An Introduction to Network Psychometrics: Relating Ising Network Models to Item Response Theory Models.

In recent years, network models have been proposed as an alternative representation of psychometric constructs such as depression. In such models, the covariance between observables (e.g., symptoms like depressed mood, feelings of worthlessness, and guilt) is explained in terms of a pattern of causal interactions between these observables, which contrasts with classical interpretations in which the observables are conceptualized as the effects of a reflective latent variable. However, few investigations have been directed at the question how these different models relate to each other. To shed light on this issue, the current paper explores the relation between one of the most important network models-the Ising model from physics-and one of the most important latent variable models-the Item Response Theory (IRT) model from psychometrics. The Ising model describes the interaction between states of particles that are connected in a network, whereas the IRT model describes the probability distribution associated with item responses in a psychometric test as a function of a latent variable. Despite the divergent backgrounds of the models, we show a broad equivalence between them and also illustrate several opportunities that arise from this connection.

Soemmering's ring, an aspect of secondary cataract: a morphological description by SEM.

A Soemmering's ring attached to a patient's I.O.L. was removed and processed for SEM-examination. The enveloping structure of the Soemmering's ring was clearly of capsular origin, adherent along a single line and forming an envelope around the inner material. The envelope had a membranous character. The contents of the ring varied from the outside to the centre. Close to the capsule envelope a more or less amorphous material was found. Most of the contents of the ring was made up of lens-fibre material. Sometimes clear lens fibres were visible with densely packed lens-fibre proteins, sometimes with clusters of recrystallized proteins, rod-shaped or spherical, surrounded by what appeared to be amorphous material. The recrystallized and the amorphous lens-fibre material are similar to material found in cataractous lenses.

Congenital absence of conjugate horizontal eye movements.

Loss of conjugate horizontal eye movements is usually due to a lesion in the pons (tumour, vascular, M.S.). However, the condition may also be congenital, either isolated or as part of Moebius's syndrome. Three patients with congenital absence of all conjugate horizontal eye movements are described. In two of them a conjugate pendular nystagmus was observed. Two of the three patients suffered from progressive scoliosis. Congenital absence of all conjugate horizontal eye movements, associated with progressive scoliosis, probably forms a separate clinical entity.