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Full text of "Emily Carr, Art Illusion and the Visual System, Science 201 (week 12)"

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Art, Illusion and the Visual System 

No electronic system or computer can match the 
ability of the human visual system to make sense of 
the countless number of images we see during our 

Our ability to make sense of the world around us is 
made possible by the brain's capacity to process 
huge amounts of information simultaneously. 

An understanding of how our brain processes visual 
information can directly be used by artists and graphic 


Research has shown that visual signals are not 
processed by a single hierarchical system but are fed 
into at least three separate processing systems in the 
brain, each with its own distinct functions: 

> One system appears to process information 
about shape perception; 

> a second system information about colour; and 

> a third, information about movement. 

It may seem strange to think of vision as a 
multifunctional system, yet anatomical, physiological 
and psychological studies strongly support this 

There is a definite functional specialization in the 
visual system. 


The process begins to take on its complexity at the 
arborization of the ganglion cells in the colour cones 
in the retina. If you recall there are two types of 
ganglion arborization: the large cells and small cells. 

The large cells do not distinguish one type of cone- 
cell signal from another; they simply add the 
information received from the three types of colour 
cone cells. Since these large cells lack colour- 
selectivity, they can be thought of as colour-blind. 

The small ganglion cells do distinguish between the 
three colours/ cone types and subtract information 
received from them. This enables the small ganglion 
cells to signal information about different colours. 

Mr- 3 

As a result of the way in which the input signals are 
processed, the signals from the small ganglion cells 
are more colour selective than the input signals they 
receive from the cone cells. 

The ganglion cells transmit their signals to the lateral 
geniculate nucleus via the optic nerve. Within the 
LGN the two types of neurons in the parvo and 
magno cellular are segregated into spatially distinct 

The small cell, parvocellular division, in the LGN 
receives input from the small ganglion cells, while the 
large cell magnocellular division, receives input from 
the large ganglion cells. 


In humans there are four parvocellular layers and two 
magnocellular layers in the LGN. 

The parvo and magno systems differ in their contrast 
sensitivity, temporal resolution and acuity. 

The magnocellular system is more sensitive to 
brightness contrast and has a faster response time 
and lower acuity than the parvocellualr system. 

The signals from the different layers in the LGN in 
turn go to different layers in the primary visual cortex. 

There is a functional segregation in the layers of the 
primary visual cortex. 





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Signals from the parvocellular-visual cortex pathway 
carries high resolution information about the borders 
that are formed by contrasting colours. 

Since much of the information about the shapes of 
objects can be represented by their borders, this 
parvocellular - visual cortex pathway appears to be 
important in shape perception. 

Information from both the parvocellular and 
magnocellular systems are combined in an 
intermediate way to assist in shape discrimination and 
some degree of depth perception, as well as 
processing information about colour and shades of 
gray, but no movement. 


Information from the magnocellular - visual cortex 
pathway carries information about movement and 
stereoscopic depth. 

Movement perception in humans is therefore colour- 
blind. Movement perception also has low acuity, 
high-contrast sensitivity and a fast response time. 

Neurons in the magnocellular - visual cortex pathway 
have a very fast response time, but their responses 
decay rapidly even when the stimulation is 
maintained, so that this system is particularly 
sensitive to moving stimulus. 

There are some interesting characteristics of these 
pathways that can be explored by artists: cells in the 
parvo cellular system can distinguish between red 
and green at any relative brightness. 


Cells in the colour-blind magnocellular system are 
analogous to a black-and-white photograph in the way 
they function: they signal about the brightness of 
surfaces but not about the surface's colour. 

For any pair of colours there is a particular brightness 
ratio at which in a black and white photograph two 
colours, for example red and green, will appear as the 
same shade of gray; hence any border between the 
two will vanish. 

Similarly, at some relative red-to-green brightness 
level, the red and green will appear identical to the 
magnocellular system. The red and green are then 
called equiluminant. 

A border between equiluminant colours has colour 
contrast but no luminance contrast. 


It is worth appreciating that the exact brightness ratio 
at which a border between two colours becomes 
invisible to the magnocellular system varies from one 
person to the next. 

This effect is explored in Op art by artists like Piet 
Mondrian in his painting Broadway Boogie Woogie. 

In Op art, the work appears jumpy, as if the colours 
are moving or are unstable, and is a phenomena 
directly related to luminance. 

In Mondrian's Broadway Boogie Woogie the yellow 
stripes in the painting have low luminance contrast 
against the off-white background. 

The impression of movement is induced by the 
lessened ability of the brain to assign a stable position 
to the yellow stripes, and so they seem to jump about. 


An object that is equiluminant with its background 
looks vibrant and unstable. 

The reason seems to be that the parvocellular system 
can signal the object's shape but the magnocellular 
system cannot see its borders and therefore cannot 
signal either the movement or the position of the 

Hence, the object appears to jump around, drift of 

The same principle can be used in other types of art, 
such as fashion, graphic design and photography. 

Such an effect has been seen since the time of the 
impressionists and the pointillists, particularly when 
the dots of colour are too small to be resolved in the 
colour system but large enough to be resolved in the 
form system. 


Another place where this effect is used is in television, 
video and DVD's. These technologies take 
advantage of the low resolution of our visual colour 
system by transmitting the colour portion of an image 
at a lower resolution than the black and white part. 

The sensitivity of the magnocellular system to 
movement and stereopsis suggest that hard to see 
objects can be made more visible by introducing 

As research has shown in the past twenty years, 
stereopsis, like movement perception is colour blind, 
as are cues about perspective and the relative size of 
objects, the relative movement of objects, as well as 
shading and gradations in texture. 

Why the magnocellular system should respond to 
both to movement and to a number of non-movement 
related cues is as yet unknown. 


From an evolutionary standpoint it appears our 
magnocellular system preceeded our parvocellular 
system, and so it seems evident from a evolutionary 
standpoint that the magnocellular system be of some 
importance in judging distance and spatial relations. 

Being able to judge distance and sort out spatial 
relationships was in times past a matter of survival. 
Today such abilities have a broader and more modern 

From the standpoint of visual understanding, most 
images contain a rich assortment of visual elements 
that includes edges of many orientations, surfaces, 
colours and textures. 

Being able to organize such a rich assortment of 
visual elements into relationships is an important 
aspect of perception. 

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There are many different theories in psychology as to 
how this organization is undertaken by our brain. One 
theory that finds direct application is the 20 th century 
school of theory known as Gestalt psychology. 

The Gestalt psychologists suggest such organization 
is possible because the brain uses certain visual 
properties to group the parts of an image together and 
also to separate images from one another and from 
their background. 

In Gestalt theory these properties include: 

> the direction and speed of motion of objects, 

> elements that move together probably 
belong together and should not be 
considered entirely separate, 

> collinearity and linear continuity, 

> depth, 

> lightness and texture. 

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The fact that these functional properties fail at 
equiluminance suggests that the ability to link 
organize parts of an image together, to discriminate 
figure from background and to perceive the correct 
spatial relationships of objects involve the 
magnocellular system. 

It can be argued that the magnocellular system 
combines, or integrates key visual properties of an 
object in such a way that it enables the brain to 
perceive the objects as a whole, thereby allowing the 
parvocellular system to focus in on the details of the 

In a subsequent class we shall explore Gestalt 
psychology in greater depth. 




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All drawings and paintings are illusions, for they are 
representations of three-dimensional objects on two- 
dimensional surfaces. 

The artist compresses a solid object to a flat surface (using 
techniques like linear perspective). In creating their drawing 
or painting some information is lost in going from three to 
two dimensions. 

When we look at a drawing or painting the brain tries to 
reconstruct the third dimension. What we in fact see is an 
illusion created by our perception. 

You construct every drawing or painting you see. So, in a 
real sense, every drawing or painting you see is subjective. 

The first serious interest in the science of illusions grew out 
of concern by 19 th century physicists and astronomers that 
observations made with optical instruments were being 
distorted by perceptual errors.