“…The least successful approaches utilized only pupil diameter achieving highly similar and low accuraries of 58.9% [42] and 59.0% [36], respectively. The most commonly used feature (eight studies) was pupil diameter [31,36,42,57,86,94,96,101], followed by fixation duration employed in four studies [31,73,84,101], and finally the least used features were pupil position [57,94] and EOG [53,54], which were used in only two studies each, respectively. The speed of the emotion recognition task was only reported in one of the studies, which could provide classification results within 2 s (with 10% variation) of the presentation of the emotional stimuli [94].…”