The challenge of measuring emotions
Action-reaction, but how to measure it if it is an emotion?
The central nervous system plays a fundamental role in detecting and understanding emotions, cognitive processes and a series of other psychosocial constructs such as trust. The responses of the nervous system are relatively specific and show different patterns of activation depending on the situations and the emotional state. Any psychological process entails an emotional experience of greater or lesser intensity and of different quality, which is why the emotional reaction is something omnipresent in every psychological process. Likewise, all emotion involves a multidimensional experience with at least three response systems: cognitive (subjective), behavioural (expressive) and physiological (adaptive). Analysing, for example, the different dimensions of anger, it is possible to distinguish between the cognitive process (feeling angry), the behavioural (frowning) and the physiological (heart rate variation, among others).
Based on this premise, there are a whole series of psychophysiological signals that have been used to identify the various emotional processes:
- Electrical activity of the brain: By means of an electroencephalogram (EEG) it is possible to capture the cortical activity of the brain using electrodes in contact with the scalp. These types of signals offer a lot of information, although their analysis can be very complex since each component (electrode) contains information of a temporal nature (its amplitude), modal (frequency of the wave) and topographic (location in the brain). Although traditionally the acquisition of this signal required precise and specialized instruments, today the necessary equipment is much more accessible.
- Cerebral blood flow: Functional magnetic resonance imaging (fMRI) is a non-invasive technique that allows the measurement of brain activity. This technique is based on detecting areas of the brain with a higher concentration of blood flow, under the premise that these specific areas have greater activity compared to others with a lower flow. Although it offers great advantages at the research level, it requires bulky equipment and very high cost.
- Variation in heart rate: It is one of the main physiological signals linked to the feeling of security or danger. At the level of analysis, the period between beats is usually considered, distinguishing between low frequency pulsations if this period is below a threshold (classically 0.15 Hz) or high frequency if said threshold is exceeded. The main techniques to obtain this metric are the electrocardiogram (ECG), which records the activity of the heart, and photoplethysmography (PPG), which uses a controlled light beam to calculate blood flow based on the amount of reflected light.
- Skin galvanic response (GSR): Also called electrodermal activity, electrodermal activity or EDA. It reflects the electrical conductance of the skin by measuring the potential difference generated between two electrodes, generally located on the phalanges of two continuous fingers of the non-dominant hand. The excitation of the skin, normally produced in stressful situations, causes a dilation of the pores which, in turn, causes a decrease in the electrical resistance of the skin. It is necessary to distinguish its tonic or basal component and its phasic component. The former undergoes slow variations and often reflects unwanted experimental changes (for example, changes in the temperature conditions of the experiment), while the latter reflects rapid changes that are linked to the study stimulus.
- Muscle activity: Muscle activity can be measured using a pair of electrodes aligned with their kinematic axis (direction in which they expand and contract). The application can be used in a variety of situations. For example, electrodes can be positioned on both sides of the eyes (either vertically or horizontally) to detect eye movements. This specific test, called electrooculography (EOG) offers very useful information when cleaning the encephalographic signal of possible unwanted artifacts. Other generic applications include the detection of involuntary movements using electromyograms (EMG) in response to certain stimuli.
- Ocular behaviour: Ocular behaviour offers very valuable information on certain aspects, both physiological (for example, dilation and contraction of the pupil, blinking frequency, etc.) and behavioural (for example, drift and gaze fixation). Depending on whether the experimental process is carried out on a fixed platform (such as a computer) or freely available (participant in movement), this information can be extracted using fixed eye tracking instruments (attached to the visualization system) or mobiles (wearable systems in the form of glasses).
In short, analysing and measuring our nervous system to study or even understand our reactions, our emotions, to certain stimuli or situations is much more than a technological challenge. Designing and correctly using elements or devices that analyse our body and its signals is a challenge for science, for technology and for its possible multisectoral applications where the basis is to know the individual and their possible behaviours.