## 2 ACT-R Modeling for Task Switching
### ACT-R Cognitive Architecture  
The ACT-R (version 7.0) architecture consists of several core modules, and each module represents a part of the brain that independently processes different types of information (see Figure 1) [7]. Perceptual-motor modules, including the vision, motor, audio, and speech modules, interact with the real environment. The goal module stores the goal information required to complete a task. The declarative module stores and processes human declarative knowledge, which is represented in terms of structures called chunks. Declarative knowledge represents facts, such as \(2+3=5\). The imaginal module temporarily stores subgoal information or new information encoded by perceptual-motor modules. The procedural module, which is also referred to as a production system, stores and processes all procedural knowledge, which is represented as the rules of human cognitive behavior, called procedural rules, in an IF-THEN form. Each procedural rule includes a set of conditions and actions.  
Each module behaves in a parallel manner and has its own buffer that can hold only one chunk associated with the module; therefore, ACT-R can represent a combination of both parallel and serial cognitive processing [11]. However, the procedural module does not have its own buffer. Instead, the procedural module performs a pattern-recognition function that continuously monitors all ACT-R buffers to identify procedural rules whose conditions match the state and content of the buffers. Following this, a conflict-resolution function is performed to select only one of the identified rules based on the highest expected utility value, and then, the prescribed actions of the selected procedural rule are executed [7]. The three functions in the procedural module represent neural functions in the basal ganglia structure, which is part of the brain. The basal ganglia evaluate the criterion satisfaction of the choice between multiple actions by interacting with the functional regions of the brain and then serve the execution of the prescribed action [33]. Thus, the basal ganglia implement the procedural rules in the procedural module by interacting with the buffers in the ACT-R modules and deciding upon the next action. In this manner, ACT-R can represent cognitive processes as symbolic structures.  
According to the explanation presented on the ACT-R homepage, ACT-R is based on symbolic and subsymbolic structures. The symbolic structure relates to the symbolic representation of cognitive processes in terms of procedural rules of processing knowledge, called chunks, between functional systems in the brain. The subsymbolic structure includes mathematical equations concerning functional systems in each module, including an abstract representation of which chunks and rules are selected and how quickly the chunks and rules are processed. Therefore, a cognitive model of a specific task can be developed by establishing the chunks and the procedural rules associated with the task, in accordance with the symbolic structure of ACT-R [34], and by supplementing additional subsymbolic equations needed to represent human cognition in the task to ACT-R. The ACT-R model can then provide a prediction of a performer's detailed cognitive process in the task in chronological order. In the next section, we describe our task-switching model in terms of symbolic and subsymbolic structures based on ACT-R.