Introduction
Humans, despite being a part of animal family do not behave similar to most of the species. The way to produce a movement is longer and more complex, simply because we undertake a lot of action consciously, or at least we think so. Although we do not know how many of the adjustments and decisions, we make in order to survive, are conscious, it is the complex thinking process that distinguishes us clearly from the rest of the species. In some situations that demand high-alertness we can still see the primitive, almost animal-like side of ourselves. We can perceive that clearly through functions that physiologically manifest our emotional expression, such as mobilizing muscles and glands and having our sensory systems on alert (Lang and Bradley, 2010). These physiological responses can tell us a lot about the experienced emotions. Each emotion is “goal-oriented”, hence we see the classic “fight or flight” sympathetic and “rest and digest” parasympathetic autonomic responses, which prepare the body to undertake a particular action (Stemmler, 2004). However, we learned how to regulate our own emotions by thinking in specific ways, to feel safe and secure.
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In order to investigate emotion-regulation and learning in high-alertness situations, the emotion has to be induced in the subject. This can be done through various means, one of which is conditioning. Pavlovian fear conditioning is a model system to study the neural mechanisms of associative learning and memory, which has been widely used for the past few decades. This type of learning uses a conditioned stimulus (CS) and pairs it with an irritating, unconditional stimulus (UCS). The outcome of this form of learning is typically a behavioural conditional response which demonstrated itself by eliciting reaction to the CS presented alone. Many evidences in non-human studies confirm that animals are able to learn the association between the CS and UCS. (Davies, 2000; LeDoux,2000; Phelps and LeDoux, 2005).
The process of “unlearning” the conditioned association human learned to self-regulate the unwanted emotions; to suppress a previously learned fear. Fear extinction thus, refers to decrement in conditioned fear responses that we observe while a person is presented with repetition of a conditioned fear stimulus (Milad and Quirk, 2012).
Otherwise, it is as a gradual process of erasing the initial association between the conditioned stimulus and unconditioned stimulus by showing the conditioned stimulus repeatedly on its own (Pavlov, 1927).These days, however, scientists are sceptical on whether the extinction erases the initial association or rather forms a completely new association that inhibits the expression of the conditioned memory.
While many researchers interested in fear extinction see the advantage of using the electric shock as the UCS paired with CS there is a good amount of studies providing the evidence that an auditory stimulus can also be a powerful unconditioned stimulus (Lang and Bradley, 2000; Baumgartner et al., 2006b).
Baumgartner et al. (2006b) showed increased activity in emotion processing brain structures when visual, emotional stimuli were combined with congruent musical excerpts compared to visual stimuli alone. The subjective and psychological variables between modalities were compared to find out that the level of involvement was higher for music than for pictures. However, the subjectively experienced emotion overlapped better with the intended emotion for pictures than for music. In terms of modality, the sound and images are very different. What is also different is that the sound lacks the clear meaning, which images convey almost immediately. In 2000, Bradley and Lang developed IADS, a database containing auditory stimuli rated for arousal, valence, and dominance. The relatively short musical excerpts carry clear inherent meaning (e.g., scream, the sound of a cheering crowd, or a gun shot). The sounds from IADS, as it was found by Bradley and Lang (2000) themselves, produced similar reactions to visual stimuli from IAPS.
In the current study, we aim to induce an emotional state of fear by presenting neutral visual stimuli (CS, yellow or blue square) paired with the auditory stimuli (UCS, scream) eliciting specific arousal levels in blocks. in a laboratory setting, we examine heart rate variability and skin conductance using the information about the onset of the stimulus.
The advantage of a quiet booth over the loud fMRI when taking simple physiological measures are multiple. First of all, the booth represents a more natural environment resembling a simplistic version a smaller size room. Participants are in a seated position, which is also a natural position for them to be in. These two factors draw out the preliminary stress associated with the unusual, unnatural setting of studies done in the scanner.
Second of all, the quality of the EDA and EEG often contain heavy noises obtained due to magnetic force which disrupts the functioning of the measurement devices. Thus, measuring these indexes of physiological activity outside the scanner almost guarantees an output free of magnetic force contamination.
To summarize, in the current study, we use blocks of bimodal stimuli to induce certain arousal levels. We determine effects of the paired stimulus valence and arousal, as well as their interaction, on pulse-derived interbeat interval and skin conductance. The following section explains the principles behind the dependent variables in the current study (pulse-derived IBI and skin conductance) in a form of a short overview. Specifically, it shows how these variables can be affected by emotional stimuli in valence- and arousal-related studies.
CARDIOVASCULAR MEASURES
Activation and suppression of sympathetic and parasympathetic system can both affect the heart rate and its variability, which can be divided into three frequency bands. Three main sources are reflected through these bands (Veltman and Gaillard, 1998): slow changes (0.02-0.06 Hz), mid-range changes (0.07-0.14 Hz) and fast changes (0.15-0.50 Hz). There are specific processes that cause each of those changes. Temperature regulation cause slow changes; resonance in the veins caused by the blood pressure regulation is related to mid-range changes; and breathing reflects the fast changes.
All three bands reflect the effects of parasympathetic system, but only rhe slow and mid frequency bands show the effects of sympathetic system (Berger et al., 1989). Heart rate adapts to the blood pressure when in resting condition. However, some particular circumstances, such as mental workload during a difficult task, can lessen this adaption, which can be reflected through decreasing heart rate variability (Aasman et al., 1987).
Heart rate measures can be affected by the sympathetic as well as parasympathetic system and other physiological processes. Heart rate acceleration was positively correlated with recall of both pleasant and unpleasant memories (Rainville et al., 2006). This suggests that arousal influences heart rate.
Heart rate deceleration was also found to be greater for high arousal unpleasant sounds in comparison with low arousal unpleasant sounds (Bradley and Lang, 2000).
In the recent paper by Chandola et al. (2010) reviewing studies that examined the heart rate variability and work stress association, work stress was associated with lower heart rate variability.
A recent review on studies that examined the association of heart rate variability and work stress concluded that reported work stress is associated with lower heart rate variability (Chandola et al., 2010). Studies on heart rate variability and emotions are mostly dealing with fear or anxiety (George et al., 1989; Friedman and Thayer, 1998; Rao and Yeregani, 2001) where heart rate variability decreases with increased levels of fear. In a study where participants relived emotions, Rainville et al. (2006) found that besides fear, also sadness and happiness decreased high frequency heart rate variability. In contrast to these studies
that suggest a negative relation between heart rate variability and arousal, studies in which emotional visual stimuli were used, report increased heart rate variability for erotic images (Ritz et al., 2005) as well as for aversive visual stimuli (Sokhadze, 2007). Whereas studies on mental workload focus their analyses on midfrequency heart rate variability (reflecting both sympathetic and parasympathetic control), studies on emotions focus on the high frequency band (only parasympathetic).
SKIN CONDUCTANCE
Electrical skin conductance varies with the moisture level of the skin. Since the sweat glands are controlled by the sympathetic part of the autonomous nervous system (Roth, 1983), skin conductance measures can be taken to indicate arousal. Indeed, a large number of studies found an increase in skin conductance with arousal (independent of valence) (Tucker and Williamson, 1984; Winton et al., 1984; Greenwald et al., 1989; Bradley et al., 1990;
Tremayne and Barry, 1990, 2001; Cook et al., 1991; Boucsein, 1992, 1999; Barry and Sokolov, 1993; Khalfa et al., 2002). As Table 1 in Chanel et al. (2009) indicates, skin conductance measures are perhaps the most popular physiological signal in studies trying to classify emotional states on the basis of (neuro)physiological signals. Arousal seems more closely associated with increases in skin conductance than heart rate (Barry and Sokolov, 1993; Croft et al., 2004; Wilkes et al., 2010). Skin conductance responses vary with rated arousal in emotional/neutral picture viewing tasks (Lang et al., 1993, 1998; Greenwald et al., 1989).
AIM AND HYPOTHESIS
We here test whether within a single group of observers rather than different groups, unpleasant sound, and neutral pictures combined will have an effect on physiological responses. Specifically, we predict for physiological responses to increase during the unpleasant sound. Moreover, we investigate whether elicited emotions (ratings) and their physiological correlates (skin conductance, pulse-derived IBI) when only the visual stimulus is present (blue or yellow square).
We predict heart deceleration and increase in skin conductance response to paired stimulus (visual (blue or yellow square)+ audio (unpleasant sound)) in the learning phase. Our second prediction states that this association will be erased during the extinction phase when the stimulus (visual (blue or yellow square)) will be repetitively shown alone.
