Controlling working memory with learned instructions.

Many neural network models of cognition rely heavily on the modeler for control over aspects of model behavior, such as when to learn and whether an item is judged to be present in memory. Developing neurocomputational methods that allow these cognitive control mechanisms to be performed autonomously has proven to be surprisingly difficult. Here we present a general purpose framework called GALIS that we believe is amenable to developing a broad range of cognitive control models. Models built using GALIS consist of a network of interacting "regions" inspired by the organization of primate cerebral cortex. Each region is an attractor network capable of learning temporal sequences, and the individual regions not only exchange task-specific information with each other, but also gate the others' functions and interactions. As a result, GALIS models can learn both task-specific content and also the necessary cognitive control procedures (instructions) needed to perform a task in the first place. As an initial test of this approach, we use GALIS to implement a model that is trained simultaneously to perform five versions of the n-Back task. Not only does the resulting n-Back model function correctly, determining when to learn or remove items in working memory, but its accuracy and response times correlate strongly with those of human subjects performing the same task. The n-Back model also makes testable predictions about how human accuracy would be affected by intra-trial changes in n's value. We conclude that GALIS opens a potentially effective pathway toward developing a range of cognitive control models with improved autonomy.

Analysis of a measles outbreak in Kent County, Michigan in 1990.

New recommendations from the Immunization Practices Advisory Committee call for a second dose of measles vaccine for school age children. The purpose of this paper is to demonstrate the need for additional measures for the protection of preschool age children. A hospital-based measles outbreak of 25 cases in Kent County, MI, in 1990 provided an opportunity to study measles transmission to preschool age children. Twenty-two (88%) were people who had never received measles vaccine. Twelve of the cases were unvaccinated preschoolers, seven of whom were older than 15 months. Three nonvaccinated, but eligible people (one philosophic exemption and two vaccine-eligible preschoolers) were the source of most of the other cases. One school age unvaccinated child died of measles pneumonitis. A telephone survey indicated that improved public education regarding indications and contraindications to vaccination might encourage vaccination of children according to public health recommendations.

Intracranial pressure reserve testing. A study in experimental animals.

Quantitating cerebral "elastance" or "ICP reserve" has added considerably to the value of continuous monitoring of intracranial pressure (ICP). Intracranial pressure reserve is a measure of the capacity of the brain's natural compensatory mechanisms for countering increases in ICP secondary to increases in intracranial volume. Intracranial pressure reserve testing was studied in dogs with known volumes of extracerebral intracranial mass, both in normal and in edematous brains and at various ICPs. Ten thousand measurements were made with five different methods of measuring ICP reserve. Testing when multiple increments of subdural saline infusion were used over a five-minute period to quantitate ICP reactivity to volumetric stress seemed most reliable and most adaptable to clinical application. Methods of measuring cerebral "elastance" using only a single subdural infusion proved to be variable and difficult to read.

Focal chemotherapy of brain tumours using semipermeable membranes.

Semipermeable silastic rubber membranes can be used to diffuse focally a variety of chemicals including antitumour drugs. Of the eight drugs tested, in vitro tests showed the best diffusion with dactinomycin, mithramycin, and oncovin, and the poorest diffusion with bleomycin, fluorouracil, and thiotepa. Biological testing was performed with mithramycin, dactinomycin, and oncovin using tissue cultures of human glioblastoma and subcutaneous implants of mouse ependymoblastoma. All three drugs caused rapid tissue culture cell death with direct injection, and impeded tumour growth when given intraperitoneally. Dactinomycin by silastic diffusion proved more effective than mithramycin against tissue cultures, but neither drug had a significant effect against local tumours treated with implanted drug capsules. Silastic diffusion of oncovin reduced tumour sizes significantly ipsilateral to the implant, compared to contralateral implants and to untreated controls.