The ethology of saccades: a non-cognitive model

It is proposed that there is a low-level automatic component of visual scanning in which saccades are triggered probabilistically in time by nonfoveal stimulus features. By ignoring cognitive factors and memory, a first-order Markov approach is taken, which is tractable for spatially homogeneous stimuli. The predicted distributions of saccade magnitude, fixation duration, fixation position, and how they are related to stimulus size are in good agreement with empirical observations by Bahill et al. (1975), Enoch (1959), and Harris et al. (1988). Ways in which this low-level process could be modulated by cognitive factors are discussed.     

DNA-protein cross-linking by chromium salts

DNA-protein cross-links were detected in several types of mammalian cells in culture when they were exposed to chromate salts. The cell types included human bronchial epithelial cells — the apparent cell type of origin of the malignancies reported in chromate workers. The level of cross-linking was proportional to the concentration of chromate used. These cross-links appeared to be persistent since no removal was seen after 12 h of repair incubation. A low level of DNA single strand breaks (SSB) were also induced after exposure of the cells to chromate but were rejoined after 4 h of repair incubation. The active form of chromium appears to be the trivalent since chromic but not chromate salts induced DNA-protein cross-links in isolated nuclei. Chromic salts also produced cross-linking between DNA and protein in solution while the hexavalent form was inactive. These data imply that chromate crosses the cell membrane, is reduced to the trivalent form and induces stable linkages of DNA to protein.     

Effects of DL–thyroxine on swimming speed in pearl danio Brachydanio albolineatus (Blyth)

Pearl Danios (Brachydanio albolineatus –Blyth) were allowed four days for acclimation in river-water filled aquaria maintained at a mean temperature of 27° C. A different concentration of DL-thyroxine (T₄) was added to each of two aquaria. A third aquarium served as a control and received no hormone. Photoperiod, feeding, aeration, and observer interference were controlled so as not to influence the results of the study. Mean swimming speeds for the fish in each aquarium were calculated. Data were subjected to analysis of variance and the Student-Newman-Keuls Test. Compared with the control, significant (P < 0.05) increases in swimming speed were found with concentrations of thyroxine in water of 1:4x 10⁶ and 1:1 x 10⁶. The difference in swimming speed between the two hormone-treated groups was not significant. The fact that exogenous thyroxine will increase swimming speed may indicate that endogenous thyroid hormone plays a significant role in such fish behaviour as migration.     

Changes in fibroblast contractility, morphology, and adhesion in response to a phorbol ester tumor promoter

We have studied the effects of the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the contractility, locomotion, morphology, and adhesion of two mammalian fibroblastic cell lines. Using the silicone rubber substratum technique, we have found that the first observable response to the tumor promoter is a rapid weakening of cell contractility (8-15 min). This is followed by gradual morphological changes, characterized by a hyperextension of the cells' leading lamellae, which stretch out to an unlimited degree, and occasionally even detach from the cell bodies. Treated cells also become able to crawl onto hydrophobic substrata which are insufficiently adhesive to support the spreading of untreated fibroblasts. We suggest that both the hyperextension and the ability to spread on nonadhesive surfaces can be explained as consequences of the reduced contractility, and that this reduced contractility may also help to explain the increased invasiveness and loss of anchorage dependence by transformed cells.