The Tetris effect (also known as Tetris Syndrome) occurs when people devote sufficient time and attention to an activity that it begins to overshadow their thoughts, mental images, and dreams. It is named after the video game Tetris.
People who play Tetris for a prolonged amount of time may then find themselves thinking about ways different shapes in the real world can fit together, such as the boxes on a supermarket shelf or the buildings on a street. In this sense, the Tetris effect is a form of habit. They might also dream about falling Tetris shapes when drifting off to sleep or see images of falling Tetris shapes at the edges of their visual fields or when they close their eyes. In this sense, the Tetris effect is a form of hypnagogic imagery.
Other examples 
The Tetris effect can occur with other video games, with any prolonged visual task (such as classifying cells on microscope slides, weeding, picking or sorting fruit, flipping burgers, driving long distances, or playing board games such as chess or go), and in other sensory modalities. In kinesthesis, a person newly on land after spending long periods at sea may move with an unbidden rocking motion, having become accustomed to the ship making such movements (known as sea legs or mal de debarquement). Computer programmers and developers sometimes have similar experiences, and report dreaming about code when they sleep. Similarly, students of mathematics may report dreaming of numbers or equations. People who play a certain game constantly for a long period of time will realize when they try to sleep their thoughts will be in a different fashion than usual.
Place in cognition 
Stickgold et al. (2000) have proposed that Tetris imagery is a separate form of memory, likely related to procedural memory. This is from their research in which they showed that people with anterograde amnesia, unable to form new declarative memories, reported dreaming of falling shapes after playing Tetris during the day, despite not being able to remember playing the game at all.
A study, conducted by Lynn Okagaki and Peter Frensch in 1994, showed that participants who played Tetris for twelve 30-minute sessions (with no previous experience of the game) did much better than the control group in both the paper-pencil test version of spatial skills as well as the computerized version. The conclusions drawn from this experiments were that video games such as Tetris had a positive effect on three areas of spatial skills including mental rotation, spatial perception and spacial visualization in those who played for a prolonged period continuously.
Another 2009 Oxford study suggests that playing Tetris-like video games may help prevent the development of traumatic memories. If the video game treatment is played soon after the traumatic event, the preoccupation with Tetris shapes is enough to prevent the mental recitation of traumatic images, thereby decreasing the accuracy, intensity, and frequency of traumatic reminders. "We suggest it specifically interferes with the way sensory memories are laid down in the period after trauma and thus reduces the number of flashbacks that are experienced afterwards," summarizes Dr. Emily Holmes, who led the study.
History of the term 
The earliest known reference to the term appears in Jeffrey Goldsmith's article, "This is Your Brain on Tetris", published in Wired in May 1994:
No home was sweet without a Gameboy in 1990. That year, I stayed “for a week” with a friend in Tokyo, and Tetris enslaved my brain. At night, geometric shapes fell in the darkness as I lay on loaned tatami floor space. Days, I sat on a lavender suede sofa and played Tetris furiously. During rare jaunts from the house, I visually fit cars and trees and people together. [...]
The Tetris effect is a biochemical, reductionistic metaphor, if you will, for curiosity, invention, the creative urge. To fit shapes together is to organize, to build, to make deals, to fix, to understand, to fold sheets. All of our mental activities are analogous, each as potentially addictive as the next. 
The term was rediscovered by Earling (1996), citing a use of the term by Garth Kidd in February, 1996. Kidd described "after-images of the game for up to days afterwards" and "a tendency to identify everything in the world as being made of four squares and attempt to determine 'where it fits in'". Kidd attributed the origin of the term to computer-game players from Adelaide, Australia. An early description of the general phenomenon appears in Neil Gaiman's SF poem "Virus" (1987) in Digital Dreams.
L'effet Tetris 
L'effet Tetris (French: the Tetris effect) is a similarly named, but quite different phenomenon found in evolutionary AI systems related to the concept of bounded rationality. L'effet Tetris then, is the effect whereby a hasty, but imprecise course of action is better than calculating an optimal move where such a calculation would not be completed in time; in short, evolutionary systems often find local rather than global optima.
Game Transfer Phenomena 
Game Transfer Phenomena (GTP) is a modern term created after the thesis written by Angelica Ortiz de Gortari, a student from the Nottingham Trent University. GTP is the set of residual feelings, thoughts and/or images which remain after playing a videogame. Examples, other than the Tetris effect, would be awareness of the absence of a head-up display in the natural human field of view after playing a first-person shooter or the urge to arrange (or command) little objects after playing a strategy game such as Starcraft.
See also 
- Domino effect
- Fixation (psychology)
- Highway hypnosis
- Video game addiction
- Earling, A. (1996, March 21–28). The Tetris Effect: Do computer games fry your brain? Philadelphia City Paper
- Daniel Terdiman (January 11, 2005). "Real World Doesn't Use a Joystick". Wired.
- Stickgold, Robert; Malia, April; Maguire, Denise; Roddenberry, David; O'Connor, Margaret (2000). "Replaying the Game: Hypnagogic Images in Normals and Amnesics". Science 290 (5490): 350–353. doi:10.1126/science.290.5490.350. PMID 11030656.
- Okagaki, L., Frensch,P. (1994). Effects of video game playing on measures of spatial performance: Gender effects in late adolescence. Journal of Applied Developmental Psychology, 15(1) 33-58.
- Holmes EA, James EL, Coode-Bate T, Deeprose C, (2009). "Can Playing the Computer Game "Tetris" Reduce the Build-Up of Flashbacks for Trauma? A Proposal from Cognitive Science". In Bell, Vaughan. PLoS ONE 4 (1): e4153. doi:10.1371/journal.pone.0004153. PMC 2607539. PMID 19127289 More than one of
- "Tetris 'helps to reduce trauma'". BBC News. January 7, 2009.
- Goldsmith, Jeffrey (May 1994). "This is Your Brain on Tetris". Wired Issue 2.05. Retrieved 20 December 2012.
- Kidd, G. (1996). Possible future risk of virtual reality. The RISKS Digest: Forum on Risks to the Public in Computers and Related Systems 17(78)
- Tetris dreams - Scientific American magazine, October 2000