Chapter 6: User Interfaces for Mobile Media 203 determine how people group objects based on proximity, similarity, symmetry, and so on. A special case in visual perception is reading, a combination of visual and cognitive processing. Similar to vision, the human ear can sense frequencies from 20 Hz to 20 000 Hz, being most sensitive between 500 and 2000 Hz, the frequency of normal speech. The human output relates mainly to motor control of effectors, such as fi ngers, limbs, and the vocal system, and depends heavily on physi- cal characteristics, skill, and practice of the user. The motor control can be measured, based on reaction and movement time. Reaction time depends on the senses. For example, the user can react to an auditory signal in 150 ms, but reacting to visual signal requires 200 ms. The second measure can be estimated by using Fitts’ 1 law (Fitts 1954). Processing the received information and translating it to motor control requires various actions related to, for example, problem solving, reasoning, and memory. Humans have a limited capacity of storing information. First, a stimulus received through the senses is buffered into sensory memory for 0.5 seconds. It is constantly overwritten by new information and only a small portion of it is transferred into working memory. Second, the size of working (or short-term) memory is limited to 7 ± 2 chunks of information (Miller 1956). Consequently, from the user interface design point of view, it is imperative to minimize the amount of information that the user needs to remember, and primarily present information that is relevant to the current task. All the things that we “know” are stored in long-term memory; transferring information from working memory to long-term memory requires learning. Naturally, the frequency of use and time required to learn affect this. Since systems are used by people that are at various stages in the learning process, all of them should be supported. Typi- cally, novice users require more navigation steps and aid when per- forming a task, which may be straightforward to complete by expert users (Dix et al. 2003; Shneiderman 1998). Related to problem solving, a central concept is a mental model, a representation of reality for understanding some phenomena based on prior knowledge. These models help to understand experiences, predict the outcomes of actions, and handle unexpected occurrences (Norman 1990). 1 Fitts’ law is used to predict movement time from a starting point to a target region with the aid of empirically determined input device -dependent constants.