Ludimar of the retina, and are too small

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. According
to Schiller and Tehovnik (2015), there are three large flaws of the Hermann
Grid. Firstly, despite our receptive fields staying the same size, when the
Hermann Grid changes in size the illusion changes the same. Secondly, the
effect can be significantly reduced or even removed entirely by distorting the
grid by even as little as 45 degrees. So, in respect to visual processing, the
grid cannot provide a fully comprehensive explanation as it only seems to work
at a particular size and orientation. In contrast, in reality we perceive
visual information that is presented in every size and orientation possible,
consequently there must be another explanation as to how we are able to do that
besides the work of our ganglion cells. Thirdly, the actual arrangement of
retinal ganglion cells and their receptive fields is not as simple as
Baumgartner supposed. Midget and
Parasol ganglion cells exist in different ratios throughout the retina.
This complicated arrangement 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. In order to find a
better explanation for visual processing, the work of James Gibson, and Richard
Gregory could be assessed.

 

James Gibson’s bottom
up theory, suggests that perception is evolutionary due to the idea that
perception is necessary for survival and without it, our ancestors would not
have been able to avoid predators or consume safe to eat fruit. 

 

Gibson first began his
theory stating that, the pattern of light reaching the eye, is known as the
optic array. 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. 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, Gibson’s work on training programs
for pilots and airports are constructed with the best possible lighting and
markings to enhance optical flow patterns for incoming pilots. 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.
Therefore, in similarity to the Hermann Grid it provides a limited explanation.
Therefore, a theory that provides evidence to show that perception can be
affected my nature influences as well as environment, may be a better
explanation for how visual processing works. 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. Visual information from our environment is often ambiguous so
to interpret it, other sources of information are required, either from past
experiences or prior knowledge. 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.

 

Further 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, as the perception is unstable and can produce two perceptions.
Gregory argued that this is because the brain develops two equally plausible
hypotheses and is unable to decide between them. And so, when the perception
changes though there is no change of the sensory input, the change of
appearance cannot be due to bottom-up processing.

 

To conclude, the
Hermann Grid tells us that visual processing happens as a result of the actions
of the ganglion cells and their receptive fields. However, as it only provides a
biological explanation, it is reductionist and limited in what it tells us.
Therefore, in order to achieve a better understanding of how visual processing
works, the work of Richard Gregory is arguably better as it provides a much
more interactionist approach and explains visual processing not only via our nature,
but how we interact the environment also.