Visual well-focused image produced and is situated
Visual perception in humans involves two main factors: foveal and peripheral vision. Foveal vision relates to the line of sight at the highest point of visual acuity. The fovea, a small pit of closely packed cones, is responsible for the well-focused image produced and is situated in the centre of the macula lutea of the retina. Peripheral vision however, refers to the area on the retina that is beyond the central 10 degrees. This part of the eye is heavily rods dominated with the peripheral region also having the lower density of cones out of the two ranges. In humans, foveal vision is responsible for detailed information about the image projected onto the fovea including shape, size, colour whereas the peripheral provides the Central Nervous System (CNS) environmental context as well as identification of movements. This indicates that both the foveal and peripheral parts of the retina have different roles during the production of our visual perception. Ultimately this alludes to the fact that each of the vision systems has distinct characteristics that result in several differences between peripheral and foveal vision including detecting motion, acuity and crowding. The difference in acuity is one key contrast between peripheral vison and foveal vision in humans. When in the periphery, the ability to see fine detail and colour is seriously impaired when compared to that in the centre of the visual field. Essentially as distance from the fovea increases, the resolution decreases rapidly. This is the result of a decrease in the density of retinal cones in addition to an increase in rod cells in the peripheral region of the retina. In 1978, Rovamo et al set up an experiment to measure psychometric performance for spatial resolution tasks in foveal and peripheral vision. The results of the research carried out by Rovamo et al produced a set of results that were represented in a graph creating a similar plot to that in Figure 1. The curve at an individual eccentricity is a contrast sensitivity function (CSF), first produced by Campbell & Robson (1968). Evidence from the research of Rovamo et al indicates that the CSFs for peripheral vision have the same shape as those observed for foveal vision. This is further illustrated in Figure 1 as the various plotlines at different eccentricities all follow the same shape. Furthermore, Figure 1 explains that as the distance from the fovea increases from 0 to 30 degrees, the peaks of the CSFs move to lower frequencies as well as a drop in acuity cut off. The optimal value of spatial frequency is also shown to decrease the further away from the fovea the CSF is measured at. Ultimately the conclusions drawn from this experiment express that peripheral acuity is much worse than foveal acuity. An explanation that may justify this is cortical magnification. The centre of the fovea on the retina represents a small range of the visual field however contains a large number of neurons. Should the same stimulus be seen in the peripheral region, the number of neurons processing the information would be significantly lower. This disparity in the density of neurons on the retina is seen as the reason for the difference in acuity between peripheral and foveal vision. One of the major differences between the two types of vision is the crowding effects. Crowding is defined as the impaired recognition of objects in a clutter. It has been shown that it is not associated with acuity as it is objects that are visible in isolation that become unclear when surrounded by flankers. The effects of crowding were recognised by Bouma (1970) during an experiment which he conducted looking at eccentricity and crowding using single letters flanked either side by distracter letters. He found that crowding was more prevalent when the flanks were around half the eccentricity away from the target. He went on to conclude that there was no significant crowding effect found in the fovea. When looking at peripheral vision, an interference zone around the given target object is defined. Flankers within this region will affect recognition and those outside it will not. Toet & Levi (1992) set up an experiment to observe the effects of eccentricity with the size of interference zones. The study involved presenting stimuli consisting of three collinear symbols in a random orientation. The task was to identify the orientation of the middle symbol at various retinal locations ranging from 0 to 45 degrees. From the results, they established that the size of the interference zones vary linearly with eccentricity. A study set up by Parkes et al (2001) asked participants to report the configuration of three tilted patches surrounded by horizontal flankers. It revealed that observes found it difficult to recognise the orientation of the target object surrounded by distractors however they were able to detect the average orientation of the whole set instead. Here, crowding is appeared to involve pooling the flankers together producing an average of the components in the section. Fundamentally, crowding changes the appearance of objects in the periphery. In addition, Freeman & Simoncelli (2011) reported that crowding gives the ‘gist’ of what is actually in the visual field as opposed to depicting each individual object in refined detail. This can be explained due to the fact there are fewer cones in the peripheral region of the retina. Consequently all aspects of the visual field cannot be represented in the same amount of detail. Both Parkes et al and Freeman & Simoncelli found that pooling is an efficient way of illustrating the information in the visual field due to limited cones. This differs vastly to foveal vision as crowding is not deemed to have a great effect on the fovea. In the periphery, the visual field in essence is made to look more alike the average whereas in the fovea tilt contrast occurs where the differences are emphasised. This is a result of the high density of cones in the fovea. Due to the number of cone cells in the fovea, the image is more finely portrayed as representation of identical information is reduced and the differences are highlighted greater. This demonstrates that both the foveal and peripheral vision is affected differently. Crowding almost solely affects the periphery as the foveal area of the retina contains a high density of cones leading to only well focused images being produced there.Additionally to how different factors are influenced in peripheral and foveal vision, both the vision systems have specific cells that enable them to be beneficial for certain activities. The periphery is very useful for detecting motion. A parasol cell is a type of retinal ganglion cell found in the retina. It is distributed in all parts of the retina and is the initial step in the magnocellular pathway. These cells collect information from a large receptive field and are responsible for the detection of motion.