Motor and perceptual responses to horizontal and vertical eye vibration in humans.

Previous studies have shown that low amplitude/high frequency mechanical vibration applied to the human eye muscles results in the illusory movement of a luminous spot fixated in total darkness. The aim of the present study was to investigate whether a vibration-induced motor response also occurs in eye muscles, and to check whether the visual illusions actually result from the proprioceptors being activated by the vibration, or whether they are simply due to the retinal slip induced by the reflex eye movement. The effects of the vibratory stimuli on the inferior rectus (IR) and lateral rectus (LR) muscles were evaluated by recording subjects' eye position changes. When applied to the IR muscle, vibration effectively elicited an upward visual illusion accompanied by a small downward ocular rotation, whereas when applied to the LR muscle, it also induced horizontal visual illusion, which was less frequent and weaker than the vertical one, but no ocular rotation. We concluded that visual illusions of this kind cannot be attributable to the retinal motion of the image of the fixated point. The difference between the vertical and horizontal vibratory motor responses is discussed as regards the particular role that oculo-muscular proprioception may play in the vertical muscles.

Shift in saccadic direction induced in humans by proprioceptive manipulation: a comparison between memory-guided and visually guided saccades.

It is nowadays generally recognized that saccades to remembered targets are planned in a craniotopic frame of reference by combining retinal input with eye position signal. The origin of the eye position signal is still a matter of controversy, however. Does it arise from an efferent copy or is it supplied by the sensory receptors with which the extraocular muscles are endowed? When applied to skeletal muscles, vibration elicits spindle responses simulating a stretching of the vibrated muscle. When vibration is applied to the inferior rectus muscle (IR), it induces the illusion that a stationary fixating point is moving upward. Here we attempted to change the initial eye position signal supplied to the oculomotor system before a memory- or visuo-guided saccade to a 10 degrees left target by applying mechanical vibration to the IR muscle. We wanted to determine whether modifying extraocular proprioceptive cues during the programming phase of a saccade might affect the latter's trajectory. In the memory-guided condition, it was observed that the saccades ended lower down when vibration was applied than in the control condition. Conversely, the visuo-guided saccades were not affected by the vibration. The above results mean first that extraocular proprioceptive cues are used as an initial eye position signal when a memory guided saccade has to be planned. Secondly, they suggest that extraocular proprioception may not be used to produce a visuo-guided saccade, or that this type of saccade is computed solely on the basis of retinal cues.

[Effects of dose rate on carcinogenesis after exposure of rats to low doses of neutron irradiation]. / Effets du débit de dose sur la cancérogenèse après exposition de rats à de faibles doses d'irradiation neutronique.

In this experiment we compared the efficiency of fission neutrons of californium 252 at doses of 25 or 53 mGy in function of the dose rate. Two groups of male Sprague-Dawley rats were exposed at 100 or 420 days of age to fission neutrons with dose rates of 950 or 760 microGy/h for 26 or 33 h, and 2 other groups were irradiated over a year from 3 months to 15 months of age with dose rates of 3.58 or 7.72 microGy/h. The 4 groups of animals were compared with a control group of 501 rats. The reduction of effectiveness on cancer induction that we have previously shown at low dose rate with rats exposed to gamma rays or to alpha particles was not observed for low dose rate exposure with fission neutrons.

[Relationship between irradiation and appearance of brain tumors in rats]. / Relations entre l'irradiation et l'apparition des tumeurs cérbrales chez le rat.

Sprague-Dawley rats were exposed to Co 60 irradiation (3 Gy) at different ages. Rats were distributed among sham-control (618 males and 120 females) and exposed groups: foetus (66 males and 65 females), 3 month-old (304 males), and 9 month-old (120 males and 60 females). The incidence of brain tumours was 5.3% in male control rats and nil in female control rats. Brain tumour incidence decreased in 9 month-old rats (3.3%), and increased from 6.6% in 3 month-old rats to 15.2% in male foetuses and 12.3% in female foetuses. Age at incidence of brain tumours decreased in irradiatiated animals. Astrocytomas were the more susceptible type of brain tumours to radiocarcinogenesis.

Mortality of a cohort of French uranium miners exposed to relatively low radon concentrations.

A cohort mortality study has been performed on French uranium miners having experienced more than 2 years of underground mining, with first radon exposure between 1946 and 1972. Vital status has been ascertained from the date of entry to the 31 December 1985 for 99% of the members of this cohort; causes of death are identified for 95.5% of the decedents. The different causes of death are compared to the age specific national death rates by indirect standardisation and expressed by standardised mortality ratios (SMR). A statistically significant excess has been observed for lung and laryngeal cancer deaths. The Poisson trend test shows a statistically significant trend for the risk of lung cancer death as a function of cumulative radon exposure, assuming a lag time of 5 years; for laryngeal cancer no significant trend has been observed. Poisson regression modelling has been applied to the following exposure groups: < 10 WLM (Working Level Month); 10-49 WLM; 50-149 WLM; 150-299 WLM; > or = 300 WLM; it indicates an increase in the SMR for lung cancer of 0.6% per WLM (standard error: 0.4%) with an estimated intercept at 0 WLM of 1.68 (standard error: 0.4). The distinction of two working periods, differing by their annual radon concentration (before/since 1956) does not modify this exposure-response relationship. This coefficient of risk per unit of exposure is lower than in most of the other uranium miners' studies but it lies in the range of the evaluation of the ICRP 50 committee and the 'BEIR IV' report of the U.S. National Academy of Science. It is observed in a cohort having experienced low cumulative exposure to radon (mean: 70 WLM) spread over a mean duration of 14.5 years. Even though occupational exposure in mines differs in several particulars from domestic exposure, this study presents characteristics of low annual exposure comparable to radon gas concentrations in houses of 500-1000 Bq.m-3, and will contribute to the evaluation of cancer risk for the public.