The Central Executive Bottleneck
Human cognitive architecture has a fundamental constraint. According to the Psychological Refractory Period (PRP) research reviewed by Pashler (1994), when two tasks simultaneously require response selection, one of them is kept waiting. This is explained by the central executive (dorsolateral prefrontal cortex) functioning as a single-channel processor. In this account, perceptual and motor execution stages can operate in parallel, but the 'what to do' decision stage proceeds sequentially. During dual-tasking, prefrontal cortex activity appears not so much to increase as to time-share between the two tasks. On this view, the brain is not so much truly parallel processing as performing rapid sequential switching.
Quantifying Task Switching Costs
Every task switch incurs cognitive costs, decomposable into two types. First, 'switch cost': time to load the new task set (rules, goals, response mappings) into working memory, which a 2006 American Psychological Association (APA) overview says is sometimes just a few tenths of a second per switch. Second, 'residual cost': time for previous task set interference to fully dissipate, which lasts longer than the switch cost. Rubinstein et al. (2001) reported from their experiments that the more complex the tasks, the more time was lost in switching. Error rates also tend to rise on trials immediately following switches. Even after 'light' interruptions like email checking, fully returning to the original task can take a while.
| Type of cost | What is happening | Magnitude |
|---|---|---|
| Switch cost | Time to load the new task set (rules, goals, response mappings) into working memory | Small (sometimes a few tenths of a second per switch) |
| Residual cost | Time until interference from the previous task set fully dissipates | Lasts longer than the switch cost |
| Increase in total work time | When you keep switching between complex tasks | Larger for more complex tasks |
| Rise in error rate | Concentrated on the trials immediately after a switch | Tends to rise |
| Return from an interruption | Even after a light interruption such as checking email, to fully get back to the original task | Can take a while |
Switch cost and residual cost are paid on every switch, so they accumulate the more often you switch. The upper 2 rows are the price paid per switch; the lower 3 are how it shows up across the whole task.
Media Multitasking and Cognitive Ability
Habitual simultaneous use of smartphones, television, and computers may have long-term cognitive effects. Research by Ophir et al. (2009) at Stanford University reported that heavy media multitaskers were more easily distracted by irrelevant information than light multitaskers and appeared to have lower attention filtering ability. They also tend to perform worse on working memory capacity tests. While the causal direction remains debated (whether multitasking degrades ability or lower ability drives multitasking), a study by Loh and Kanai (2014) reported that people who media-multitask more had smaller gray matter density in the anterior cingulate cortex. Since this region is involved in attention control and conflict resolution, the structural difference may be related to the weaker function.
Conditions Where Parallel Processing Is Possible
Not all multitasking is equally inefficient. When one task is highly automated, near-parallel processing can become achievable. Walking while conversing or cooking while listening to music work because motor control is automated, which makes central executive competition less likely. According to Wickens' Multiple Resource Theory, task combinations using different sensory modalities (visual vs. auditory), different processing stages (perception vs. response), and different processing codes (spatial vs. verbal) produce less interference. However, when both tasks demand the same resource pool (e.g., visual attention plus verbal processing), interference tends to be large. Smartphone use while driving is extremely dangerous partly because both tasks simultaneously demand visual attention and judgment.
The hidden cost of doing things alongside
So-called doing one thing alongside another hides a cost the person finds hard to notice. For example, writing text while listening to music with lyrics makes the brain region governing language contended for by two tasks, and the quality of both tends to drop. On the other hand, a stimulus that does not compete with language, such as music without lyrics, has little effect. Whether doing things alongside becomes a problem depends largely on whether the two tasks fight over the same resource of the brain. When you want to concentrate on a task, choosing an environment that does not compete with it makes quality easier to maintain.
Practicing Single-Tasking and Achieving Deep Focus
Understanding multitasking's inefficiency, concrete methods exist for practicing single-tasking. Time blocking (assigning single tasks to time periods) minimizes switching frequency. The Pomodoro Technique (25 minutes focus plus 5 minutes rest) provides sustainable concentration units. Environmental design includes complete notification blocking, closing irrelevant tabs, and physically partitioning workspaces. There is a view that entering deep focus (deep work) takes a certain ramp-up period. Frequent interruptions during this ramp-up can mean the day ends without ever reaching deep focus. For MyRank tests as well, performing 5 minutes of single-task concentration before testing (such as attending to breathing) reduces residual switching costs, making it easier to approach testing in a calm state.