The effects of similarity and viewpoint on the recognition of objects

Purpose: In human vision, it is generally assumed that the process involved in identifying individual objects is different from that involved in classifying objects. It has been reported that the identification of objects is largely view-dependent. Conversely, studies have shown that the classification of objects, at the basic level at least, is view invariant. This dual-process assumption has been challenged recently and it has been argued that the same process is involved in both identification and classification: The similarity between objects is measured and an increase in inter-item similarity leads to stronger view-dependent recognition performance. But how is similarity measured by the visual system? More specifically, is the measurement scale absolute or is it dependent on the object set? Method: Subjects had to identify a target object from among a set of distractor objects in a series of experiments. The number of target objects and the objective similarity of the target objects to the distractor objects were manipulated across experiments. Recognition performance was measured in terms of error rates and recognition times. Results: The results confirmed the notion that high inter-item similarity affects view-dependent performance since the identification of a single target was view-invariant when distractors were from a different basic-level class (Exp. 1), and view-dependent when distractors were from the same class (Exp. 2). However, when the task involved detecting a target object from a single similar distracror then recognition performance was view-invariant (Exp. 3). Recognition performance was view-dependent when the number of dissimilar target objects increased (to 4 or 6) although each target object had only one similar distractor object (Exp. 4 and 5). Conclusions: The results indicate that viewpoint-invariant recognition, in general, is affected by the level of similarity between objects but that the scale for measuring perceptual similarity between representations can be relative, in that, an increase in the number of targets reduces the sensitivity of the similarity scale and recognition becomes view-dependent.