REPRESENTATION OF OBJECTIVE SIMILARITY AMONG 3-DIMENSIONAL SHAPES IN THE MONKEY

Humans have been recently shown to represent parameterized three-dimen sional objects in a manner that preserves relative similarities (as me asured by parameter-space distances) among the objects (Cutzu and Edel man 1996). We show that the representation of objects in the monkey vi sual system is similarly faithful to the parametric variation built in to the stimulus set, A monkey (Macaca fuscata) performed a delayed mat ching-to-sample task with 28 images (4 views x 7 objects). Stimuli in each of the two experiments were seven computer-rendered parameterized animal-like shapes, arranged in a low-dimensional configuration (name ly, a two-dimensional TRIANGLE) in a common 56-dimensional parameter s pace. The monkey's task was to match objects (not views). Each experim ent lasted for 3 - 4 weeks after the introduction of the stimulus set to the subject. Error rates were entered into a 7 x 7 object confusion matrix and submitted to nonmetric multidimensional scaling (MDS). In both experiment 1 (mean correct rate 69.7%) and experiment 2 (mean cor rect rate 59.9%), the MDS solutions resembled closely the low-dimensio nal parameter-space patterns built into the stimuli, in the sense that the point corresponding to the central one in the original pattern wa s inside the other six points and that the order of the six points in angular positions around the center point was preserved, A simulation study showed that the resemblance could not be due to chance. These re sults demonstrate the possibility of veridical representation of param etric similarity among complex objects in the monkey.