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PubMed1997

Visuelle Welt: a Windows program for demonstrating visual-perception phenomena.

The Windows program Visuelle Welt allows the demonstration of various visual phenomena such as geometrical-optical illusions, subjective contours, apparent movement, and Gestalt principles. One of the program's most interesting features is that the illusion-inducing context can be switched off, and in addition for many phenomena parameters can be varied interactively.

Computer Terminals↗
PubMed2002

The Pulfrich phenomenon in a large population of young healthy subjects.

BACKGROUND: To determine whether the Pulfrich phenomenon, an optical illusion occurring in many ophthalmological diseases, is perceived equally in both eyes in a large group of healthy medical students. SUBJECTS AND METHODS: A pendulum bob swinging perpendicular to the direction of observation was observed with either the right or the left eye covered with neutral density filters (50, 80 or 90 % absorption) and the apparent elliptical pendulum movement measured in depth. Interocular time delay was calculated from depth. Data from 65 individuals were included based on having: completed all 7 determinations of depth, a visual acuity of >/= 20/20 on both eyes and an intact stereoscopic perception (Titmus stereotest, acuity of >/= 20/25). RESULTS: All subjects perceived the phenomenon. Depth perception was not significantly different (p > 0.05; MANOVA) between the two eyes (depth in [mm]; mean +/- sem): OD: 8.5 +/- 0.38, 23.7 +/- 0.53, 36.3 +/- 0.81; OS 9.4 +/- 0.50, 24.1 +/- 0.69, 35.6 +/- 0.92; for 50, 80 and 90 % absorption of the filter respectively. At 0 % absorption the pendulum was seen in average at positive values (0.9 +/- 0.23 mm; p > 0.05). Calculated interocular time delay (ms) was: OD: 8.0 +/- 0.18, 5.1 +/- 0.12, 1.8 +/- 0.08; OS: 7.46 +/- 0.15, 5.0 +/- 0.11, 1.9 +/- 0.10. The average depth perceived without filter corresponded to a time delay for the right eye of 0.2 +/- 0.05 ms. Correction for the depth perception perceived without filter did not alter statistical significance. CONCLUSIONS: The Pulfrich phenomenon is perceived equally in both eyes. Depth perception without filters was not significantly different from zero. The illusion has clinical utility, since in normal subjects reliability of measurements is good and size of the illusion (without filters) small.

Adult↗
PubMed1988

Mach bands in mammography.

The radiologic appearance of tissue surrounding a tumor often influences the characterization of a cancerous lesion. An example is the radiolucent halo that is sometimes seen around breast cancers. Mammograms of 108 malignant tumors (74 scirrhous tumors, 17 partially scirrhous nodular tumors, and 17 nodular tumors) and eight benign tumors that showed peritumoral fat were retrospectively evaluated. All the scirrhous and partially scirrhous nodular cancers were more or less surrounded by a 5-10-mm-wide hyperlucent halo; this corona sign was also found in 14 nodular cancers. The halo sign, a hyperleucent 1-mm band, was observed around all benign lesions and immediately adjacent to sharply delineated cancers. These radiologic findings are an optical illusion (Mach band, background contrast effect). Densitometric measures and isoluminous analysis did not demonstrate the presence of a true hyperlucent zone.

Absorptiometry, Photon↗
PubMed1993

The structural basis of Felice Fontana's spiral bands and their relationship to nerve injury.

In 1779, Felice Fontana, using a six-power hand-held magnifying lens, described what appeared to be spiral bands surrounding the peripheral nerve. He hypothesized that these bands were due to an optical illusion related to the underlying undulations of the individual nerve fibers, which he was the first to observe with an early microscope. The present study examines the historical basis of Fontana's work, confirms with intrafascicular dissection that the bands are an illusion created by unstretched nerve fibers, and relates their clinical disappearance to current concepts of the pathophysiology of chronic nerve compression and nerve injury.

Animals↗
PubMed1983

Cervical burnout and Mach band: two shadows of doubt in radiologic interpretation of carious lesions.

The factors that produce cervical burnout images in the film emulsion and Mach band effects in the retinal receptors have been discussed. In the former, overpenetration of thinner cervical tooth areas produces radiolucent areas that have rounded, diffuse inner borders, but that show intact tooth edges. Mach bands, produced by lateral inhibition of neural receptors in the eye, are optical illusions that may appear in the dentin along the proximal DEJ, or in dentinal peaks bounded by occlusal and proximal enamel. Not every observer has the same ability to perceive Mach bands, and perception is modified by projection, contour, and film and object density. A masking technique has been described to facilitate differential interpretation between the Mach apparition and actual tooth decalcification.

Dental Caries↗
PubMed1989

An alternate strategy for reducing the large nasal base.

The position of the dorsum directly influences the apparent size of the nasal base: the higher the bridge, the smaller the nasal base seems. This powerful optical illusion provides the surgeon with an alternate strategy for creating harmony in the nose with a low, straight dorsum and a disproportionately large nasal base. Instead of reducing the lower nose, the surgeon can augment the upper nose (and make whatever nasal base changes are independently necessary). This alternate strategy limits the amount of nasal skeletal reduction necessary, thereby limiting the potential for postoperative change and soft-tissue distortion and directly increasing the surgeon's control over the result.

Adult↗
PubMed1996

The contribution of videostroboscopy in daily ENT practice.

Videostroboscopy has now left specialized voice laboratories and must be made available to general ENT practice. It is based on an optical illusion giving the impression of slow motion from a series of successive vocal cycles but it is the least awkward system to examine the movement of vocal folds while waiting for the development of very high speed video recordings which would enable to visualize a real single cycle. The equipment comprises a stroboscope, a CCD type of camera, a video recorder, a microphone, a monitor or television screen and rigid optics and/or a fiberscope. An examination lasts about 1/2 an hour and must follow a protocol for which we have developed a simplified index card based on the one proposed by HIRANO and B less. This simplified version has the advantage of being easily integrated into daily ENT practice. A visual examination of the larynx was carried out on 732 patients. Videostroboscopy was undertaken in 69.2% of cases and was found to be useful or essential in 68% of the cases where it was carried out. Furthermore, it enabled to correct a diagnosis in 13% of these cases. It was especially performed in the assessment of benign organic pathologies where associated pathologies have to be looked for (nodules--87.7%; REINKE's edema--72.2%; mucosal cysts--92.6%; sulcus and scars--83.3%), as well as for tumors (73.7%). Among the dysfunctional pathologies, it was principally carried in cases of hypokinetic dysphonia (87.7%). Videostroboscopy is a pertinent examination because, when it was used, it enabled to modify the initial diagnosis in 17% of dysfunctional dysphonias, 20% of nodules, 23% of REINKE's edemas and 17% of granulomas.

Fiber Optic Technology↗
PubMedDate not supplied

Image rotation and reversal--major obstacles in learning intracorporeal suturing and knot-tying.

BACKGROUND AND OBJECTIVES: A major stumbling block to teaching and learning the finer skills of laparoscopy is related to the "optical illusions" the video camera plays on the surgeon's eyes. Until now, the belief was that lack of coordination was the result of depth perception deficiencies resulting from the two dimensional plane of the video monitor. In reality, this is a minor problem that is easily surmounted with practice. A closer analysis of how organ orientation at the operative site compares to the video camera's fields of focus reveals the real problem: the major optical difference between laparotomy and laparoscopy involves rotation of the images received by the brain. CONCLUSIONS: There are four major operating positions in laparoscopy: camera position, right camera position, left camera position and opposite camera position. The object in front of the camera has two components; the first, a reality image, which results from light reflected off the object as it exists in time and space. The second, a visual image, which represents the actual light entering our eyes. At right camera position the visual image is a 90 degrees counter-clockwise rotation of the reality image. At the left camera position the visual image is a 90 degrees clockwise rotation of the reality image. At opposite camera position, a 180 degrees rotation and complete reversal of the reality image occurs. It is only at camera position that the visual image is equal to the reality image, and we approach a scenario similar to that found in laparotomy. Every other position will be unlike what we were accustomed to in open surgery.

Clinical Competence↗
PubMed1995

Anosognosia in parietal lobe syndrome.

Patients with right parietal lesions often deny their paralysis (anosognosia), but do they have "tacit" knowledge of their paralysis? I devised three novel tests to explore this. First, the patients were given a choice between a bimanual task (e.g., tying shoe laces) vs a unimanual one (e.g., threading a bolt). They chose the former on 17 of 18 trials and, surprisingly, showed no frustration or learning despite repeated failed attempts. I conclude that they have no tacit knowledge of paralysis (or, if such knowledge exists, it is not available for this particular task). Second, I used a "virtual reality box" to convey the optical illusion to the patient that she was moving her paralyzed left hand up and down to the rhythm of a metronome, and yet she showed no sign of surprise. Third, I irrigated patient BM's left ear canal with cold water, a procedure that is known to shift that patient's spatial frame of reference by stimulating the vestibular system. Surprisingly, this allowed her "repressed" memory of the paralysis to come to the surface; she said she had been paralyzed continuously for several days. I suggest that the vestibular stimulation produces these remarkable effects by mimicking REM sleep. These patients also employ a whole arsenal of grossly exaggerated Freudian "defense mechanisms" to account for their paralysis. To explain this, I propose that in normal individuals the left hemisphere ordinarily deals with small, local anomalies by trying to impose consistency but, when the anomaly exceeds threshold, an interaction with the right hemisphere forces a "paradigm shift." A failure of this process, in patients with right hemisphere damage, might partially account for anosognosia. Finally, I present a new conceptual framework that may help link several psychological and neurological phenomena such as Freudian defense mechanisms, vestibular stimulation, anosognosia, memory repression, visual illusions, anterograde amnesia, REM sleep, dreaming, and humor.

Adaptation, Psychological↗
PubMed1979

Apparent movements induced by stroboscopic illumination of stabilized images.

Stroboscopic illumination of stabilized images causes habituation effects and illusory phenomena which, at low stimulation frequencies, show up as brighter patches. When a line is used as a stimulus, these patches are observed as brighter spots in the line surrounded by dimmer patches in the background. These locally brighter parts function as sources from which brightness spreads in the direction of the line at light-on and as sinks to which the brightness retracts again after light-off. This spread and retraction of the brightness induces the perception of movements. Higher stimulation frequencies or a diminished stimulus-background contrast enables the brightness to spread also in the direction perpendicular to the line contour, i.e., into the background. Again a perception of movement is induced, local displacements of a part of the field are observed as a result of the brightness spread. Tiny pinpoints of light, the smallest foveal perceptive elements, visible in lines narrower than 10 min arc (Gerrits, 1978), are also observed when these lines are illuminated with stroboscopic light. These tiny elements do not spread their brightnesses and so also no movement is induced. The results are discussed in relation to the properties of the perceptive elements and the cells activating them (habituation, barriers, filling-in).

Afterimage↗