Operationalization of mental effort

The amount of mental effort mobilized during a cognitive task cannot be measured directly. Thus, studies that investigated effort have used several methods to measure and quantify effort, such as self-reports, or performance. These methods appear to be problematic for several reasons (see Gendolla, 2004; Gendolla & Richter, 2010, for a review). Even though self-report measures have been used for decades for quantifying effort (e.g., Efklides, Kourkoulou, Mitsiou, & Ziliaskopoulou, 2006), they are notably influenced by self-presentation—by how people present themselves. For many researchers, performance seemed to be a prominent indicator of effort mobilization (e.g., Aarts, Custers, & Marien, 2008). However, effort and performance are not conceptually interchangeable. Effort refers to the mobilization of resources to carry out instrumental behavior, whereas performance refers to the outcome of the instrumental behavior. Wright and his colleagues have demonstrated in a series of studies on ability beliefs (see Wright, 1998; Wright & Kirby, 2001),

that people with high ability to perform a task will mobilize little effort but though perform well, whereas people with low ability will mobilize high effort to perform on the same level (see also Hockey, 1997). For this reason, performance cannot systematically reflect effort.

Consequently, the present thesis will instead operationalize effort as cardiovascular reactivity—changes in the cardiovascular system during task performance with reference to cardiovascular baseline activity at rest—which is a more valid and reliable indicator of effort intensity. The departure point of this idea goes back to the work by the psychophysiologist Paul Obrist (1981) who has suggested cardiovascular activity as an indicator of task engagement (i.e., effort). More specifically, Obrist demonstrated that when organisms can have control over their behavior’s outcomes—active coping situation—the β-adrenergic influence exerted on the myocardial muscle by the sympathetic nervous system is proportional to task demand. It is of note that Obrist’s approach applies to cognitive challenges, suggesting β-adrenergic activity on the myocardium as a measure of mental effort.

Wright (1996) combined motivational intensity theory (Brehm & Self, 1989) with the active coping approach (Obrist, 1981) and posited that β-adrenergic sympathetic impact on the heart responds proportionally to the level of experienced task demand as long as success is possible and the necessary effort is justified.

1.4.3.1. Cardiovascular measures for the operationalization of mental effort Among the non-invasive measures of β-adrenergic sympathetic activity on the myocardium, the most sensitive indicator is cardiac pre-ejection (PEP) period (Kelsey, 2012).

PEP is defined as the time interval between the onset of the heart’s left ventricular depolarization (Q peak) and the opening of the aortic valve (B point) with the ejection of the blood into the aorta. Since it is difficult to locate the Q-point, we take into account the R-onset instead of the Q-peak (Figure 4). Thus, cardiac PEP is determined as the interval (in ms) between ECG R-onset and ICG B-point (Berntson, Lozano, Chen, & Cacioppo, 2004). PEP is measured in milliseconds (ms), reflects the contractility force of the left ventricle, and takes on average 100 ms at rest in healthy persons. The shorter the PEP, the stronger is cardiac contractility and thus the influence of the sympathetic nervous system on the myocardium.

As PEP is directly influenced by β-adrenergic impact, this measure is our primary measure of mental effort.

Figure 4. R-onset and B-point localization for the measure of PEP from the electrocardiogram (ECG) and the first derivative of the impendence cardiography (ICG (dZ/dt)) signals (Figure taken from Lozano et al., 2007).

Cardiac contractility has a systematic influence on systolic blood pressure (SBP) via its impact on cardiac output (CO). SBP is defined as the maximal arterial pressure in the vascular system between two heartbeats and is measured in millimeters of mercury (mmHg). Several studies have used SBP as an indicator of effort mobilization (see Gendolla & Richter, 2010;

Gendolla, Wright, & Richter, 2012; Wright & Kirby, 2001). Even though SBP is systematically influenced by β-adrenergic activity, it is also influenced by total peripheral vascular resistance, which is not systematically influenced by β-adrenergic impact (Levick, 2003). Diastolic blood pressure (DBP), which is defined as the minimal arterial pressure between two heartbeats, is also measured in mmHg and is even more strongly related to vascular resistance than SBP.

Given that both SBP and DBP measures are influenced by total peripheral vascular resistance, they are less reliable indicators of mental effort than PEP. However, blood pressure measures are essential to control for an afterload effect—aortic diastolic pressure—which can potentially influence PEP (Sherwood et al., 1990). For instance, if a decrease in PEP is observed together with a decrease in DBP, this means that PEP is influenced by peripheral resistance—

afterload effect. By contrast, observing an increased or stable DBP coupled with a decrease in PEP can be interpreted as indicating an absence of such an effect.

Another, frequently used indicator of effort mobilization is heart rate (HR) (e.g., Brinkmann & Gendolla, 2007; Eubanks, Wright, & Williams, 2002, Richter & Gendolla, 2007), which is defined as the number of heartbeats per minute (bpm). Given that HR is determined

by both sympathetic and parasympathetic nervous system activity, HR is a less reliable index of β-adrenergic activity and should reflect effort mobilization when the sympathetic impact is stronger. However, HR is also assessed to control for a preload effect—left ventricular filling during diastole—which can potentially influence PEP (Sherwood et al., 1990). Increased preload can decrease PEP due to higher ventricular filling during diastole. Observing, however, an increased or stable HR together with a decrease in PEP indicates the absence of a preload effect.