Is attentional selection continuous or discrete? Is it deterministic or variable between trials? Is the distribution of responses across trials representative of selection on a given trial? Most cognitive psychology experiments measuring selection average responses across trials to determine the distribution of selected items. From these data, authors infer the properties of selection on any one trial. However, the assumptions underlying this inference may not be justified. We devise a novel experiment and analysis technique: multiple probes are used on each trial, allowing us to assess the degree of between-trial variability and within-trial spread of selection that contribute to the final distribution of reports. Our analyses show little, if any, variability between trials. Further, we define a model of an observer that deterministically selects many items to varying degrees on every trial, and randomly samples from this selected subset to produce reports. This model provides an excellent fit with the data. We conclude that selection of items from an RSVP stream is continuous (in that many letters are selected at once to varying degrees), that selection is deterministic (in that there is little or no variability in what is selected across trials), and that subjects make responses by sampling from the deterministically selected distribution. |