) or with simple sensory filters. Instea d, probes were apparently selected by object files. The interference is not simply due to an inability to process 2 objects at once but requires close spatial proximity of incompatible motion signals.