Essay Deepth Perception
An experimental design was used in which a sample of 20 participants were asked to judge which of two identical pens was closer to them on a 10cm depth scale. The independent variable was whether participants were in the binocular or monocular condition and the dependent variable was the accuracy of depth perceptions. Correct or incorrect responses were recorded on a response scale. The results showed that participants in the binocular condition produced more correct answers than participants in the monocular condition, supporting the experimental hypothesis.
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This corresponds with previous research. The Effect of Monocular and Binocular Vision when Judging Depth Perception Depth perception in relation to physical stimuli comes from the study of psychophysics. Depth perception is the ability to see the world in three-dimensions and the distances of objects. The retina receives information in only two-dimensions, but the brain elicits information about depth so that we can perceive the world in three dimensions. It does this via visual and oculomotor cues. Some depth cues only require input from one eye; these are known as monocular depth cues.
They include motion parallax; when moving forward, the way objects move past you can inform you as to how far they are, aerial perspective; objects become clearer the closer they are to you, relative size; if there are two objects that you know are the same size, the smaller one must be further away, interposition; if one object overlaps another you can judge which object is closer to you, texture gradient; the texture of an object is more difficult to see the further away it is, elevation; objects appearing higher in the visual field are perceived as being further away, shading, shadows of objects can help the brain determine how far away they are, and linear perspective; the perception that parallel lines angle towards each other as they recede into the distance (Schiffman, 1997). However, some sources of information require input from both eyes; this is known as binocular depth cues. These include binocular disparity; using two images of the same scene from slightly different angles. And convergence; the movement of eyes turning inwards, depending on the distance of the object your observing. The ability to perceive relative depth using binocular vision is known as stereopsis.
Oculomotor cues derive from the act of muscular contraction, either from the focus of the lens or the position of the eyes (Sekuler, 2002). Oculomotor cues include convergence and accommodation. Accommodation is the process of your eyes adjusting to keep an object in clear focus. But why do humans have two eyes? Some animals, such as herbivores, have eyes on the side of their head. Evolutionary psychologists would argue that this enables them to notice the approach of predators and ensure survival. However, most predators need binocular vision and therefore have eyes in the front of their head enabling them to judge distances when looking for pray. Previous research has found that binocular depth cues provide more accurate information about depth.
Mckee and Taylor (2010) investigated monocular and binocular depth cues when judging a pair of metal rods in a natural setting either in isolation, or surrounded by other objects, and found that the presence of objects and textures improved monocular judgements; however, binocular judgments were still more accurate. Similarly, Barlow, Blakemore and Pettigrew (1967) investigated monocular and binocular vision by recording the action potentials of neurons in the primary visual cortex of cats, and found that the retinal images of an object must be correctly placed in both eyes to elicit the most accurate response, as opposed to one eye. However, it could be said that the experiment may not be generalizable to humans as it was conducted on cats. Although many studies show support that binocular vision is more accurate in judging depth than monocular vision, some studies show support for how monocular judgements are just as important.
Bruno and Cutting (1967) investigated monocular depth cues; relative size, elevation, interposition, and motion parallax, on computer generated stimuli. The results showed that perceivers use monocular depth cues to judge relative depth, and the more cues there are, the more depth is revealed. The findings from this study illustrate the importance of monocular depth cues, however, computer generated stimuli cannot generalise to real world stimuli. Based on previous research, this experiment aimed to investigate the effect of binocular vision and monocular vision when judging which of two objects is closer in humans and in real life objects. The current study had one hypothesis: two eyes would be better than one eye when judging depth perception. Method Design The study was an experiment.
It used a between participants design. Participants were allocated to one of two conditions, monocular vision or binocular vision. The independent variable was whether participants used monocular or binocular vision to judge the depth of the objects and the dependent variable was the accuracy of depth perceptions. Participants The participants included an opportunistic sample of students from Southampton Solent University and friends and family of the experimenters. Participants were recruited on an opt in basis and were all over 18 years of age. 20 participants were recruited. Materials The researchers completed an ethics form (appendix A) before the study.
The participants were given an information sheet (appendix B), including brief and consent, and a debrief (appendix C), which they received after completing the experiment. A 10cm depth scale (appendix D) with a gap of 1cm was used and results were recorded on a response scale (appendix E) which consisted of correct or incorrect responses from each participant for each trial. The objects used consisted of two identical pens. Procedure Participants were given an information sheet, briefing them on the experiment, and asked to sign their consent. The experimenter then read the standardised instructions to the participant. Participants were assigned to one of two conditions; monocular vision and binocular vision.
Each participant was asked to judge which of two objects was closer to them from at a 2m distance. The objects consisted of two identical pens which were placed on the 10cm depth scale. Psychophysical techniques used included Just Noticeable Difference (JND) and the stimuli were kept constant. The order of trails was randomised. The experiment was counterbalanced with half of participants using their left eye, and half using their right eye in the monocular condition. Results were recorded on a response scale which consisted of correct or incorrect responses from each participant for each trial. This is known as a 2 adjustment forced choice procedure (2AFC).
After the experiment, participants were debriefed and explained the aims and hypothesis of the study and inviting questions. Ethical Considerations An ethics form was completed and approved before the experiment took place. Participants were briefed on the experiment, given a consent form and then debriefed after the experiment. Participants’ right to withdraw from the study was indicated on the information sheet, as well as being reminded verbally by the experimenters. All participants were recruited on an opt in basis. No names or participant data were collected. Responses were therefore anonymous. Results Hypothesis: two eyes are better than one when judging depth perception.
Mean and standard deviation was calculated for each experimental condition: monocular vision and binocular vision, as shown in table 1. Raw data can be seen in appendix G. Table 1 The mean and standard deviation according to each experimental condition Experimental Condition| Mean| Standard Deviation| Monocular vision| 15. 70| 2. 58| Binocular vision| 19. 30| 1. 34| Table 1 shows that the mean (19. 30) for binocular vision was higher than the mean (15. 70) for monocular vision. This indicates that participants in the binocular vision experimental condition had more correct answers than participants in the monocular vision condition, supporting the hypothesis. Calculations from the output can be seen in appendix F. Figure 1. Mean total scores of correct answers.
Figure 1 shows that the mean total scores of correct answers were higher for participants in the binocular condition than participants in the monocular condition. Having used a between participants design, an unrelated t test was conducted to test the directional hypothesis. The output from the calculations can be seen in Appendix F. The results were as follows: t(18) = -3. 912, the significance of the t value equals . 0005, meaning that p<0. 01. This means that the results were significant to 1% and thus participants in the binocular condition had significantly more correct answers than participants in the monocular condition. Discussion The results of this experiment do support the predicted hypothesis that two eyes are better than one when judging depth perception.
Participants in the binocular condition had more correct answers and were able to judge which of the two pens was closest to them than participants in the monocular condition. These findings can be explained by binocular disparity; each eye has a slightly different perspective of the same image, due to the eyes being slightly separated. The brain then combines information from the two eyes into one three-dimensional image, and obscures the differences (Sekuler, 2002). The brain uses binocular disparity to get depth information and judge the distances of objects. Figure 2. A diagram of binocular disparity. Figure 2 demonstrates how the left and right eyes have different visual views of the same image.
If the object is far away, the disparity of that image will be small, if the object is close, the disparity will be large. Therefore, participants in the binocular condition used the images in their left and right eye, to determine the disparity of the objects and thus judge which pen was closest to them. In the monocular condition, the use of Just Noticeable Difference (JND), the smallest change in the physical stimuli, made it more difficult for participants to use the monocular depth cue of interposition to judge which pen was closest to them. The closer the objects were to each other (e. g. 5 and 6 on the depth scale), the harder it was for participants to judge which one was closest to them.
This is because when the objects were further apart from each other on the depth scale, one would overlap or block the other, and the fully exposed object was perceived as being closer to the participant. However, when the objects were closer to each other, neither object concealed the other and the ability to use interposition was eliminated, therefore producing incorrect responses. One possible limitation of this study is that it used a between participants design. This may have reduced the validity of the study as participants may have had different visual abilities. For example, people with ocular conditions, such as amblyopia, have poor stereo vision and thus perform worse in depth discriminations (Thompson & Nawrot, 1998).
This may have confounded the results and therefore further research should use a within participants design testing the same participants in both conditions to see if the findings are consistent and if different visual capabilities affect the ability to perceive depth. Furthermore, it could be said that judging which of two pens is closer to you in experimental conditions with high control may not generalise to real live situations. It can be concluded that binocular vision is better than monocular vision when making depth perceptions. It may be valid to conclude that the results from this study are only valid within the context it was conducted and that further research should look into people with visual impairments to see if their depth perceptions are affected. References Schiffman, H. R. (1997). Sensation and Perception: an Integrated Approach. (pp. 215-227). New York: Wiley Sekuler, R (2002).