## 2 Theoretical Background
### Present Study
#### 2.3.1 How Does Similarity Depend on the Task Used to Elicit Human Attention Maps?  
Our tasks represented three points on a continuum of intentionality (see Figure 1). On the low end, we simply tracked participants' _spontaneous gaze_ during categorization. The aim of this task was to obtain fixations on areas that participants actually used, not confounded by participants being unaware of their true information needs. At the same time, this task comes with three risks. First, it might produce only few fixations because eye movements are not needed for rapid gist perception. If participants do move their eyes, this leads to the second risk, namely that fixations might mainly reflect response selection processes (i.e., remembering the key mapping). In the best case, participants might look at the category-defining areas while selecting their response. In the worst case, it might lead to the third risk, namely that eye movements reflect task-irrelevant biases. In sum, attention maps generated via spontaneous gaze might provide little information about the areas actually needed for categorization.  
Given these risks, attention maps can be elicited by moving to the other end of the continuum of intentionality: manual selection. This was implemented in a task we will call _drawing_: participants used their mouse to draw a polygon around the most relevant area. This avoids the risks of spontaneous gaze but comes with the risk of participants not being aware of their inner processes. This could take two forms. First, participants might select the whole area that defines a category (e.g., all the sand in a desert). While this choice is valid given that natural scenes are defined by global properties, it might include areas that people would never actually attend to. Second, the opposite risk is for participants to select arbitrary areas (e.g., a small patch of desert sand). Presumably, these areas would be different for each participant, providing little generalizable information.  
Considering the complementary risks of spontaneous gaze and drawing, we introduced a third task to combine the benefits and mitigate the costs: _gaze-pointing_. We instructed participants to intentionally fixate the areas most relevant for categorization. This task was inspired by two previous findings. First, tracking participants' eyes while they explained why an image matched a category label led them to focus on key features [51]. Second, gaze-pointing caused eye movements to be more focused on relevant parts of a scene than free viewing [60]. Gaze-pointing still is likely to include fixations on task-irrelevant areas but might compensate for this by putting additional weight on task-relevant areas that participants would not spontaneously look at (e.g., desert sand). From a practical perspective, gaze-pointing can tell us whether attention maps benefit from additional instruction or whether spontaneous gaze is sufficient to elicit useful attention maps.  
Concerning the similarity of human attention maps to CNN, different outcomes are conceivable. On the one hand, if our concerns about spontaneous gaze and drawing are warranted, this should lead to higher similarity with gaze-pointing than the two other tasks. On the other hand, if the concerns about one or both tasks are unwarranted, we might find different results, depending on which task actually is problematic. However, we also expected these effects to be highly dependent on image type, which is why we consider the interaction to be most informative.