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ROLE OF COGNITIVE EFFORT
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Lacey, 1970) showed that when people engage in internal cognitive
activities, such as mental arithmetic, heart rate tends to increase.
Indeed, the amount of heart rate during a mental task appears to be
directly related to the amount of cognitive effort expended during it
(Kahneman, Tursky, Shapiro, & Crider, 1969).
Several studies have shown a fairly close relationship between heart-
rate increases and graded level of psychological difficulty on mental
tasks. For instance, Ginsberg, Heslegrave, Scher, Wong, and Furedy
(1980) presented participants with eight trials of an easy task and eight
trials of a difficult task. The task involved subtraction of a one-digit
number from a two-digit number paced at either four subtractions per
trial (easy) or ten subtractions per trial (difficult). They found that heart
rate increased significantly as a function of task difficulty. Similarly,
Scher, Furedy, and Heslegrave (1984) administered two levels of diffi-
culty of a backward digit span task, which involved auditory presenta-
tion of a predetermined number of single digits at 1-second intervals.
Participants were then required to cognitively manipulate the numbers
during a 15-second interval so they could subsequently repeat them in
reverse order. During the difficult trials, two more digits were presented
than during the easy trials, and the absolute lengths of the spans were
adjusted for individual subjects. The researchers found that heart-rate
accelerations during the 15-second cognitive manipulation intervals
were significantly greater on the difficult trials than on the easy trials.
Carroll, Turner, and Hellawell (1986) also examined the relationship
between task difficulty and heart-rate acceleration. They administered
two cognitively challenging tasks (mental arithmetic and Raven matri-
ces), each with three levels of difficulty (easy, difficult, and impossible).
In addition, they calculated expected heart rates in each task condition
using regression equations based on graded exercise tasks in which oxy-
gen-consumption data were collected for various exercise loads for each
participant. Knowing the actual oxygen consumption during each psy-
chological task, the researchers calculated the “additional heart rate” as
the difference between the predicted and actual heart rates. They found
that cardiac activity was sensitive to variations in difficulty level
whether represented as “additional heart rate” or simply as the differ-
ence between task and resting heart rates. Heart rates increased much
more during the difficult and impossible conditions than during the
easy conditions. Likewise, with the recent development of “affective
computing” (Picard, 1997), there is considerable interest in the impact of
cognitive effort on physiological indices, including heart rate, which can
be monitored continuously by a computer system interacting with a
human user and used to characterize how the person is regulating effort
(e.g., Fairclough & Venables, 2006; Hockey, 1997).
In summary, the more cognitive effort a mental task requires, the
greater the increase in heart rate. Accordingly, the present study
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