Relationship of Eccentricity and Visual Acuity - Lab Report Example

Visual Acuity That would be referring to the ability of the person to discern the fine spatial details. Aculity threshold would be the minimum size of the feature that could be resolved. The metric that is used to quantify this would be the visual angle. Methods Participants group of 5 individuals had participated in this experiment. There was an observer, data recorder, stimulus holder whose job was to change letter C according to different angles to observer, distance recorder, and a controller who would be responding to the whole experiment process. There was an Observer, male, height: 1.9m, normal eyesight, 21 years old. The right eye of observer was used to observe stimulus. The stimulus went to the left side of observer, in case stimulus falling on the observer’s blind spot. Materials Place: A lab room in Psychology building of The Australia National University, with ordinary lighting (bit dimmer towards the wall with the stimulus) had been arranged. A 6 pieces of white cards with capital C and 1piece of card with a black spot as fixation point would be shown to observer on the wall and that would be placed at a height of 1.5m, right in front of the Observer. All of the cards were of same size and having square shape. There would be a Ruler, a string, protractor, some blue tack, a white tape, pen and data recording table. Procedure: 1). Pilot Testing Method This has been used to determine the size of letter “C” that has to be used in different degree angle and that would be suitable for the observer from a approximately distance. The subjects in the pilot tests were asked to cover their left eye with a piece of paper and only examine stimuli with their right eye. The subject remained in the same position, 3meters from the fixation point. The stimuli of different sizes of were presented at different angles of eccentricity, 0°, 10°, 20°, 30°, 40°, 50°, 60°, while the subject focused on the fixation point. The method of constant stimuli was be used throughout this experiment. This hopefully gave us the full psychometric curve with focus on the region around the absolute threshold point. The stimuli were of a standardized form, that is, all of a black colour, uniform font on a plain white, untutored background. They were uniformly rationed in terms of height width and gap forming the C. The fixation point was also on the same plain card. The cards of stimuli were numbered for ease during the experimental and reporting process (1 = largest stimulus through to 6 = smallest stimulus).. Six different sizes of the stimuli were presented and the subject was asked to indicate whether the gap in the letter C was up or down. This 2AFC would ensure that the stimuli were actually seen by the subject and minimize any false reports. Different presentations of the sizing enabled us to mark correct and incorrect responses on a table, allowing us to determine the approximate threshold point at different angles of eccentricities. The sizes determined to be approximately the absolute threshold point would be used as the stimuli size for each trial in the actual experiment. This ensured that the experiment time was being spent on values close to the actual absolute threshold point, narrowing down our study area interest in terms of values. For each angle of eccentricity, the figure of each size of stimulus and the distance of best sight would be noted down. Result: Degree Usage of C 0 degree use smallest “C”, 0.1cm 10 degree 0.2cm “C” 20 degree 0.3cm “C” 30 degree 0.4cm “C” 40degree 0.6cm “C” 50 degree the biggest “C”, 2cm 60 degree, the biggest “C”, 2cm 2) Main Experiment Method Presentation conditions were the same in the main experimental section as what they were in the pilot testing section in terms of lighting. This is inferred as the main experiment was conducted directly after the completion of the pilot testing. It was still dark outside and the lighting in the room used had not changed. We also conducted the main experiment in the same position in the room therefore the amount of source of light was also the same. In this experiment of relationship between visual acuity and eccentricity, the absolute threshold of every 10 degree away from the observer’s sight of line was tested. A string was attached with the protractor, and the other end attached by blue tack between observer and stimulus, to make sure observer’s eyesight face on the exact angle and record the exact distance. To make sure the accuracy, 2AFC and 2 up 1 down staircase methods were used to hold the experiment, thus, the staircase gives a performance level of ~71% was used to find the thresholds. Firstly, 0° eccentricity was tested using the smallest letter. The observer’s left eye was asked to be covered by a piece of card, and look steadily on the piece of card with letter C in front of him, to tell which side the letter faces (Up or Down). The letter was showed from a distance that could be discerned easily, and moving backward, till incorrect answer was told. To avoid guessing, a 2 up 1 down staircase method was used to investigate the absolute threshold. The distance of letter from observer was adjusted every 10cm. In the “threshold area”, there was 8 reversals, and average of the last 4 reversals. After 8 reversals, the distance that observer could just notice the letter with a performance level of 75 was inferred from the data, which was the absolute threshold of the distance the observer could notice the letter in 0° eccentricity. Following that, protractor was used to find 10° away from observer towards his left. The protractor will be set on 10°angle from observer. The observer was asked to focus on the fixation point straightly towards him, and tell which side the letter C was facing. It also started from a distance could be easily perceived by observer and moving backwards (the approximate distance was known form pilot testing method). Every method was the same except the stimuli was changed to the second smallest C, for a better vision. After the absolute threshold of 10° was investigated, 20° was found by protractor and the third smallest C was used to find the absolute threshold in eccentricity of 20°. In this way, as the eccentricity increasing every degree, the stimulus increase one size up, except the 50° and 60° eccentricity both using the biggest C. During the experiment, the observer was asked to close his eye to have a rest every 10° eccentricity test finished, and in the whole experiment, he was not told if his answer was correct. Result Using Tan a = H/D, visual angel can be result from the height of stimulus and the distance of the absolute threshold of each eccentricity. the distance of the absolute threshold of each eccentricity was investigated and they were Degree Angle Threshold 0 0.731 4.7m 10 3.125 2.2m 20 4.125 2.5m 30 5.093 2.7m 40 12.133 1.7m 50 25.464 2.7m Compare to the previous study The Current Study The Previous Study Discussion Visual acuity could be considered as the spatial resolving capacity of the visual system. This could be thought of as the ability of the eye to see fine detail. There would be various ways to measure and specify visual acuity that would be depending on the type of acuity task that has been used. Visual acuity has been limited by diffraction, aberrations and photoreceptor density in the eye (Curcio C. A., Sloan K. R., Kalina R. E. et al,1990). The number of factors that usually affect visual acuity would be refractive error, illumination, contrast and the location of the retina being stimulated.( Kolb H., Linberg K.A., Fisher S. K.,1992) The limitations of this type of study have been that detecting the smallest point or the finest detail to be resolved, and that would require a good optical system and there would be appropriately spaced detectors. The Objects that will be looked an imaged would be at back of the eye. That would be at taken a point source, the image would be distributed on the retina as a point spread function due to distortions created by the optics of the eye. The Refractive errors would be affecting the visual acuity by causing defocus at the retina. Defocus would blur out fine detail, sharp edges and contrast sensitivity by affecting its point spread function. There would be Refractive errors such as myopia and hyperopic that would be causing the point spread function to spread more laterally.( Roorda A and Williams D. R. ,1999) That would be affecting the resolution.( Roorda A and Williams D. R. ,1999) The image at the back of the eye of an object has been focused sharply on the retina in a myopic eye. In a myopic eye, the optical system could be considered to be too strong, and the image would be focused in front of the retina. (Roorda A and Williams D. R. ,1999)The reverse would occur with a hyperopic eye where the optical system is too weak and the image would be focused behind the retina.( Roorda A and Williams D. R. ,1999) The size of the pupil would be an important factor affecting visual acuity. (Mills S. L., and Massey S. C.,1999)Large pupils would be allowing more light to stimulate the retina and that would reduces diffraction but resolution would be affected by aberrations of the eye. (Mills S. L., and Massey S. C.,1999)There would be a small pupil that would reduce optical aberrations but resolution would be diffraction limited.( Mills S. L., and Massey S. C.,1999) Therefore, a mid-size pupil of about 3 mm to 5 mm would be optimal, as this would compromise between the diffraction and aberration limits. (Mills S. L., and Massey S. C.,1999)As noted earlier, pupil area would be affecting the size of the point spread function, and hence resolution.( Mills S. L., and Massey S. C.,1999) For recognition tasks, visual acuity would be greatly affected by the level of background luminance. Two branches would be evident, with the lower belongs to the rod function and the upper to the cone function. The cone branch has a long "linear" range of about 3 log units which asymptote at the photopic level of about 300 cd/m2.( Vaney D. I. ,1994) There would be quantal availability; quantal capture would be more probable in the para-central and peripheral retinal due to greater spatial summation. That means photoreceptors density in the area would be low, and resolution would be poorer. When light levels increase, quantal capture would occurs more successfully at the central retinal .A higher level of visual acuity is achieved due to the high photoreceptor density.( Vaney D. I. ,1994) To detect a small bright spot, detection would be greatly dependent on the quantity of light rather than the exposure time. To detect a line, the acuity, would be proportional to the exposure time. There would be no simple acuity-exposure time relationship for the resolution of the target. Visual acuity would be the greatest at the centre of fixation. At a distance of 5 minutes of arc from the centre of fixation, there would be a measurable loss in visual acuity. At 10 minutes of arc (1/6 of a degree) from fixation, there would be a 25 % loss of visual acuity. Highest level of visual acuity would be achieved if the eye has adapted to the same level as the test luminance for test luminances of 34 cd/m2 to 34,000 cd/m2. For test luminance less than 34cd/m2, adapting to a lower luminance would be a having a slightly better acuity. During steady fixation, the eyes are in constant motion. Under these conditions, retinal images traverse a distance of about 3 minutes of arc in one second. Contrast would be an important parameter in assessing vision. Visual acuity measurement in the clinic would be using high contrast, that is, black letters on a white background. In reality, objects and their surroundings would be of varying contrast. References Curcio C. A., Sloan K. R., Kalina R. E. et al,1990, Human photoreceptor topography, J Comp Neurol. Vol.292,pp. 497-523 Kolb H., Linberg K.A., Fisher S. K.,1992, Neurons of the human retina: a Golgi study, J Comp Neurol,Vol. 318,pp.147-187 Mills S. L., and Massey S. C.,1999, AII amacrine cells limit scotopic acuity in central macaque retina: A confocal analysis of calretinin labeling, J Comp Neurol, Vol.411,pp.19-34 Roorda A and Williams D. R. ,1999, The arrangement of the three cone classes in the living human eye. Nature Vol.11,pp.520-522 Vaney D. I. ,1994, Patterns of neuronal coupling in the retina ,Prog Retinal Res, Vol.13,pp. 301-355 Read More

The method of constant stimuli was be used throughout this experiment. This hopefully gave us the full psychometric curve with focus on the region around the absolute threshold point. The stimuli were of a standardized form, that is, all of a black colour, uniform font on a plain white, untutored background. They were uniformly rationed in terms of height width and gap forming the C. The fixation point was also on the same plain card. The cards of stimuli were numbered for ease during the experimental and reporting process (1 = largest stimulus through to 6 = smallest stimulus).. Six different sizes of the stimuli were presented and the subject was asked to indicate whether the gap in the letter C was up or down.

This 2AFC would ensure that the stimuli were actually seen by the subject and minimize any false reports. Different presentations of the sizing enabled us to mark correct and incorrect responses on a table, allowing us to determine the approximate threshold point at different angles of eccentricities. The sizes determined to be approximately the absolute threshold point would be used as the stimuli size for each trial in the actual experiment. This ensured that the experiment time was being spent on values close to the actual absolute threshold point, narrowing down our study area interest in terms of values.

For each angle of eccentricity, the figure of each size of stimulus and the distance of best sight would be noted down. Result: Degree Usage of C 0 degree use smallest “C”, 0.1cm 10 degree 0.2cm “C” 20 degree 0.3cm “C” 30 degree 0.4cm “C” 40degree 0.6cm “C” 50 degree the biggest “C”, 2cm 60 degree, the biggest “C”, 2cm 2) Main Experiment Method Presentation conditions were the same in the main experimental section as what they were in the pilot testing section in terms of lighting.

This is inferred as the main experiment was conducted directly after the completion of the pilot testing. It was still dark outside and the lighting in the room used had not changed. We also conducted the main experiment in the same position in the room therefore the amount of source of light was also the same. In this experiment of relationship between visual acuity and eccentricity, the absolute threshold of every 10 degree away from the observer’s sight of line was tested. A string was attached with the protractor, and the other end attached by blue tack between observer and stimulus, to make sure observer’s eyesight face on the exact angle and record the exact distance.

To make sure the accuracy, 2AFC and 2 up 1 down staircase methods were used to hold the experiment, thus, the staircase gives a performance level of ~71% was used to find the thresholds. Firstly, 0° eccentricity was tested using the smallest letter. The observer’s left eye was asked to be covered by a piece of card, and look steadily on the piece of card with letter C in front of him, to tell which side the letter faces (Up or Down). The letter was showed from a distance that could be discerned easily, and moving backward, till incorrect answer was told.

To avoid guessing, a 2 up 1 down staircase method was used to investigate the absolute threshold. The distance of letter from observer was adjusted every 10cm. In the “threshold area”, there was 8 reversals, and average of the last 4 reversals. After 8 reversals, the distance that observer could just notice the letter with a performance level of 75 was inferred from the data, which was the absolute threshold of the distance the observer could notice the letter in 0° eccentricity. Following that, protractor was used to find 10° away from observer towards his left.

The protractor will be set on 10°angle from observer. The observer was asked to focus on the fixation point straightly towards him, and tell which side the letter C was facing. It also started from a distance could be easily perceived by observer and moving backwards (the approximate distance was known form pilot testing method).

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