Recent work with brain-damaged pationts has provided evidence for a tentative neuroanatomical localization of mental image generatino in the posterior left hemisphere. This evidence will be briefly summarized and critiqued. And a new test of the localization, using normal subjects, will be presented. When mental images of stimuli were used a templates to facilitate a visual discrimination, the effect of imagery was greater for stimuli presented in the right visual field (left hemisphere) than in the left visual field (right hemisphere). This result is discussed in relation to earlier claims about the hemisphericity of imagery.

Evidence suggests that an early representation in the visual processing of orthography is neither visual nor phonological, but codes abstract letter identities (ALIs) independent of case, font, size, etc. How could the visual systeai come to develop such a representation? W e propose that, because many letters look similar regardless of case, font, etc., different visual forms of the same letter tend to appear in visually similar contexts (e.g., in the same words written in different ways) and that correlationbased learning in visual cortex picks up on this similarity among contexts to produce ALIs. W e present a simple selforganizing Hebbian neural network model that illustrates how this idea could work and that produces ALIs when presented with appropriate input.

Letters are detected more efficiently among digits than among letters. This alphanumeric category effect suggests an architectural distinction between letter and number representation in human vision and dissociations between letter and number recognition following brain damage support this interpretation. Because letter and number recognition are not innate, this implies that experience can shape the functional architecture of vision. A possible explanation is that letters co-occur with letters in the environment while numbers co-occur with numbers; such statistics cause segregation of letter and number representations in artificial neural networks. To test the general hypothesis that environmental statistics affect the architecture of vision, and the specific hypothesis that within-category cooccurrence causes the alphanumeric category effect, we measured the effect in foreign mail sorters who process Canadian zip codes (which violate the co-occurrence statistics) and in control subjects. As predicted, foreign mail sorters showed a smaller category effect

A computer model is presented which performs four different types of tasks sometimes impaired by frontal damage: the Wisconsin Card Sorting Test, the Stroop task, a motor sequencing task and a context memory task. Patterns of performance typical of frontal-damaged patients are shown to result in each task from the same type of damage to the model, namely the weakening of associations among elements in working memory. The simulation shows how a single underlying type of damage could result in impairments on a variety of seemingly distinct tasks. Furthermore, the hypothesized damage affects the processing components that carry out the task rather than a distinct central executive responsible for coordinating these components.