No abstract

Brain-stem auditory-evoked potentials were recorded in neurosurgical patients from surface electrodes applied to the VIIIth nerve, medulla, pons, midbrain and cortex; from depth electrodes in the thalamus; and from a movable electrode in the IVth, IIIrd, and lateral ventricles. The potentials recordable over the scalp within 10 ms after click stimulation are characterized by a slow positive wave (peaking at 5 to 6 ms) and a negative wave (8 to 10 ms) with 7 small positive wavelets superimposed upon them. The sources of these components have been identified by observing their increased amplitude in depth recordings, and by tracing the potentials from their intracranial maxima to the scalp. Wave I is generated within the most distal portion of the VIII nerve; Wave V in the midbrain (inferior colliculus); and Wave VI the medial geniculate body. Both low positive and negative components have their origins in the inferior colliculus. Intracranially-recorded brain-stem auditory-evoked potential showed very rapid changes in amplitude within the brain-stem but only slight changes in the more rostral regions, although their amplitude gradients varied in the different components. They also demonstrated minor but systematic shifts in latency with distance from the potential sources, reflecting a significant overlap of separate potentials. This effect must be taken into account in the interpretation of a 'concurrent' intracranial potential as the source of a far-field surface-recorded potential.

Differential dissolution in NaOH solution was investigated as a means of purification of clay. It was discovered that a large quantity of allophane and free alumina and silica were dissolved from clay preparations in 0.5 N NaOH solution by boiling for as little as 2.5 min, provided the ratio of clay weight to solution volume was kept less than 100 mg to 100 ml. Reprecipitation of dissolved silica occurred if a more prolonged digestion or higher sample to solution ratio was employed. Also, prolonged digestion in the NaOH solution (for 80 min) dissolved as much as 50 percent of kaolinite (Merck) and 25 percent of Wyoming montmorillonite, but only a small quantity of these crystalline minerals was dissolved during the 2.5 min digestion period. Subsequent dithionite-citrate-bicarbonate treatment removed the released iron. Marked improvement of x-ray diffraction patterns of the clays resulted. This rapid and selective dissolution of allophane and free oxides greatly improves the quantitative analysis of crystalline minerals by conventional methods.After dehydroxylation at 500°C, kaolinite and halloysite became amorphous and also dissolved by the same differential dissolution procedure. Heat-stable (aluminous) montmorillonite and chlorite were only slightly dissolved by this treatment. The selective removal of the 1 : 1 layer aluminosilicates greatly improves the quantitative analysis of the remaining crystalline components of clays by conventional methods.