illusions : an illusion object or drawing
which appears to have properties which are physically impossible, deceptive,
illusory contour figures : radial line segments whose inward-pointing
end produce the illusion of a circle or other figure. The apparent figure
has the same color as the background, but appears brighter
Kanizsa triangle : the eye perceives a white upright equilateral
triangle where none is actually drawn.
impossible figure : a class of illusion
in which an object which is physically unrealizable is apparently depicted.
Fraser's spiral : An optical illusion named after British psychologist
James Fraser, who first studied the illusion in 1908 (Fraser 1908). The
illusion is also known as the false spiral, or by its original name, the
twisted cord illusion. While the image appears to be a spiral formed by
a rope containing twisted strands of two different colors, it actually
consists of concentric circles of twisted cords. The visual distortion
is produced by combining a regular line pattern (the circles) with misaligned
parts (the differently colored strands). Zöllner's illusion and the
café wall illusion are based on a similar principle, like many other
visual effects, in which a sequence of tilted elements causes the eye to
perceive phantom twists and deviations
3-arc illusion : the larger an arc is, the smaller its radius
appears. For example, the 3 arcs illustrated above belong to the same circle
Ehrenstein illusion : an illusion studied by the psychologist
Walter Ehrenstein in which the sides of a square placed inside a pattern
of concentric circles take an apparent curved shape. The name Ehrenstein
is also associated with one of the illusory contour figures
café wall illusion : an optical illusion produced by
a black and white rectangular tessellation when the tiles are shifted in
a zigzag pattern, as illustrated above. While the pattern seems to diverge
towards the upper and lower right corners in the upper figure, the gray
lines are actually parallel. Interestingly, the illusion greatly diminishes
if black lines are used instead of gray. Gregory and Heard (1979) first
noticed the illusion on the wall decoration of a café in Bristol,
England. The Café wall illusion is only one among many visual distortion
effects involving parallel lines. The most famous example of this kind
is Zöllner's illusion.
Continuous line illusion : while the above figure appears to
be a sequence of nested squares, it actually consists of a single square
bullseye illusion : although the inner shaded region has the
same area as the outer shaded annulus, it appears to be larger. Since the
rings are equally spaced, Ainner = p.
32 = 9 . p = p.
52 - p. 42 = 9 .p
In the above illustration, black dots appear to form and vanish at
the intersections of the gray horizontal and vertical lines. When focusing
attention on a single white dot, some gray dots nearby and some black dots
a little further away also seem to appear. More black dots seem to appear
as the eye is scanned across the image (as opposed to focusing on a single
point). Strangely, the effect seems to be reduced, but not eliminated,
when the head is cocked at a 45° angle. The effect seems to exist only
at intermediate distances; if the eye is moved very close to or very far
away from the figure, the phantom black dots do not appear. The illusion
is known as the scintillating grid, and was discovered by E. Lingelbach
In the illusion illustrated above, when the concentric square borders
with rounded edges are rotated slowly, the entire pattern appears to pulsate
A perception illusion in which the brain switches between seeing a
rabbit and a duck.
Ouchi illusion : the central disk seems to float above the checkered
background when moving the eyes around while viewing the figure. Scrolling
the image horizontally or vertically give a much stronger effect. The illusion
is caused by random eye movements, which are independent in the horizontal
and vertical directions. However, the two types of patterns in the figure
nearly eliminate the effect of the eye movements parallel to each type
of pattern. Consequently, the neurons stimulated by the disk convey the
signal that the disk jitters due to the horizontal component of the eye
movements, while the neurons stimulated by the background convey the signal
that movements are due to the independent vertical component. Since the
2 regions jitter independently, the brain interprets the regions as corresponding
to separate independent objects (Olveczky et al. 2003)
The illusion illustrated above in which the bounding rectangle and inner
square both appear distorted
Jastrow illusion : the left edges of the laminas A and B are
colinear, creating an illusion of different size. However, the two laminas
are actually identical.
Hermann grid illusion : a regular 2D arrangement of squares
separated by vertical and horizontal "canals." Looking at the grid produces
the illusion of gray spots in the white area between square vertices.
goblet illusion : the eye alternately sees 2 black faces, or
a white goblet
Bonneh's illusion (motion-induced
blindness) : look steadily at one stationary point, such as one of the
yellow discs, as blinks and sudden eye movements destroy the illusion.
Notice that 1, 2 or all of the yellow discs will disappear and reappear.
We can make the disappearing yellow dots reappear by disrupting the
activity of the left hemisphere using TMS. Alternatively, we can increase
the duration of the disappearance by disrupting teh activity of the right
hemisphere with precisely time pulses (the right hemisphere seems to be
much more picky about the prcise timing of teh TMS pulse than the left,
perhaps associated with the large blocks of time that the left deals in).
The left hemisphere is "into denial" as Ramachandran showed in patients
suffering from right hemisphere damage, who felt less compelled to deny
their paralysis when right hemisphere function was temporarily improved
by caloric stimulation. The illusion and perceptual rivalry may share the
same competitive tussle between the right and left sides of the brain.
The disappearance of the yellow discs may thus represent the ascendancy
of the left, while their reappearance may represent the ascendancy of the
Right. Critics have pointed out that there is only one left hemisphere
while there are a number of different possible patterns of disappearance
of the yellow discs (8 to be precise). This may be a little pedantic, since
all of these patterns can be subsumed under one rubric, viz:- disappearance.
Multi-stable perceptions where > 2 alternatives are possible do not seem
to present fatal difficulties to the hemispheric switching idea so far.
In any case, multistable rivalries are relatively rare compared to the
vast majority of rivalries, which are bistable. Another apparent difference
between binocular rivalry and Bonneh's MIB is that increasing the contrast
seems to increase the duration of the dominant phase in opposite directions,
since brighter yellow dots increase the disappearance phase while increasing
contrast shortens the suppressed phase but has not effect on the dominant
phase of binocular rivalry. It is premature to make too much of this difference,
since there are some forms of stimulus manipulation (e.g. changing the
context) that CAN increase the dominance phase duration in binocular rivalry.
The more monophasic form of the oscillation in Bonneh's MIB might align
it more apporpriately with only one phase of the binocular rivalry oscillation,
for example. In addition, brightening the yellow discs might be seen as
a contextual stimulus in the sense that this would further separate the
blue swirl from the discs and therefore tend to reject the hypothesis that
the yellow discs were "connected" to the blue swirl. Meditation can focus
the mind in a measurable way, according to a study of Buddhist monks. In
a visual test designed to confuse the brain, the monks were able to stave
off confusion more easily than those not trained in the contemplative arts.
Researchers studied 76 Tibetan Buddhist monks taking a test of 'perceptual
rivalry', in which 2 conflicting images are presented, one to each eye.
This usually causes the brain to switch back and forth between the images
every few seconds as it struggles to make sense of what it is seeing. Monks
skilled in the art of 'one-point' meditation - which involves focusing
all of one's attention on a single object or thought - were able to slow
this switching down or even stop it completely. In their study, they asked
monks with training ranging from to 5 to 54 years to practise different
forms of meditation and then don a set of goggles which displayed 2 different
images: horizontal bars to one eye, and vertical bars to the other. The
most experienced one-point meditators, who had spent > 20 years in isolated
retreats, were able to resist visual switching for the whole 5 minutes
of the experiment. According to the monks' self-reported assessment, they
saw only a single stable image with one set of bars dominant. There was
no noticeable improvement for monks who were practising 'compassion' meditation,
which involves contemplating the suffering of others. The monks were also
given another test, of 'motion-induced blindness'ref,
which involves staring at a stationary dot in the midst of a pattern of
swirling dots, until the other stationary dots in the picture seem to disappear.
Monks maintained this 'blindness' state for an average of 4.1 seconds,
compared to just 2.6 seconds for ordinary people. The most experienced
meditator managed to uphold the optical illusion for more than 12 minutes.
The discovery supports that idea that meditation calms the mind and allows
it to focus more clearly. Monks appear to be able to control the rate and
content of thoughts flowing through their 'stream of consciousness'ref.
Meditation could conceivably help people with depression, or who have recently
suffered a trauma, to stop their minds constantly dwelling on negative
thoughts. It has long been claimed by practitioners of meditation that
when faced with bad news or tragic events they are able to acknowledge
the tragedy, but rather than dwell on the situation they have the capacity
to redirect their thoughts to other, more positive directions : this is
something that the average person cannot do. Too much introspection and
contemplation might be dangerous for someone struggling with depression.