The visual system is very good at noticing a new object coming into view. However, the system isn’t perfect. If a second object appears near the first one, it takes a little longer to spot it. This phenomenon, known as inhibition of return, has been well-documented. We discussed it in a 2005 post:

If an object appears in one part of our field of view, it temporarily delays our ability to detect another object appearing near it. The effect begins about a third of a second after the first object appears and lasts about a second. If the second object appears sooner than that, we actually notice it quicker. Subsequent research revealed that the effect became progressively smaller at greater and greater distances from the spot where the first object appeared–surprisingly, we’re quicker to spot other objects appearing farther away from the original object.

Recently some researchers have suggested that inhibition of return might make us better food foragers. If we’ve already looked for, say, a grub, in a particular spot, then it’s more efficient for us to focus our attention on a new spot. The problem with this explanation is that foraging is typically much slower than the observed effect. When you think about it, the typical inhibition of return experiment, with letters or other symbols rapidly flashing at viewers, isn’t really much like foraging at all.

To do a realistic foraging experiment, you’d need something like a holodeck — the computer-controlled room in Star Trek which allows simulation of nearly any sort of environment.

At the Beckman Institute of the University of Illinois they’ve actually got a piece of equipment that comes close: a room with translucent walls, floor and ceiling. Each wall can have an image projected on it to simulate a complete immersion in a three-dimensional world. Here’s a schematic of the room, called a Virtual Reality Cube:

A team led by Laura Thomas projected images of tree leaves on one wall of the cube and asked college students to “forage” for fruit hiding behind the leaves using a virtual wand. They pointed to the leaves one at a time, clicking on a response box to “lift” the leaf and see if there was any fruit behind it. The researchers weren’t actually interested in what happened when fruit was found, but instead wanted to see how quickly they could return their attention to leaves they’d already searched. Every so often fruit was found to keep the students interested in the task, but the students were also told that if one of the leaves began to flash, they should press a second button on their response box. In fact, most of the time there was no fruit to be found at all.

The question is, do foragers also demonstrate inhibition of return — when they haven’t found anything yet, do they respond more slowly to flashing leaves they’ve already searched, compared to new flashing leaves? Here are the results:

Foragers were slower to respond to the previous leaf they had just searched (“1 leaf back”) when it flashed, compared to a new leaf an equivalent distance away from where they were pointing when the flashing started [this difference is only marginally significant -- see Ryan's comment below]. The same results held two leaves previous, but the difference in reaction times was not significant for three leaves back.

Previous inhibition of return studies typically only found an effect 1.6 seconds or less from the appearance of an object, so this result is dramatically different, with significant differences in reaction times as long as 3.4 seconds after a leaf was initially searched. Thomas et al. argue that the result strongly suggests that inhibition of return may be related to adaptations favoring more efficient foraging.

But why hadn’t previous researchers found an effect so long after the object in question appeared? It may be that inhibition of return changes based on the relative speed of the search, so that in fast-paced searches the effect is faster, while in slow searches like foraging, the effect lingers. Thomas’s team makes a convincing case that this apparent deficit in visual processing may actually enhance our ability to forage for food.

I’d be interested to know if better foragers — the fastest berry-pickers, for example — actually have a more pronounced inhibition of return effect.

Thomas, L.E., Ambinder, M.S., Hsieh, B., Levinthal, B., Crowell, J.A., Irwin, D.E., Kramer, A.F., Lleras, A., Simons, D.J., & Wang, R.F. (2006). Fruitful visual search: Inhibition of return in a virtual foraging task. Psychonomic Bulletin & Review, 13(5), 891-895.