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Integrating the Numerical Pain Rating Scale (NPRS) with an Eye Tracker: Feasibility and Initial Validation

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Yoram Braw, Motti Ratmansky and Itay Goor-Aryeh

Submitted: 06 May 2023 Reviewed: 17 May 2023 Published: 21 June 2023

DOI: 10.5772/intechopen.111897

Pain Management IntechOpen
Pain Management From Acute to Chronic and Beyond Edited by Theodoros Aslanidis

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Pain Management - From Acute to Chronic and Beyond

Edited by Theodoros Aslanidis and Christos Nouris

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Abstract

This chapter details the integration of a Numerical Rating Scale (NPRSETI) with a portable eye tracker, enabling the assessment of pain in conditions in which verbal communication and use of hands are limited (e.g., advanced Amyotrophic lateral sclerosis, ALS). After detailing the construction of the NPRSETI, we describe its validation in an outpatient pain clinic. More specifically, thirty chronic pain patients performed the NPRSETI and filled a conventional NPRS (order was pseudo-randomized). Eye movements, including gaze direction and additional eye movement measures (e.g., saccade rate), were recorded, while participants rated their pain using the NPRSETI. The study’s findings indicated no significant differences in pain severity ratings of the NPRSETI and conventional NPRS. Notably, ratings using the two scales were highly correlated (r = 0.99). NPRSETI’s ratings were also strongly associated with participants’ currently experienced pain rating using the Brief Pain Inventory (BPI). The findings provide initial proof of concept for integrating conventional pain rating scales with an eye tracker and validate the NPRSETI compared with the well-validated and commonly used NPRS. Enhanced usability and decreasing costs of eye trackers will ease the additional research mandated to validate these preliminary findings and hopefully advance their integration into clinical practice.

Keywords

  • pain
  • evaluation
  • numerical pain rating scale
  • eye movements
  • validation

1. Introduction

An accurate assessment of pain is the basis for the treatment of pain in a systematic manner [1]. However, pain is a subjective experience that is challenging to evaluate in a clinical setting [2, 3]. Unidimensional pain scales were developed as a quick and efficient method to assess a vital aspect of the subjective experience of pain, its intensity [4]. They are well-established and extensively used in the clinical care of wide-ranging patient populations [5, 6, 7] and ages [8, 9]. As noted in a recent review, they are quick to administer and do not encroach on the time required for usual care [7]. Unidimensional pain scales, however, have several limitations. In addition to requiring intact comprehension, they necessitate a graphomotor response or speech production for the patient to input their response. While some physical limitations may be overcome (e.g., using nurse-administered pain scales [10]), others present a more formidable challenge. For example, pain was not consistently evaluated in Amyotrophic Lateral Sclerosis (ALS) studies despite its prevalence [11], partially due to using instruments that were not adapted to ALS-related impairment [12]. Other conditions, such as mechanically ventilated patients [13], also necessitate adapted pain assessment methods.

Eye tracking technology allows us to capture and record an individual’s gaze and other key eye movement measures [14]. It was implemented in studying cognitive functioning and process in diverse academic fields such as industrial engineering, marketing, and psychology [15]. More recently, eye movement analysis was also used to enhance clinical practice and effort that coincides with the growing exploration of biomarkers in various neuropsychiatric disorders [16, 17]. Eye movements offer two advantages for assessing pain. First, eye movement-directed pain scales may allow patients to provide pain ratings without a motor activity (i.e., use of the patient’s hands) or verbal response. In other words, an eye tracker may be used to input their ratings; individuals use their gaze as they would use a computer mouse or other means of responding (e.g., marking with a pencil or providing a verbal response). Challenging medical patients may particularly benefit from such human-computer interactive systems. For example, Ull, Weckwerth [18] assessed the technology’s utility for treating mechanically ventilated patients in an intensive care unit (ICU). These patients were able to learn eye movement-directed responding to indicate their basic needs, respond to rating scales, as well as respond to quality of life and self-esteem questionnaires [18]. Similarly, eye tracking was used to control household electric appliances by ALS patients [19]. Second, eye movements indicate what the examinee is paying attention to while engaging with a task and offering a glimpse into both cognitive and processes [20, 21]. The latter are sensitive to aspects of cognitive processing (e.g., experienced cognitive load), as well as affective states such as anxiety [22]. Eye movement analysis, therefore, offers to advance clinical practice by enhancing clinicians’ ability to diagnose medical conditions, especially those impacting brain functioning. For example, Tomer, Lupu [23] indicated the utility of integrating an eye tracker with the Word Memory Test (WMT), a well-established test for assessing feigning during cognitive assessments [24]. In addition to enhancing the detection of feigning when combined with conventional accuracy measures, Tomer, Lupu [23] suggested a three-stage cognitive process involved in feigning cognitive impairment. Similarly, we integrated an eye tracker with the MOXO-dCPT, an established continuous performance test (CPT) and found support for the utility of combining conventional and eye movement measures to enhance the diagnosis of ADHD and detection of feigned ADHD-associated cognitive impairment [25, 26, 27]. Overall, eye trackers-integrated medical appliances and similar novel technologies offer to provide objective psychophysiological data that may aid clinical practice, a prospect we hoped to capitalize on in the current project [28, 29, 30].

The current research project aimed to assess the utility and validity of an eye tracker-integrated Numerical Pain Rating Scale (NPRSETI). The conventional NPRS utilizes a paper-and-pencil format with numerals that reflect a range of pain intensities. As thoroughly reviewed earlier, it is one of the most commonly used unidimensional pain intensity scales in clinical settings, a fact that is related to its ease of administration and scoring [2, 4]. It also has higher compliance rates—particularly among older adults—than the Verbal Analog Scale (VAS), a unidimensional pain scale in which patients mark their experienced pain on a straight line with anchors at each edge. Its measurement properties are robust and well established across multiple patient populations, and it has slightly superior measurement properties—i.e., reliability, validity, and responsiveness—compared to other scales. Key figures in the field and relevant guidelines (e.g., Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials; IMMPACT) recommended its use [2, 4]. Importantly, NPRS allows the patient to verbally report their pain rating without using their hands, making it amendable for integration with an eye tracker. These advantages led us to choose the NPRS for integration with an eye tracker.

The following sections will detail the construction of the NPRSETI and its initial validation in an outpatient pain clinic. We compared patients’ ratings using the NPRSETI to those using a conventional paper-and-pencil NPRS. Ratings using the NPRSETI were also compared to an NPRS embedded in the Brief Pain Inventory (BPI), correlated with key demographic and clinical variables, and qualitatively analyzed using a relative gaze duration heatmap and general eye movement measures (i.e., saccade rate and pupil size).

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2. Methods

2.1 Participants

Participants were recruited from the outpatient pain unit of Tel-Hashomer Rehabilitation Hospital (N = 30). Inclusion criteria were: (a) Adult age (18 to 65). (b) Pain persisting > 3 months, based on the definition by the International Association for the Study of Pain (IASP; [31]). (c) Normal or corrected vision (candidates with a severe visual impairment such as nystagmus and hemianopsia were excluded). Exclusion criteria were: (a) Significant developmental, neuro-psychiatric disorders, or other medical conditions deemed to impair cognitive functioning and thereby impact their ability to provide pain ratings or comply with the experimental procedures (e.g., major neurocognitive disorder), according to the treating physician or electronic medical records. (b) Drug or alcohol dependence. (c) Poor comprehension of Hebrew.

The study was approved by the Tel-Hashomer Rehabilitation Hospital’s Institutional Review Board (IRB) committee, with all participants signing a written informed consent form before participating in the study.

2.2 Tools

2.2.1 Construction of the numerical rating scale eye tracker-integrated (NPRSETI)

The NPRSETI was constructed to maintain the critical characteristics of the conventional NPRS corresponded to those NPRS versions presented in the literature (e.g., [32]) with pain ratings that ranged from 0 (“no pain at all”) and 10 (“unbearable pain”). The first version of the NPRSETI included five stages: (a) A welcome screen requesting participant code and date of birth to minimize later errors in participant identification. (b) A screen presenting the NPRSETI and the method of responding to it (i.e., gazing at the number corresponding to the pain rating will select the rating). This screen also included an illustration of the NPRSETI so participants could conceptualize the scale and raise any questions regarding its use. (c) A blank screen with a black circle appearing in its center was presented. The circle followed the gaze of the participant, and the participant was instructed to experiment with moving it around the screen using their gaze. Verbal instruction was: “The marker, a black circle, indicates where you are gazing on the screen. The pain scale we presented before will soon appear again. When it appears, gaze for a few seconds at the number that best fits the pain you are currently experiencing.” (d) Once the participant felt confident using the gaze-directed marker and understood the NPRSETI instructions, a blank screen with a fixation cross at its center appeared. The participant was then instructed to gaze at the fixation point. (e) The actual NPRSETI appeared at the center of the screen, with instructions for its use appearing in its upper section. The participant then provided their response by fixating for 250 ms on a number corresponding to their currently experienced pain intensity. The color of the box surrounding the selected rating then changed to red, indicating response selection. Next, the question “Are you confident with this choice?” and two response boxes (“Yes, I’m sure” and “No, I want to change my response”) appeared at the bottom of the screen. If the participant was satisfied with their choice, a thank you screen appeared (i.e., “Thank you for participating in this study”). Otherwise, the pain rating procedure was repeated until the participant approved of their response. This initial NPRSETI version was tested on six healthy participants leading us to modify the NPRSETI since the program combined the eye movement data of the separate trials when participants altered their response (e.g., the NPRSETI was performed twice). Five other healthy participants were assessed using this newer NPRSETI version. This pretest indicated that participants tended to erroneously select a pain rating, usually “5” because the fixation point was located in the center of the screen (i.e., the location in which the NPRSETI later presents the “5” pain rating). We, therefore, constructed a third and final NPRSETI version in which the minimal fixation duration required for responding was set to 500 ms, thereby eliminating further errors in pain rating selection. See Figure 1 for a diagram portraying the pain measurement process by NPRSETI.

Figure 1.

NPRSETI design. Notes: Eye tracker-integrated Numerical Pain Rating Scale = NPRSETI.

2.2.2 Eye-tracking apparatus and eye movement measures

Binocular eye movements were recorded at a sampling rate of 250 Hz using the Portable Duo eye-tracking system (SR Research Ltd., Mississauga, Canada). The eye-tracking apparatus has an accuracy of approximately 0.5° and includes a host PC and display PC; the first tracks and computes participants’ gaze position, and the latter displays the stimuli (i.e., the NPRSETI). Eye movements were analyzed using SR research event detection algorithm, widely used in academic research [33, 34] and by our research team in earlier studies [23, 25, 26, 27]. Stimuli were presented on an Alienware OptX AW2310, 23″ display screen (1920 x 1080 resolution) with a 120 Hz refresh rate. Eye-to-screen viewing distance was approximately 65 cm with the participants’ eyes recorded in remote monocular mode using a designated sticker. Figure 2 presents the experimental setup.

Figure 2.

The experimental setting. Notes: The actual experiment was performed with the computer located behind the participant and out of their field of view. The photographed person was part of the research team and not an actual participant in the experiment. Eye tracker-integrated Numerical Pain Rating Scale = NPRSETI.

The participants’ field of view (FoV) was divided into three areas of interest (AOIs) to assess the extent that their visual attention was directed toward the pain scale and other relevant stimuli (i.e., instructions for rating pain and anchors) vs. task-irrelevant regions: (a) NPRSETI AOI: The AOI included the screen region in which the pain scale appeared. It was located at the center of the screen (length = 1540 pixels). Each of its squares was 140*140 pixels. (b) Anchors AOI: The AOI includes two regions in which the anchors of the pain scale were located (“no pain at all” and “unbearable pain”). (c) Instructions AOI: The region in which the NPRSETIinstructions were presented. See Figure 3 for the NPRSETI’s AOIs. Based on these AOIs, NPRSETI dwell time (%), anchors dwell time (%), and instructions dwell time (%) were calculated. These measures reflected the percent time spent gazing at each AOI out of the total task duration (i.e., time from the appearance of the scale to the selection of pain rating using the participant’s gaze). Two additional general eye movement measures were calculated: (b) Saccade rate (no.): The number of saccades that participants performed per second while providing the pain rating. Saccades are ballistic motions during which the eyes rapidly move between fixations, periods in which information is being processed, and the eyes remain fairly still [15]. (c) Pupil size (no.): Average number of camera pixels occluded by the pupil while the participant provided the pain rating.

Figure 3.

AOIs of the NPRSETI. Notes: Blue regions delineate AOI in the participants’ FoV as described in the Methods section (dark shade = NPRSETI AOI, medium intensity shade = instructions AOI, light shade = anchors AOI,) areas of interest = AOI; Eye-integrated Numerical Pain Rating Scale = NPRSETI; field of view = FoV.

Paper-and-pencil Numerical Pain Rating Scale (NPRS): The NPRS (length = 10 cm) was presented on an A4 paper. It was identical to the NPRSETI (e.g., pain scale length, instructions) except for the use of a paper-and-pencil format. Participants marked using a pen their currently experienced pain on a scale ranging from 0 (“no pain at all”) and 10 (“unbearable pain”).

Self-report questionnaires: The following self-report questionnaires were used: (a) Brief Pain Inventory (BPI): A commonly used questionnaire that assesses the severity of pain and its impact on daily functioning [35, 36]. It includes a screening question, inquiring about the presence of pain and a body chart that is used to indicate locations of experienced pain. The next six items are NPRS scales that are scored from 0 to 10, with higher scores indicating worse pain or more substantial functional impact. The BPI also includes an item rating the pain relief that the patient experienced by medications or other treatments. (b) Patient Health Questionnaire (PHQ-9): A nine-item self-report questionnaire of depressive symptoms [37, 38]. Each response range was from 0 (“not at all”) to 3 (“nearly every day”). (c) General Anxiety Disorder-7 (GAD-7): A seven-item questionnaire of anxiety [39]. Responses range was 0 (“not at all”) to 3 (“nearly every day”), with the participants rating their anxiety levels during 2 weeks preceding the study. (d) Debriefing survey: A survey assessing participants’ motivation to undergo the experiment. It used a 1–7 Likert scale, with higher scores indicating stronger motivation.

Wechsler Adult Intelligence Scale-III (WAIS-III) digit span task: A short task derived from the WAIS-III which assesses attention and other cognitive functions such as working memory and executive functions [40]. As part of the task, the participant repeats increasingly longer strings of numbers that are read aloud by the examiner, in forward and reverse order. This continues until two consecutive strings of the same length are missed, or the longest sequences are successfully repeated.

2.3 Procedure

All participants were given a general description of the study and signed a written informed consent form. They then filled out a demographic-medical questionnaire, the BPI, PHQ-9, and GAD-7. Next, they reported their currently experienced pain using the NPRSETI and pencil-and-paper NPRS and with their order pseudo-randomized. The participant underwent a nine-point calibration procedure before performing the NPRSETI (five-point calibration was performed when encountering difficulties calibrating the participant). Participants then completed the WAIS-III digit-span task and filled out the debriefing survey. As in earlier studies (e.g., [41]), the participants were not permitted to compare their second ratings with those of the first one as this may influence patients’ ratings.

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3. Results

The participation of five participants was discontinued due to either inability to calibrate the eye tracker (n = 3) or complaints regarding pain and discomfort (n = 2). The final sample included 25 participants, seven of which had learning disabilities or suspected they had such a disability, three were previously diagnosed with attention-deficit/hyperactivity disorder (ADHD), and one participant had a CNS neuropathology that did not impact neuropsychological functioning. Besides comorbid affective depression and anxiety, psychiatric comorbidity included personality disorder (n = 1) and adjustment disorder (n = 1). Nineteen (76%) patients rated their current pain in the BPI (6th item) as of at least moderate severity using a ≥ 4 cut-off that was chosen in accordance with earlier studies [32, 42]. All participants reported adequate motivation to follow the experimental procedures, scoring ≥4 on a 7-point Likert scale in which higher scores indicated stronger motivation (as used in [43]). See Table 1 for demographic and clinical data.

VariableParticipants (n = 25)
Parametric dataMean (SD)
Age (years)46.92 (11.12)
Education (years)12.92 (2.13)
Motivation level (1–7)6.68 (0.80)
PHQ total score (no.)15.0 (7.29)
GAD score (no.)10.12 (6.92)
WAIS-III digit span (SS)8 (2.36)
NPRSETI pain rating (0–10)6.4 (2.62)
BPI- 6th item pain rating (0–10)5.88 (3.07)
Paper-and-pencil NPRS pain rating (0–10)6.4 (2.66)
Relative Dwell Time (%)NPRSETI AOI81.83 (29.04)
Instructions AOI7.15 (19.29)
Anchors AOI2.19 (6.15)
Saccade rate (no. per sec.)1.77 (0.99)
Pupil size (no.)393.42 (92.97)
Nonparametric dataNumber of cases
Gender (female / male)10 / 15
Marital status (married / other)19 / 6
Birth country (Israel / other)22 / 3
Medical Cannabis license (yes / no)11 / 14
Methamphetamine medication (yes / no)2 / 23

Table 1.

Demographic and clinical data.

Notes: Area of interest = AOI; Brief Pain Inventory = BPI; Eye tracker-integrated Numerical Pain Rating Scale = NPRSETI; General Anxiety Disorder Questionnaire 7-item = GAD-7; Patient Health Questionnaire-9 = PHQ-9, Scaled Score = SS.

A comparison of the first and second pain assessments, regardless of the utilized method, revealed that four participants rated their pain one point lower in the second assessment compared to the first. Intra Class Correlation, ICC, calculated according to procedures described by Shrout and Fleiss [44], was 0.99 (95% CI: 0.98–0.99). This indicates excellent agreement based on the interpretative categorization of Koo and Li [45]. Next, we evaluated the match between the NPRSETI and paper-and-pencil NPRS, with two participants rating their pain as more severe in the former than the latter and two participants showing an opposite pattern (i.e., rated their pain as lower in the NPRSETI). In all cases, the deviations in absolute numbers were one NPRS point. In other words, the scores of 16% of the sample deviated by one point between the two NPRS methods without a trend to these deviations. The match between the NPRSETI and paper-and-pencil NPRS was additionally assessed using the following methods: (a) Group comparisons: Assumptions for parametric analyses for the pain ratings in each measurement (NPRSETI, NPRS, and BPI 6th item) were checked using Shapiro-Wilk test. This analysis revealed the pain ratings distributions deviated from the normal distribution (ps < 0.05) for the NPRSETI (W = 0.91), NPRS (W = 0.91), and BPI 6th item (W = 0.91). Correspondingly, skewness and kurtosis indicated that the distributions of NPRS, NPRSETI, and BPI 6th item were asymmetrical (S = −0.84, K = 0.37; S = −0.94, K = .43; S = −0.68, K = −0.41; respectively). These findings were expected considering the participants’ high pain ratings, as reported earlier. We, therefore, performed a Wilcoxon signed-rank test which indicated no significant difference in the reported pain severity using NPRSETI (Mdn = 7) and paper-and-pencil NPRS (Mdn = 7), T = 5, p = 1. (b) Correlation between NPRS methods: There was a very strong correlation [46] between the NPRSETI and paper-and-pencil NPRS (ICC = 0.99, 95% CI: 0.98–0.99). Pain severity as evaluated using the NPRSETI was also strongly correlated with participants’ pain rating using the BPI (6th item; ICC = 0.93, 95% CI: 0.84–0.97). The correlation matrix can be found in Table 2.

1.2.3.4.5.6.7.8.9.
1. Age (years)_____________________________________________
2. Education (years)0.11________________________________________
3. PHQ-9 (no.)−0.14−0.06___________________________________
4. GAD-7 (no.)−0.09−0.230.84**______________________________
5. Pain rating (BPI 6th item; no.)0.20−0.090.180.05_________________________
6. Pain rating (paper-and-pencil NPRS; no.)0.33−0.020.04−0.040.89**____________________
7. Pain rating (NPRSETI; no.)0.36−0.020.07−0.010.89**0.99**_______________
8. Average pupil size (no.)0.05−0.060.220.130.02−0.010.01__________
9. Saccade rate (no. per second)0.002−0.01−0.24−0.09−0.27−0.10−0.12−0.02_____
10. WAIS-III digit span (SS)0.15−0.03−0.51**−0.39−0.13−0.05−0.08−0.350.25

Table 2.

Pearson product-moment correlations among demographic, clinical, and pain ratings of patients (n = 25).

Note:*p < 0.05, **p < 0.01.

Brief Pain Inventory = BPI; Eye-tracker integrated Numerical Pain Rating Scale = NPRSETI; General Anxiety Disorder Questionnaire = GAD-7; Patient Health Questionnaire = PHQ-9; Scaled score = SS.

Bland-Altman analysis is an efficient approach for quantifying the limits of agreement between two measurements [47]. However, it could not be used in this study as the differences between the two NPRS scales were not normally distributed [48]. More specifically, assumptions for parametric analyses for the differences between the two measurements (NPRSETI minus paper-and-pencil NPRS chosen pain ratings) were checked using the Shapiro-Wilk test (W = 0.57, p < 0.001) and histogram and quantile-quantile (Q-Q) plots. These indicated deviations from the normal distribution, an expected finding considering that the differences between the NPRSETI and paper-and-pencil NPRS data were expected to be minimal (based on [49]). Correspondingly, kurtosis revealed a heavy-tailed distribution that deviated from the customarily used +1.96 and − 1.96 z-score range, though skewness did not indicate a deviation from a symmetrical distribution (K = 4.29, S = 0).

Additional analysis and general remarks: While most participants noted that NPRSETI gaze-directed response that was chosen corresponded to their intended rating (n = 17), six participants requested to alter their response, while two additional participants revised their selected response twice. Figure 4 presents a relative gaze duration heat map that visualizes the proportion of accumulated gaze duration at each AOI. Analyses were performed using IBM SPSS Statistics version 29.

Figure 4.

Relative gaze duration heatmap; visualizing the proportion of accumulated gaze durations relative to the total duration for rating pain using the NPRSETI (i.e., time until providing the pain rating).

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4. Discussion

Earlier studies by our research team indicated the feasibility and utility of integrating an eye tracker with commonly used neuropsychological tests [23, 25, 26, 27]. These studies, as well as additional efforts in recent years by other researchers [50, 51], highlighted the potential of this novel technology to enhance daily clinical practice and was the impetus for the current project. We, therefore, set to integrate an eye tracker (i.e., Portable Duo eye-tracker by SR Research Ltd.) with an NPRS. This self-report pain intensity scale is well-established and commonly used in pain clinics [2, 4, 32]. The eye tracker-integrated scale, NPRSETI, is unique as it allows the patient to provide pain ratings without manual manipulation (i.e., use of hands). It thus paves the way for incorporating eye tracker-integrated pain scales in clinical settings (e.g., when evaluating ALS patients with severe physical limitations). The integrated system then underwent an initial validation study in a pain outpatient clinic in which participants filled both the NPRSETI and conventional paper-and-pencil NPRS in a pseudo-randomized order (N = 25).

The study’s findings provide initial validation for the use of the NPRSETI. Overall, excellent agreement was evident between pain ratings made using the NPRSETI and the gold standard selected for this study, the conventional paper-and-pencil NPRS (ICC = 0.99). A closer inspection reveals that 21 of the participants (84%) rated their pain identically using NPRSETI and the paper-and-pencil NPRS. The scoring of four participants deviated by one point between the two scales. Significantly, these deviations were not related to the type of scale that was used; two participants rated their pain as weaker, while two others rated it as stronger when using the NPRSETI. A closer inspection of the data revealed that in all cases, the second pain assessment’s rating was lower than the first (i.e., two participants were first evaluated using the NPRSETI, and two others were first evaluated using the paper-and-pencil NPRS). Order rather than method, therefore, seemed to have impacted the pain ratings, with a minority of participants decreasing their pain ratings when undergoing temporally close consecutive pain assessments. Though the small number of participants that showed this trend limits speculations regarding its source, a similar decrease was found in other studies in which ratings of pain were performed with a short time interval between them. For example, a similar decline was evident in Bergh, Sjostrom [41] when geriatric patients rated their pain twice (5-minute interval) using the VAS and Graphic Rating Scale (GRS), though not when filling the NRS. As patients’ experience of pain likely does change during such short periods [41], any change likely stems from other factors. More specifically, pain is a multidimensional subjective phenomenon [52] that is influenced by environmental factors. These include boredom and loneliness, which patients may experience and consequently view experiments and being attended by the research team as a pleasant experience [41]. This may consequently translate to a decrease in pain ratings, speculation that necessitates further research. Next, we evaluated the match between the NPRSETI and paper-and-pencil NPRS by both group comparisons and by correlating pain scores obtained using the two NPRS types. The first method revealed that that NPRSETI did not significantly differ from that of the paper-and-pencil NPRS, while the correlation matrix indicated a very strong correlation between the two methods (ICC = 0.99). Similarly, pain severity as rated using the NPRSETI was strongly correlated with those made using the BPI (6th item; r ICC = 0.93). Overall, pain ratings using the NPRSETI and conventional unidimensional scales using the NPRS format were almost identical with deviations likely stemming for momentary changes in experienced pain or other factors (e.g., fluctuating anxiety levels). This corresponds to recent studies, which found similar equivalence between conventional unidimensional pain rating scales and their digital equivalents. For example, Escalona-Marfil, Coda [49] electronic VAS was operated using a tablet and found it to be both highly reliable and interchangeable with the paper-and-pencil VAS (for additional examples, see [53, 54]). Overall, we can conclude that the NPRSETI’s reliability is adequate.

We also performed an initial inspection of eye movement patterns rating their pain using the NPRSETI. First, we produced a relative gaze duration heatmap. Heatmaps visualize the proportion of accumulated fixations—periods in which the eye is relatively still and information is extracted from the scene—at each AOI [55]. Heatmaps, therefore, allow easy visualization of the information that the participant pays attention to. The heatmap reflecting participants’ eye movements while rating their pain using the NPRSETI indicated most fixations were made on the pain scale itself and were concentrated on pain ratings ranging from 5 to 7. Participants’ gaze was also directed toward the rightward anchor (“unbearable pain”) and keywords in the instructions that were presented above the scale (i.e., “mark”, “pain severity”, and “gaze”). In other words, participants mostly gazed at pain ratings that corresponded to the pain ratings their later chose (i.e., Median pain rating was 7) and to critical elements in the instructions. Regarding the latter, visual attention tends to be maintained on pain cues that signal potential personal discomfort [56]. It would therefore be of interest to assess in future studies whether patients tended to gaze more at verbal stimuli due to their content (e.g., “unbearable pain,” which constitutes the rightward anchor) and whether this correlates with their pain ratings. In addition to analyzing spatial attention based on gaze direction, we analyzed general eye movement measures, saccade rate, and pupil size (see correlation matrix in Table 2). The first provides an estimate of periods in which information is being processed [15], while the latter is impacted by various relevant factors; the pupil dilates in response to painful stimuli as well as the effort exerted in a task and other cognitive processes [57]. Both saccade rate and pupil size were not significantly correlated with pain ratings using the unidimensional pain scales used in the study, including the NPRSETI (see Table 2). However, the variability of the pain ratings and the relatively small sample size of this study may have masked associations of interest. Further research is therefore called for.

Several limitations of this study can be mentioned, notably its modest sample size. Repeated measurements using the NPRSETI ‘s and paper-and-pencil NPRS would have enabled the evaluation of both intermethod and intramethod reliabilities, as research design which would have been preferable in hindsight (as performed by [49, 54]). The median pain ratings (Mdn = 7) of the patients in this study may also be in the higher range of patients treated at an outpatient clinic based on our impression of earlier studies (e.g., [5]). This stresses the need to further validate the NPRSETI using larger samples. This will enable comparisons of different pain etiologies (e.g., neuropathic vs. musculoskeletal) and thereby clarify the generalizability of the findings, as well as evaluating the contribution of factors such as pain medications and sleep quality [58, 59] on NPRSETI pain ratings. More strongly powered studies will also allow as well a more thorough assessment of the clinical utility, reliability, and validity of the NPRSETI [1, 60]. Assessing NPRSETI’s usability is also of utmost importance. The comfort and usability of medical devices, with their increasing human-machine interface, have been a focus of increasing research attention [30, 61]. This is especially important as eye tracker-integrated pain scales necessitate calibration and additional components (e.g., gaze-directed pain rating may be incorporated in the scale) and are, therefore, more cognitively demanding than conventional unidimensional pain scales. It is also more challenging for those with visual impairment, which may not be assessed reliably. As part of this study, the participation of three participants was discontinued due to the inability to calibrate the eye tracker. Eight participants requested to alter their response at least once, suggesting that that gaze-directed pain rating may not have been user-friendly. This is disconcerting as these factors already limit the usability of conventional scales. For example, visual and hearing impairment, as well as physical restrictions, led to an inability of about a third of orthopedic post-surgery patients to undergo the VAS [62]. Eye-tracking technology has considerably advanced in recent years with increasingly affordable, precise, and faster eye trackers [63]. Hopefully, this will offset any usability issues that may plague eye trackers such as the one used in this study.

Beyond the earlier mentioned suggestions, several lines of research may be proposed based on the findings of the current study. First, comparisons with other pain scales is of importance, including digital versions that are similar in design to the NPRSETI (e.g., [64]), and validated pain scales that use a different format (McGill Pain Questionnaire, MPQ; [65]), and more uniquely designed paper-and-pencil scales (e.g., [66]). Second, eye movement analysis can explore cognitive and affective factors associated with rating pain using unidimensional pain scales. Eye movements are sensitive to the effects of cognitive load [67, 68, 69], with gaze aversion interpreted in an earlier study performed in our laboratory as an attempt to lessen the cognitive load stemming for attempting to simulate cognitive impairment [23]. Simulators in this study also tended to gaze significantly more at the instructions of the task, which were speculated to serve as “attentional hooks” due to the automatic processing of language and its ability to involuntary draw attention (for a more thorough discussion, see [23]). This is somewhat reminiscent of the pattern evident in this study, with participants gazing toward the marking on the scale’s right anchor (“unbearable pain”) and keywords in the instructions, as noted earlier. Eye movements may therefore be used to explore the association between experienced pain, cognitive load, and likely other cognitive processes involved in rating pain. Eye movements are also impacted by anxiety and other negative affective states [70, 71], as well as stress levels (e.g., stress leads to shorter fixation durations [33]). Regarding the latter, pain is associated with the activation of a stress response—at least in its acute stages—and involves those related to heart rate, respiration, sweating, and muscle tension [1]. Overall, analysis of eye movements constitutes a window through which researchers can explore this various pain-related cognitive and affective factors. Eye movements are only under partial volitional control and were there studies as measures of deception and malingering [23, 26, 72]. Studying participants attempting to feign pain is scales such as the NPRSETI is therefore also of value, and the interested reader is encouraged to further read about relevant research designs [73]. Finally, researchers are encouraged to integrate other unidimensional pain scales with eye trackers. For example, it was noted in a recent review of adult postoperative patients [7] that older adults and children who have less abstract thinking ability might prefer a categorical scale (e.g., verbal rating scale, VRS), which are easier use for them. This may prove useful as recent research tentatively suggests that the NPRS may have lowered utility in populations of individuals from developing countries with low literacy rates [2]. We hope that researchers can draw ideas from the above-mentioned suggestions, devising comprehensive research plans which may promote the integration of the NPRSETI or similar eye tracker-integrated pain scales in clinical practice. Recent publications may be useful in devising a comprehensive research plan [15, 74, 75, 76].

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5. Conclusions

Technological advancements in recent years have enabled clinicians to incorporate increasingly affordable and progressively convenient psychophysiological biomarkers, as also evident in pain research (e.g., [77, 78]). Eye movement analysis provides a window to cognitive and affective processes [75, 76], and as with other biomarkers, there are initial explorations of their utility for evaluating pain (e.g., [79, 80, 81]). This study is part of these recent efforts, indicating that the integration of an eye-tracker integrated unidimensional pain rating scale, the NPRSETI, in a hospital-based pain clinic is both feasible and can be used with a relatively diverse adult chronic pain patient population. The study’s findings also provide initial validation of the NPRSETI. More specifically, participants’ pain ratings were very similar to those performed by the conventional NPRS, with no bias in pain ratings associated with the NPRS method that was used. The participants’ gaze indicated that their visual attention was directed toward the relevant range of pain ratings in the scale, as well as the NPRSETI’s anchors and keywords in its instructions. These findings should, however, be considered as preliminary and await further validation using larger samples. These studies can further assess the validity, reliability, and additional key attributes that are mandated from pain scales [1, 60]. Analysis of eye movement also opens a myriad of other research opportunities, ranging from assessing its usability to the exploration of feigning based on the fact that eye movements are under lessened volitional control. The former is particularly important considering the possible deleterious impact of visual and cognitive impairment on utilizing a more complex device such as the NPRSETI. The reduced costs of eye trackers and enhanced usability evident in recent years [21] will hopefully encourage such research. Pending further validation, the NPRS NPRSETI may find a place among the more established pain assessment tools. The path toward this goal, however, will necessitate a concerted effort by researchers and clinicians.

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Acknowledgments

We thank Mr. Lior Golan, Miss Anastasya Garnik, and Miss Astar Lev for assisting in patient recruitment and performing the experimental procedures. We also thank Mrs. Oria Appel for helping with statistical analyses. The project would not have been possible without their aid.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Yoram Braw, Motti Ratmansky and Itay Goor-Aryeh

Submitted: 06 May 2023 Reviewed: 17 May 2023 Published: 21 June 2023

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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