PubMed Health⌕ Search

Biomedical subjects

C Bonnet

Publications and source records attributed to C Bonnet.

At least 91 records · Page 5Linked to original sources
PubMed1993

Perceived speed of moving lines depends on orientation, length, speed and luminance.

In this study, the perceived speed of a tilted line translating horizontally (for a duration of 167 msec) is evaluated with respect to a vertical line undergoing the same translation. Perceived speed of the oblique line is shown to be underestimated when compared to the vertical line. This bias increases: (1) when the line is further tilted, (2) with greater line lengths, (3) with lower contrasts, and finally (4) with a speed of 2.1 deg/sec as compared to a higher speed of 4.2 deg/sec. These results may be accounted for by considering that two velocity signals are used by the visual system to estimate the speed of the line: the translation of this line (this signal does not depend on the line's orientation) and the motion component normal to the line (this signal depends on orientation). We suggest that these two signals are encoded by different types of units and that the translation signal is specifically extracted at the line endings. We further suggest that these signals are integrated by a weighted average process according to their perceptual salience. Other interpretations are considered at the light of current models dealing with the two-dimensional integration of different velocity signals.

Differential Threshold↗
PubMed1993

The inverse intensity effect is not lost with stimuli in apparent motion.

The inverse relationship between the visible persistence of a briefly presented stimulus and its intensity is well established for static displays. However, with non-static displays, this relationship is only partially reported by previous studies. In order to clarify this topic, we investigated the effect of luminance on the visible persistence of a stimulus in apparent motion. Assuming that persistence duration is a normally distributed random variable, we studied whether the mean persistence of a stimulus could be systematically varied by varying its luminance. Our paradigm permits evaluation of this effect without changing the temporal interval between two successive presentations of the stimulus, thus avoiding the potential influence of this latter factor on persistence. Our results show that the inverse intensity effect still occurs at each of the successive locations of a stimulus in apparent motion. In addition, we provide evidence that increasing the spatial separation between the successive presentations, and decreasing the background luminance, result both in longer persistence duration. Altogether, these findings favour the hypothesis that persistence is actively suppressed by inhibitory interactions between adjacent neural zones.

Afterimage↗
PubMed1993

Sugar composition of dietary fibre and short-chain fatty acid production during in vitro fermentation by human bacteria.

The aim of the present study was to assess the relationship between the disappearance of dietary fibre sugars and the production of individual short-chain fatty acids (SCFA). The bacterial degradation of five dietary fibres whose sugars were quantified was investigated in vitro using a human faecal inoculum. Involvement of the main fibre sugars in SCFA production was evaluated by a stepwise multiple linear regression. The results show first that the nature and chiefly the associations between the fibre sugars were key variables in the fermentability. Second, the nature and the amounts of SCFA produced were closely related to the in vitro fermentation of the main sugars available: uronic acids seemed to be principally involved in the production of acetic acid whereas the production of propionic acid could be promoted by the fermentation of glucose and, to a lesser extent, by that of xylose and arabinose. Xylose tended to have a greater impact than uronic acids and glucose on the production of butyric acid. Thus, it would be possible to predict which SCFA could be specifically produced during the fermentation of a fibre, as far as the chemical composition and structure of this fibre are known.

Acetates↗
PubMed1993

Properties of curvilinear vection.

Approximately linear relationships were observed between contrast, spatial frequency, temporal frequency, or velocity of stimulation and perceived velocity of curvilinear vection--that is, a visually induced self-motion in a curved path. Similarly, linear relationships were also found between the perceived degree of curvature of curvilinear vection and spatial frequency or velocity of stimulation. Since the perceived velocity of curvilinear vection varies with contrast, spatial frequency, temporal frequency, and angular velocity, and the perceived degree of curvature of curvilinear vection varies only with spatial frequency and angular velocity, peripheral vision is not sufficient for computing accurately the curvilinear component of induced self-motion in a curved path. Concurrently, it was shown that the perceived direction of curvilinear vection is not always unambiguously perceived (Sauvan & Bonnet, 1989). Consequently, it is suggested that two different types of visual processing, which involve the peripheral or the central vision, underlie the processing of curvilinear vection.

Adult↗
PubMed1992

Adolescent osteoporosis disclosing familial osteopenia.

The observations of familial juvenile osteoporosis, presumably of genetic origin are exceptional. The authors report the observation of a 16-year old adolescent suffering from osteoporosis, confirmed by histomorphometry and decrease in bone density (lumbar vertebrae 0.79 g/cm2 and femoral neck 0.88 g/cm2: LUNAR DPX). We prescribed fluorine and calcium therapy. Lumbar bone density increases by 11% and bone density of the thighbone neck by 7.6%. We cannot rule out growth as a factor in the changes observed, given that the propositus is only 16. A densitometric investigation performed in 4 of his 12 brothers shows a decrease in the lumbar bone mineral content (from 61 to 94% expressed as Z score). A genotypic origin seems to be conceivable, especially since no other cause could be considered (endocrinal, alimentary...). On the other hand, there is no argument in favour of osteogenesis imperfecta disease. The bone densitometry is a useful diagnostic means to detect familial forms of osteoporosis.

Adolescent↗
PubMed1992

[Nicardipine vs trinitrin for controlled hypotension in maxillo-facial surgery].

In this study we examined perioperative blood losses during maxillo-facial surgery comparing 2 techniques for controlled hypotension using nitroglycerin (NTG) and nicardipine (NICAR). 37 ASA I and II patients electively scheduled for rhinoseptoplasties under general anaesthesia were randomly divided into 2 groups. General anaesthesia was induced with intravenous thiopental and vecuronium following a neuroleptanalgesia consisting of fentanyl (0.053 +/- 0.004 microgram.kg-1.min-1) IV and droperidol (1.98 +/- 0.35 microgram.kg-1.min-1) IV given in bolus doses. The patients were intubated and ventilated (N2O in O2) to maintain normocarbia. Controlled hypotension to maintain a mean arterial pressure (MAP) between 50 and 55 mmHg was induced using an IV infusion of NTG 6 micrograms.kg-1.min-1 in group I and NICAR 10 micrograms.kg-1.min-1 over 10 minutes followed by an infusion at 4 mg.kg-1.min-1 for group II. Measurements of heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), MAP using an automated blood-pressure cuff and end tidal carbon dioxide (ETCO2) were recorded prior to induction (PREIND), intraoperatively (PEROP) at 15, 30, 40 and 120 minutes post-induction, and at 15, 30 and 60 minutes postoperatively (POSTOP). Blood loss, graded as minimal or frank, was assessed by the surgeon during the operative period, post-extubation and 3 days postoperatively at dressing removal. The surgeon was unaware of the randomization schedule. Student's test was applied for statistical analysis of measured variables and Fisher's test for qualitative parameters. No statistically significant differences were noted between both groups with respect to demographic data, duration of surgery and total doses of anaesthetic agents (Table I).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Psychophysical evidence for low-level processing of illusory contours and surfaces in the Kanizsa square.

Increment thresholds were measured on either side of one of the illusory contours of a white-on-black Kanizsa square and on the illusory contour itself. The data show that thresholds are elevated when measured on either side of the illusory border. These elevations diminish with increasing distance of the target spot from the white elements which induce the illusory figure. The most striking result, however, is that threshold elevations are considerably lower or even absent when the target is located on the illusory contour itself. At an equivalent position in a control figure where no illusory contour is visible, such a threshold decrease does not occur. The present observations add empirical support to low-level explanations of illusory contour perception.

Female↗
PubMed1991

A study of the effect of structural information and familiarity in form perception.

Two experiments were carried out to investigate the influence of structural information and familiarity on the processing of visual forms. Pairs of "well" structured and nameable and "poorly" structured and non-nameable fragmented forms were employed as stimuli. The effects of structure and familiarity were assessed by manipulating the visual hemifield of presentation and the task. In Experiment 1 stimuli were judged as being either in the same orientation or mirror-reversed, a task that does not require high-level semantic information to be processed. Experiment 2 required physically identical forms to be matched, which may use either physical or name information. In Experiment 1 "same" judgements were equivalent for both types of stimuli, and "different" judgements were longer for the "poorly" structured (non-nameable) forms. In Experiment 2 there was little overall difference between "well" and "poorly" structured forms, though response times to "well" structured (nameable) forms were slowed for right-visual-field presentations. It is suggested that familiarity may not be sufficient to provide a perceptual advantage for nameable forms, as the advantage for nameable stimuli was confined to "same" judgements in Experiment 1 and response times were shorter for non-nameable stimuli in Experiment 2. Rather, performance depends upon factors such as the computation of global shape (due to structural properties of collinearity and closure) and on the use of different kinds or representations (physical versus name) in matching.

Adult↗
PubMed1991

Changes in left ventricular ejection fraction during REM sleep and exercise in chronic obstructive pulmonary disease and sleep apnoea syndrome.

Nine patients, 4 with chronic obstructive pulmonary disease (COPD) and 5 with obstructive sleep apnoea syndrome (OSAS) were monitored during sleep, rest and exercise. Left ventricular ejection fraction (LVEF) was investigated using gated equilibrium 99mtechnetium ventricular scintigraphy during rapid eye movement (REM) sleep, during exercise, and during wakeful rest. Control wakeful rest periods used for comparison with a study state (either REM sleep or exercise) were always selected during the same circadian segment as that state. Myocardial stress thallium-201 scintigraphy was performed during, and 4 h after, exercise, and results were compared to a daytime rest period. Several patients had myocardial hypoperfusion despite a normal electrocardiographic (ECG) treadmill test. During REM sleep, all patients exhibited a significant change in LVEF (greater than 5%) compared to wakefulness. During exercise, 5 subjects increased their LVEF normally (greater than 5%) and 4 (1 COPD, 3 OSAS) decreased it. All patients had a similar change (increase or decrease) during REM and at maximal exercise. Our results suggest that REM sleep in COPD and in OSAS can produce a myocardial stress as great as that produced by exercise. We conclude that REM sleep, like exercise, is a state in which morbidity may become higher and that it may account for mortality in COPD and OSAS patients with compromised myocardial circulation.

Adult↗