visual hallucination

hallucination

[hah-loo″sĭ-na´shun] a sensory impression (sight, touch, sound, smell, or taste) that has no basis in external stimulation. Hallucinations can have psychologic causes, as in mental illness, or they can result from drugs, alcohol, organic illnesses, such as brain tumor or senility, or exhaustion. When hallucinations have a psychologic origin, they usually represent a disguised form of a repressed conflict. adj. adj hallu´cinative, hallu´cinatory.auditory hallucination a hallucination of hearing; the most common type.gustatory hallucination a hallucination of taste.haptic hallucination tactile hallucination.hypnagogic hallucination a vivid, dreamlike hallucination occurring at sleep onset.hypnopompic hallucination a vivid, dreamlike hallucination occurring on awakening.kinesthetic hallucination a hallucination involving the sense of bodily movement.olfactory hallucination a hallucination of smell.somatic hallucination a hallucination involving the perception of a physical experience occurring within the body.tactile hallucination a hallucination of touch.visual hallucination a hallucination of sight.

visual hallucination

The sensation of seeing objects that are not really there. This is a hallmark of alcohol and drug withdrawal and of other medical illnesses that adversely affect the brain. See also: hallucination

illusion

A false interpretation of an object or figure presented to the eye (visual illusion). Illusions can occur with each of the senses. See ambiguous figure.
autokinetic visual illusion The apparent motion of a luminous object fixated in the dark, or in a large blank field. It is not due to eye movements and the illusion disappears as soon as the ambient luminance increases so that other objects become visible. Syn. visual autokinesis.
Baldwin's illusion 1. Illusion in which a line connecting two large squares appears shorter than a line connecting two smaller squares (Fig. I1). 2. Illusion in which a dot placed halfway between a large disc and a smaller disc appears to be nearer the large one.
café wall illusion An illusion induced by a pattern of alternating columns of black and white rectangles (or squares) placed in such a way that the lines that they compose do not appear to be parallel. Syn. Munsterberg illusion. A variant of this illusion consists of hollow squares without alternating colour and is called a 'hollow square illusion'.
Cornsweet illusion See Craik-O'Brien-Cornsweet effect.
corridor illusion Illusion in which images of equal size in a perspective figure of a corridor, appear to be of different sizes. The figure that seems further away appears larger than the one in the foreground (Fig. I2).
Craik-Cornsweet illusion See Craik-O'Brien-Cornsweet effect.
Delboeuf illusion Illusion in which a circle surrounded by a slightly larger concentric circle appears larger than another circle of the same size surrounded by a much larger concentric circle (Fig. I3).
Ebbinghaus illusion Illusion in which a circle usually appears larger when surrounded by smaller circles than by larger circles (Fig.I4).
Ehrenstein's brightness illusion Illusion in which the erased area at the intersection of radial (or horizontal and vertical) lines appears to be brighter than the background and with an illusory contour (Fig. I5).
floating-finger illusion Illusion noted when fixating a point in the distance while the forefingers of each hand are held horizontally about 30 centimetres in front of the eyes, with the fingertips nearly touching. A small, disembodied finger with two tips appears floating in between and can be shortened or lengthened by varying the distance between the fingertips. It is a peculiar illustration of physiological diplopia (Fig. I6).
frequency doubling illusion Illusion in which a grating pattern appears to have twice as many black and white bars as it actually has. This happens when a sinusoidal grating with a low spatial frequency (less than 4 c/deg) flickers in a counterphase fashion (i.e. light bars become dark and vice versa) at a high temporal frequency (more than 15 Hz). This type of stimulation is assumed to stimulate the non-linear mechanism within the magnocellular visual system. See frequency doubling perimetry.
Helmholtz illusion See irradiation.
Hering's illusion Illusion in which a pair of parallel lines appear bent when placing diagonal lines across them. This illusion is most noticeable when radiating lines are crossing two parallel lines on opposite sides of the point of radiation. In this case, the two parallel lines appear to bend away from each other (Fig. I7). See Wundt's illusion.
Hermann's illusion See Hering-Hermann grid.
hole in the hand illusion See hole in the hand test.
hollow square illusion See café wall illusion.
horizontal-vertical visual illusion Illusion in which the vertical line appears longer than the horizontal line when two lines of equal length are placed with the vertical line at the midpoint of the horizontal. See top hat illusion.
Jastrow illusion Illusion in which two identical curved and tapering ring segments placed one above the other appear unequal in size, the band nearer the centres of curvature appearing to be the longest (Fig. I8).
Kundt's illusion Illusion occurring when one attempts to bisect a horizontal line with only one eye and the segment on the temporal side of the visual field is then larger than the other.
moon illusion Illusion in which the moon appears much larger at the horizon than when viewed high in the sky. In fact, the actual size of the moon remains constant as does its distance from the earth. One possible explanation is that at the horizon there are many other cues in the field of view (e.g. houses, mountains) that make the moon appear to be much closer than when it is high in the sky and thus should be larger. See Ames room.
Müller-Lyer illusion Illusion in which a line with outgoing fins on both ends appears longer than another of equal length but with arrowheads on both ends (Fig. I9).
Munsterberg's illusion See café wall illusion.
oculogyral illusion Illusion of apparent movement of viewed objects when the body is subjected to rotary acceleration. The initial apparent movement is opposite to that of the direction of rotation of the body and is followed by an apparent movement in the same direction.
Oppel-Kundt illusion Illusion in which a divided, interrupted or filled area appears to be larger than an empty area of equal size.
optical illusion See visual illusion.
Orbison illusion Illusion of a distorted geometric figure such as a square or a circle drawn on a background of radiating lines or concentric lines.
Poggendorff's illusion Illusion in which two visible portions of a diagonal line overlaid by a rectangle do not appear to be continuous (Fig. I10).
Ponzo illusion Illusion in which two parallel lines of equal length do not appear equal when they are surrounded by two radiating straight lines, one on each side. The parallel line nearer the point of radiation appears to be longer (Fig. I11).
Schroeder's staircase visual illusion See Schroeder's staircase.
top hat illusion Illusion in which a top hat drawn with equal vertical and horizontal dimensions appears to be much greater vertically than horizontally. It is closely related to the horizontal-vertical illusion (Fig. I12). See horizontal-vertical visual illusion.
visual illusion Perception of an object or a figure that does not correspond to the actual physical characteristics of the stimulus. Syn. optical illusion; geometrical optical illusion.
waterfall illusion See waterfall after-effect.
Wundt's illusion Illusion in which a pair of parallel lines appear bent towards each other when crossed by lines radiating from two points, one on each side of the parallel lines. See Hering's illusion.
Zollner's illusion Illusion in which a series of parallel lines appear to converge or diverge from each other when crossed by short diagonal lines.