Expressive EMG activity to stim 1: Sigur Ros’s VaruoPosted: November 8, 2013
One clue as to how we feel while listening to music is in how we move, from dance-like motions to brows furrowed in concentration. Surface electromyography sensors (sEMG) are very useful in picking up muscle contractions which generate these expressive behaviours. In the solo response project, two typical sensors on the face to capture brow furrowing and smiling (see the set-up post) and an additional sensor tracking contractions of the trapezius muscle up the back of my neck, to capture tension in the back and head nodding. While there is much to say about each signal, this post is about what these sensors measured of my responses to Varuo, by Sigur Ros.
In the analysis post, I mentioned that the B section, starting around 220s, was really intense, and often overwhelming. These sensors agree with that, all shwing more instances of high muscle activity across sessions from 250s to 350s. Sublime face, yawning, and an explanation of how to read this graph below the cut.
The above figure is composed of two types of plots summarising sEMG sensors data. The first of the pairs is a heat map in the same vein of the ratings data: it is a heat map of the sensor activity over the course of the stimulus (x axis) and across sessions (y axis). The first plot has many horizontal stripes of red, indicating that in some listenings, my eyebrows were lightly furrowed for sustained periods. Just after 150s, there are some black spots, indicating stronger contractions of the corrugator supercilius around the same moment, but only in a few listenings. White horizontal lines follow every fifth session to identify individual listenings, though magnification may sometimes be necessary to see these marks.
The second plots of the pairs reports activity-levels over successive thresholds. The top of the yellow band reports the proportion of sessions showing more than a half mean of muscle activity in each half second time frame. The proportion above average is the orange interval and below, and so on, with the proportion of sessions showing activity above 3 means/standard deviation reported in black. Besides being colourful, this plot makes it possible to recognize similar types of response events (contraction of a relevant muscle/muscle region) while keeping track of different intensities. So at 280s, my brow made a sharp contraction in nearly half the sessions, but less dramatic contractions also occured at the same spot in several more instances.
Both of these types of graphs are descriptive, without claims to the significance or robustness of the behaviours measured. The following interpretation of the muscle activity recorded is informed by the patterns of results measured to other stimuli.
The first pair of plots show the corrugator activity. The corrugator supercilius has often been associated with negative valence responses, such as dislike, but the prominence of high activity to this piece of music challenges that link. Consider the interval around 100s: this is the first instance of the singer’s long warning calls. This moment usually evoked positive feelings and the sessions of high corrugator activity coincide with some of these. Most sessions seem to show increased activity with the onset of this call, according to the second plot. More on that under sublime face.
In contrast the corrugator, my smiling muscle stays pretty calm for the beginning half of the piece. The first and second set of calls show zygomaticus activity in some listenings, in agreement with me reports of finding these pleasurable. The big ramp up in the B section is very activity, except for a few sessions (15 – 20). Zygomaticus activity is associated with positive valenced responses, but my reported emotions were not particularly positive through this section.
While there is a clear tactus through this music, its not super groovy. The session wise heat map for the trapezius (plot 5) whos a bit more red after 125, the second time through A so likely this added line encouraged more head motion. The intensity of the B section also shows up here, with both more sessions showing activity and more intensity of muscle contractions between 250 and 350. Two particular listenings stand out here: 11 and 19 both have a lot of black though the B period. The notes from these listenings mark exceptionally intense responses here, including many successive chills. Session 21 did not show such high sustained activity in the trapezius (under the last white line), but it was the listening which left me nauseous with too much “vibrating”.
While the corrugator and zygolaticus have often been used as indications of opposite valenced responses, these listenings have many instances of both being active at the same time. Here, I think they might be capturing my “sublime face” (I’m open to suggests for a better name): when I find a sound particularly lovely, something delicately beautiful (like Jonsi falsetto), I tend tense my forehead, squint a little, smile to some degree, and lift my face (contracting the trapezius). All three sEMG sensors show interesting increases in activity at 100 (second of first pair of calls) and 175 s and 190s (the second pair of calls), and that particular beatific expression may be responsible. I can’t say how common or idiosyncratic this expression is, but I expect the feeling it communicates is typical for those who enjoy Sigur Ros.
It is likely some instances of concurrent corrugator and zygomaticus activation (and potentially trapezius too) are due to less musically relevant causes. According to the session notes, I often sneezed and yawned during this piece. The last listening shows many moments of high muscle activation in both facial sensors, and these are likely almost all yawns. The string of late session high activity after 310s may also be yawns, though yawning at that point in the music felt like a bodily intervention, a deep breath to move me back to a calmer state, than an expression of fatigue.
We express feelings through our faces selectively, though that selection is not always a conscious choice. Facial expressions and head motions are specifically communicative, but in this piece, a particularly introspective and yet deliberately overwhelming style of music which is also, inhibitions on expression are dissolved with attentive listening. I can’t imagine measuring the same responses to this music were I not alone or in an explicitly permissive environment like a dark and loud performance venue with the rest of the audience similarly inwardly oriented. Expressive behaviours, even without anyone to notice, felt necessary for the development of such strong responses: a funny paradox of the modern musical experience.