1. An article, comprising: a computing device and a memory; wherein the memory stores a set of computer instructions that, if executed by the computing device, cause the computing device to perform operations comprising: defining at least one first layer for spatial perception; defining at least one second layer for at least partial spatial perception relative to the at least one first layer; causing a first perceptible item to be associated with one of the at least one first layer; and pursuant to a select action, dynamically causing a second perceptible item to be associated with one of the at least one first layer; wherein the computer instructions, if executed by the computing device, further cause the computing device to make the first perceptible item and the second perceptible item perceptible in sequence, with only one of the first and second perceptible items perceptible at a time, so that at least one object in a set of items including the first and second perceptible items is perceptible as moving.