Frontal eye field inactivation alters the readout of superior colliculus activity for saccade generation in a task-dependent manner

Saccades require a spatiotemporal transformation of activity between the intermediate layers of the superior colliculus (iSC) and downstream brainstem burst generator. The dynamic linear ensemble-coding model (Goossens and Van Opstal 2006) proposes that each iSC spike contributes a fixed mini-vector to saccade displacement. Although biologically-plausible, this model assumes cortical areas like the frontal eye fields (FEF) simply provide the saccadic goal to be executed by the iSC and brainstem burst generator. However, the FEF and iSC operate in unison during saccades, and a pathway from the FEF to the brainstem burst generator that bypasses the iSC exists. Here, we investigate the impact of large yet reversible inactivation of the FEF on iSC activity in the context of the model across four saccade tasks. We exploit the overlap of saccade vectors generated when the FEF is inactivated or not, comparing the number of iSC spikes for metrically-matched saccades. We found that the iSC emits fewer spikes for metrically-matched saccades during FEF inactivation. The decrease in spike count is task-dependent, with a greater decrease accompanying more cognitively-demanding saccades. Our results show that FEF integrity influences the readout of iSC activity in a task-dependent manner. We propose that the dynamic linear ensemble-coding model be modified so that FEF inactivation increases the gain of a readout parameter, effectively increasing the influence of a single iSC spike. We speculate that this modification could be instantiated by FEF and iSC pathways to the cerebellum that could modulate the excitability of the brainstem burst generator.

     

Durable Interactions of T Cells with T Cell Receptor Stimuli in the Absence of a Stable Immunological Synapse

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Environmental and management determinants of vegetation state on protected areas in the eastern Lowveld of South Africa

Principal driving determinants (rainfall, geology, soil, tree density and canopy cover, animal numbers and feeding classes, and fire) of vegetation structure and function in the Lowveld savanna in South Africa were grouped for a 7‐year period to establish their influence on the limiting herbaceous layer. Grass type, abundance and cover were examined (450 sites; approximately 4000 km²). Using ordination, the variation and differences in the herbaceous‐response variables viz. perennial composition and cover allowed for the broad environmental grouping of areas of similar ecological potential. We demonstrate that areas of higher ecological potential carried higher densities of large herbivores without detrimentally affecting herbaceous composition and cover. The results have implications for land users and policy makers in terms of setting animal stocking density guidelines.     

The effect of low temperature on sucrose,ATP and potassium concentrations and fluxes in the sieve tubes of willow

Summary Experiments have been performed on the effect of localised low (0°C) temperature application on solute concentration and fluxes in the sieve elements of willow. Sieve tube exudate was obtained via the severed stylets of the aphid Tuberolachnus salignus (Gmelin). In stem segments, low temperature caused a fall in both the concentration and flux of sucrose. No recovery was observed during a 24 h cold application period. The concentrations of ATP and potassium were generally also reduced, though the effect on the fluxes of these solutes was not as marked. Both ATP and potassium appear to be translocated along the sieve tubes of stem segments as evidenced by girdling experiments. In leafy cuttings low temperature consistently reduced the concentration of sucrose in the sieve tube exudate. These data are discussed in relation to previous work on low temperature effects on the phloem transport system of willow.     

The expression of PS integrins in Drosophila melanogaster imaginal disc cell lines

Summary Drosophila imaginal disc cell lines show a characteristic pattern of aggregation in culture, which appears to be due to cell-cell rather than cell-substrate interactions. We have examined the distribution of PS integrins in wing and leg cell lines, and find that these integrin homologues are expressed preferentially in aggregates. Cell sheets, small cell clumps and chains of cells express antigen at points of cell-cell contact only.