Reported by Ludimar Hermann, the Hermanngrid is characterized by “ghostlike” grey blobs perceived at the intersectionsof a white grid on a black background, (Spillmann & Levine, 1971). Baumgartner believedthat the effect is due to inhibitory processes in the retinal ganglion cells,the neurons that transmit signals from the eye to the brain, (Baumgartner1960).

 However, the Hermann grid alone onlyprovides a biological explanation visual processing and so in attempt toexplain visual processing fully, we must search for explanations that includethe environment as part of the explanation also.  At the center of an intersection there ismore light in its inhibitory surround than the receptive field locatedelsewhere along the same line. More light in the inhibitory surround means thatthere is more lateral inhibition at the intersection.  Lateralinhibition disables the spreading of action potentials from excitedneurons to neighbouring neurons in the lateral direction, (Yantis & Steven,2014).  This creates a contrast instimulation that allows increased sensory perception.  An important featureof the Hermann Grid is that when staring directly at intersection, no grey spotwould appear but rather would see them in peripheral vision. This is explainedas receptive fields in the central fovea are much smaller than in the rest ofthe retina, and are too small to span the width of an intersection.

 Conversely, theHermann grid only provides a limited explanation for visual processing. Schillerand Tehovnik (2015) cite three main flaws. Firstly, the illusion persists when increasingthe size of the figure despite the fact that receptive fields are of a fixedsize. Secondly, the illusory effect can be greatly diminished or even removedentirely by skewing or otherwise distorting the grid by even as little as 45degrees. Thirdly, the actual arrangement of retinal ganglion cells and theircorresponding receptive fields is not as simple as Baumgartner supposed.

Midget and Parasol ganglion cellsexist in different ratios throughout the retina, the latter having much largercentre-surround receptive fields than the former. This complicated arrangementof excitatory centres and inhibitory surrounds, operating across variousdistances on the 2-D retinal image, means that Baumgartner’s localized retinalprocesses cannot explain the Hermann grid effect (Schiller and Carvey2005).  Therefore, it can beconcluded that visual processing cannot only be explained by lateralinhibition, and thus there must be alternate explanations. Cognitiveexplanations suggest that we process visual information through cognitiveprocesses such as attention and retention. The two main cognitive explanationsfor visual processing include the work of James Gibson, and Richard Gregory James Gibson’s bottomup theory, suggests that perception involves innate mechanisms forged byevolution and that no learning is required. This suggests that perception isnecessary for survival because without perception the environment would be verydangerous. Our ancestors would have needed perception to escape from harmfulpredators and to know which fruit is poisonous and which is safe to consume, thussuggesting perception is evolutionary.   The starting point forGibson’s Theory was that the pattern of light reaching the eye, known as theoptic array, containing all the visual information necessary for perception.

Thisoptic array provides unambiguous information about the layout of objects inspace. Changes in the flow of the optic array contain important informationabout what type of movement is taking place. The flow of the optic array willeither move from or towards a particular point.

 If the flow appears to becoming from the point, it means you are moving towards it. If the optic arrayis moving towards the point you are moving away from it.  A strength of Gibson’stheory would be a large number of applications can be applied in terms of histheory. For example, when painting marking onto the floor of a runway forpilots, the lines can gradually decrease in length or width to indicate inwhich direction the pilot should drive in. Gibson’s theory is also very generalizableacross different species as it highlights the richness of information in opticarray, and provides an account of perception in animals, babies andhumans.  However, his theory isreductionist as it seeks to explain perception solely in terms of theenvironment. There is strong evidence to show that the brain and long-term memorycan influence perception.

For instance, the work of Richard Gregory shows thatour pre-existing schemas help to process new visual information in relation towhat we already have experienced.  Richard Gregory arguedthat perception is a constructive process which relies on top-downprocessing.  Stimulus information from our environment is frequentlyambiguous so to interpret   it, othersources of information is required, either from past experiences or storedknowledge in order to makes inferences about what is being perceived. In orderto provide evidence to support his hypothesis, Gregory conducted the Hollow Faceexperiment.

He used the rotation of a Charlie Chaplin mask to explain how weperceive the hollow surface of the mask as protruding based on our expectationof the world. Our prior knowledge of a normal face is that the nose protrudes.So, we subconsciously reconstruct the hollow face into a normal face.  Evidence to supportGregory’s idea that perceptions are often ambiguous is provided by the Neckercube. When staring at the crosses on the cube the orientation can suddenlychange, or ‘flip’. It becomes unstable and a single physical pattern canproduce two perceptions.

Gregory argued that this object appears to flipbetween orientations because the brain develops two equally plausiblehypotheses and is unable to decide between them. When the perception changesthough there is no change of the sensory input, therefore the change ofappearance cannot be due to bottom-up processing. It must be set downwards bythe existing perceptual hypothesis of what is near and what is far. 


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