Cardiovascular response to Stroop: Effect of verbal response and task difficulty
Introduction
The Stroop task measures the ability to selectively attend to the color of a word while filtering out its meaning. The task engages control processes (color naming) and automatic processes (reading). The strong interference of word reading upon color naming is called the Stroop effect (Stroop, 1935). Basically, participants take longer to name the color of the ink that color words are written in when the ink color and color word do not match. This effect has fascinated researchers interested in the fundamental processes of attention for over 60 years (MacLeod, 1991).
Theories underlying the Stroop phenomenon have included relative speed of processing (Fraisse, 1969), automaticity (Hunt and Lansman, 1986), and parallel distributed processing (Cohen et al., 1990). The latter appears to incorporate components of relative speed of processing and automaticity without some of their limitations (MacLeod, 1991). The parallel processing model discards the idea of a limited capacity response stage. Thus, rather than viewing processing as sequential and involving bottlenecks, parallel distributed processing sees capacity limitations as system-resource limits (Cohen et al., 1990). This model views the Stroop effect as a decision process gathering evidence, whereby parallel processing of multiple sources of relevant and irrelevant information determines the length of processing time needed to respond to the Stroop words (Logan, 1980).
The Stroop task has been used extensively as a laboratory stressor in cardiovascular reactivity research (Steptoe and Vogele, 1991) and typically results in increased cardiac, renal, and splanchnic nerve activity with simultaneous skeletal muscle vasodilation (Freyschuss et al., 1988). Cardiovascular hyper-reactivity (a heightened cardiovascular response in relation to resting values) to mental challenge is thought to contribute to the development of cardiovascular disease (Krantz and Manuck, 1984, Matthews et al., 1993, Treiber et al., 2003). Although mechanisms underlying the reactivity/disease relationship are unclear exaggerated skeletal muscle blood flow in animals (Folkow, 1990) and humans (Miller and Ditto, 1991) has been shown to be implicated in the development of hypertension. Catecholamines have also been shown to increase during mental challenge and circulating epinephrine is one of the mechanisms mediating mental challenge-induced skeletal muscle vasodilation (Freyschuss et al., 1988). The Stroop is attractive for use as a laboratory stressor because of the absence of a significant learning effect (MacLeod, 1991) and its consistent influence on autonomic reactivity. For example, after 10 min of Stroop performance heart rate (HR), blood pressure, and skeletal muscle vasodilation remained significantly elevated compared to baseline (Hamer et al., 2003).
Stroop autonomic reactivity may be brought about by a number of influences such as social context, task difficulty, and verbal response. For example, Stroop is performed at a pace that makes perfect performance difficult whilst under the scrutiny of an experimenter. The pressure to perform in this kind of situation generates both cardiovascular reactivity and negative affect (Brown et al., 1988). Also increasing the difficulty of a mental task may produce higher reactivity response. For example, Brown et al. (1988) found that increasing the difficulty of a mental arithmetic task resulted in a higher HR response. Other studies have reported higher HRs to more difficult forms of Raven's Matrices (Steptoe et al., 1990) and other mental stressors (Carroll et al., 1986, Obrist et al., 1978). In contrast, Light and Obrist (1980) used tasks with a monetary reward and found that cardiovascular response to an easy version of a reaction time task was similar to the response to a moderately difficult version. Therefore, the influence of task difficulty on HR reactivity may be influenced by the nature of the task. Thus, greater HR responses are sometimes, but not always, associated with greater task difficulty.
Verbally responding to slides appears to cause a physiological response by itself. For example, reading colored slides without Stroop challenge produced significant increases in HR and blood pressure (Brown et al., 1988, Stein and Boutcher, 1993). The verbal component influencing reactivity produced by mental arithmetic has been shown to contribute to the HR response to mental arithmetic (Brown et al., 1988, Seraganian et al., 1997, Sloan et al., 1991, Tomaka et al., 1994). Thus, vocalizing numbers or words appears to produce a significant increase in HR (Friedmann et al., 1982, Linden and Estrin, 1988).
There is also the possibility that motor involvement is necessary to bring about Stroop-induced cardiovascular reactivity. For example, excitation of sympathetic centres may only occur when participants performing the Stroop have to generate a motor response such as speaking (recruiting facial muscles to speak the color of the slide) or hand responding (moving fingers to press a switch in the reaction time Stroop). A test of Stroop-induced motor influence on sympathetic reactivity could be achieved by having participants perform the Stroop without any muscle recruitment. Thus, participants could “think” a response to Stroop slides without being required to physically respond whilst multiple measures of cardiovascular reactivity are being monitored.
Whether non-verbal performance of the Stroop and verbal response to slide reading brings about differing cardiovascular reactivity to that of the traditional Stroop is undetermined. Therefore, the purpose of this study was to compare multiple measures of cardiovascular reactivity and perceived task difficulty to versions of the Stroop task containing differences in verbal involvement and task difficulty. Three versions of the task: (1) the traditional Stroop, (2) the verbal component without cognitive challenge, and (3) the cognitive challenge without the verbal component were employed. It was hypothesized that the traditional version of the Stroop (Stroop 1) would result in significantly greater forearm blood flow and epinephrine reactivity compared to passively reading monochrome words (Stroop 2) or performing Stroop without a verbal response (Stroop 3).
Section snippets
Participants
Participants were 13 healthy young males aged between 18 and 25 years old, were normotensive, and possessed no background of family hypertension. All procedures and potential risks were explained, and participants gave their written informed consent. This study was approved by a University Human Experimentation Ethics Committee.
Anthropometric
Anthropometric measures included height, weight, and skinfolds to determine body fat percentage (Durnin and Womersley, 1974). Three bipolar electrodes of ECG were
Participant characteristics
Participant characteristics are shown in Table 1.
Heart rate
Heart rate (HR) was significantly different, F(1.7,20.3) = 11.8, p = 0.001 (Greenhouse-Geiser adjustment), with an effect size (ES) of 0.38, throughout the three different types of Stroop. HR during Stroop 1 (70.7 ± 3.1 b min−1) was significantly higher (p < 0.01) compared to HR during Stroop 3 (65.4 ± 2.1 b min−1) (Fig. 1a). There were no significant differences in SV and CO throughout the three different types of Stroop (Table 2).
Heart period variability
Results showed that there
Discussion
The present study investigated whether versions of the Stroop task containing variations in task difficulty and verbal responding produced differing patterns of cardiovascular reactivity and perceived task difficulty. The traditional version of the Stroop, containing both high task difficulty and verbal response, produced significantly greater cardiovascular reactivity than either reading monochrome words or performing the Stroop without speaking. In contrast, participants rated the level of
References (41)
Why is naming longer than reading?
Acta Psychologica
(1969)- et al.
Interdimensional interference in the Stroop effect: uncovering the cognitive and neural anatomy of attention
Trends Cognitive Science
(2000) - et al.
The effect of vocalization on the heart rate response to mental arithmetic
Physiology and Behaviour
(1997) - et al.
Components of heart rate reactivity during mental arithmetic with and without speaking
Physiology and Behaviour
(1991) - et al.
Effects of vocalization on cardiovascular and electrodermal responses during mental arithmetic
International Journal of Psychophysiology
(1994) - et al.
Anterior cingulate cortex, conflict monitoring, and levels of processing
Neuroimage
(2001) - et al.
Conflict monitoring versus selection-for-action in anterior cingulate cortex
Nature
(1999) - et al.
Physical versus psychological determinants of heart rate reactivity to mental arithmetic
Psychophysiology
(1988) - et al.
The counting Stroop: an interference task specialized for functional neuroimaging-validation study with functional MRI
Human Brain Mapping
(1998) - et al.
Heart rate and oxygen consumption during active psychological challenge: the effects of level of difficulty
Psychophysiology
(1986)