This is something that is very familiar at academic eye tracking conferences in fact every other paper refers to it, but I don’t ever recall seeing it in the commercial sector.
In 1967 Yarbus eye tracked people viewing the painting “They did not expect him” by Repin. The instructions given to the participants varied from, among other, free examination, to the people’s ages, and how long the visitor had been away.
In the gaze plots below, you will see significant differences depending on the instruction given.

At this point, probably needless to say, when using eye tracking in market research, it is very important to give appropriate instructions to the participants, and when analysing heatmaps to know what instruction was given.
In our ThinkPack eyetracking test for the FMCG and CPM market, we show participants two planograms, but give them different tasks with each one as we are investigating two different shopping experiences and elements of the pack performance. With the first planogram we are interested in the customer’s free shopping behaviour to see how well the product stands out on the shelf: Do people notice the product? With the second planogram we are interested in how easy it is to find the specific product: Do people confuse the product with another one? Does the product branding stand out?
In the first heatmap below, participants were asked to shop for deodorant. They were to take a look at the shelf and click on the product they would choose to purchase.
In the second heatmap, participant were again asked to shop for deodorant, but to buy “Formula 1 Performance” deodorant as their friend had recommended it. You can see two very different heatmaps are produced.

Just to reiterate the message here; when designing eye tracking research be careful what task you give to the participants, and when drawing conclusions from eye tracking outputs be aware of the task given. It is not easy to control your eyes, but it is actually fairly easy to control what people look at during an eye tracking exercise simply by giving particular tasks. That said, you need to be careful that the outputs are not a result of inappropriate tasks.