Fundamental visual factors

Visual perception cannot be defined simply by what we see. Three different processes are involved in the science of vision. These are the physical, the perceptual and the cognitive processes. These three processes will be briefly described in this section as a simplification of the broad understanding of how visual perception works. Many disciplines, such as physics, psychology, neuroscience, and biology, have an ongoing research focus on the science of vision. Due to this interdisciplinary nature, visual perception can be studied at different levels dissimilar to the ones exposed here. However, these three processes cannot be omitted when discussing visual perception.

Physiologically speaking, in simple words, light goes through the pupil while the lens focuses the image on the retina. The retina is a layer of tissue located at the back of the eyeball that receives different kinds of light information along parallel tracks. This information is received by different kinds of neurons connected to

Chapter 2. Theoretical Background  

photoreceptor cells located in the retina: rods and cones. Although the retina is less than half a millimetre thick, it contains many layers. Photoreceptor cells are found at the outermost layer of the retina. This means that the light passes through different layers and other types of cells before reaching the rods and cones. Although both rods and cones absorb light, they differ regarding their sensitivity to light and the type of information they carry. The rods, about 95% of the photoreceptors, are located in the periphery of the retina (Valberg, 2005). They are more sensitive than cones and function in scotopic vision (i.e. low light conditions). The cones are located in the centre of the gaze and function in photopic vision (i.e. high light conditions). Within the retinal layers the retinal ganglion cells are also encountered. Conventional ganglion cells lacking melanopsin do not present intrinsic light responses, whereas retinal ganglion cells containing melanopsin present intrinsic light responses (Hattar et al., 2002). These light-sensitive cells are called intrinsically photoreceptive retinal ganglion cells (ipRGCs). However, unlike the rods and cones, the ipRGCs do not contribute directly to vision, but are responsible for helping human circadian rhythms remain on a diurnal light and dark pattern cycle and for initiate other non-image-forming visual functions, such as the pupillary light reflex.

Figure 2.1: Anatomy of the human eye6 _{and schematic cross section of the retina}7_. At this point, it is important to remember that what we see is not limited to the visual scenes collected in our human physiological stages. The information acquired by the visual system needs to be processed. This is when the perceptual process starts.        6  "Three Main Layers of the Eye" by Artwork by Holly Fischer ‐ http://open.umich.edu/education/med/resources/second‐look‐ series/materials ‐ Eye Slide 3. Licensed under CC BY 3.0 via Wikimedia Commons ‐  http://commons.wikimedia.org/wiki/File:Three_Main_Layers_of_the_Eye.png#mediaviewer/File:Three_Main_Layers_of_the_Ey e.png  7  “Simple organization of the retina” by Helga Kolb. “For non‐commercial, academic purposes, images and content from the  chapters portion of Webvision may be used with a non‐exclusive rights under a Attribution, Noncommercial, No Derivative Works  Creative Commons license.” Source: http://webvision.med.utah.edu/imageswv/schem.jpeg 

Chapter 2. Theoretical Background  

The retina cells send neural signals via the optical nerve, first to the primary visual cortex of the brain (also known as V1, located in the occipital lobe) and then to higher cortical regions (V2 to V5, located in the parietal and temporal lobes). The information extracted from an environment is thus processed in the visual cortex, and scene details such as pattern recognition, colour perception, spatial organisation, motion and depth are perceived. A useful model of visual perception was first presented by Ungerleider and Mishkin (1982). This model divides the kinds of information processing into two areas of the brain: the dorsal stream and the ventral stream. These are more commonly known as the Where System and the What System respectively. The Where System is responsible for the perception of motion, space, position, depth, and figure/ground segregation. This system is colour-blind, has a lower acuity than the What System and operates quickly. The What System is responsible for object and face recognition and colour perception (Livingstone, 2008). An example of this division can be explained through a common human experience. We are travelling in a car and have a first glance of a known person. We still do not know who that person is, but we know his location. Our head then turns rapidly back to that person’s position to recognise his face. The Where System has already told us where that person is because it operates faster than the What System. The What System is slow, and that is why it takes longer to recognise the face of the person. Our visual perception is already processing the information we receive through our eyes.

Figure 2.2: ‘What and Where Systems’. From Livingstone (2008).8

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Chapter 2. Theoretical Background 

Finally, after the information is processed in the visual cortex, and once a scene has been identified and recognised, the cognitive process can use the stored information from past experiences to acquire the meaning of what a person sees. This means that, for example, two healthy persons with no functional visual impairment can perceive the same kind of information through the visual system (eye and brain). However, due to the different experiences, memories, context, and knowledge of each person, the information can be understood in two different ways.

After understanding the fundamental visual factors, and for the purpose of the present research, it seems essential to understand how humans perceive light and colour.