Academic journal article Attention, Perception and Psychophysics

Haptic Pop-Out of Movable Stimuli

Academic journal article Attention, Perception and Psychophysics

Haptic Pop-Out of Movable Stimuli

Article excerpt

Published online: 18 October 2011

© The Author(s) 2011. This article is published with open access at

Abstract When, in visual and haptic search, a target is easily found among distractors, this is called a pop-out effect. The target feature is then believed to be salient, and the search is performed in a parallel way. We investigated this effect with movable stimuli in a haptic search task. The task was to find a movable ball among anchored distractors or the other way round. Results show that reaction times were independent of the number of distractors if the movable ball was the target but increased with the number of items if the anchored ball was the target. Analysis of hand movements revealed a parallel search strategy, shorter movement paths, a higher average movement speed, and a narrower direction distribution with the movable target, as compared with a more detailed search for an anchored target. Taken together, these results show that a movable object pops out between anchored objects and this indicates that movability is a salient object feature. Vibratory signals resulting from the movable ball were found to be a reasonable explanation regarding the sensation responsible for the pop-out of movability.

Keywords Haptic search . Pop-out . Motion perception . Exploratory movements

Humans are able to recognize many common objects and discriminate them from others by just touching them. An everyday example of this is the search for your keys in a full bag. When trying to find the keys, one searches for certain characteristic features of the keys that set them apart from the other things in the bag-for example, the coldness or hardness of the metal. When objects are searched for, some searches are easier because certain features are more distinctive than other features and have the property to "stand out" from others. This phenomenon is called the pop-out effect. In this case, the target property that is to be found pops out from distractor items that do not exhibit the target property (Treisman & Gelade, 1980). The pop-out effect has been found not only in vision (Treisman & Gelade, 1980; Treisman & Souther, 1985; Wolfe, 1998), but also in haptic search (Lederman & Klatzky, 1997; Plaisier, Bergmann Tiest, & Kappers, 2008, 2009). An easy visual example of this is the pop-out of a red dot among green dots. When the task is to find the red dot, it is immediately seen, without the need for a thorough search. In contrast, when a slanted line is searched for among upright lines, the search is more difficult. The slanted line is less distinctive from the upright line distractors.

Another interesting phenomenon is when the pop-out effect disappears when the properties of the target and distractor are interchanged; this is known as a search asymmetry. For example, the search is more efficient when a rough item is searched for than when a smooth item is the target (Plaisier et al., 2008). In other words, the rough item pops out from among the smooth items, but a smooth item does not pop out from among rough items.

In the situations described in the examples above, often the distinction between a parallel and a serial search strategy is made. In the case of a serial strategy, the search is more difficult, and all items must be explored one by one to find out whether it has the target property. In a parallel search, multiple items can be searched at once, which makes the search much easier. In an experiment, typically, reaction times are measured to illustrate the difference between these two search strategies. Reaction times are longer in the serial strategy and, because additional time is needed for each extra item, reaction time tends to increase with increases in the number of distractors. In the case of a parallel strategy, the reaction time is independent of the number of items, and the target pops out. If a feature pops out, it is believed to be a salient feature. …

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