Psychologists have been studying visual search in the lab for decades, in order to understand how we might have developed real-world ‘foraging’ behaviour. But just how similar are the two?
One of the big questions in vision research over the past 40 years has asked how we effectively search around our visual environment. Search is something that we unwittingly engage in every day of our lives – whether it’s looking for our car keys, scrabbling around for a lost contact lens, or rummaging around in a bag for a lost pen lid. But the way in which researchers have classically tested the limits of visual search have looked very different to what we might think of as search in the real world.
Try it yourself
In a typical lab setting, participants might be presented with an image like the one below, and asked to find the target T in and among a number of distractor Ls:
People are usually pretty fast at this version, because the target and distractors only differ on one feature. However, search becomes much harder if you have to take into account more than one feature – say, trying to find a red L in and among red Ts and blue Ls:
Among many features of visual search, what typically happens in these cases is that the amount of time it takes to search through the display increases systematically depending on how many items there are in the search array. This makes intuitive sense; if you only have to ‘search’ among two items, you’ll find the target quickly. Ramp that up to 10,000 items, and you would be there a while:
Visual search versus foraging
There are also differences in this relationship depending on whether there’s a target present in the display, or no target is there. But aside from this, back in 2001, researchers at the University of Bristol asked a more fundamental question of the visual search paradigm: Does it actually resemble real-life foraging? Before that point, many visual search studies had used this as a motivation for using a computer-based task (or equivalent): visual search is an important real-world behaviour, and by constraining it to a simple experimental paradigm, it was argued, we could better understand the way that the visual system works. No one had ever really checked that claim out, though.
To answer that question, Iain Gilchrist, Alice North and Bruce Hood developed a simple but ingenious experiment: instead of getting people to search through a computer display, they got them to forage around a grid of film canisters laid out on a floor. The task was to find the ‘target’ – in this case, a marble hidden inside one of the canisters. Just like the computer-based versions, the task could have either a target-present or a target-absent condition, and the number of items in the display could be varied.
…memory processes might therefore play an important role in foraging.
Gilchrist and his team found some similarities to traditional visual search, but also some important differences. As you might expect, search time increased linearly as the number of items in the display increased. However, whereas in a computer-based search task you might see participants returns fairly frequently to items that they’ve looked at before, in the foraging experiment revisits were relatively rare. Gilchrist’s team argued that memory processes might therefore play an important role in foraging. This makes sense, because it takes a lot more effort (not least in terms of physical exertion) to search through a real-world foraging display than it does a computer screen – so it pays to be selective in terms of the locations you revisit.
This idea was explored further in a follow-up study in 2005 in a newly-developed foraging lab at Bristol. Instead of film canisters, targets and distractors were represented by lights and switches embedded in a floor. All of the lights were initially green, and the children had to search for a hidden red light by pressing switches at each of the item locations, to see if the light changed colour. Critically, the amount of effort required to do the task was manipulated by having the participants use either their dominant (less effortful) or non-dominant (more effortful) hand to search with. When they had to use their non-dominant hand, children tended to make more revisits to item locations that they had already searched – suggesting that the amount of effort the task required was interfering with memory for those previously visited locations.
The original Gilchrist experiment was a simple extension of a much-researched topic, but one with important consequences for the way psychologists approached foraging. The development of a well-controlled experimental lab that more realistically mimicked how people might actually search for things in the real world was a necessary milestone in accurately modelling and furthering our understanding about a deeply-ingrained human behaviour – one that feels almost effortless to us on a day-to-day basis.
Pete Etchells is a lecturer in biological psychology at Bath Spa University.
This article appeared in theguardian.com on 4 April 2017