Hermann Grid's Review

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 of visual processing and so in an 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 neighboring neurons in the lateral direction, (Yantis & Steven, 2014). This creates a contrast in stimulation that allows increased sensory perception.

An important feature of the Hermann Grid is that when staring directly at the 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 center-surround receptive fields than the former. This complicated arrangement of excitatory centers 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 an 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 are required, either from past experiences or stored knowledge in order to make 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 on 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.