If you want to measure gaze points (where someone is looking at) or eye movements, there is no alternative to eye tracking. It is hard to consciously control your eyes, which makes this measure a very attractive tool for quantitative research. It allows you to tap into subconscious processes and decisions of your audience and understand what elements of your products or services such as advertisements, websites, layouts and models etc. trigger the fundamental brain circuits responsible for attention, cognition and emotion. Also, you gain insights into individual preferences and decision strategies of your target group of interest.
In the following, the most common metrics and terminologies in eye tracking will be presented:
1. Fixations and gaze points
When we talk about eye tracking, most likely fixations and gaze points are the most used terms in the conversation. Gaze points show towards which elements of a stimulus the eyes are looking at. If your eye tracker collects data with a sampling rate of 60 Hz, you will end up with 60 individual gaze points per second. If a series of gaze points is very close – in time and in space – this gaze cluster constitutes a fixation, denoting a period where the eyes are locked towards an object. The eye movements between fixations are generally referred to as saccades. When we read, for example, our eyes don’t travel smoothly. We lock our eyes towards every third or fourth word. The term “visual span” refers to how many words we can read before and after the currently fixated word. Trained readers have a higher visual span, allowing them to cover more text with less fixations. By contrast, when we watch a distant car driving by, our eye movements are quite different. Here, we fixate constantly the tracked object, and there won’t be any obvious saccades (this is generally referred to as “smooth pursuit”). Fixations and saccades are excellent measures of visual attention and interest, and research in this field is experiencing a significant boom.
2. Heat maps
Heat maps are visualizations which show the general distribution of fixations and gaze points. They are therefore indicators of someone’s attention, with red areas suggesting a high number of gaze points (increased level of attention), followed by yellow and green. Heat maps can be generated for single respondents as well as for a full study of several participants. In this case, eye tracking heatmaps are an excellent method to visualize which elements attract more attention than others. iMotions software even allows to draw a heat map for the first 500 milliseconds, which then visualizes the subconscious processing or attention.
3. Areas of Interest (AOI)
An area of interest, also referred to as AOI, is a tool to select subregions of the displayed stimuli, and to extract metrics specifically for these regions. For instance, if you show pictures of a person, it is possible to draw separate AOIs around the body and the face. You will then be able to display metrics for each region separately, e.g., how much time from stimulus onset passed until respondents looked into the region (time to first fixation = TTFF), how much time your respondents spent in the region, how many fixations were counted, how many people looked away and back (revisits). These metrics come in handy when evaluating the performance of two or more areas in the same video, picture, website or program interface.
3.1 Time to first fixation (TTFF)
The time to first fixation indicates the amount of time that it takes a respondent (or all respondents on average) to look at a specific AOI from stimulus onset. TTFF can indicate both bottom-up stimulus driven searches (e.g., a flashy company label) as well as top-down attention driven searches (e.g., when respondents actively decide to focus on certain elements or aspects on a website or picture). TTFF is a basic yet very valuable metric in eye tracking.
3.2 Time spent
“Time spent” quantifies the amount of time that respondents have spent on an AOI. Time spent often indexes motivation and top-down attention, since respondents have to blend out other stimuli in the visual periphery that could be equally interesting. Long prevalence at a certain region clearly indicates a high level of interest, while shorter prevalence times indicate that other areas on screen or in the environment might be more interesting.
The ratio allows extracting information about how many of your respondents actually guided their gaze towards a specific AOI. In market research it might be relevant to optimize an advertisement so that more people are “drawn” towards a specific region in a picture or ad. A higher ratio might indicate that fixations and gaze points are driven rather by external aspects in the stimulus material (bottom-up), or that the target group is very consistent in looking towards a specific AOI while ignoring others.
4. Fixation sequences
Based on fixation position (where?) and timing information (when?), you can generate a fixation sequence. As your eyes screen an environment, they wander, and with this your attention. Dependent on where people look and how much time they prevail, you can build an order of attention, telling you where people looked at first, second, third etc. Order of attention is a commonly used marker in eye tracking research, since it reflects a person’s interest as well as “salient” elements in the display or environment (i.e., elements that stand out in terms of brightness, hue, saturation etc.). AOIs which respondents look at first are typically visually more appealing (more salient) and are therefore of more interest.
These basic terms and metrics in eye tracking are key to get started. They quantify visual attention which is driven by cognitive and emotional processes as well as object characteristics. Often eye tracking is combined with other biometric sensors such as EEG, GSR, ECG/EMG and facial expression analysis as well as surveys to get a more complete idea of why people gaze towards certain things and what happens when they do.