INDUCING VARIABILITY IN COMMUNICATIVE GESTURES USED BY SEVERELY RETARDED INDIVIDUALS

Handicapped individuals who have been taught spontaneous gesture requests often use only a small part of their vocabulary. Procedures to recover the unused part of this vocabulary have not been documented. This study was designed to identify procedures for increasing the proportion of gestures used spontaneously. Six mentally handicapped individuals served as subjects. After a baseline phase during which spontaneous gesture requests were reinforced, consequences were withheld for high-rate gesture requests. This led to an increase in different gesture requests. Although gesture requests did not return to baseline levels during a reversal condition, functional control was demonstrated by way of a multiple baseline across subjects. The absence of a reversal effect suggests enduring effects of the procedure.     

Local and global mechanisms of one- and two-dimensional orientation illusions.

One-dimensional (1-D) orientation illusions induced on a test grating by a tilted and surrounding 1-D inducing grating have a well-known angular function that exhibits both repulsion and attraction effects. Two-dimensional (2-D) orientation illusions are those induced on a test grating by 2-D image modulation, such as a pair of superimposed inducing gratings at different orientations, usually orthogonal (a plaid). Given the known angular functions induced by the plaid component gratings, two hypotheses were developed that predicted different plaid-induced illusion functions. Hypothesis 1 states that the 1-D component-induced effects simply add linearly; Hypothesis 2 states that there is an additional mechanism that responds to the virtual axes of mirror symmetry of the plaid and adds to the effect. The data of two experiments were consistent with the predictions from the second hypothesis but not the first. Possible neural substrates of mechanisms that extract axes of symmetry are discussed; it is suggested that such global symmetry axes may underlie the perceived orientation of complex shapes.     

Analysis of the components of disparity in Wilde's dot-row phenomenon

Summary In this study on Wilde's phenomenon (Wilde 1950) the two components of disparity, one of them processing displacement, and the other one apparent rotation, are analysed in terms of dependence on the disparity of the end-lines of the pattern (), and on the percentage of magnification (M) of one of the monocular patterns in relation to the other one. It was found that the component of disparity for displacement ' can be expressed as a linear regression equation '=–a+b.The component of disparity for rotation, expressed as a percentage of magnification effective for rotation (M) can be expressed as M=a–b₁+b₂M.It was concluded that the two components of disparity are processed through independent parallel channels, the processing of the component of disparity for displacement being the faster process, accounting for the larger part of the total disparity.     

Maximum likelihood estimation for Luce's choice model

A terminology for general choice models based on the choice axiom is given. It applies to all kinds of choice experiments, such as confusion choice experiments, paired comparisons, triadic comparisons, directional rankings, scores on binary test items, and others. Maximum likelihood estimation for such general choice models is considered. Conditions for the uniqueness of maximum likelihood estimates are given, and it is shown that the estimates can be derived by iterative proportional fitting. This offers the opportunity of a general test of the choice axiom for all kinds of choice experiments using the likelihood ratio. The estimation and testing procedure is applied to data from a form recognition experiment, reported by W. A. Wagenaar (Nederlands Tijdschrift voor de Psychologie, 1968, 23, 96–108).