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Lien, M., Ruthruff, E., and Johnston, J. (2006). Attentional Limitations in Doing Two Tasks at Once: The Search For Exceptions. Current Directions in Psychological Science. Vol 15 (2). 89--93.

  author = 	 {Lien, M., Ruthruff, E., and Johnston, J.},
  title = 	 {Attentional Limitations in Doing Two Tasks at Once: The Search for Exceptions},
  journal = 	 {Current Directions in Psycholoigcal Science},
  year = 	 {2006},
  volume = 	 {15},
  number = 	 {2},
  pages = 	 {89--93}

Author of the summary: Mark Fortney, 2006, mark.d.fortney@gmail.com

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It has been proposed that dual task interference is the result of a central bottleneck that only allows humans to process one task at a time. The central processing occurs in between perceptual encoding and response processing; the theory is that if central processing for two tasks overlaps, a central bottleneck will occur and create dual task interference.

This paper provides an overview of previous studies in dual task interference, specifically, where an exception to the central bottleneck was discovered, and incorporates the authors’ own research from 2005 concerning ideomotor compatible tasks. The conclusion is that although the central bottleneck has been bypassed in controlled laboratory conditions, in a few studies, it would be difficult to create the exact conditions that are necessary to bypass it in real-world scenarios.

Subjects in the studies were required to quickly respond to Task 1 and Task 2; the time between them was changed to measure a baseline time to do the tasks and various levels of interference.

Possible exceptions to the bottleneck:

(1) Ideomotor compatible tasks: ideomotor compatible tasks are tasks where the ‘stimulus resembles sensory feedback from the response’ (Greenwald and Schulman, 1973). It was proposed that these tasks might use the same mental codes that represented the stimuli and thereby bypass central processing. This theory was accepted for a time but was recently challenged (Lien, McCann, Ruthruff, and Proctor, 2005; Lien, Proctor, and Allen, 2002). One exception, where idoemotor compatible tasks did bypass the central bottleneck, involved an experiment where Task 1 was judging a tone as high/low and Task 2 was keeping a circle over a moving cross. Johnston & Delgado (1993) speculated that Task 2 had ‘preauthorized’ a joystick response and thereby avoided the need for central processing.

(2) Practicing tasks has been found to reduce dual task interference, but generally not to entirely eliminate it. One study found that when Task 2 (a tone judgement) was easier and practiced for 8 sessions, a minority of participants bypassed the bottleneck - Ruthruff, Van Selst, Johnston, and Remington (in press).

(3) Special Response Systems: Perhaps general purpose tools, like the hands and voice, require central processing, while narrow-purpose tools such as the eyes do not. Pashler, Carrier and Hoffman (1993) found that focusing the eyes on an object for Task 2 bypassed the bottleneck. Two possible explanations: eye focus is partly reflexive, and might have its own circuitry apart from central resources, or focusing the eyes is so highly practiced that it easily bypasses the bottleneck.

Summary author's notes:

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