Brain-Computer Interface: Usability Evaluation of Different P300 ...

Brain-Computer Interface: Usability Evaluation of Different P300 Speller Configurations: A Preliminary Study Liliana Garcia1(), Véronique Lespinet-Najib1, Sarah Saioud2, Victor Meistermann2, Samuel Renaud2, Jaime Diaz-Pineda3, Jean Marc André1, and Ricardo Ron-Angevin4 1

Team CIH - Laboratory IMS CNRS UMR 5218, Bordeaux, France {liliana.garcia,veronique.lespinet,jean-marc.andre}@ensc.fr 2 ENSC - Bordeaux INP, Bordeaux, France {sarah.saioud,victor.meistermann,samuel.renaud}@ensc.fr 3 CATIE - Information and Electronic Technology Center of Aquitaine, Aquitaine, France [email protected] 4 Dpto. Tecnología Electrónica, Universidad de Málaga, Málaga, Spain [email protected]

Abstract. Brain–Computer Interface (BCI) is particularly relevant as a new way to interact with the outside world for disabled people. Based on P300 event-related potentials (ERPs) BCIs have been frequently used for communication purposes, being the first P300-based BCI paradigm developed by Farwell and Donchin for visual speller. P300-BCI speller studies require a significant attentional demand during sustained long times which could represent fatigue and feeling of increasing workload. The evaluation of workload while using P300-BCI speller requires taking into account the cognitive, emotional and physical state of participant during task. This would help to improve usability of the system. The objective of the study is to evaluate, through objective and subjective measures, three different size of speller in order to analyze effectiveness, cognitive load and user comfort. Three healthy subjects took part in the experiment. The preliminary results suggest that speller size can have different effects on user performance and represent important workload for subjects. Keywords: Brain-computer interface (BCI) · Usability · Speller · P300 · Matrix · Size

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Introduction

Brain-computer interface (BCI) systems [1, 2] are devices that transform a user’s brain activity into commands that are interpreted by a machine. Such systems offer a non-muscular channel for users to interact with their environment. This is particularly useful for people suffering from neurodegenerative disorders, such as amyotrophic lateral sclerosis, as they can eventually present severe motor disabilities, to the point of losing control of the muscles that are responsible of voluntary body movements, including eye movement and eventually breathing. In some cases, BCIs turn out to be © Springer International Publishing Switzerland 2015 I. Rojas et al. (Eds.): IWANN 2015, Part I, LNCS 9094, pp. 98–109, 2015. DOI: 10.1007/978-3-319-19258-1_9

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the only way for patients to gain some degrees of communication and autonomy in their daily lives. The most widely used BCI systems are those based on electroencephalographic (EEG) signal recording, due to its non-invasiveness, but also to its good temporal resolution and ease of use. Three types of EEG-based BCI systems have been used for communication purposes, namely those based on: (a) slow cortical potentials (SCPs), (b) P300 event-related potentials (ERP), and (c) sensorimotor rhythms (SMR) [3]. BCIs based on SCP and SMR requires extensively trained users before they could show sufficient control of their brain activity. In contrast, BCIs based on P300 require minimal training as they rely on a common expected human response to infrequent target stimuli, usually visual. The P300 signal, recorded over the central and parietal regions, is a positive deflection of brain wave at a latency of about 300 ms after stimulus presentation. The main applications of P300-based BCI systems are aimed at communication purposes. They are based on the P300 speller first developed by Farwell and Donchin [4], which is still referenced and intensely studied [5, 6, 7, 8]. In this BCI, a 6 x 6 matrix of letters, arranged in rows and columns, is shown to the subject. The user focuses his/her attention on the matrix element he/she wishes to select as each row and column is flashed (i.e., intensified) randomly, one after the other. After a number of flashes, the symbol that the user has supposedly chosen is presented on screen. The effectiveness of the P300-based BCI speller system is guaranteed by a number of studies carried out not only on healthy subjects [9, 10] but also on subjects affected by some motor disability [11]. Overall, these studies conclude that the P300 speller processor is an effective communication tool for people who have lost or are losing their ability to write or speak. However, it is still needed to improve the usability of these BCI speller systems. The current ISO definition of usability (9241-11) includes three measures: effectiveness (accuracy and completeness of users achieving set goals), efficiency (the resources expended to complete goals), and satisfaction (the users’ attitude) [12, 13, 14]. Frokjaer et al. argue that these components should be considered as separate and independent aspects of usability [15]. The efficiency and satisfaction can be measured through several subjective variables: mental workload, fatigue, motivation, comfort, pleasure to use, etc [16, 17, 18]. Some factors, such as the mental fatigue induced by a long use [19, 20], the sustained attention at a symbol on screen [21], the user´s motivation [6], [19] or his/her frustration due to a mistake can influence the amplitude and latency of the P300 component (See [22] for a review). In this regard, the influence on performance of the temporal and spatial aspects of the user interfaces of these systems is increasingly drawing the attention of researchers [23, 24, 25]. Although much research has been studied parameters that could influence user performance in P300-based BCI applications, some research are still required, particularly regarding to speller size. Some studies have showed how matrix size affects user task performance. Allison et al. [26] carried out a study comparing three different matrix sizes (4x4, 8x8, 16x16). Results indicated that larger matrices evoked larger P300 amplitude, and the matrix size did not significantly affect performance or preference. In this study, size symbol was the same for the three matrices, thus the larger

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matrices appeared larger on the screen. On the other hand, a study comparing two different matrix sizes concluded that accuracy was higher for the 3x3 matrix, whereas that P300 amplitude were higher for the 6x6 matrix condition [8]. In spite of these studies related to matrix sizes, there have been no studies in the literature investigating the effect of speller size. In one study, three different screen sizes were investigated: a computer monitor, a global positioning system (GPS) screen, and a cell phone screen [27]. However no information related to size symbol is provided. According to the screen resolution and the different distances from the participants to the screen, visual fields are 6,4º, 3,7º and 3,56º for computer monitor, GPS and cell phone screen respectively, being almost the same for the two smallest screens. Actually, the main objective of this study was to evaluate BCI performance using these three specifically screens, but not to study screen size effect. In order to study the effect of speller size in P300-based BCI performance, different visual fields should be proposed, as they would be very important to justify the different size speller proposals. The purpose of this study is to evaluate different speller sizes in order to analyze effectiveness (accuracy and completeness of users achieving set goals), efficiency (the resources expended to complete goals), and satisfaction (the users’ attitude). We hypothesized that these three factors are different when using small, intermediate or larger speller size.

2

Methods

2.1

Participants

Three healthy subjects (2 Male, 1 female, aged 20.4 years, SD =0.89) took part in the experiment. None all them had previous experience with BCI systems. All subjects had normal vision and gave informed consent through a protocol reviewed by the ENSC-IMS Cognitive team. 2.2

EEG Data Acquisition and Processing

EEG was recorded using gold electrodes placed at positions Fz, Cz, Pz, Oz, P3, P4, PO7 and PO8, according to the 10/20 international system (see Fig. 1). All channels were referenced to the right earlobe, using FPz as ground. The EEG was amplified through a 16 channel biosignal amplifier (g.BSamp, Guger Technologies). The amplifier settings were 0.5 and 100 Hz for the band-pass filter, the notch (50 Hz) was on, and the sensitivity was 500 µV. The EEG was then digitized at a rate of 256 Hz by a 12-bit resolution NI-USB-6210 data acquisition card (National Instruments). All aspects of EEG data collection and processing were controlled by the BCI2000 system.

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Fig. 1. 1 Electrodes montage for EEG recording

2.3

Experimental Setup p

Subjects were seated in a co omfortable chair at a distance about 60 cm from the screeen. Before the beginning of thee experiment, instruction regarding the procedure and B BCI speller management was given in verbal form. They were presented with the classical Farwell & Donchin [4] speeller, which consists on a 6 x 6 matrix of symbols (366 alphanumeric letters and num mbers) arranged within rows and columns (Fig. 2).

Fig. 2. Schemaatic representation of a classical P300 speller BCI

Each subject participated d in a 2 hours session. During this session, subjects w were exposed to the three condiitions (ie., three different speller size : small; middle and large), whereby 10-15 min break were given between conditions. To avoid effects due to learning, the order in whiich participants test the three speller sizes was randomizzed. Each condition was divided into two phases (see Fig. 3): a first one for calibrattion purposes and a second on ne to evaluate the interface. In the calibration phase, the researchers told the particip pant that he/she would see a number of consecutive randdom sequences of 20 row and co olumn short intensifications (i.e., flashes). These sequennces would be separated by sho ort intervals with no intensifications. To spell a letter oor a number, the participant onlly had to mentally count how many times the desired ccharacter had been intensified d during a sequence. After these instructions, participaants copy-spelled, without receiv ving any feedback, two French words of 4 characters annd a number. These words weree “lune”, “feux” “kilo” and the number “2015”. EEG w was recorded for offline analysis.

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After this runs, we performed a stepwise linear discriminant analysis (SWLDA) of the data from the last three runs to obtain the weights for the on-line P300 classifier. After calibration and training of the classifier, the evaluation phase started. Participants had to spell 2 words and a number: “CHAT”, “PURE” and “1935”, without spaces. They were instructed not to correct and to continue if the classifier chose a wrong letter. In order to help subject, each word (or number) was showed in the screen for 1 second before to start to spell it. In this phase, the characters typed by the user were displayed inside a text box below the keyboard.

Fig. 3. Experimental design showing the temporal sequence of writing words

Considering that the main objective of the study was to evaluate the performance of three different speller sizes, it was important to take into account the location of the different letters. In this way, if we consider the distance from the letter to the matrix center, 6 different areas are possible (see Fig. 4). The 3 sequences (2 words and 1 number) to spell were chosen such that each area was covered 2 times (by 2 characters per area).

Fig. 4. Area distribution according to the distance from center (left). Letter distribution of the 3 words used: CHAT-PURE-1935 (right)

The values of the temporal parameters for all spellers were based on those used by [9]. Briefly, each row and column was randomly flashed 10 times. Therefore, each character was randomly intensified 20 times. The duration of each flash was 125 ms and the inter-stimulus interval (ISI) between flashes was also 125 ms. There was a pause of 6 s after each sequence of flashes (i.e., after a character had been selected).

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This pause gave the subject time to shift attention and gaze to the new character. According to these temporary parameters, the time needed to select a symbol is 36 s – the duration of flashing six rows and six columns each during 125 ms with an ISI of 125 ms, plus the 6s of pause -, being the time to spell the 3 words of the test phase of 7 min and 12 s. After the evaluation phase of a specific speller, subjects were asked to complete the NASA-TLX test [28, 29] (see Fig. 3). At the end of the session, users were asked to express their preference between the interfaces. 2.4

Speller Size

Three different speller sizes were investigated. All were presented on a 17” TFT screen with a refresh rate of 60 Hz and a resolution of 1440 x 900 px2. For each speller, the 6 x 6 matrix of 36 characters was centered on the screen. The biggest size was chosen according to the speller size used in [30], and frequently used by other researchers, i.e. [31]. As in [30], the matrix subtended ±6.98º of the visual field both horizontally and vertically, and symbols were arranged on a grid with a size of 500 x 500 px2. As proposed in [30], the size of each character was 1.12º (40 px), being the distance (horizontally and vertically) between characters 1.46º (54 px). The smallest size was chosen according to the smallest symbol size to use without lost of performance. As it is reported in [32], a small symbol size could substantially reduce the spelling performance, being the smallest symbol size recommended 0.4º (25 px). This visual field corresponds to 0.7 cm when situated at 100 cm in front of the screen [32]. Comparing to the symbol size of the previous matrix (1.12º), this small symbol size (0.4º) represents a reduction of 35.89%. In order to maintain the same speller size proportion, the matrix of the smallest speller subtended ±2.51º (185 x 185 px2), being the distance between characters 0.52º (19 px). The intermediate size was chosen to be the middle size between the two other spellers. Then, the matrix subtended ±4.75º (350 x 350 px2), being the intermediate symbol size 0.75º (27 px) and the distance between characters 1º (37 px). The metrics used for each speller can be seen in Fig. 5.

Fig. 5. Size metrics for each speller size

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Effectiveness Dimension - Objective Measures

To carry out direct comparison across BCIs, performance measures based on classification accuracy of correct target selection with the P300 speller have been considered. 2.6

Efficiency and Satisfaction Dimensions - Subjective Measures

Regarding the degree of efficiency, two variables are evaluated: mental workload and fatigue felt. • Mental workload: Participants were asked to rate subjective workload using validated NASA Task Load Index (NASA-TLX) [28, 29]. This test represents 6 factors (mental, physical and temporal demands, as performance, effort and frustration) scored between 0-100 in which higher values are indicative of higher workload. Each factor is rated by participant at the end of each matrix speller. The test is composed of 2 parts, in the first part, participants should rate to give a magnitude to each dimensions, then the six subscales were combined into 15 pairs, and for each pair of scales, the subjects were asked to indicate and identify the factor that contributed more to their workload. A weighted average technique was used to compute an overall measure of workload (between 0 and 100) and for analyze the relative contribution of each subscale. • Fatigue: Simultaneous with NASA TLX test subjects executed the visual analogue scale to evaluate fatigue [33]. Regarding the degree of efficiency, at the end of the session, a final questionnaire based on the comparison of the three speller sizes has been proposed allowing the subject to express their opinions on satisfaction of the interface in terms of fun, ease of use, complexity, etc. A comparative questionnaire adapted from the SUS (System Usability Scale) allowed us to evaluate four dimensions (fatigue, complexity, attractiveness and overall preference). For each dimension, the subject had to rank the three keyboards between them.

3

Results

3.1

Impact of Speller Size on Effectiveness

Performance accuracies for each chosen word and speller size are shown in Table 1. The highest overall accuracy is obtained for large speller size (94.4%). High accuracy variability between words is observed. Effectively, similar accuracy performance is obtained for the first word (CHAT) for all speller sizes (91.6%). On the contrary, different performances are obtained for the third word (1935) for each speller size, being very low when intermediate speller size is used (41.6%).

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Table 1. Accuracy performance p (%) for each chosen word and speller size Large Size Middle Size Small Size Words First: “CHA AT” 91.6 % 91.6 % 91.6 % Second: “P PURE” 91.6 % 100.0 % 75.0 % Third: “193 35” 100,0 % 41,6 % 83,3 % Overall 94,4 % 77,7 % 83,3 %

In order to better understtand these results, the total number of wrong selections for each location area and spelller size is shown in Fig. 6. So, a higher number of wroong selections are associated to o the intermediate speller size, getting 3 letters, locatedd at the bottom, incorrectly seleected by two subjects. Regarding the small size speller, the wrong selected characters are a more regular distributed over the areas. Only two wroong selections are obtained with h the large speller size.

Fig. 6. Total wrong selectio ons according to area location. “+”: one error; “++”: two errorss

3.2

Impact of Speller Size S on Efficiency

Average scores obtained for f workload (NASA-TLX) and fatigue (visual analogue scale) are shown in Table 2, for each speller size. The results represent an averrage value, according to type off keyboard and show that the cognitive load is moderattely high compared to the task. They seem indicate that speller size did not significanntly affect workload scores neith her fatigue scores. Table 2. Averag ge workload (/100) and fatigue (/10) scores obtained Workload Score Fatigue Sccore

Large Size Middle Size Small Size 53,57 54,33 51,23 4,3 4 5

However, as it is shown in Figure 7, three out of six factors considered by NAS SATLX test have an importan nt contribution in workload (score higher than 50): menntal, temporal demands and efforrt.

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Fig. 7. Average workload w scores obtained for each factor of workload

3.3

Impact of Speller siize on Satisfaction

Preference for any speller size is very dependent on subjects. The first subject expresses no preference for any a speller, the second subject preferred the intermeddiate size speller and the third one o the biggest size. The sample used for this study is too small to advance some concclusion about size speller preference however, the obtaiined results seem to show indiv vidual and specific preference (satisfaction) and so beeing independent of performancee (effectiveness), workload and fatigue (efficiency).

4

Discussion

In this study, three differentt P300-BCI spellers have been compared through objecttive and subjective measures. The T usability of interface has bean evaluated by users. T The three factors of usability were w evaluated: effectiveness, efficiency and satisfacttion [15]. The results suggest thatt accuracy seems to be dependent on speller size, gettting lower efficiency when usin ng intermediate or small speller size. Although if these are preliminary results, the treends of measures suggest that the large speller size haas a better performance, suggestting a higher ability to visual speller task when the subjject can realize gaze movement,, as it is the case here. Moreover, a comparison n between the numbers of errors between words show ws a worsening on efficiency, probably due to user’s fatigue or to a lack of attention but only with intermediate and d smaller speller size. However, in terms of fatigue exxperienced (with analogue visu ual scale), there is no difference between the types of sppeller size. The level of fatiguee experienced by the subject is relatively small. In the same way, we obsserved that most of the character errors in each session ccorresponds rather to the inteermediate and smaller speller size, this later presentinng a more sparse distribution in n the different areas of the speller (as shown in Figuree 6). Interestingly, the intermed diate and small BCI spellers showed greatest numberr of errors particularly with the numeric characters.

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We observe that visual features dimensions are important in the visual BCI spellers design possibly because using bigger size allows enhanced contrast. These results are in agreement with previous authors, which have showed that large elements can be identified more easily in the periphery [29]. Subjective analysis using the NASA TLX suggest that cognitive factors such as temporal and mental demands, both with effort, could have great contribution on the global workload. Furthermore, although this remains to be demonstrated by additional experiments, frustration level does not seem to have influenced dramatically the execution of the task. This may be explained by the fact that subjects were advised that making mistakes does not mean they were unable to realize task and that the aim of experiment was rather to assess the effectiveness of the interface. Those overall preliminary results suggest that despite using such interface requires a high degree of concentration; the subjects have a good degree of satisfaction and acceptability. Our results are coherent with the literature, which establish that the three factors of usability (effectiveness, efficient and satisfaction) are relatively independent [15].

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Conclusion

We have performed one comparative analysis of performance and usability of different size of P300 BCI speller. The aim of this preliminary work was to identify the best parameters to be considered to enhance adaptability and to develop methods to communicate with the outside world. Although the obtained results are preliminary, these one suggest that subjects’ performance during the execution of the task is higher when larger speller size is used. This could be implemented later for people with severe motor disabilities. We noted relative contribution in workload particularly on mental, temporal demand and effort. Further studies will be needed in order to establish the most important advantages for the users. Acknowledgements. This work was partially supported by the University of Málaga, by the Spanish Ministry of Economy and Competitiveness through the project INCADI (TEC 201126395), by the European Regional Development Fund (ERDF). This project benefit from specific funding through The Excellence Initiative of the University of Bordeaux (IdEx Bordeaux). We would like to thank the volunteers for their time and contribution to the experiment.

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