Haptic perception of mutiple objects

Abstract

In this thesis a series of investigations into haptic (touch) perception of multiple objects is presented. When we hold a collection of objects in our hand, we can extract different types of information about these objects. We can, for instance, identify which objects we are holding. The first chapters of this thesis aim at providing insight into how fast humans can find a certain object among other objects using touch and which specific features make an object stand out among the other objects. To this end human subjects were instructed to respond as fast as possible whether a certain target item was present among a varying number of distractor items. This way response times were measured as a function of the number of items. In chapters 2 and 3 subjects were asked to search a plane on which items could be placed. The results show that a rough item is highly salient among less rough items (chapter 2) and that in this produces ‘pop-out’ effect. In chapter 3 it is shown that very poor visual information can already guide haptic exploration effectively. In chapters 4 and 5 items consisted of three-dimensional shapes (spheres, cubes, tetrahedrons, cylinders and ellipsoids) that could be grasped together in the hand. We show that shapes with edges are highly salient and that there is a whole range of search slopes depending on the target –distractor combination. In addition to identifying the object we may hold in our hand, we can also determine how many objects we are holding. In chapters 6 to 8 we investigated haptic numerosity judgement. From vision it is known that numerosity judgment is fast and error-free up to 3 or 4 items, while for larger numbers response times and error-rates increase rapidly. The process used for assessing small numerosities has been labeled ‘subitizing’, while the process for larger numerosities is referred to as ‘counting’. In chapter 6 we show that subitizing also occurs in haptics when subjects are asked to determine the number of spheres grasped in their hand. A visual study was conducted in chapter 7 to show that visual and haptic numerosity judgment is comparable. In chapter 8 we show that numerosity judgment can three times faster when the items are distributed over the two hands compared when they are all held in the same hand. Finally, in chapter 9 we show that numerosity judgement is not affected by size or shape differences between the objects in a set. The studies in this thesis show that haptic perception can be fast and that not all items have to be necessarily explored sequentially. Haptic information can be processed in parallel across the different items in a set when searching for a salient target item or when judging small numbers of items.