1. Introduction
Any comprehensive model of wellness should account for a complex mix of cognitive, affective, behavioral, and physiological factors that contribute to individual differences in health and disease [
1]. These individual differences related to blood and pulse pressures are often related to autonomic balance and may influence the cognitive performance [
1]. In fact, there are some scientific evidences that highlighted the relationship between vagally mediated heart-rate variability (HRV) and good performance in cognitive tasks that require the use of mental functions [
1,
2]. HRV is a widely used measure because of its convenience and noninvasive features, and because associated to an activation of the SNS, with a decrease of its high frequency components (and a relevant increase of low frequency ones) when the sympathetic nervous system (SFS) is activated [
3,
4].
The heart rate variability was also found associated to the performance at the Stroop task [
5] that is a task frequently used to represent the ability to manage interfering information at cognitive level [
6]. In the Stroop task the subject is asked to verbally read a word, that is the name of a colour, that could be written in the same colour semantically represented by that word (congruent condition) or in another colour (incongruent condition). It is a widely used cognitive test assessing the ability to regulate thoughts and actions in accordance with internally maintained behavioral goals, by the activation of a cognitive control [
7].
In the last decades, there has been an increasing diffusion of wearable devices allowing to measure cardiac functions, electrodermal activity, skin temperature and electromyographic activity and also kinematic parameters of trunk movements thanks to embedded inertial sensors [
8,
9]. More recently, researchers tried to use these wearable devices to investigate the complex relationships between cardiac functions, cognitive aspects and control of movements [
10]. In fact, the control of movements requires the management of sensorial feedback together with an internal body representation and often with superior cognitive processes. For example, it has been observed that the higher is the speed at which subjects walk, the higher is the coupling with cardiac rhythm and also with cognitive performances [
10].
According to the idea of a comprehensive model, the relationship between physiological factors related to the autonomic system and cognitive performance should also take into account motor and behavioral aspects. But despite this growing body of literature, there were not protocols for testing if in the coupling between HRV and Stroop task performance also the motor control can play a role. Conversely, there are many direct and indirect pathways linking the frontal cortex to autonomic motor circuits responsible for both the sympatho-excitatory and parasympatho-inhibitory effects on the heart [
1]. On the other hand, a wide range of inhibitory processes across cognitive, motor, and affective tasks are related to the same brain region: the right prefrontal cortex [
1]. It may explain the reason for which the motor awareness can be reduced in presence of a cognitive load [
11].
Most of the studies investigate the unidirectional effects of cognitive load on heart rate variability [
5,
12] but not how the heart rate variability, and also the motor behavior, may affect the cognitive performance. To study the complex system including cognitive, motor and cardiac functions is fundamental to identify the basic components transversally involved in the three systems.
The aim of this study is to propose a simple protocol to identify the principal components of this complex bidirectional system integrating heart, motor control and cognitive functions. According to the literature this protocol is based on the analysis of HRV and its relationship to the results of Stroop task, adding the analysis of trunk rotations, performed using a wearable inertial unit containing a triaxial accelerometer, a triaxial gyroscope and a magnetometer for the measure of the range of motion [
8,
9]. From a bioengineering point of view a device including inertial sensors for analyzing trunk movements and electrodes recording the cardiac signal for computing the heart rate variability has been proposed. Then, we tested if this sensorized protocol can be sensitive to assess the changes induced by a specific intervention focused on putting attention to the perception-action link that is at the basis of the embodied cognition [
13]. Because some previous studies showed gender differences both in trunk accelerations [
14] and in the performance at the Stroop task [
15], in this first study about this topic, we focused our analysis only on women. Then, because recently a new frontier of data analysis was based on the use of artificial neural network [
16], also in the analysis of heart rate variability [
17,
18], we used an ANN to verify the relationship between principal components and the cognitive outcome.
4. Discussion
A sensorized quantitative assessment of heart rate variability has often been put in relationship to cognitive performances, in particular with that at the Stroop task (in athletes [
22], people with post traumatic stress syndrome [
23], and in healthy subjects managing the daily stress [
24]) and to motor control (also in terms of range of motion and interoceptive accuracy [
25]), but a protocol considering these three domains together was still lacking. For this reason, the first aim of this study was to define a simple protocol to identify the components aggregating heart rate variability, trunk motion variability and the cognition parameters obtained with the Stroop task.
The protocol proposed in our study used a sensorized band worn at trunk level and measuring the temporal distance between R waves and the trunk left and right rotations using an embedded inertial device. This protocol involved a PCA that correctly identified three components related to the above domains: cardio, cognition, motion. As confirmatory analysis we used an approach based on the assessment of weights in the prediction of changes in cognitive performance using an artificial neural network, already used in literature [
16].
According to the previous literature [
26], the HRV-LF was found associated not only to cardiac parameters but also to cognitive ones [
5] and, as we suggested, also to kinematic parameters. In healthy subjects, to perform a symmetric wide rotation not only depends on the kinematic functions but also to the perception of the trunk mid-line [
21], to the afferent feedback and to the perception-action link [
13].
The second aim was to verify if and how it was possible to measure the alteration of these relationships after a specific intervention focused on the embodiment of sensations and cognition. After this intervention, only two components were identified, one related to high and very low frequencies of heart rate variability, and another one putting together HRV-LF, the parameters related to the Stroop task performance and those related to the execution of symmetric wide trunk rotations.
This result is not surprising. After many years in which cognition and motor control were separately investigated, with a sharp distinction between mind and body, many authors started to criticize this view. Damasio talked about Descartes’ error of dividing mind and body [
27], Clark claimed at the need of putting brain, body and world together again in neuroscience [
28], and Berthoz defined the brain’s sense of movement [
29].
More recently, the 4E theory of cognition proposed that the body is a constituent of the mind, and cognition is strictly related to physiological parameters and performed motor actions [
30]. The theory of embodied cognition and, furtherly, the “4E” approach suggested that the cognition does not occur solely in the brain, but is also embodied, embedded, enacted, or extended by way of body structures, functions and processes [
30].
According to these recent theories, a recent research reported that the cognitive performance of executive function tasks that evoke attentional control partially depend on the responsiveness of autonomic control parameters that could be assessed by heart rate variability [
31].
The results of our PCA were furtherly confirmed by those of the analysis conducted using artificial neural network to assess the weight of the changes in HRV and trunk movements on the changes in the Stroop Task performance. We have used an artificial intelligence tool already used in some different studies [
16,
32,
33]. The input associated to a higher relative importance were the changes in trunk range of motion, trunk movement symmetry and those in HRV-LF. These results confirmed those found by PCA, and were able to provide a high predictability of the observed outcome (as shown in
Figure 3) Furthermore, artificial neural network is an emerging approach for identify more complex relationship among variables, especially when those are not simply linear as in this case. For these purposes, the artificial neural network analyses seemed particularly adapted for investigating the parameters extracted by inertial magnetic unit such as that used in this study, as previously done in clinical contexts [
32].
De Bartolo and colleagues claimed the need of evaluation methods for a quantitative assessment of cardiac, cognitive and motor interactions that could be helpful in physiological research, sport training of athletes and, more specifically, for rehabilitation purposes [
10]. In fact, neurorehabilitation may benefit of an integrated approach not aiming at the separate recovery of specific single functions, but curing and caring the patient as a whole person [
34]. Even more, the investigation of this connection and of the brain’s sense of movement are particularly interesting in developmental age [
35] when the trunk mental representation could be altered affecting the symmetry of posture and movements [
36].
The use of a multimodal assessment based on principal component analysis on heart rate variability and brain data is not new [
37,
38], but in this study we proposed a combined approach to quantify the level of overlapping of cognitive, motor and cardiac functions. We could define this approach as “embodimetrics” because focused on the assessment of embodiment, as well as psychometrics is the discipline concerning the quantitative measurement of psychological aspects [
39].
The results of this study should be read at the light of its limits: only female gender was involved in this study, the adherence to the intervention was verbally reported by participants but not quantitatively assessed, the environment was not taken into account despite it was seen it may act as a selective tuning between different strategies in cognition and motor coupling [
40]. Further researchers on this topic should be conducted on the male gender and including other tests assessing cognitive functions. Finally, as previously reported the artificial neural network analysis, despite provide good results in term of accuracy, often lacks of a high reliability [
33].
In conclusion, our study using the combined assessment of HRV and trunk mobility identified the level of connection with the cognitive aspects measured by Stroop task, providing a useful approach for measuring how much strong the relationship between cognitive functions, autonomic cardiac functions and body movements is. Then, according to the emerging literature about the embodied cognition [
13] and the theory of 4E [
30] we have suggested the definition of a new term for defining the field of research within psychometrics concerning the techniques and properties of objective measurements, assessments and analyses related to the latent construct of the embodied cognition and with all the aspect related to the 4E theory: Embodimetrics.
Figure 1.
Percentages of Heart Rate Variability (HRV) in low frequencies (LF, x-axis) and high frequencies (HF, y-axis), pre (black dots) and post (grey dots) intervention, with the relevant regression lines.
Figure 1.
Percentages of Heart Rate Variability (HRV) in low frequencies (LF, x-axis) and high frequencies (HF, y-axis), pre (black dots) and post (grey dots) intervention, with the relevant regression lines.
Figure 2.
Architecture of ARIANNA (Artificial Intelligence Assistance for Neural Network Analysis). Δ: change in the parameters post vs. pre, HRV: Heart Rate Variabilit, VLF: very low frequency, LF: low frequency, HF: high frequency, ROM: Range of motion of the trunk, SI: Simmetry Index, NTCT: Normalized time to complete the Stroop task.
Figure 2.
Architecture of ARIANNA (Artificial Intelligence Assistance for Neural Network Analysis). Δ: change in the parameters post vs. pre, HRV: Heart Rate Variabilit, VLF: very low frequency, LF: low frequency, HF: high frequency, ROM: Range of motion of the trunk, SI: Simmetry Index, NTCT: Normalized time to complete the Stroop task.
Figure 3.
Predictions of the change in the performance (Δ) at the Stroop Task on the basis of the parameters reported in
Table 4. NTCT: Normalized time to complete the task.
Figure 3.
Predictions of the change in the performance (Δ) at the Stroop Task on the basis of the parameters reported in
Table 4. NTCT: Normalized time to complete the task.
Table 1.
Mean ± standard deviation of the measured variables pre- and post- intervention with the p-value obtained by Wilcoxon rank test (in bold if p<0.05). HRV: heart rate variability, VLF: very low frequency, LF: low frequency, HF: high frequency, ACC: Stroop task accuracy, NTCT: normalized time to complete the Stroop task, ROM: range of motion, SI: symmetry index.
Table 1.
Mean ± standard deviation of the measured variables pre- and post- intervention with the p-value obtained by Wilcoxon rank test (in bold if p<0.05). HRV: heart rate variability, VLF: very low frequency, LF: low frequency, HF: high frequency, ACC: Stroop task accuracy, NTCT: normalized time to complete the Stroop task, ROM: range of motion, SI: symmetry index.
Variable |
Pre |
Post |
p-value |
HRV-VLF (%) |
26.3±10.2 |
29.2±10.7 |
0.096 |
HRV-LF (%) |
33.1±7.5 |
34.2±7.9 |
0.522 |
HRV-HF (%) |
40.8±11.1 |
36.2±10.5 |
0.002 |
HF/LF |
1.3±0.6 |
1.0±0.5 |
0.001 |
Stroop task NTCT (s) |
18.8±5.2 |
16.4±3.9 |
<0.001 |
Stroop task ACC |
96.6±5.2 |
98.9±2.7 |
<0.001 |
Trunk ROM (deg) |
110.0±21.5 |
127.0±28.0 |
<0.001 |
Trunk SI (%) |
87.4±9.1 |
87.9±10.0 |
0.743 |
Table 2.
Weights of the variables assessed at baseline (pre-intervention) on the components obtained with a principal component analysis (in bold if their absolute value is >0.25).
Table 2.
Weights of the variables assessed at baseline (pre-intervention) on the components obtained with a principal component analysis (in bold if their absolute value is >0.25).
Variable |
Component 1 |
Component 2 |
Component 3 |
HRV-VLF |
-0.20 |
0.84 |
-0.19 |
HRV-LF |
0.34 |
0.36 |
0.43 |
HRV-HF |
-0.03 |
-0.99 |
-0.06 |
Stroop task NTCT |
0.87 |
-0.11 |
-0.07 |
Stroop task ACC |
-0.83 |
0.04 |
-0.02 |
Trunk ROM |
0.12 |
-0.09 |
0.72 |
Trunk SI |
-0.13 |
-0.02 |
0.77 |
Table 3.
Weights of the variables assessed post-intervention on the components obtained with a principal component analysis (in bold if their absolute value is >0.25).
Table 3.
Weights of the variables assessed post-intervention on the components obtained with a principal component analysis (in bold if their absolute value is >0.25).
Variable |
Component 1 |
Component 2 |
HRV-VLF |
-0.24 |
0.92 |
HRV-LF |
0.62 |
-0.01 |
HRV-HF |
-0.19 |
-0.94 |
Stroop task NTCT |
0.80 |
0.03 |
Stroop task ACC |
-0.53 |
0.05 |
Trunk ROM |
-0.27 |
0.14 |
Trunk SI |
0.35 |
0.04 |
Table 4.
Results of Artificial Neural Network Analysis in predicting the Stroop Task normalized time to complete the task and accuracy. Δ: change in the parameters post vs. pre, HRV: Heart Rate Variabilit, VLF: very low frequency, LF: low frequency, HF: high frequency, ROM: Range of motion of the trunk, SI: Simmetry Index, NTCT: Normalized time to complete the Stroop task.
Table 4.
Results of Artificial Neural Network Analysis in predicting the Stroop Task normalized time to complete the task and accuracy. Δ: change in the parameters post vs. pre, HRV: Heart Rate Variabilit, VLF: very low frequency, LF: low frequency, HF: high frequency, ROM: Range of motion of the trunk, SI: Simmetry Index, NTCT: Normalized time to complete the Stroop task.
Input layer parameters |
Importance of the input layer in the output prediction |
Raw Weight |
Relative |
Normalized |
ΔHRV-VLF |
0.191 |
19.1% |
81.9% |
ΔHRV-LF |
0.214 |
21.4% |
91.8% |
ΔHRV-HF |
0.166 |
16.6% |
71.4% |
ΔTrunk ROM |
0.233 |
23.3% |
100% |
ΔTrunk SI |
0.196 |
19.6% |
84.4% |