Excellent observations. It is better to avoid simultaneous stimuli. However, multimodal stimuli may be used:
1. If one stimulus supplements the other and adds to multisensory integration and gives a holistic experience of cognition to the user.
2. when there is room for multisensory enhancement. That is if only one stimulus (any one of visual, auditory or tactile) is used and it results in very small response from the user. Then it clearly shows that more than one stimuli may be used for achieving the desirable user response. But at the same time, care should be taken to ensure that the modalities supplement one another to create multisensory enhancement. But the downside of multimodal stimuli is that it can lead to intersensory redundancy – example – a presenter reading the content on the slide shown on the screen. The audience can also read the same content. This leads to intersensory redundancy and leads to poor UX.
Choice of stimuli should depend on the context & user’s capabilities. Through contextual enquiry, the UX researcher needs to identify the user’s context-dependent-constraints of information processing and decide the stimuli. For example, if the person is using a mobile app while commuting with train’s noise all around, and the mobile device is in his trouser pocket, the only way the app will get attention would be through haptic stimuli because of his auditory and visual constraints to receive the stimuli. If the same app throws an alert when at home with mobile device carelessly placed on the table and the user watching TV on the couch, then auditory stimuli will be best suited than visual or tactile. Please see the supplementary video and article attached with the lesson for better understanding of the concept.