Mind-wandering (sometimes referred to as task-unrelated thought) is the experience of thoughts not remaining on a single topic for a long period of time, particularly when people are not engaged in an attention-demanding task.
Mind-wandering tends to occur during driving, reading and other activities where vigilance may be low. In these situations, people do not remember what happened in the surrounding environment because they are pre-occupied with their thoughts. This is known as the decoupling hypothesis. Studies using event-related potentials (ERPs) have quantified the extent that mind-wandering reduces the cortical processing of the external environment. When thoughts are unrelated to the task at hand, the brain processes both task relevant and unrelated sensory information in a less detailed manner.
Mind-wandering appears to be a stable trait of people and a transient state. Studies have linked performance problems in the laboratory and in daily life. Mind-wandering has been associated with possible car accidents. Mind-wandering is also intimately linked to states of affect. Studies indicate that task-unrelated thoughts are common in people with low or depressed mood. Mind-wandering also occurs when a person is intoxicated via the consumption of alcohol.
It is common during mind-wandering to engage in mental time travel or the consideration of personally relevant events from the past and the anticipation of events in the future. Poet Joseph Brodsky described it as a “psychological Sahara,” a cognitive desert “that starts right in your bedroom and spurns the horizon.” The hands of the clock seem to stop; the stream of consciousness slows to a drip. We want to be anywhere but here.
Studies have demonstrated a prospective bias to spontaneous thought because individuals tend to engage in more future than past related thoughts during mind-wandering.
The history of mind wandering research dates back to 18th century England. British philosophers struggled to determine whether mind-wandering occurred in the mind or if an outside source caused it. In 1921, Varendonck published The Psychology of Day-Dreams, in which he traced his "trains of thoughts' to identify their origins, most often irrelevant external influences." Wallas (1926) considered mind-wandering as an important aspect of his second stage of creative thought - incubation. It wasn’t until the 1960s that the first documented studies were conducted on mind wandering. John Antrobus and Jerome Singer developed a questionnaire and discussed the experience of mind wandering. This questionnaire, known as the Imaginal Processes Inventory (IPI), provides a trait measure of mind wandering and it assesses the experience on three dimensions: how vivid the persons thoughts are, how many of those thoughts are guilt or fear based, and how deep into the thought a person goes. As technology continues to develop, psychologists are starting to use Functional magnetic resonance imaging to observe mind wandering in the brain and reduce psychologists reliance on verbal reports.
Jonathan Smallwood and colleagues popularized mind-wandering using thought sampling and questionnaires. Mind-wandering is studied by finding lapses of external attention in participants. One method to study mind-wandering is the SART (sustained attention to response) task. In a SART task there are two categories of words. One of the categories are the target words. In each block of the task a word appears for about 300 ms, there will be a pause and then another word. When a target word appears the participant hits a designated key. About 60% of the time after a target word a thought probe will appear to gauge whether thoughts were on task. If participants were not engaged in the task they were experiencing Task Unrelated Thoughts (TUTs), signifying mind-wandering. Another task to judge TUTs is the experience sampling method (ESM). Participants carry around a personal digital assistant (PDA) that signals several times a day. At the signal a questionnaire is provided. The questionnaire questions vary but can include: (a) whether or not their minds had wandered at the time of the (b) what state of control they had over their thoughts and (c) about the content of their thoughts. Questions about context are also asked to measure the level of attention necessary for the task. One process used was to give participants something to focus on and then at different times ask them what they were thinking about. Those who were not thinking about what was given to them were considered “wandering”. Another process was to have participants keep a diary of their mind wandering. Participants are asked to write a brief description of their mind wandering and the time in which it happened. These methodologies are improvements on past methods that were inconclusive.
Mind-wandering is important in understanding how the brain produces what William James called the train of thought and the stream of consciousness. This aspect of mind-wandering research is focused on understanding how the brain generates the spontaneous and relatively unconstrained thoughts that are experienced when the mind wanders. One candidate neural mechanism for generating this aspect of experience is a network of regions in the frontal and parietal cortex known as the default network. This network of regions is highly active even when participants are resting with their eyes closed suggesting a role in generating spontaneous internal thoughts. One relatively controversial result is that periods of mind wandering are associated with increased activation in both the default and executive system a result that implies that mind-wandering may often be goal oriented.
In addition to neural models, computational models of consciousness based on Bernard Baars' Global Workspace theory suggest that mind-wandering, or "spontaneous thought" may involve competition between internally and externally generated activities attempting to gain access to a limited capacity central network.
There are individual differences in some aspects of mind-wandering between older and younger adults. Although older adults reported less mind-wandering, these older participants showed the same amount of mind-wandering as younger adults. There were also differences in how participants responded to an error. After an error, older adults took longer to return focus back to the task when compared with younger adults. It is possible that older adults reflect more about an error due to conscientiousness. Research has shown that older adults tend to be more conscientious than young adults. Personality can also affect mind-wandering. People that are more conscientious are less prone to mind-wandering. Being more conscientious allows people to stay focused on the task better which causes fewer instances of mind-wandering. Differences in mind-wandering between young and older adults may be limited because of this personality difference.
Mind-wandering and working memory
One important question facing the study of mind-wandering is how it relates to working memory capacity. Recent research has studied the relationship between mind wandering and working memory capacity. This relationship requires more research to understand how they influence one another. It is possible that mind-wandering causes lower performance on working memory capacity tasks or that lower working memory capacity causes more instances of mind-wandering. Also, reports of task unrelated thoughts are less frequent when performing tasks that do not demand continuous use of working memory than tasks which do. Moreover, individual difference studies demonstrate that when tasks are non-demanding, high levels of working memory capacity are associated with more frequent reports of task unrelated thinking especially when it is focused on the future. By contrast, when performing tasks that demand continuous attention, high levels of working memory capacity are associated with fewer reports of task unrelated thoughts. Together these data are consistent with the claim that working memory capacity helps sustain a train of thought whether it is generated in response to a perceptual event or is self-generated by the individual. Therefor, under certain circumstances, the experience of mind-wandering is supported by working memory resources. Working memory capacity variation in individuals has been proven to be a good predictor of the natural tendancy for mind-wandering to occur during cognitively demanding tasks and various activities in daily life.  Mind-wandering sometimes occurs as a result of saccades. In an antisaccade task, for example, subjects with higher working memory capacity scores resisted looking at the flashing visual cue better than participants with lower working memory capacity. Higher working memory capacity is associated with fewer saccades toward environmental cues. Mind-wandering has been shown to be related to goal orientation; people with higher working memory capacity keep their goals more accessible than those who have lower working memory capacity, thus allowing these goals to better guide their behavior and keep them on task.
Another study compared differences in speed of processing information between people of different ages. The task they used was a go/no go task where participants responded if a white arrow moved in a specific direction but did not respond if the arrow moved in the other direction or was a different color. In this task, children and young adults showed similar speed of processing but older adults were significantly slower. Speed of processing information effects how much information can be processed in working memory. People with faster speed of processing can encode information into memory better than people that have slower speed of processing. This can lead to memory of more items because more things can be encoded.
Mind-wandering and retention
Mind wandering affects retention where working memory capacity is directly related to reading comprehension levels. Participants with lower working memory capacity perform worse on comprehension-based tests. When investigating how mind wandering affects retention of information, experiments are conducted where participants are asked a variety of questions about factual information, or deducible information while reading a detective novel. Participants are also asked about the state of their mind before the questions are asked. Throughout the reading itself, the author provides important cues to identify the villain, known as inference critical episodes (ICEs). The questions are asked randomly and before critical episodes are reached. It was found that episodes of mind wandering, especially early on in the text led to decreased identification of the villain and worse results on both factual and deducible questions. Therefore, when mind wandering occurs during reading, the text is not processed well enough to remember key information about the story. Furthermore both the timing and the frequency of mind wandering helps determine how much information is retained from the narrative 
Mind-wandering and reading comprehension
Reading comprehension must also be investigated in terms of text difficulty. To assess this, researchers provide an easy and hard version of a reading task. During this task, participants are interrupted and asked whether their thoughts at the time of interruption had been related or unrelated to the task. What is found is that mind-wandering has a negative affect on text comprehension in more difficult readings. This supports the executive-resource hypothesis which describes that both task related and task unrelated thoughts (TUT) compete for executive function resources. Therefore, when the primary task is difficult, little resources are available for mind-wandering, whereas when the task is simple, the possibility for mind-wandering is abundant because it takes little executive control to focus on simple tasks. However, mind-wandering tends to occurs more frequently in harder readings as opposed to easier readings. Therefore it is possible that similar to retention, mind-wandering increases when readers have difficulty constructing a model of the story.
Mind-wandering and happiness
Matthew Killingsworth invented an iPhone app that captured user’s feelings in real time. The tool alerts the user at random times and asks: "How are you feeling right now?" and "What are you doing right now?" Killingsworth and Gilbert's analysis suggested that mind-wandering was much more typical in daily activities than in laboratory settings. They also describe that people were less happy when their minds were wandering than when they were otherwise occupied. This effect was somewhat counteracted by people's tendency to mind-wander to happy topics, but unhappy mind-wandering was more likely to be rated as more unpleasant than other activities. The authors note that unhappy moods can also cause mind-wandering, but the time-lags between mind-wandering and mood suggests that mind-wandering itself can also lead to negative moods. Furthermore research suggests that regardless of working memory capacity, participants participating in mind wandering experiments report more mind wandering when bored, stressed, unhappy.
Mind-wandering and Executive Functions
Executive functions (EFs) are cognitive processes that make a person pay attention or concentrate on a task. Three executive functions that relate to memory are inhibiting, updating and shifting. Inhibiting controls a person's attention and thoughts when distractions are abundant. Updating reviews old information and replaces it with new information in the working memory. Shifting controls the ability to go between multiple tasks. All three EFs have a relationship to mind-wandering.
Executive functions have roles in attention problems, attention control, thought control, and working memory capacity. Attention problems relate to behavioral problems such as inattention, impulsivity and hyperactivity. These behaviors make staying on task difficult leading to more mind wandering. Higher inhibiting and updating abilities correlates to lower levels of attention problems in adolescence. The inhibiting executive function controls attention and thought. The failure of cognitive inhibition is a direct cause of mind wandering. Mind wandering is also connected to working memory capacity (WMC). People with higher WMC mind wander less on high concentration tasks no matter their boredom levels. People with low WMC are better at staying on task for low concentration tasks, but once the task increases in difficulty they had a hard time keeping their thoughts focused on task. Updating takes place in the working memory, therefore those with low WMC have a lower updating executive function ability. That means a low performing updating executive function can be an indicator of high mind wandering. Working memory relies on executive functions, with mind wandering as an indicator of their failure. Task Unrelated Thoughts (TUTs) are empirical behavioral manifestations of mind wandering in a person. The longer a task is performed the more TUTs reported. Mind wandering is an indication of an executive control failure that is characterized by TUTs.
- McVay, Jennifer C.; Kane, Michael J. (2009). "Conducting the train of thought: Working memory capacity, goal neglect, and mind wandering in an executive-control task.". Journal of Experimental Psychology: Learning, Memory, and Cognition 35 (1): 196–204. doi:10.1037/a0014104.
- Smallwood, J., Obonsawin, M.C., & Heim, D. (2003) Task Unrelated Thought: the role of distributed processing. Consciousness and Cognition. 12(2), 169-189.
- Smallwood, J., Beech, E.M., Schooler, J.W. & Handy, T.C. Going AWOL in the brain – mind wandering reduces cortical analysis of the task environment. Journal of Cognitive Neuroscience. 20(3), 458-469.
- Kam, J.W.Y., Dao, E., Farley, J., Fitzpatrick, K., Smallwood, J., Schooler, J.W., & Handy, T.C. (2010). Slow fluctuations in attentional control of sensory cortex. Journal of Cognitive Neuroscience. 0(0), 1-11.
- Braboszcz, C. and Delorme, A. (2011) Lost in thoughts: neural markers of low alertness during mind wandering. Neuroimage, 54(4):3040-7.
- Smallwood, J., Davies, J. B., Heim, D., Finnigan, F., Sudberry, M.V., O'Connor, R.C. & Obonsawain, M.C. (2004). Subjective experience and the attentional lapse. Task engagement and disengagement during sustained attention. Consciousness and Cognition, 13(4), 657-690.
- McVay J.C., Kane M.J., Kwapil T.R. (2009). Tracking the train of thought from the laboratory into everyday life: an experience-sampling study of mind wandering across controlled and ecological contexts. Psychon Bull Rev. 16(5):857-63. PMID 19815789
- Galéra C, Orriols L, M'Bailara K, Laborey M, Contrand B, Ribéreau-Gayon R, Masson F, Bakiri S, Gabaude C, Fort A, Maury B, Lemercier C, Cours M, Bouvard MP, Lagarde E.Mind wandering and driving: responsibility case-control study. BMJ. 2012 Dec 13;345:e8105.
- Smallwood, J., Fitzgerald, A., Miles, L., & Phillips, L. (2009). Shifting moods, wandering minds: negative moods lead the mind to wander, Emotion. 9(2), 271-276.
- Smallwood, J., O'Connor, R.C., Sudberry, M.V. & Obonsawin, M.C. (2007). Mind wandering & Dysphoria. Cognition & Emotion, 21(4), 816-842.
- Finnigan, F., Schulze, D. & Smallwood, J. (2007). Alcohol and the wandering mind – a new direction in the study of attentional lapses. International Journal of Disability and Human Development, 6(2), 189–199.
- Lehrer, Jonah The Importance of Mind-wandering
- Smallwood, J., Nind, L. & O'Connor, R.C. (2009) When is your head at? An exploration of the factors associated with the temporal focus of the wandering mind. Consciousness & Cognition. 18(1), 118-125.
- Varendonck, J. 1921, The Psychology of Day-Dreams London:Allen & Unwin
- Wallas,G. 1926, The Art of ThoughtLondon:Johnathon Cape
- Baumeister, R. (2007). Encyclopedia of social psychology ([Online-Ausg.]. ed.). Thousand Oaks, Calif.: Sage Publications. pp. 574–577. ISBN 978-1-4129-1670-7.
- Antrobus J.S., Singer, J.L., Goldstein, S. and Fortgang, M. (1970). Mind-wandering and cognitive structure. Transactions of the New York Academy of Science 32(2):242-252. PMID 5265228
- Craighead, edited by Irving B. Weiner, W. Edward (2010). The Corsini encyclopedia of psychology (4th ed. ed.). Hoboken, N.J.: John Wiley. ISBN 9780470479216.
- Kane, M. J.; Brown, L. H.; McVay, J. C.; Silvia, P. J.; Myin-Germeys, I.; Kwapil, T. R. (1 July 2007). "For Whom the Mind Wanders, and When: An Experience-Sampling Study of Working Memory and Executive Control in Daily Life". Psychological Science 18 (7): 614–621. doi:10.1111/j.1467-9280.2007.01948.x. PMID 17614870.
- Unsworth, Nash; McMillan, Brittany D.; Brewer, Gene A.; Spillers, Gregory J. (2012). "Everyday attention failures: An individual differences investigation.". Journal of Experimental Psychology: Learning, Memory, and Cognition 38 (6): 1765–1772. doi:10.1037/a0028075.
- Unsworth, Nash; Brewer, Gene A.; Spillers, Gregory J. (July 2012). "Variation in cognitive failures: An individual differences investigation of everyday attention and memory failures". Journal of Memory and Language 67 (1): 1–16. doi:10.1016/j.jml.2011.12.005.
- Mason, M.F., Norton, M.I., Van Horn, J.D., Wegner, D.M., Grafton, S.T., Macrae, C.N. (2007). Wandering minds: the default network and stimulus-independent thought. Science 315(5810):393-395. 17234951
- Christoff, K., Gordon, A.M., Smallwood, J. Smith, R. & Schooler, J.W. (2009). Experience sampling during fMRI reveals default network and executive system contributions to mind wandering. Proceedings of the National Academy of Sciences of the United States of America. 106(21), 8719-24.
- Gusnard, D.A. & Raichle, M.E. (2001). Searching for a baseline: functional imaging and the resting human brain. Nature Reviews Neuroscience. 2(10):685-694. doi:10.1038/35094500 PMID 11584306
- Bar, M., Aminoff, E., Mason, M., Fenske, M. (2007). The units of thought. Hippocampus. 17(6):420-428. PMID 17455334
- Smallwood, J. & Schooler, J.W. (2006). The Restless Mind. Psychological Bulletin, 132(6), 946-958.
- Miyake, A.; Friedman, N. P. (31 January 2012). "The Nature and Organization of Individual Differences in Executive Functions: Four General Conclusions". Current Directions in Psychological Science 21 (1): 8–14. doi:10.1177/0963721411429458.
- Rodríguez-Villagra, Odir Antonio; Göthe, Katrin; Oberauer, Klaus; Kliegl, Reinhold (2013). "Working memory capacity in a go/no-go task: Age differences in interference, processing speed, and attentional control.". Developmental Psychology 49 (9): 1683–1696. doi:10.1037/a0030883.
- Baars, Bernard (1988), A Cognitive Theory of Consciousness (Cambridge, MA: Cambridge University Press)
- Baars, Bernard (1997), In the Theater of Consciousness (New York, NY: Oxford University Press)
- Dehaene, S. & Changeux, J.-P. (2005). Ongoing spontaneous activity controls access to consciousness: A neuronal model for inattentional blindness. PLoS Biology, 3(5):e141. PMID 15819609
- Kane, M. J.; McVay, J. C. (1 October 2012). "What Mind Wandering Reveals About Executive-Control Abilities and Failures". Current Directions in Psychological Science 21 (5): 348–354. doi:10.1177/0963721412454875.
- Jackson, Jonathan D.; Balota, David A. (2012). "Mind-wandering in younger and older adults: Converging evidence from the sustained attention to response task and reading for comprehension.". Psychology and Aging 27 (1): 106–119. doi:10.1037/a0023933.
- Schooler, Jonathan W. (2007). Encyclopedia of Social Psychology. SAGE Publications. pp. 562–564.
- Smallwood, Jonathan (February 2011). "Mind-wandering While Reading: Attentional Decoupling, Mindless Reading and the Cascade Model of Inattention". Language and Linguistics Compass 5 (2): 63–77. doi:10.1111/j.1749-818X.2010.00263.x.
- Levinson, D, Smallwood, J., Davidson, R.J. (2011). "The persistence of thought". Psychological Science 23 (4): 375–380. doi:10.1177/0956797611431465. PMID 22421205.
- Baird, B, Smallwood, J., Schooler, J.W. (2011). "Back to the future: auto-biographical planning and functionality of mind wandering". Consciousness & Cognition 20 (4): 1604–1611. doi:10.1016/j.concog.2011.08.007. PMID 21917482.
- Smallwood, J. (2013). "Distinguishing how from why the mind wanders: a process occurrence framework for self-generated thought". Psychological Bulletin 139 (3): 519–535. doi:10.1037/a0030010. PMID 23607430.
- Alloway, T.P.; S.E. Gathercole, H. Kirkwood, J. Elliott (Mar–Apr 2009). "The cognitive and behavioral characteristics of children with low working memory". Child Development 80 (2): 606–621. doi:10.1111/j.1467-8624.2009.01282.x. PMID 19467014.
- Gathercole, S.E.; Alloway TP, Kirkwood HJ, Elliott JG, Holmes J, Hilton KA (2008). "Attentional and executive function behaviours in children with poor working memory". Learning and Individual Differences 18: 214–223.
- Kane, M.J; Brown LH, McVay JC, Silvia PJ, Myin-Germeys I, Kwapil TR (July 2007). "For whom the mind wanders, and when: An experience-sampling study of working memory and executive control in daily life". Psychological Science 18 (7): 614–621. doi:10.1111/j.1467-9280.2007.01948.x. PMID 17614870.
- McVay, JC; MJ Kane (May 2012). "Drifting from slow to "D'oh!": working memory capacity and mind wandering predict extreme reaction times and executive control errors". Journal of Experimental Psychology: Learning, Memory, Cognition 38 (3): 529–549. doi:10.1037/a0025896. PMID 22004270.
- Kane, MJ; Bleckley MK, Conway AR, Engle RW (June 2001). "A controlled-attention view of working-memory capacity". Journal of Experimental Psychology: General 130 (2): 169–183. PMID 11409097.
- Unsworth, N; Schrock JC, Engle RW (November 2004). "Working memory capacity and the antisaccade task: individual differences in voluntary saccade control". Journal of Experimental Psychology: Learning, Memory, and Cognition 30 (6): 1302–21. doi:10.1037/0278-73220.127.116.112. PMID 15521806.
- Vaughan, Leslie; Giovanello, Kelly (2010). "Executive function in daily life: Age-related influences of executive processes on instrumental activities of daily living.". Psychology and Aging 25 (2): 343–355. doi:10.1037/a0017729. PMID 20545419.
- Zanto, Theodore P.; Toy, Brian; Gazzaley, Adam (January 2010). "Delays in neural processing during working memory encoding in normal aging". Neuropsychologia 48 (1): 13–25. doi:10.1016/j.neuropsychologia.2009.08.003.
- Smallwood, Jonathan; McSpadden, Merrill; Schooler, Jonathan W. (September 2008). "When attention matters: The curious incident of the wandering mind". Memory & Cognition 36 (6): 1144–1150. doi:10.3758/MC.36.6.1144.
- Smallwood, J. "Mind Wandering and oOther Lapses". Encyclopedia of Consciousness.
- Feng, S.; D'Mello, S., & Graesser, A. C. (2013). "Mind wandering while reading easy and difficult texts". Psychonomic Bulletin & Review 20 (3): 586.
- Matthew A. Killingsworth and Daniel T. Gilbert (2010). "A Wandering Mind Is an Unhappy Mind". Science 330 (932). doi:10.1126/science.1192439. PMID 21071660.
- Diamond, Adele (3 January 2013). "Executive Functions". Annual Review of Psychology 64 (1): 135–168. doi:10.1146/annurev-psych-113011-143750.
- Barry, Danielle. "Executive Function." The Gale Encyclopedia of Mental Health. Ed. Kristin Key. 3rd ed. Vol. 1. Detroit: Gale, 2012. 592-594. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- Schnitzspahn, Katharina M.; Stahl, Christoph; Zeintl, Melanie; Kaller, Christoph P.; Kliegel, Matthias (2013). "The role of shifting, updating, and inhibition in prospective memory performance in young and older adults.". Developmental Psychology 49 (8): 1544–1553. doi:10.1037/a0030579.
- Friedman, N. P.; Haberstick, B. C.; Willcutt, E. G.; Miyake, A.; Young, S. E.; Corley, R. P.; Hewitt, J. K. (1 October 2007). "Greater Attention Problems During Childhood Predict Poorer Executive Functioning in Late Adolescence". Psychological Science 18 (10): 893–900. doi:10.1111/j.1467-9280.2007.01997.x. PMID 17894607.
- "Neuropsychological Assessment in Schools." Encyclopedia of Applied Psychology. Ed. Charles D. Spielberger. Vol. 2. Amsterdam: Elsevier, 2004. 657-664. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- "Executive Functions." The SAGE Glossary of the Social and Behavioral Sciences. Ed. Larry E. Sullivan. Thousand Oaks, CA: SAGE Reference, 2009. 191. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- Sala, Sergio Della, and Robert H. Logie. "Working Memory." Encyclopedia of the Human Brain. Ed. V. S. Ramachandran. Vol. 4. Amsterdam: Academic Press, 2002. 819-830. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- "Attention." Encyclopedia of Gerontology. Ed. James E. Birren. 2nd ed. Vol. 1. Amsterdam: Elsevier, 2007. 120-129. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- Cooke, D. Tighe. "Executive Functioning." Encyclopedia of Human Development. Ed. Neil J. Salkind. Vol. 1. Thousand Oaks, CA: SAGE Reference, 2006. 486-487. Gale Virtual Reference Library. Web. 5 Dec. 2013.
- Schooler, Jonathan W., and Jonathan Smallwood. "Meta-Awareness." Encyclopedia of Social Psychology. Ed. Roy F. Baumeister and Kathleen D. Vohs. Vol. 2. Thousand Oaks, CA: SAGE Publications, 2007. 562-564. Gale Virtual Reference Library. Web. 5 Dec. 2013.