Keeping a diary is a very traditional way of lifelogging. Some people tend to write down in their diaries all the details of what they saw and did, while others like to note moods and emotions they had during a day. Presently, there are various kinds of lifelogging tools (e.g., [1–3]) that have been developed to assist people with recording their life experiences. However, these tools can only record the surrounding environment of people, which ultimately includes everything that they encounter, but not the internal world, which comprises moods, thoughts and emotions. Therefore, current lifelogging tools do not provide people with a possibility to keep records of their mental life, which is crucial for some people who keep diaries [4, 5].
To offer capabilities that are superior to diaries, lifelogging applications should try to capture the complete experiences of people including data from both their external and internal worlds. Since mental experiences of people are too broad and complex to start with, it is reasonable to focus on the individual components of one’s internal world. We propose to first consider emotions because they are undoubtedly one of the most important components of mental life. Moreover, emotions can be recognized from physiological signals of a human body [6–9]. This latter fact is important for the development of an automatic lifelogging tool. It should be noted that questions about emotions are fundamental in psychology and play an important role in understanding mind and behavior . Recently, an interesting idea about this role was presented: emotion is a medium of communication between the unconscious and the conscious in the human mind . According to this idea, emotion is seen as the conscious perception of the complex mapping processes from the unconscious space into the low-dimensional space of the conscious.
Recent studies have proved that it is possible to recognize emotion based on physiological signals. This research direction looks rather promising  and the primary focus there is on the conscious emotions, which people are aware of and can report. However, Berridge and Winkielman  argue that emotion can be unconscious as well. According to Kihlstrom [14, pp. 432], “explicit emotion’ refers to the person’s conscious awareness of an emotion, feeling, or mood state; ‘implicit emotion’, by contrast, refers to changes in experience thought or action that are attributable to one’s emotional state, independent of his or her conscious awareness of that state”. For this reason, we believe that lifelogging tools should take into account both conscious and unconscious emotions. Unconscious emotions might even be of higher interest and importance than conscious emotions for some users of lifelogging tools because they are hidden from their conscious awareness.
To the best of our knowledge, recognition of unconscious emotions has not been studied yet, and, thus, it is necessary to investigate if physiological signals can be used for this purpose. In our study, we focused on the signal of heart rate, which has been proved to be one of the physiological signals that are related to emotional states [15–18]. Importantly for development of lifelogging tools, heart rate can be unobtrusively measured with wearable sensors.
In emotion recognition studies various sets of stimuli are utilized to elicit emotions in participants. For this purpose, specialized databases of stimuli have been developed and validated [19, 20]. However, in case of unconscious emotions such sets of stimuli have not been clearly identified yet. Therefore, in our study, we had to introduce archetypal stimuli  as a new kind of stimuli that are applied to evoke unconscious emotions.
Another difference of our study from the large part of the previous work in this direction is that we targeted five different emotional states, while other studies tended to focus on a fewer number of emotions .
Based on the aforesaid, an experiment was set up in a laboratory setting for elicitation of emotions (both conscious and unconscious) with visual and auditory stimuli and for measurement of heart rate changes of participants in response to presentation of the stimuli. Conscious emotions were included in the experiment for control purposes. This experiment should (1) clarify if different types of emotional stimuli evoke diverse cardiovascular responses and (2) if unconscious emotions can be recognized from heart rate.
Visual and auditory affective stimuli
To elicit emotional feelings under laboratory conditions, it is common to use visual and auditory stimuli. Based on the previous work in this field, there are publicly available databases with affective pictures and sounds that cover the most popular emotions and have been successfully tested [19, 20, 23]. Unfortunately, there are no databases that contain stimuli capable of eliciting unconscious emotional experiences. In view of the aforementioned, for the experiment, it is necessary to pick appropriate content from one of the available databases for regular emotion elicitation and to choose stimuli that are capable of evoking unconscious emotion.
Affective pictures and sounds
In the field of emotion research, International Affective Picture System (IAPS)  and the International Affective Digital Sound System (IADS)  are widely used to investigate the correlation between self-reported feelings of subjects and the stimuli that are exposed to them. In our study, IAPS and IADS were selected as the sources of experimental material due to the facts that these databases consistently cover the emotional affective space, have relatively complete content including pictures and sound clips and provide detailed instructions about usage of the databases.
Now, we have to identify the remaining stimuli for unconscious emotion.
Archetypal pictures and sounds
Jung  postulated the concept of collective unconsciousness, arguing that in contrast to the personal psyche, the unconsciousness has some contents and modes of behavior that are identical in all individuals. This means that the collective unconsciousness is identical in all human beings and, thus, constitutes a common psychic substrate of a universal nature which is present in every human being. Jung further posited that the collective unconsciousness contains archetypes: ancient motifs and predispositions to patterns of behavior that manifest symbolically as archetypal images in dreams, art or other cultural forms . According to Jung’s personal confrontation with the unconsciousness, he tried to translate the emotions into images, or rather to find the images that were concealed in the emotions . According to the record of Jung’s patients, archetypal symbols are essential for representation of one’s emotions at an unconscious level. Jung further argued that mandala (Fig. 1b, c), a circular art form, is an archetypal symbol representing the self and wholeness [24, 26]. The fundamental and more generic form of mandala consists of a circle with a dot at its center (Fig. 1a). This pattern can also be found in different cultural symbols, such as the Celtic cross, the aureole and rose windows.
Since Jung’s argument, mandala drawings have been applied for practical use in the art and psychotherapeutic fields as basic tools for self-awareness, self-expression, conflict resolution and healing [29–33]. Recent studies have discovered that mandala could be a promising tool for non-verbal emotional communication [32, 34–37]. For patients with post-traumatic stress disorder (PTSD), therapists can diagnose patients’ emotional statuses through the mandalas drawn by them, while these patients are not willing or not able to discuss sensitive information regarding childhood abuse . Furthermore, in another case concerning breast cancer patients, mandala drawings, as a non-invasive assessment tool, allowed the physician to extract valuable information that may have been otherwise blocked by conscious processes . The above studies have shown the potential of mandala to be a promising tool to convey unconscious emotions.
Based on the work of Jung, the Archive for Research in Archetypal Symbolism (ARAS) was established . ARAS is a pictorial and written archive of mythological, ritualistic and symbolic pictures from all over the world and from all epochs of human history . Therefore, we assumed that the archetypal content of ARAS might enable us to elicit unconscious emotion and included the archetypal symbols in our experiment.
About archetypal sounds, very little information is available. We found out that ‘Om’ and Solfeggio frequencies are considered to be archetypal sounds [39, 40]. ‘Om’ or ‘Aum’ represents a sacred syllable in Indian religions . “Om” is the reflection of the absolute reality without beginning or end and embracing all that exists . Next, Solfeggio frequencies are a set of six tones that were used long ago in Gregorian chants and Indian Sanskrit chants. These chants had special tones that were believed to impart spiritual blessings during religious ceremonies . Solfeggio frequencies represent the common fundamental sound that is both used in Western Christianity and eastern Indian religions; therefore, we considered them as archetypal sounds.
In addition to the archetypal symbols, we also included the archetypal sounds in our experiment to take into account the effect of auditory stimuli. Our expectations are that unlike the content of IAPS and IADS, the visual and auditory archetypal stimuli might evoke unconscious emotions.