How moving visual stimuli modulate the activity of the substantia nigra pars reticulata.

The orientation of spatial attention via saccades is modulated by a pathway from the substantia nigra pars reticularis (SNr) to the superior colliculus, which enhances the ability to respond to novel stimuli. However, the algorithm whereby the SNr translates visual input to saccade-related information is still unknown. We recorded extracellular single-unit responses of 343 SNr cells to visual stimuli in anesthetized cats. Depending on the size, velocity and direction of the visual stimulus, SNr neurons responded by either increasing or decreasing their firing rate. Using artificial neuronal networks, visual SNr neurons could be classified into distinct groups. Some of the units showed a clear preference for one specific combination of direction and velocity (simple neurons), while other SNr neurons were sensitive to the direction (direction-tuned neurons) or the velocity (velocity-tuned neurons) of the movement. Furthermore, a subset of SNr neurons exhibited a narrow inhibitory/excitatory domain in the velocity/direction plane with an opposing surround (concentric neurons). According to our results, spatiotemporally represented visual information may determine the discharge pattern of the SNr. We suggest that the SNr utilizes spatiotemporal properties of the visual information to generate vector-based commands, which could modulate the activity of the superior colliculus and enhance or inhibit the reflexive initiation of complex and accurate saccades.

Spatial and temporal visual properties of single neurons in the suprageniculate nucleus of the thalamus.

The spatial and temporal visual sensitivity to drifting sinusoidal gratings was studied in 105 neurons of the suprageniculate nucleus of the feline thalamus. Extracellular single-unit recordings were performed in halothane-anesthetized, immobilized, artificially ventilated cats. Most suprageniculate nucleus cells were strongly sensitive to the direction of drifting gratings. The suprageniculate nucleus units had a clear preference for very low spatial frequencies with a mean of 0.05 cycle/deg. The spatial resolution was also very low with a mean of 0.16 cycle/deg. Most of the cells displayed low-pass spatial tuning characteristics, while the remainder of the units were band-pass tuned. The suprageniculate nucleus units were extremely narrowly tuned, to spatial frequencies with a mean spatial bandwidth of 1.07 octaves. A majority of the units responded optimally to high temporal frequencies, with a mean of 8.53 Hz. The temporal frequency tuning functions predominantly revealed a band-pass character, with a mean temporal bandwidth of 1.66 octaves. These results demonstrate that the neurons in the suprageniculate nucleus display particular spatial and temporal characteristics. The spatial and temporal tuning properties of the suprageniculate nucleus neurons are very similar to those of the superior colliculus and the anterior ectosylvian cortex, structures that provide the main visual afferentation toward the suprageniculate nucleus. This suggests their common function in motion perception, and especially in the recording of movements of the visual environment relative to the body, and the related behavioral action.

Factors affecting the secretion of staphylococcal enterotoxin A.

The biosynthesis of enterotoxin A by replicating and nonreplicating cells was investigated. Unlike enterotoxin B, a secondary metabolite, enterotoxin A secretion resembled that of a primary metabolite by being secreted during the exponential phase of growth. The amount of toxin produced per unit of growth was not influenced by NaCl, NaNO(2), or NaNO(3). Several surfactants increased toxin secretion. Toxin secretion by nonreplicating cells was inhibited by chloramphenicol and 2, 4-dinitrophenol but not by streptomycin or penicillin G. The optimal pH for enterotoxin A production was 6.5 to 7.0. The findings suggest a number of possible reasons for the higher incidence of food poisonings caused by enterotoxin A as compared to enterotoxin B.

Enterotoxin B synthesis by replicating and nonreplicating cells of Staphylococcus aureus.

Although 95% of the enterotoxin B produced by Staphylococcus aureus appears during the latter part of the exponential phase of growth, growth per se is not necessary for toxin synthesis. A procedure is described whereby a concentrated suspension (at least 6 x 10(10) cells per ml) of a 16-hr culture of S. aureus was found to be capable of producing toxin, without replication, when air and glucose were present. This technique allows the growth requirement to be separated from toxin formation. Although higher (100 mug/ml) concentrations of toxin appeared in the medium when nitrogen was present, lower levels (30 mug/ml) were produced in the absence of N-Z-amine A. Toxin production proceeded without any net increase in deoxyribonucleic acid, ribonucleic acid, or protein. Chloramphenicol did not inhibit toxin formation in a nitrogen-free medium. The optimal pH for toxin production in a nitrogen-free medium was 8.0 to 8.5; for synthesis in a medium where nitrogen was available, the optimal pH was 7.0 to 7.5. Increasing the rate of aeration increased toxin release during growth, but decreased the amount of toxin subsequently produced when the bacteria were resuspended. These results suggest the presence of a precursor pool in the cells collected after 16 hr of growth.