phenomenon

[G. phainomenon, fr. phainō, to cause to appear]

[G. phainomenon, fr. phainō, to cause to appear]

1. A remarkable event or appearance. 2. A fact or an occurrence that can be described or explained. Plural: phenomena.
Abney's phenomenon A slight change in hue resulting from a change in saturation. This is especially noticeable when white light is added to a monochromatic blue or green light.
Aubert's phenomenon If, in the dark the head is tilted slowly to one side while looking at a bright vertical line, this line will appear to tilt in the opposite direction. This phenomenon is due to the absence of compensatory postural changes. Syn. Aubert's effect.
Aubert-Förster phenomenon When targets (e.g. letters) of different sizes are placed peripheral to the foveal region and at different distances from the observer, visual acuity is better for the smaller targets nearer the observer than for the larger targets farther than the observer, although they subtend the same visual angle. Syn. Aubert-Förster law.
Bell's phenomenon An outward and upward rolling of the eyes when closing, or attempting to close the eyelids. See Bell's sign.
Bezold-Brücke phenomenon A change in perceived hue of some spectral colours with a change in intensity. However, some wavelengths, such as 478, 503 and 578 nm, remain a constant hue with varying intensity. These are called invariant wavelengths or unique hues. Syn. Bezold-Brücke effect.
Bielschowsky's phenomenon In alternating hypertropia, occluding one eye leads to its rotation upward, and then placing a neutral density filter in front of the other eye gives rise to a downward movement of the occluded eye. See Bielschowsky's phenomenon test.
Broca-Sulzer phenomenon See Broca- Sulzer effect.
Brücke-Bartley phenomenon See Brücke- Bartley effect.
crowding phenomenon A difficulty or inability to discriminate small visual acuity tests when they are presented next to each other in a row, thus inducing contour interaction, whereas the same sized acuity symbols presented singly against a uniform background are resolved. Although this phenomenon may be experienced by normal patients, it is most often characteristic of amblyopic eyes and of people with reading difficulties. Syn. crowding effect. See morphoscopic visual acuity; amblyopia.
doll's head phenomenon Reflex movement of the eyes in a direction opposite to the direction of a rapid head turn, followed by a return towards the original position. These vestibular-elicited eye movements are aimed at maintaining fixation. The phenomenon can be used to assess the integrity of the vestibulo-ocular response system (doll's head test). If the eye movements do not accord with the above, it may indicate a brainstem defect. Syn. doll's eye sign.
entoptic phenomenon See entoptic image.
extinction phenomenon A condition in which individual stimuli placed in the visual field are seen, but when the nasal field of one eye and the temporal field of the other eye are stimulated simultaneously the subject fails to see one of the stimuli. This condition is common following a stroke. Syn. pseudo-hemianopia. See visual neglect.
Fick's phenomenon See Sattler's veil.
jack-in-the-box phenomenon When wearing very high positive lenses (e.g. in aphakia) there exists an area in the periphery situated between the outer extent of the field seen through the lens and the field beyond the edge of the lens, which is not seen (ring scotoma). This phenomenon refers to the disappearance and sudden reappearance of an object when the eye moves from the periphery to the centre passing over the ring scotoma. This phenomenon can be avoided by turning the head rather than the eye for peripheral viewing or by correcting with contact lenses. Modern aspheric lenses minimize this phenomenon as they have reduced peripheral power. See real field of view; ring scotoma.
jaw-winking phenomenon An abnormal condition associated with congenital ptosis, characterized by the elevation of the ptotic eyelid when the mouth is opened or the jaw is moved laterally to the side opposite to the ptosis. The eyelid droops again if the jaw maintains its new position or is closed. The condition often diminishes with time, otherwise surgery is the main treatment. Syn. Marcus Gunn phenomenon; Marcus Gunn jaw-winking syndrome.
Marcus Gunn phenomenon See jaw-winking phenomenon.
Mizuo's phenomenon The appearance of a golden brown colour of the retina as it adapts to light, in Oguchi's disease. When adapted to darkness the fundus has the normal red appearance. Syn. Mizuo's sign.
phi phenomenon See phi movement.
Pulfrich phenomenon See Pulfrich stereophenomenon.
Purkinje's phenomenon See Purkinje shift.
Riddoch phenomenon Ability to perceive the motion of an object while being unable to detect any other features of that object, such as its colour or its form. This may occur in a scotomatous area of the visual field caused by a lesion somewhere in the visual pathway from the lateral geniculate body to the occipital and temporal cortex.
Troxler's phenomenon An image in the periphery of the retina tends to fade or disappear during steady fixation of another object. This phenomenon is rarely noticed due to the involuntary eye movements. When these are neutralized optically, as in stabilized retinal imagery, the phenomenon occurs readily even in central vision. See fixation movements; stabilized retinal image.
Uhthoff's phenomenon See Uhthoff's symptom.

A symptom; an occurrence of any sort, whether ordinary or extraordinary, in relation to a disease.

[G. phainomenon, fr. phainō, to cause to appear]