Ludimar means that Baumgartner’s localized retinal processes cannot explain

Ludimar Hermann first observed the Hermann
Grid, and characterized it by “ghostlike grey blobs perceived at the intersections
of a white grid on a black background”, (Spillmann & Levine, 1971). Baumgartner believed
that the effect is due to inhibitory processes in the retinal ganglion cells,
the neurons that transmit signals from the eye to the brain, (Baumgartner
1960).  However, the Hermann grid alone only
provides a biological explanation visual processing and so in attempt to
explain visual processing fully, we must search for explanations that include
the environment as part of the explanation also.

 

At the center of an intersection there is
more light in its inhibitory surround than the receptive field located
elsewhere along the same line. More light in the inhibitory surround means that
there is more lateral inhibition at the intersection.  Lateral
inhibition disables the spreading of action potentials from excited
neurons to neighbouring neurons in the lateral direction, (Yantis & Steven,
2014).  This creates a contrast in
stimulation that allows increased sensory perception.

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An important feature
of the Hermann Grid is that when staring directly at intersection, no grey spot
would appear but rather would see them in peripheral vision. This is explained
as receptive fields in the central fovea are much smaller than in the rest of
the retina, and are too small to span the width of an intersection.

 

Conversely, the
Hermann grid only provides a limited explanation for visual processing. Schiller
and Tehovnik (2015) cite three main flaws. Firstly, despite our receptive
fields staying the same size, when the Hermann Grid changes in size the
illusion changes the same. Secondly, the illusory effect can be greatly
diminished or even removed entirely by skewing or otherwise distorting the grid
by even as little as 45 degrees. Thirdly, the actual arrangement of retinal
ganglion cells and their corresponding receptive fields is not as simple as
Baumgartner supposed. Midget and
Parasol ganglion cells exist in different ratios throughout the retina,
the latter having much larger centre-surround receptive fields than the former.
This complicated arrangement of excitatory centres and inhibitory surrounds,
operating across various distances on the 2-D retinal image, means that
Baumgartner’s localized retinal processes cannot explain the Hermann grid
effect (Schiller and Carvey 2005). 

 

Therefore, it can be
concluded that visual processing cannot only be explained by lateral
inhibition, and thus there must be alternate explanations. Cognitive
explanations suggest that we process visual information through cognitive
processes such as attention and retention. The two main cognitive explanations
for visual processing include the work of James Gibson, and Richard Gregory

 

James Gibson’s bottom
up theory, suggests that perception involves innate mechanisms forged by
evolution and that no learning is required. This suggests that perception is
necessary for survival because without perception the environment would be very
dangerous. Our ancestors would have needed perception to escape from harmful
predators and to know which fruit is poisonous and which is safe to consume, thus
suggesting perception is evolutionary. 

 

The starting point for
Gibson’s Theory was that the pattern of light reaching the eye, known as the
optic array, containing all the visual information necessary for perception. This
optic array provides unambiguous information about the layout of objects in
space. Changes in the flow of the optic array contain important information
about what type of movement is taking place. The flow of the optic array will
either move from or towards a particular point. If the flow appears to be
coming from the point, it means you are moving towards it. If the optic array
is moving towards the point you are moving away from it.

 

A strength of Gibson’s
theory would be a large number of applications can be applied in terms of his
theory. For example, when painting marking onto the floor of a runway for
pilots, the lines can gradually decrease in length or width to indicate in
which direction the pilot should drive in. Gibson’s theory is also very generalizable
across different species as it highlights the richness of information in optic
array, and provides an account of perception in animals, babies and
humans. 

 

However, his theory is
reductionist as it seeks to explain perception solely in terms of the
environment. There is strong evidence to show that the brain and long-term memory
can influence perception. For instance, the work of Richard Gregory shows that
our pre-existing schemas help to process new visual information in relation to
what we already have experienced.

 

Richard Gregory argued
that perception is a constructive process which relies on top-down
processing.  Stimulus information from our environment is frequently
ambiguous so to interpret   it, other
sources of information is required, either from past experiences or stored
knowledge in order to makes inferences about what is being perceived. In order
to provide evidence to support his hypothesis, Gregory conducted the Hollow Face
experiment. He used the rotation of a Charlie Chaplin mask to explain how we reconstruct
information of the present based off information from previous experiences. Our
prior knowledge of a normal face is that the nose protrudes, therefore, we
subconsciously reconstruct the hollow face into a normal face.

 

Evidence to support
Gregory’s idea that perceptions are often ambiguous is provided by the Necker
cube. When staring at the crosses on the cube the orientation can suddenly
change, or ‘flip’. It becomes unstable and a single physical pattern can
produce two perceptions. Gregory argued that this object appears to flip
between orientations because the brain develops two equally plausible
hypotheses and is unable to decide between them. When the perception changes
though there is no change of the sensory input, therefore the change of
appearance cannot be due to bottom-up processing. It must be set downwards by
the existing perceptual hypothesis of what is near and what is far.