2. “ILLUSION”
-MISREPRESENTATION OF A “REAL” SENSORY STIMULUS..!!
INTRODUCTION:
•Visual illusions have the potential to offer great insight into our visual perception. Illusions
have been extensively studied by psychologists, as a method of deducing the assumptions that
the brain makes and how we process visual information.
•One classical illusion known to induce misjudgment, is the MÜLLER-LYER ILLUSION (MLI).
•In the MLI, the perceived length of a line is affected by arrowheads or arrow tails placed at the
ends of the line. Specifically, the line appears elongated in the arrow tails and contracted with
arrowheads. Behavioral studies have shown that the strength of the illusion is correlated with
factors including shaft length, fin angle and inspection time.
•A variation of this effect consists of a set of arrow-like figures. Straight line segments of equal
length comprise the "shafts" of the arrows, while shorter line segments (called the fins)
protrude from the ends of the shaft.
•The fins can point inwards to form an arrow "head" or outwards to form an arrow "tail". The
line segment forming the shaft of the arrow with two tails is perceived to be longer than that
forming the shaft of the arrow with two heads.
3. Two sets of arrows
that exhibit the
MÜLLER-LYER
OPTICAL
ILLUSION.
The set on the
bottom shows that
all the shafts of
the arrows are of
the same length.
The ML illusion in various forms.
A: The classical four-wing form illustrates the perceptual effect of the top line
appearing shorter than the bottom line, even though the lines are of equal
length. B: Terminating circles still induce a perceptual effect of line length
misjudgment. C: The effect persists even when shafts are removed from the
original figure.
•Many theories have been put forward to explain the MLI, such as misapplied size constancy
scaling, the statistics of image-source relationships and the filtering properties of signal
processing in primary visual areas.
•Although many theories have been put forward to explain the MLI, there is ongoing debate as
to the source of the MLI. Originally, the illusion was explained as a combination of two
opposing factors: ‘confluxion’ and ‘contrast’. These terms were later interpreted into more
modern concepts of lateral inhibition and contour repulsion.
•Explaining the illusion has proven difficult because the effect persists even when the wings of
the illusory figure are replaced with other terminating shapes, such as circles or squares. Even
without the shaft, the perceptual effect remains. These variants demonstrate the persistence
of line length misjudgment and rule out simple explanations for the cause of the illusion.
4. TYPES OF ILLUSION:
•POGGENDORFF ILLUSION:-
•The Poggendorff illusion is a geometrical-optical illusion that involves the misperception of the
position of one segment of a transverse line that has been interrupted by the contour of an
intervening structure.
•It is named after Johann Christian Poggendorff, the editor of the journal, who discovered it in the
figures Johann Karl Friedrich Zöllner submitted when first reporting on what is now known as
the Zöllner illusion, in 1860.
•Many detailed studies of the illusion, including "amputating" various components point to its
principal cause: acute angles in the figure are seen by viewers as expanded though the illusion
diminishes or disappears when the transverse line is horizontal or vertical.
A straight black and red line is obscured by a grey rectangle. The
blue line, rather than the red line, appears to be a continuation of
the black one, which is clearly shown not to be the case on the
second picture. Instead there is an apparent position shift of the
lower portion of the line
5. •PONZO ILLUSION:-
•The Ponzo illusion is a geometrical-optical illusion that was first demonstrated by the Italian
psychologist Mario Ponzo (1882–1960) in 1911.
• He suggested that the human mind judges an object's size based on its background.
•He showed this by drawing two identical lines across a pair of converging lines, similar to
railway tracks. The upper line looks longer because we interpret the converging sides according
to linear perspective as parallel lines receding into the distance.
• In this context, we interpret the upper line as though it were farther away, so we see it as longer
– a farther object would have to be longer than a nearer one for both to produce retinal images of
the same size.
An example of
the Ponzo
illusion.
Both of the
horizontal
yellow lines
are of the
same length.
6. •APPARENT-MOVEMENT ILLUSION:-
•The term illusory motion, also known as motion illusion, is an optical illusion in which a static
image appears to be moving due to the cognitive effects of interacting color contrasts, object
shapes, and position.
•Apparent motion is the most common type of illusory motion and is perceived when images are
displayed in succession at a specific frame rate such as in a movie.
• It is an illusion of movement where two or more still images are combined by the brain into
surmised motion.
The static image
appears to be
moving due to
the cognitive
effects of
interacting color
contrasts, object
shapes, and
position.
7. •VERTICAL-HORIZONTAL ILLUSION:-
•The vertical–horizontal illusion is the tendency for observers to overestimate the length of a
vertical line relative to a horizontal line of the same length.
•This involves a bisecting component that causes the bisecting line to appear longer than the line
that is bisected. People often overestimate or underestimate the length of the bisecting line
relative to the bisected line of the same length. This even happens if people are aware that the
lines are of the same length.
•Cross-cultural differences in susceptibility to the vertical–horizontal illusion have been noted.
People from Western cultures and people living in urban landscapes show more susceptibility
than those living in eastern or open landscapes
