By running fingers over a surface, texture perception results. The size, shape, arrangement, and density of small elements that protrude from, or indent into, the surface affect the resulting perceived texture. The duplex theory states that coarser textures (larger elements) are mainly perceived by spatial cues, whereas finer textures are mainly perceived through temporal cues [125,142]. By spatial cue, it means that the structure can be inferred by pressing the finger against the surface. By temporal cue, the finger is slid across the surface, resulting in a pressure vibration that can be sensed by the Pacinian and Meissner corpuscles. For a finer texture, a slower motion may be necessary so that the vibration frequency remains below 250 to 350 Hz. Recall from Section 12.1 that people can learn to improve their texture perception and acuity when reading Braille. Thus, perceptual learning may be applied to improve tactile (touch) perception.