A monkey was trained to respond on the basis of the serial position of a test stimulus in a sequence. First, three stimuli were presented successively on a circle. Then one of them (except the last) changed color (test stimulus) and served as the go signal: The monkey was required to produce a motor response in the direction of the stimulus that followed the test stimulus. When the test stimulus was the second in the sequence, there was a change in motor cortical activity from a pattern reflecting the direction of this stimulus to the pattern associated with the direction of the motor response. This change was abrupt, occurred 100 to 150 milliseconds after the go signal, and was evident both in the activity of single cells and in the time-varying neuronal population vector. These findings identify the neural correlates of a switching process that is different from a mental rotation described previously.

No abstract

Transmission and scanning electron microscopy were utilized to follow the degeneration of bile ducts of lampreys (Petromyzon marinus L.) during metamorphosis. The convoluted bile ducts of larval lampreys are surrounded by rich sinusoids, but this intimate biliovascular relationship is lost during metamorphosis because the bile duct degeneration is accompanied by the development of thick periductal fibrosis. Lipocytes, which are present not only in the parenchyma but also in the interstitial tissue of the liver, increase in number in the periductal fibrous tissue, and their processes are directly opposed to collagen fibrils. Fibrillar materials in the dilated cisternae of the rough endoplasmic reticulum and the nuclear envelope of lipocytes are believed to be excreted by exocytosis in a manner similar to such excretion by fibroblasts. The findings suggest that lipocytes are responsible for the periductal fibrosis during biliary atresia in lampreys. This animal might prove to be an interesting model in which to study the biology and fibrogenic potential of lipocytes.

Measurements of plating efficiency, accumulation of metaphases and generation times have shown that fibroblast from patients with Fanconi anemia (FA) have decreased probability of completing a further division after successful mitosis. Thus FA cells show decreased growth rates and increased generation times. We have also measured the survival of FA fibroblasts and lymphoblasts after treatment with a variety of mutagens. All FA cells show an increased sensitivity to drugs such as MMC and psoralen plus long wave length UV which cause DNA interstrand crosslinks. FA strains show varying degrees of sensitivity to these drugs and the extent of this sensitivity seems to be characteristic of each patient. FA cells are equal to controls in their sensitivity to other alkylating agents such as ethyl methane sulfonate, N-methyl-N1-nitro-N-nitrosoguanidine and actinomycin D. Both the decreased growth and increased drug sensitivity may result from defect in DNA replication or repair.