Cognitive Load Theory
Cognitive load is the amount of stress our ability to store information in memory is placed under by any given task. Classic cognitive load theory defines three main types of cognitive load.
Intrinsic load is defined by the complexity of the information to be learned. The amount is determined by the interactivity of the elements being learned. So for example learning words in a foreign language is relatively straightforward, however learning grammar is a lot harder, due to the interactivity of the words in a phrase.
Extraneous load is caused by inappropriate display of information, or irrelevant information, or having to combine spatially different information. So if an operator of a power plant has to apply safety information displayed in one format into an emergency happening in another format, then there is an increased extraneous load placed on him or her.
Germane cognitive load is essentially the idea of practicing things to get better. In this case a certain amount of load is required to construct and automate "schemas" which allow for easier completion of the task in the future.
There is some concept of multimodality in most things we do, and this might be because it is actually a way in which we deal with cognitive load. Think of an example of giving instructions to a tourist, usually we point at things, we wave and create gestures as a way to get a more complex message across
As Sharon Oviatt says
“Users respond to dynamic changes in their own working memory limitations and cognitive load by shifting to more multimodal communication as load increases with task difficulty as a result, a flexible multimodal interface supports users in self-managing their cognitive load and minimizing related performance errors while solving complex real-world tasks”.