# What Makes a UI Simple? Difficulty and Complexity in Tasks Engaging Visual-Spatial Working Memory
## 2 Methods and Related Work
### Human Information Processing  
In the classical work on [PERSON]'s law, the coefficient \(1/b\), essentially information processing speed in choice reaction tasks, was found to be equal to about 6.7 bit/s. This value can vary significantly due to characteristics of the subjects and the stimuli type [35]. The Human Processor model (MHP) that was introduced in the 1980s but remains influential till the present day details the information processing in several interlinked sub-systems [36]: perceptual, cognitive, and motor one (Figure 1).  
MHP allows prediction of the time required to perform HCI-related tasks, e.g., with GOMS model, as the sum of the times in the involved sub-systems. However, these may differ significantly for various user groups, tasks, contexts, etc.: e.g., a cognitive processor cycle duration varies in the range of 25-170 ms. The experimental values for human information input (Rin) and output (Rout) speeds reported in different research publications are not entirely consistent, particularly since different information measurement units had been used and the experimental setup wasn't always specified in enough detail to convert to a single unified unit.  
Still, available sources seem to be in agreement that the upper limit of human vocal cords' long-term information output is about Rout = 5 syllables/s, i.e., about 12.5 sounds/s for English (for other languages, this value may differ). For humans equipped with instruments or tools, the values are generally presented in bit/s or chars/s. Handwriting is limited at 3.5 chars/s, while average typing speed is about 6.7 chars/s, with 10 chars/s being the top record. In any case, maximum speed of human as information transmitter is estimated at 40 bit/s. The values for conscious information reception are only marginally higher: Rin for speaking comprehension is estimated at 3-4 words/s, for reading comprehension at 2.5 words/s or 18 bit/s, for reading aloud at 30 bit/s, for visual recognition of familiar letters and digits at 55 bit/s. Most sources agree that although Rin for reading reaches about 45 bit/s, the average capability of human visual perception in long-term operation is only around 8 bit/s [37].  
The information processing speed can be enhanced using more complex [38] and multi-dimensional signals--e.g., size, brightness, and color, instead of just one varying facet. Another factor that is known to increase the speed is training, which is partially related to the fact that the operator can process information with various degrees of memory engagement. For tasks that do require considerable information exchange with the memory, the performance speed is reported to decrease by the order of magnitude, to about 2-4 bit/s [37]. The comparative analysis of numerous publications on working memory mechanisms presented in [37] (pp. 201-206), suggests that in most cases, information processing speed amounted to 3-5 units/s (sic!--not bits, but various units, such as figures). Similar quantitative estimations of human information processing capabilities and working memory performance can also be traced back to [39] or [40].  
Overall, it is believed that the analytical information-theoretic paradigm that does not consider spatial structures is fundamentally limited in explaining human perception that is mostly top-down, i.e., focused on higher-order images and structures. An important advantage in the field was the development of Algorithmic Information Theory (AIT) in the 1980s that allowed bonding Information Theory with the integral Gestalt approach, which is focused on organization of visual sensory input into a percept [41].