Academic journal article Canadian Journal of Experimental Psychology

Visual Search for Features and Conjunctions in Development

Academic journal article Canadian Journal of Experimental Psychology

Visual Search for Features and Conjunctions in Development

Article excerpt

Abstract Visual search performance was examined in three groups of children 7 to 12 years of age and in young adults. Colour and orientation feature searches and a conjunction search were conducted. Reaction time (RT) showed expected improvements in processing speed with age. Comparisons of RTs on target- present and target-absent trials were consistent with parallel search on the two feature conditions and with serial search in the conjunction condition. The RT results indicated searches for feature and conjunctions were treated similarly for children and adults. However, the youngest children missed more targets at the largest array sizes, most strikingly in conjunction search. Based on an analysis of speed/accuracy trade-offs, we suggest that low target-- distractor discriminability leads to an undersampling of array elements, and is responsible for the high number of misses in the youngest children.

Visual search paradigms are ideal for studying perceptual and cognitive factors in visual selective attention. In a typical experiment, observers determine whether or not a specified target exists in an array of distractors. Array size is varied and the complexity of target-distractor relationships is manipulated to vary search difficulty. For example, in easy feature searches, the target generally differs from the distractors on the basis of a single dimension such as colour (e.g., a search for a red bar among blue and green bars), or contains a feature not present in the distractors (e.g., a "Q" among "Os"). Search is more difficult when the target is defined by a conjunction of features present in the &tractors (e.g., size and colour: a small red circle among large red circles and small blue circles) or is defined as the absence of a feature present in the distractors (e.g., an "O" among "Qs").

Search difficulty and attentional resources required are typically measured by differences in the slopes of search reaction times (RT) across array size. These RT slopes tend to fall into two categories -- shallow and steep. Shallow slopes are taken to indicate fewer attentional resources are required to detect the presence or absence of the target. Steeper slopes occur when the search is more difficult and reflect the degree to which attentional resources are allocated. In easy searches, target-absent and target-present trials produce shallow, parallel slopes. In difficult searches, target-absent slopes are frequently twice as steep as target-present slopes. These task-- related differences between slopes have supported the hypothesis of parallel, exhaustive search in the first case, and serial, self-terminating search in the second (see Townsend & Ashby,1983, for a discussion of alternative models).

In spite of considerable interest in the development of selective attention (for reviews see Cooley & Morris, 1990; Halperin,1996; Plude, Enns, & Brodeur, 1994), surprisingly few studies have used visual search tasks in children. Early work varied the types of distractors or number of targets in an array and the findings demonstrated age-related improvements in search speed and decreased interference from distractors (e.g., Day,1978; Gibson & Yonas,1966a; Gibson & Yonas,1966b). Ruskin and Kaye (1990) manipulated stimuli based on the separability of their features (e.g., Gamer,1974) and examined feature and conjunction searches in 5- to 12year-old children. At all ages, stimuli made from two easily separable components (circle size and line orientation) generated steep (serial, > 20 ms/item) slopes in conjunction search and flat (parallel, < 20 ms/item) slopes in feature search. However, the slopes in the feature search decreased from 13 ms/item in the 7- to &year-olds to 7 ms/item in the 11- to 12-year-olds, and from 37 ms/item to 25 ms/item in the conjunction search; and findings for RT (slope intercept) were similar. As both RT and RT slopes decreased with increased age in both types of search, this indicates the younger children were globally less efficient than older children. …

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