## 4 Discussion  
This study was the first to assess the user experience with a human-machine interface, in which voice is used for a simple confirmation of selection operation based on eye tracking, just as a mouse click is used to confirm selection made with a computer mouse. Since voice has a capacity to carry rich information from a human to a machine, making it to play such an oversimplified role can be perceived as counterintuitive and boring, especially to healthy users. This could be one of the reasons why in all previous attempts to combine voice and gaze in a hybrid human-machine interface, more complex voice commands were explored [12; 13; 14; 15; 16; 17; 18; 19; 20]. However, the participants of our study successfully fulfilled multiple selections on a computer screen using the simple \"voice-as-click\" approach, only slightly slower than with mouse-click confirmation. They rated their experience with \"voice-as-click\" not much below than with the conventional mouse-click confirmation: The convenience of selection was rated \(8.0\pm 2.1\) (M \(\pm\) SD, on a 10-point scale) for the most convenient variant of voice confirmation (condition N), while the score was \(9.2\pm 1.1\) for the mouse-click confirmation (condition M). Selection in the condition T (confirmation by pronouncing the word \"ty\") was rated as less convenient compared to the condition N (confirmation by pronouncing the ball's number). Most likely, this was related to the congruence between the subtask of pronouncing the number and the subtask of remembering the number of the current ball to select in the condition N.  
Voice confirmation was a new way of selection confirmation to all our participants, while they were routinely using a mouse-click confirmation in their everyday tasks, and some of them were also frequently using it while playing games. It is possible that with gaining more experience with the voice confirmation, they could make a selection with the same speed or even faster than with mouse clicks. While manual actions are naturally well controlled, the \"voice-as-click\" confirmation may have features making them more compatible with gaze-based selection. One of them is related to spatial attention. In manual confirmation, hand spatial position is far from the position of the object to be selected. Unlike in the conventional computer mouse use, when cursor movement is driven by hand movement, and their spatial coordinates are well correlated, in gaze-based selection, the position of eye-gaze is not just correlated, but coincide with the object to be selected, however, the hand position is not coordinated with it at all. It seems that \"voice-click\" may fit gaze-based selection much better, because it does not lead to the division of spatial attention between some remote position (where a mouse-click is executed) and object position. With the automation of the mouse-click use, the need to direct attention to hand may disappear, but probably not in full. As discussed in the Introduction, gaze and voice are naturally coordinated in human-to-human communication. The target object can be considered as an analog of the addressee of the message sent by the user with their voice and gaze. Here, the target object is evidently the only focus of attention, and the act of selection should be executed in a natural and efficient way. Several limitations of the current study should be considered. First, in applications of gaze-based selection designed for people with motor disabilities, the most common way to confirm the selection is not manual confirmation (which many of such people cannot use), but a prolonged eye fixation (dwell) or/and an additional dwell on a designated screen button. We did not include such type of confirmation in our experimental conditions. However, it seems not reasonable to expect that these ways of confirmation could be much more effective than voice confirmation, because short dwell