 The psychological definition characterizes mental imagery is defined negatively: it is perceptual representation not triggered directly by sensory input. This leaves open the question what it is triggered by. Often it is triggered by higher level cognitive processes – this is the case when you count to three and visualize an apple. But it can also be triggered laterally by different sense modalities (see Section 2.2 below). And it can also be triggered by sensory input but in an indirect manner (see Section 2.1 below).