1.1 Executive functions
A number of important features of the human mind are summarized under the
terms “executive control” or “executive functions”. These aspects can be regarded as top down effects in contrast to bottom up effects, that only represent stimulus driven processes. Executive functions include the ability to initiate, control or discontinue action, to useinformation flexibly, to make reasonable inferences, to think abstractly, to respond to novel information and
situations, to sequence information and to direct behavior in a goal-directed manner (e.g. Baddeley, 1996; Lezak, 1982; Logan, 1985a; Stuss & Benson, 1984). Welsh and Pennington (1988, pp. 201-202) define executive functions the following way:
“[Executive functions are] ... the abilityto maintain an appropriate problem-solving set for attainment of a future goals. This set can involve one or more of the following: (a) an intention to inhibit a response or to defer it to a later more appropriate time, (b) a strategic plan of action sequences, and (c) a mental representation of the task, including the relevant stimulus information encoded into memory and the desired futuregoal-state.”
The term executive function covers many abilities and, as such, is a concept for
which providing a precise theoretical or operational definition is difficult. It is interesting that in order to define executive functions, authors usually refer to supposed abilities being executive functions or describe situations in which
executive functions are likely to be needed (see Table 1.1).That those characterizations are necessary reveals a key aspect about executive functions.
As Burgess (1997, p.84) puts it: “Neuropsychologists would hardly feel it necessary to define the circumstances under which speech production processes are likely to be needed.” The provisional and underspecified definition of executive function in both, neuropsychology and cognitive psychology, is due toseveral reasons.
In comparison to other cognitive functions, executive functions are much less
well understood (e.g. Burgess, 1997). Furthermore, the term executive functions is often associated with a homunculus (Pennington & Ozonoff, 1996, for homunculus conceptions see “supervisory attentional system”, SAS, Norman
& Shallice, 1986; Shallice, 1988; Shallice & Burgess, 1991; or“central executive”, Baddeley, 1986). In addition, there is no clear empirical distinction between executive and non-executive function, those can rather be regarded as
a continuum than as separate entities (Rabbitt, 1997).
This state of affairs is reflected in the heterogeneous picture how researchers approach the issue of executive functions. Some study specific functions (e.g. control of motorresponses), other base their research on a test-oriented approach (e.g. the Wisconsin Card Sorting Test) and still other address more abstract concepts like self-awareness (see Stuss, 1991). Currently, because the term executive function has no operational definition and entails varying lists of functions, researchers have begun, rather than studying “executive functions”, to give detailed analysisof certain types of functions, for example confabulation, concept formation and response inhibition. This will also be the approach in this dissertation. The function that will be studied in the present work is the inhibition of ongoing responses.
1.2 Executive functions and the frontal lobes
The terms “executive functions” and “frontal functions” are often used interchangeably. However, theterm “frontal functions” refers to a structural entity, the anterior one-third of the brain, but does not emphasize that the brain
is a integrated functioning unit. The term “frontal system” reflects a more interactive approach, but again emphasizes the anatomical base (Rabbitt, 1997). Therefore the term “executive functions” is preferred. This term makes
the attribution to the frontal lobes,...