Colliding saccades evoked by frontal eye field stimulation: artifact or evidence for an oculomotor compensatory mechanism underlying double-step saccades?

 What happens when the goal is changed before the movement is executed? Both the double-step and colliding saccade paradigms address this issue as they introduce a discrepancy between the retinal images of targets in space and the commands generated by the oculomotor system necessary to attain those targets. To maintain spatial accuracy under such conditions, transformations must update ‘retinal error’ as eye position changes, and must also accommodate neural transmission delays in the system so that retinal and eye position information are temporally aligned. Different hypotheses have been suggested to account for these phenomena, based on observations of dissociable cortical and subcortical compensatory mechanisms. We now demonstrate how a single compensatory mechanism can be invoked to explain both double-step and colliding saccade paradigm results, based on the use of a damped signal of change in position that is used in both cases to update retinal error and, thereby, account for intervening movements. We conclude that the collision effect is not an artifact, but instead reveals a compensatory mechanism for saccades whose targets appear near the onset of a preceding saccade. Received: 14 February 1996/Accepted in revised form: 17 September 1996     

Calcium-sensitive cells of the pars intermedia and osmotic balance in the eel

Summary Freshwater eels were adapted to calcium-free sea water (SW) or 1/3 Ca-free SW. Survival was generally poor in Ca-free SW, although three eels were still in good condition after 19–30 days; survival in 1/3 Ca-free SW was excellent. Osmotic disturbances (increase of plasma osmolarity and sodium levels), which initially occur in Ca-free SW, were no longer detectable after 19–30 days, or in eels in 1/3 Ca-free SW after one month. Plasma calcium sharply decreases initially; it is less depressed after 19–30 days and in 1/3 Ca-free SW. Alterations in the mucus production may be involved in the osmotic changes. Under these conditions no clear stimulation of the calcium-sensitive (Ca-s) cells of the pars intermedia was registered, but in Ca-free SW (1/3 or full strength) the inhibitory effect normally observed in SW does not occur. In a hyperosmotic environment, other ion(s), possibly magnesium, may reduce the response of the Ca-s cells to a lack of environmental calcium.     

Serotonin and MSH secretion: Effect of Parachlorophenylalanine on the pituitary cytology of the eel

Summary Parachlorophenylalanine (pCPA), an inhibitor of tryptophan hydroxylase which depletes brain serotonin in higher vertebrates, was injected into freshwater eels. After 4 or 6 injections (200 mg/kg/day) or 10 injections (100 and 140 mg/kg/day), the animals are paler, with a low melanophore index. In the pituitary gland, granules tend to accumulate in the basal part of the MSH cells and in the perinuclear area. Cells appear smaller with a decreased nuclear area (P< 0.001). In the neurohypophysis, the amount of neurosecretory material is often reduced. Conversely, injections of 5-hydroxytryptophan induce a strong darkening, a result similar to that previously reported in some amphibian species and in one lacertilian species. These data substantiate the hypothesis of a stimulatory influence of 5-hydroxytryptamine on MSH release and possibly its synthesis in the eel and other lower vertebrates.     

The structure of Sipunculus nudus erythrocyte chromatin

The structure of the Sipunculus erythrocyte chromatin has been characterized by electron microscopy and nuclease digestion (staphylococcal nuclease and pancreatic nuclease). Contrary to previous results [1], we were able to isolate and characterize a histone H_2B in sipunculid nuclei. Though the histones H_2A and H_2B were markedly different from their vertebrate homologues, the subunit structure of the chromatin is the same. But the length of the repeat unit of DNA in the chromatin, is 177 ± 5 bp for the sipunculid erythrocyte nuclei, close to that reported for the chromatin of some lower eukaryotes.     

Action de la prolactine chez l'Anguille

Résumé Les modifications histologiques de la thyroïde de l'Anguille traitée avec la prolactine ovine indiquent une forte activation qui paraît liée à une hypersécrétion d'hormone thyréotrope, les cellules adénohypophysaires élaborant cette stimuline étant très actives.L'emploi de l'iode radioactif ¹³¹I permet de préciser que la prolactine provoque une augmentation de la fixation thyroïdienne de l'isotope, et l'histoautoradiographie met en évidence un accroissement marqué de la synthèse des composés organiques iodés. On observe également une chute plus rapide de la radioactivité sérique, le rapport thyroïde/sérum étant beaucoup plus élevé chez les animaux traités que chez les témoins, par contre l'excrétion de l'iode dans le milieu externe est ralentie.Ces diverses données permettent donc d'établir que la prolactine ovine n'agit pas chez l'Anguille comme une substance antithyroïdienne et ne confirment pas l'hypothèse d'une action goitrogène formulée par Etkin et Gona chez divers Amphibiens. Au contraire, ces résultats sont similaires à ceux de Mazzi et ses collaborateurs sur un autre Amphibien, montrant que la prolactine stimule l'activité fonctionnelle de la thyroïde; cette action s'exerce par l'intermédiaire de l'hypophyse et paraît offrir une analogie avec celle d'un TRF, toutefois le mécanisme initial de ce processus reste à élucider.

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Action of prolactin in eelsIV. Thyroidal metabolism

Summary The histological changes described in the thyroid gland of ovinprolactin treated eels indicate a strong stimulation (Figs. 1 and 2) which seems correlated with a hypersecretion of thyrotrophic hormone as the adenohypophyseal cells elaborating this stimulin are highly activated.In prolactin injected eels, the utilization of radioiodine ¹³¹I demonstrates that the thyroidal iodine uptake increases (Fig. 7) and histoautoradiography shows a significant increase in the synthesis of iodinated organic compounds (Figs. 3 to 6). The thyroid/serum ratio has much higher values than in control animals, in correlation with a decrease in serum radioactivity (Fig. 8); the iodine excretion in water is reduced.These various data establish that ovin prolactin has no antithyroid effect in the eel, although the hypothesis of a goitrogenic action has been formulated for some amphibians by Etkin and Gona. On the other hand, these results are similar to those of Mazzi and his school, obtained in another amphibian, which show that prolactin stimulates the thyroidal functional activity. This action seems mediated by the pituitary gland and appears to be somewhat analogous with that of a thyrotropic releasing factor; however, the initial step of this process remains to be investigated.

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Nous remercions très vivement le NIH de Bethesda, Endocrinology Study Section, qui nous a généreusement offert la prolactine ovine utilisée dans ces expériences, Mr. F. Lachiver et Melle F. Boulu pour les mesures de la radioactivité sérique et de la fixation thyroïdienne totale qu'ils ont bien voulu effectuer, et Meile J. Olivereau, du CNRS, pour son assistance technique dans la réalisation de ce travail.     

Social responses to variation in food supply and nest suitability in ants (Tapinoma erraticum)

The social organization of Tapinoma erraticum colonies has been studied in two environmental contexts: those of mild starvation and of disturbance inducing a complete change of nest site. Trophallactic activity was compared with brood transport behaviour in tests performed repeatedly on the same group of insects. Workers were individually marked enabling interactions to be analysed within the group. An age-dependent difference in behaviour exists during feeding. However, when the colony changed its location, certain workers were found to be transport specialists, whereas others remained inactive: the transport behaviour of T. erraticum workers does not appear to be agedependent, unlike feeding behaviour. A principal components analysis of the results indicated no link between these two types of activities at the individual level. This study indicates the existence of types of social organization which are specific to each of the activities performed.