The main interest of this dissertation was to examine the role of attention in early auditory processing using event-related potential (ERP) recording techniques. The Mismatch Negativity (MMN), a component of the ERP, is an index of early auditory deviance detection. In the present research, the MMN was used to probe the extent to which auditory stimuli were processed when these stimuli were or were not within the focus of attention. Four experiments were carried out. The aim of Experiments 1 and 2 was to investigate whether the demands of common diversion tasks modulated the MMN. In these experiments, subjects were instructed to ignore the auditory sequence and engage in tasks that varied in the amount of attention required to complete them. The results of Experiment 1 indicated that the MMN elicited by small frequency deviants was sensitive to the nature of the task (although this effect could not be clearly related to attention). In contrast, the MMNs elicited by small intensity deviants in Experiment 2 did not vary based on the diversion task demands. A limitation of Experiments 1 and 2 was the absence of an independent measure of task demands. This limitation was addressed in Experiments 3 and 4. A visual discrimination task was employed that permitted quantification of task demands. The visual task was either easy or difficult to perform. Hence, in Experiments 3 and 4, the extent to which the attentional demands of the visual task affected the passively-elicited MMN could be more rigorously examined. Importantly, the MMNs elicited by small frequency and intensity deviants were not significantly modulated by task difficulty, in spite of a wide variation in performance measures between the easy and difficult visual tasks. Experiments 3 and 4 also investigated whether the direction of attention had an effect on the MMN. Subjects were subsequently asked to divide their attention between the visual and auditory channels and to detect all visual and auditory deviant stimuli. This allowed a comparison of the MMN when it was elicited by stimuli within or outside the focus of attention. The results of Experiment 3 indicated that the frequency MMN was unaffected by the direction of attention; the intensity MMN, however, was larger during active attention of the auditory sequence. The attentional modulation of the intensity but not frequency MMN may be due to specific deviant features being more sensitive to the effects of attention. Alternatively, these results could be explained by the difference in perceptual discriminability of the intensity and frequency deviants from the standard (the frequency deviants were detected more accurately and more rapidly than the intensity deviants). Experiment 4 tackled this issue by increasing the discriminability of the intensity deviant while decreasing the discriminability of the frequency deviant. The results showed that the frequency MMN was significantly affected by the direction of attention, whereas the intensity MMN was not. Together, the results suggest that the attentional modulation of the MMN may be limited to the processing of small stimulus changes. Attention may be viewed as a means to sharpen the memory trace of the attended stimuli and consequently enhance the processing of small auditory changes. This explanation would also be consistent with the present findings that the passively-elicited MMN is insensitive to variations in visual task difficulty.

Cell free extracts of glucose grown cultures of P. aeruginosa were prepared by exposure to sonic vibration. These sonicates were capable of oxidizing glucose-6-phosphate, ribose-5-phosphate, glucose, gluconic acid and gluconolactone. Treatment of the sonicate with (NH₄)₂S0₄ resulted in the formation of precipitates which possessed the ability to oxidize glucose, gluconolactone and gluconic acid. After (NH₄)₂SO₄ treatment neither the supernatant nor the precipitate could oxidize the phosphorylated compounds. Since 30.0% (NH₄)₂SO₄ or centrifugation for one half hour at 25,000X g precipitated the enzymes they were considered to be insoluble. Precipitation with different concentrations of (NH₄)₂S0₄ and MnSO₄ failed to separate the glucose and gluconate enzymes from one another. Extraction with bile salts solubilized the gluconate enzyme, but precipitated the glucose enzyme along with a high proportion of the gluconate system. An increase in concentration of the bile salts destroyed the glucose enzyme while a reduction left a large percentage in solution. The addition of glycine to the sonicate solubilized the enzymes but did not aid in their separation. When added before the sonic treatment, the enzymes became labile to protein precipitants. Ethyl alcohol, dioxane and acetone destroyed glucose oxidizing and gluconate oxidizing enzyme activity while ethyl ether destroyed only the glucose system. The use of a growth substrate other than glucose resulted in the formation of a reduced gluconic acid system, but the results were not uniform in that the gluconate enzyme frequently was very active. The glucose system was sensitive to KCN and NaN₃, and 8-hydroxy-quinoline but not to NaF. The 8-hydroxy-quinoline inhibition could be overcome by Mg⁺⁺. Adenosinetriphosphate, flavine adenine dinucleotide, diphosphopyridinonucleotide, triphosphopyridinonucleotide, had no effect on the glucose oxidizing system. Methylene blue, brilliant cresyl blue and pyocyanin had no ability to act as a hydrogen acceptor. However 2.6 dichlorobenzenoneindophenol stimulated the reaction. When this dye was added in the presence of MgS0₄ a 300.0% increase was noted. The product of the reaction was determined by paper chromatography to be gluconic acid. These data indicate that the glucose dehydrogenase differs from any previously described glucose dehydrogenase and that some unknown hydrogen transport system apparently functions in the transport of electrons to the cytochromes. The data support earlier conclusions that the reaction does not involve phosphorylation. / Land and Food Systems, Faculty of / Graduate

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