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.

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.

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.

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.

[The image of intravertebral somatic gaseous dissection. Report of 9 cases one of which has a triple localization and review of the literature]

We report a series of 9 cases of vertebral somatic gaseous dissection studied between 1978 and 1989. We find most of the characteristics established from the literature data: advanced age, possible medullar compression contrary to the vertebral compression by common osteoporosis, predilection for the dorso-lumbar junction, frequency of the osteoporosis, possibility of metabolic disorders, of occurrence after radiotherapy. The context of the occurrence and the paraclinical examinations of the whole have enabled us to eliminate, in this series, a malignant pathology. However, when there exists a doubt on a malignant origin, it is preferable to carry out a vertebral biopsy with a trocar. In fact, the image may not be pathognomonic of a benign vertebral osteoporosis. Finally, we give the first description of a triple localization of vertebral gaseous dissection.

Apparent brightness enhancement in the Kanizsa square with and without illusory contour formation

The perceived strength of darkness enhancement in the centre of surfaces surrounded or not surrounded by illusory contours was investigated as a function of proximity of the constituent elements of the display and their angular size. Magnitude estimation was used to measure the perception of the darkness phenomenon in white-on-grey stimuli. Darkness enhancement was perceived in both types of the stimuli used, but more strongly in the presence of illusory contours. In both cases, perceived darkness enhancement increased with increasing proximity of the constituent parts of the display and with their angular size. These results suggest that the occurrence of darkness (or brightness) enhancement phenomena in the centre of the displays is not directly related to illusory contour formation.

Visual adaptation to a spatial contrast enhances visual evoked potentials

The effects of adaptation to the visual contrast of a counterphase grating were studied with visual evoked potentials (VEPs). The spatial frequency of the grating was 4 cpd, and its temporal frequency was 4 or 12 Hz. Steady-state VEPs were analyzed through an FFT (fast Fourier transform) algorithm. In the first experiment, contrast thresholds rose strongly just after the end of adaptation and declined regularly over time. The VEPs recorded in the medio-occipital lead showed an initial decrease in amplitude after adaptation, followed by an enhancement well above the preadaptation level and then a return to that level. The paradoxical enhancement of the VEP was found at both high and low contrast in both the medio-occipital lead and the right temporal lead of a right-handed subject. The left temporal leads showed a VEP enhancement at high contrast and a decline at a low value.

Influence of spatial frequency and handedness on hemispheric asymmetry in visually steady-state evoked potentials

Recent suggestions on the involvement of the spatial frequency of visual stimuli in the hemispheric lateralization were investigated by recording steady-state evoked potentials in two groups of subjects: five right-handers and five left-handers. Sinusoidal gratings at spatial frequency of 0.5, 1, 3, 4, 6, 8, 10, 12 or 16 cpd were phase reversed at 4 Hz or 12 Hz. Evoked potentials recorded from temporal leads over each hemisphere were submitted to a FFT analysis. Results concern the amplitude of the fundamental component. In right-handers, the temporal frequency was the deciding factor of the lateralization: the evoked activities were greatest in the RH at 4 Hz and in the LH at 12 Hz. This effect was obvious for the range of spatial frequencies from 3-12 cpd. Results, discussed in terms of global/local information, suggested the existence of two transient and sustained systems. In left-handers, both the spatial and temporal parameters were relevant to the lateralization. A spatio-temporal interaction was observed which was reversed at 6 cpd.

Hemispheric asymmetries in the visual evoked potentials to temporal frequency: preliminary evidence

Steady-state evoked potentials were recorded in eight adult subjects from occipital and temporal leads of both hemispheres to investigate the effect of temporal frequency on the hemispheric specialization for basic visual information. A 3 cycles deg-1 grating was phase-reversed at different temporal frequencies (from 4 to 18 Hz), and the frequency spectrum of evoked potentials was computed by means of a fast Fourier transform program. Significant results were obtained for the component at twice the temporal frequency of stimulation. Occipital evoked potentials did not show hemispheric asymmetry, whereas temporal evoked potentials showed an interaction between hemisphere and temporal frequency: right and left hemispheres were respectively prominent for low (4 and 6 Hz) and for high (8-18 Hz) temporal frequencies. The results are discussed in the context of current research on hemispheric specialization for basic spatiotemporal parameters of visual information processing.

Discrimination of velocities and mechanisms of motion perception

In a first experiment, velocity discrimination thresholds were measured in twenty-one experimental conditions by presentation of successive pairs of standard/comparison motions of a single spot target. Exposure times between 300 to 900 ms did not affect velocity discrimination. However, very slow velocity increased discrimination thresholds. Velocity discrimination is improved by the presence of stationary references and decreased by eccentricity. At slow velocities, in the absence of a motion phase, a discrete mode of presentation of the translation leads to higher discrimination thresholds than those observed with the continuous and stop-go-stop modes of the same translation. In a second experiment, it was shown that for shorter exposure time (100 ms), discrimination thresholds are much higher for the discrete mode. It is concluded that at long exposure time, velocity discrimination thresholds are essentially independent of the presence of a motion phase.

Visual motion detection models: features and frequency filters

Two kinds of models have been proposed for taking into account the sensory processes at work in the detection of visual motion: the feature model and the frequency-filter model. The problem of the complementarity of these models is raised. On the basis of empirical data, it is proposed that they are consistent.

Apparent size and duration of a movement after-effect

Four experiments were performed to study the relationship between Emmert's law and the duration of the movement after-effect (MAE). The duration of the MAE increased with increased distance of the test field; this result was shown to be produced by the correlative change in apparent size of the after image. The effect did not occur when cues for distance judgments were reduced. Reducing the duration of the MAE suppressed the variation in its duration at varying distances of the test field. Some implications for the mechanism of the MAE are discussed.