John G. Neuhoff

 

Interacting and Separable Perceptual Dimensions

Previous research using discrete static stimuli has shown that when subjects evaluate a stimulus on a dimension of interest, they sometimes encounter interference from orthogonal variation of an irrelevant dimension. The two dimensions interact with each other and are considered "integral dimensions" (Garner 1974). For example, using "converging operations," (i.e. speeded sorting, restricted classification, and dissimilarity scaling), subjects who evaluate the brightness of a stimulus suffer interference (i.e. speed and accuracy deficiencies) if the irrelevant dimension of saturation is varied orthogonally. Conversely, "separable dimensions" such as brightness and size show no such interference (Attneave, 1950; Garner, 1974; Gottwald & Garner, 1975; Handel & Imai, 1972).

Garner has suggested, and others (Grau & Kemler-Nelson, 1988; Melara & Marks, 1990) have demonstrated that the dimensions of pitch and loudness are integral. According to the traditional view, stimuli consisting of integral dimensions are initially perceived as dimensionless, unanalyzable, holistic "blobs" (Garner, 1974; Lockhead, 1972, 1979). The individual dimensions constituting the stimuli in effect are not perceived. The psychological distance between stimuli can best be described by a Euclidean metric, and stimuli themselves are processed in a holistic, "unitary" manner (Shepard, 1964). In other words the subject does not have primary access to dimensions in question, and cannot selectively attend to one dimension. Rotation of these dimensional axes therefore, does not change in a meaningful way the psychological distance between two stimuli. This is not to say that a dimensional structure cannot be extracted from integral dimensions, but that it is a more derived and secondary cognitive process (Garner 1974; Kemler-Nelson, 1993).

Alternatively, Melara and Marks (1990; Melara, Marks, & Potts, 1993) have advanced a model of dimensional interaction that proposes a primary orientation of the dimensional axes, and mandatory immediate access to interacting dimensions. This access to primary axes is called attribute-level processing, since subjects extract individual attributes from the dimensions of interest. With interacting dimensions, the extraction of a dimensional attribute creates a context in which attributes of the other dimension are perceived. This influence of context is called stimulus-level processing. In the case of interacting dimensions then, the perception of an attribute on one dimension is influenced by the context created by an attribute in the other dimension. In the words of Melara and Marks, "the attribute high pitch has one perceptual meaning when paired with the attribute loud but a different meaning when paired with the attribute soft. Context established by loudness values thus acts to weight perceptually the extraction of pitch information; this is stimulus-level processing." (Melara & Marks, 1990, p.399.)

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