The outcome of many court cases rely on eyewitness identification, which in criminal law, is evidence received from a witness “who has actually seen an event and can so testify in court”. In recent years many US Courts have noted on the inherently suspect qualities of eyewitness identification evidence, and described the evidence as “notoriously unreliable”. In the United Kingdom, the Criminal Law Review Committee stated that cases of mistaken identification “constitute by far the greatest cause of actual or possible wrong convictions”.

In many of these cases the persons providing the eyewitness accounts were convinced of what they saw. So how is it possible that mistaken identities can occur from people who actually saw what happen?

The Visual Pathway

The answer lies in the fact that vision is a complicated process that requires numerous components of the human eye and brain to work together. The visual system is the most complex neural circuitry of all the sensory systems in the human body with the optic nerve alone containing over a million nerve fibres.

What humans perceive as vision starts with light rays being focused clearly and accurately on to the back of the eye on an area called the retina. Interestingly the image focused on the retina is inverted relative to the actual object. This means that what we perceive as upright is actually focused upside down on the retina.

Due to the optics of the eye, the focused image on the retina of the eye is inverted

The retina itself consists of nine layers of nerve cells and one layer of photoreceptor cells called rods and cones. The neural signals from the rods and cones undergo processing by the layers of neurons of the retina and is finally output to the retinal ganglion cells whose axons form the optic nerve.

The human retina is made up out of ten nerve layers

From the optic nerve (or cranial nerve II) of each eye, the visual signals are send to the back of the brain via the visual pathway. The visual information from the left side of each eye’s retina (what you will perceive as your right side of vision) will go to the left side of the visual area of the thalamus and visual information of the right side of both eye’s retina (your perceived left visual field) will go to the right side of the visual area of the thalamus. The cross-over of the nerve fibres carrying the visual information from the nasal side (nose side) of the two eye’s retina occur in an area in the brain called the optic chiasm.

Schematic showing the visual pathway from the eyes to the visual cortex

Vision, as we perceive it, is interpreted by the visual cortex situated in the 2 occipital lobes, in the back of the brain.

Picture showing how the human visual field is transmitted to the ocipital visual cortex

The left hemisphere visual cortex receive visual signals from the right visual field (left side of the retinas) and the right visual cortex will receive visual signals from the left visual field (right side of retinas).

The primary visual cortex transmits visual information to the ventral and dorsal pathways or streamsThe primary visual cortex (V1) of each hemisphere transmits visual information to two primary anatomical pathways, called the ventral and the dorsal stream:

  • The ventral stream (also called the “What Pathway”) send information to the inferior temporal cortex and is associated with conscious perception like form recognition and object representation and also storage of long term memory.
  • The dorsal stream (also called the “Where Pathway” or “How Pathway”) sends information to the posterior parietal cortex and is associated with automatic, unconscious action like representation of object locations, motion and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.

The primary visual cortex (V1) of each hemisphere transmits visual information to two primary anatomical pathways, called the ventral and the dorsal stream:

Some visual scientist believe that this dichotomy of the visual cortex makes it possible for certain optical illusions to distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs. Others believe that optical illusions are possible due to the fact that the brain receives two-dimensional images from the retina that has no real fixed relationship with their three-dimensional source. In theory this means that a number of real world scenarios can generate a similar single image on the retina and that this creates a certain ‘open licence’ for the brain to interpret the images.

An example of this is where human vision will perceive horizontal lines shorter than vertical lines when they are the same length as is illustrated in the picture below.

Horisontal lines equal in length to vertical lines are perceived as shorter

So when you think you are drawing a perfect square of equal length you might be wrong. A square that is perfect will always appear slightly taller than it is wide, due to the nature of the brain to add length to vertical line.

Equal Suares will appear taller than wide due to the brain adding length to vertical lines

Other examples of optical illusions where the ‘perceived’ reality is different to the ‘actual’ reality include:

Café wall illusion

In the Café wall illusion the grey horisontal lines appear sloped when placed in between staggered rows of alternating black and white blocks. In reality the lines are actually straight.

Café wall illusion

 Ebbinghaus illusion

In the Ebbinghaus illusion or Titchener circles, two circles of identical size (orange in colour) are placed next to each other. One of the orange circles is surrounded by large light blue circles while the other orange circle is surrounded by small light blue circles. By the juxta-positioning of the big or small light blue circles around the central orange circles, the brain perceives the first orange circle to be smaller in size compared to the second orange circle, although both are in reality the same size.

Two equal sized circles will apear to be different in size when placed inside a circle of either big or small circles

Spinning dancer Illusion

The Spinning Dancer is a moving bistable (resting in two states) optical illusion of a pirouetting female dancer. Some observers initially see the figure as spinning clockwise and some counter clockwise. The illusion is possible due to the lack of visual cues for depth and that all silhouettes are ambiguous. Our brain tries to reconstruct the third dimension (space) from the flat two dimensional image in our eyes, adding information which is usually realistic, but not really there. And in the case of a silhouette, there are two equally likely interpretations, leading to perceptual rivalry.

Bistable spinning sillouette appear to be spinning either clockwise and anticlockwise depending on the observer

Figure ground Organisation

Figure ground organization or perception illusions like the Faces-Vase drawing work on how the visual brain assigns edges and its effect on recognizing shapes. In the Faces-Vase picture below, the perceived shape depends on the direction in which the border (edge) between the black and white regions are assigned. If you focus on the white edges inwards, you will perceive a white vase. When you focus on the dark edges outwards, you will perceive the silhouette of two black faces.

Either a white vase or two silhouette faces can be seen depending on how the brain assigns the black and white edges

Physiological illusions

Physiological illusions, such as afterimages following bright lights, or adapting to stimuli of excessively longer alternating patterns, are presumed to be the effects on the eyes or brain of excessive stimulation or interaction with contextual or competing stimuli of a specific type—brightness, colour, position, tile, size, movement, etc. The theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing, and that intense or repetitive activity in that or interaction with active adjoining channels cause a physiological imbalance that alters visual perception.

In the Grid Illusion below the shape, position, colour, and 3D contrast of the black squares, grey lines and white dots converge to produce the illusion of black dots at the line intersections.

Black spots can be seen in the grid when in reality they don't exist

Afterimage Illusions are images that remain in your vision after its initial exposure is gone. If you were to stare at the image below for 30 to 60 seconds and then look at a blank white screen you will see a clear picture of man.

There are 2 main things that cause this type of effect. The first is the adaptation in the firing rate of ganglion cells in the retina. When ganglion cells become excited or unexcited for a long period of time, they produce a rebounding effect, firing these ganglion cells at a faster or slower rate than normal. The second reason for this occurrence is because of the eye’s photoreceptors, mainly the cone cells, becoming over stimulated and then desensitized or fatigued.

In the after image of this picture the face of a man/Jesus can be seen

Looking at the after image of the picture above some would say it is a picture of Jesus. If you have said this, you are most likely a Christian believer or have been exposed to the Christian faith. Which brings up another interesting point about vision. Visual perception is also learned and culturally based.

As another example, imagine what the associated visual response would be showing the three silhouetted pictures below to children from South Africa, Australia, UK and America.




Colour and Brightness constancy

Colour constancy and Brightness constancy are responsible for the fact that a familiar object will appear the same colour regardless of the amount of or colour of light reflecting from it. In the colour constancy illusion below, the coloured circle regions appear rather different, roughly orange and brown. In reality however they are the same colour but the brain changes its assumption about the circles colour due to the global interpretation of the surrounding image. Also, the white tiles that are shadowed are the same colour as the grey tiles outside the shadow.

Image showing the effect of colour and brightness constancy where the brain perceives one coloured circle (in the shadow of a cylinder) brighter than the other (not in the shadow of teh cylinder), when in reality they are of equal colour

Hybrid Images

A hybrid image illusion like this Marilyn Monroe-Albert Einstein hybrid image is interpreted in one of two different ways, depending on the viewing distance. Hybrids images are made by combining the low-spatial frequencies of one picture with the high spatial frequencies of another picture to produce an image which is interpreted differently by the visual brain depending on the viewing distance.
In this hybrid image the high spatial frequency picture of Albert Einstein is seen close up. From a far distance the low spatial frequency image of Marilyn Manroe is seen.

When you look at the picture from a short distance, you will see the sharp or high spatial frequency image of Albert Einstein, with only a hint of blurry distortion hinting at the presence of an overlaid image. Viewed from a distance of about 5m in which the fine detail blurs, the low spatial frequency image Marilyn Monroe appears.

Seeing is believing, or is it?

So the next time you hear abut an eye witness account of an event ask yourself the question: “How reliable do you think is eyewitness testimony? Did it really happen or was it all an illusion?”