## 4 Discussion
### About Signifiers of AD Usability  
A detailed examination of rubber's role as a signifier for each affordance (Figure 3) reveals several positive effects. Models with rubber generally received higher ratings from on-screen assessment, particularly where rubber's physical properties were most advantageous. For instance, rubber enhanced _robustness_ of the grip area, especially in model 6, by providing a sense of increased thickness. _Comfort_ improved in the rubber versions (significantly in models 1, 5, and 6), likely due to the perceived softness, reducing pressure points and providing greater comfort. Rubber also clarified the intended grip area, as in models 1, 2, and 6, resulting in perceiving an improved ease of grip. In model 1, the rubber version was rated higher for _lid slippery_, as the enhanced grip is perceived to reduce slippage and facilitate tighter sealing. Additionally, the rubbered model 1 was also rated more positively for _easiness to use_, suggesting that the defined grip area clarified how to operate the model. Finally, no significant differences were observed between rubber and non-rubber versions for the affordance of _effort level_, which was one of the least effectively conveyed. To conduct a more detailed study of other design elements that function as signifiers, a qualitative analysis by affordance has been carried out, supported by participants' visual attention from heatmaps (Figure 4). Comments follow:  
_Robustness._ Participants focused primarily on the placement area on the lid, highlighting its importance in evaluating this affordance. Material and thickness notably influenced perceptions: in models 2 and 3, the steel material served as a positive signifier, while in model 4, the thick plastic conveyed solidity. Additionally, the solid and somewhat bulky shape of model 4 may have further reinforced the perception of _robustness_. _Comfort._ The grip area received the most attention, particularly in models 1 and 6, which are elongated, plastic, and feature large grip areas, characteristics that likely act as _comfort_ signifiers, especially with rubber reinforcement. In contrast, model 2, rated lowest for _comfort_, showed fewer fixations in its rubber version, possibly due to its slim design and steel material, which may have been perceived as uncomfortable when pressing the hand. _Easiness to grip._ Attention across AoIs varied by model: in model 1, the grip area received most attention; in models 5 and 6, focus centred on the placement area on the lid, while in model 3, both areas were equally observed. Models 2 and 4 showed similar pattern, with attention distributed between both areas. The handle design in model 3, with its familiar shape and material, likely acted as a signifier of _easiness to grip._ In contrast, model 2, perceived as the hardest to grasp, received less attention, possibly due to its wire-like shape and limited support surface, leading participants to dismiss it early. _Lid slippery._ Attention primarily focused on the placement area on the lid, with minimal focus on the grip area. Higher-rated models displayed features enhancing adhesion perception: models 2 and 3, with metal contact areas and protruding tabs, were likely perceived as less slippery due to these elements (Figure 6). Similarly, models 4 and 6, featuring soft, rubber-like plastic in the contact area, acted as a positive signifier of adhesion. In contrast, model 1 was perceived as more slippery due to its smooth hard plastic and model 5 due to its toothless metal surface (Figure 6). _Effort level._ Model 2, one of the best rated, attracted more attention to the grip area, while the lower-rated models were more observed in the placement area on the lid. Regarding signifiers, model 2's metallic and lever-shaped design suggests reduced effort. However, model 1, despite a similar lever shape, was rated as requiring more effort, possibly due to  
Figure 6: Material and shape details for models 1, 2, 3, and 5.  
its plastic material, perceived as a negative signifier.