under naturalistic conditions. As Sellen and Norman (1992, p. 334) pointed out,
many naturally occurring action slips occur:
... when a person is internally preoccupied or distracted, when both the
intended actions and the wrong actions are automatic, and when one is
doing familiar tasks in familiar surroundings. Laboratory situations offer
completely the opposite conditions. Typically, subjects are given an un-
familiar, highly contrived task to accomplish in a strange environment.
Most subjects arrive motivated to perform well and ... are not given to
internal preoccupation.... In short, the typical laboratory environment is
possibly the least likely place where we are likely to see truly spontane-
ous, absent-minded errors.Theories of Action Slips
At a general level, most theorists (e.g. Reason, 1992; Sellen & Norman, 1992)
have assumed that action slips occur in part because there are two modes of
control:
.An automatic mode, in which motor performance is controlled by sche-
mas or organised plans; the schema that determines performance is the
strongest available one..A conscious control mode based on some central processor or atten-
tional system; it can oversee and override the automatic control mode.
Each mode of control has its own advantages and disadvantages. Automatic
control is fast and it permits valuable attentional resources to be devoted to
other processing activities. However, automatic control is relatively inflexible,
and action slips occur when there is undue reliance on this mode of control.
Consciouscontrolhastheadvantagesthatitislesspronetoerrorthanauto-
matic control and it responds flexibly to environmental changes. However, it
operates relatively slowly, and is an effortful process.
It follows from this theoretical analysis that action slips occur when an indi-
vidual is in the automatic mode of control and the strongest available schema
or motor programme is not appropriate. The involvement of the automatic
mode of control can be seen in many of Reason’s (1979) action slips. One com-
mon type of action slip involves repeating an action unnecessarily because the
first action has been forgotten (e.g. attempting to start a car that has already
started, or brushing one’s teeth twice in quick succession). We know from
studies in which listeners attend to one message and repeat it back while
ignoring a second message presented at the same time, that unattended infor-
mation is held very briefly and then forgotten. When the initial starting of a car
or brushing one’s teeth occurs in the automatic mode of control, it would be
predicted that subsequent memory for what has been done should be extremely
poor, and so the action would often be repeated.
Sub-routine failures occur when a number of distinct motor programmes
needtoberunoffinturn.Althougheachmotorprogrammecanbecarriedout
without use of the conscious mode of control, a switch to that mode is essential
at certain points in the sequence of actions, especially when a given situation is
common to two or more motor programmes, and the strongest available motor
Attention and Performance Limitations 391