Too little, too late: reduced visual span and speed characterize pure alexia

Whether normal word reading includes a stage of visual processing selectively dedicated to word or letter recognition is highly debated. Characterizing pure alexia, a seemingly selective disorder of reading, has been central to this debate. Two main theories claim either that 1) Pure alexia is caused by damage to a reading specific brain region in the left fusiform gyrus or 2) Pure alexia results from a general visual impairment that may particularly affect simultaneous processing of multiple items. We tested these competing theories in 4 patients with pure alexia using sensitive psychophysical measures and mathematical modeling. Recognition of single letters and digits in the central visual field was impaired in all patients. Visual apprehension span was also reduced for both letters and digits in all patients. The only cortical region lesioned across all 4 patients was the left fusiform gyrus, indicating that this region subserves a function broader than letter or word identification. We suggest that a seemingly pure disorder of reading can arise due to a general reduction of visual speed and span, and explain why this has a disproportionate impact on word reading while recognition of other visual stimuli are less obviously affected

The Statistics of the Environment Affect the Functional Architecture of Vision in Adulthood: A Reduce d Alphanumeric Category Effect in Canadian Mail Sorters

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 effec

A Simple Co-Occurrence Explanation for the Developmen t of Abstract Letter Identities

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

Toward a Unified Theory of Immediate Reasoning in Soar

Soar is an architecture for general intelligence that has been proposed as a unified theory of human cognition (UTC) (Newell, 1989) and has been shown to be capable of supporting a wide range of intelligent behavior (Laird, Newell & Rosenbloom, 1987; Steier et al, 1987). Polk & Newell (1988) showed that a Soar theory could account for human data in syllogistic reasoning. In this paper, we begin to generalize this theory into a unified theory of immediate reasoning based on Soar and some assumptions about subjects' representation and knowledge. The theory, embodied in a Soar system (IR-Soar), posits three basic problem spaces (comprehend, test-proposition, and build-proposition) that construct annotated models and extract knowledge from them, learn (via chunking) from experience and use an attention mechanism to guide search. Acquiring task specific knowledge is modeled with the comprehend space, thus reducing the degrees of freedom available to fit data. The theory explains the qualitative phenomena in four immediate reasoning tasks and accounts for an individual's responses in syllogistic reasoning. It represents a first step toward a unified theory of immediate reasoning and moves Soar another step closer to being a unified theory of all of cognition

Paroxysmal Atrial Fibrillation in a Mission-Assigned Astronaut

This presentation will explore the clinical and administrative conundrums faced by the flight surgeon upon discovering asymptomatic paroxysmal atrial fibrillation seven months prior to scheduled long duration spaceflight. The presenter will discuss the decision-making process as well as the clinical and operational outcomes