###### Abstract  
Voice- and gaze-based hands-free input are increasingly used in human-machine interaction. Attempts to combine them into a hybrid technology typically employ the voice channel as an information-rich channel. Voice seems to be \"overqualified\" to serve simply as a substitute of a computer mouse click, to confirm selections made by gaze. It could be expected that the user would feel discomfort if they had to frequently make \"clicks\" using their voice, or easily get bored, which also could lead to low performance. To test this, we asked 23 healthy participants to select moving objects with smooth pursuit eye movements. Manual confirmation of selection was faster and rated as more convenient than voice-based confirmation. However, the difference was not high, especially when voice was used to pronounce objects' numbers (speech recognition was not applied): Score of convenience (M \(\pm\) SD) was \(9.2\pm 1.1\) for manual and \(8.0\pm 2.1\) for voice confirmation, and time spent per object was \(1269\pm 265\) ms and \(1626\pm 331\) ms, respectively. We conclude that \"voice-as-click\" can be used to confirm selection in gaze-based interaction with computers as a substitute for the computer mouse click when manual confirmation cannot be used. hands-free interaction; gaze-based interaction; voice; voice command; selection; moving objects; smooth pursuit; eye movements; human-machine interaction +
Footnote †: journal: Medical  
Finding an appropriate substitute is one of the crucial problems in designing effective and user-friendly gaze-based interaction [1; 3]. An extended eye dwell is often used for this purpose. This solution has a serious drawback: A high dwell time threshold makes interaction slow and tedious, while an insufficiently high threshold leads to frequent false selections because of uncontrollable, vision-related eye fixations (the so-called Midas touch problem, [4]). The most straightforward approach to complete the selection made by gaze, as in the case of using a computer mouse, is to use a button press [1]. However, the use of the manual button press for selection makes the selection process not fully hands-free, so it cannot be used by people with severely paralyzed hands and by other people (including those who are healthy) when their hands are both busy. Moreover, the correspondence (mapping) between the movement of a hand and the movement of a cursor on the screen, which may help to perceive the relation between hand and cursor locations in the case of using a computer mouse, is not held for the manual confirmation, so a button press may distract spatial attention from the screen object to be selected by gaze, thus, interfering with gaze control. Voice is another hands-free modality of the human-machine interaction (e.g., References [5; 6; 7]). Unlike the gaze-based control, it is already used by many healthy users, e.g., in interaction with virtual assistants [8]. The gaze is often used together with a voice in natural communication, e.g., in joint attention [9], in turn-taking [10], and in direct gaze interaction [11]. Attempts were made to combine gaze- and voice-based interaction in a single technology [12; 13; 14; 15; 16; 17; 18; 19; 20]. Indeed, it seems natural to use voice communication as a most natural and information-rich channel, and to supplement it with gaze pointing whenever spatial information is helpful, e.g., to quickly indicate where or on which object an action should be applied [21]. No dissociation or distraction of spatial attention is needed, as voice and gaze may address the same \"recipient of the message\". Nevertheless, such combined technology has not become widely adopted so far. One possible reason could be different time scales of voice and gaze channels: Commands given through natural speech or even through short, predefined phrases are often relatively long compared to comfortable gaze delays (below 1 s).