Physio Lab PBL

VISUAL ACUITY FOR FAR VISION

Tabulate results: OS (left eye) 20/20 -4

OD (right eye) 20/20 +3

Discussion: 1. What is emmetropic vision? Do you have it? - The condition of the normal eye when parallel rays of light are focused exactly on the retina and vision is perfect Eyes that have emmetropia do not require vision correction. When a person has emmetropia in both eyes, the  person is described as having ideal vision. Some of the students in the group have emmetropic vision while others require the use of correction lens. 2. What is the purpose of doing the pinhole test? - The purpose of pinhole test is that it differentiates vision that can be corrected with lenses from vision that cannot . The pinhole test helps to determine whether reduced visual acuity is due to a refractive error (an inability of the cornea and lens of the eye to bend light rays into proper focus foc us on the retina) or to an organic vision disorder. VISUAL ACUITY FOR NEAR VISION

Tabulate results: Jaeger score result (left eye) J 1+

(right eye) J 1+

Discussion: 1. How does the eye use its refractive power to see near objects? - The refractive index of the eye’s lens system allows the eye to produce sharply focused images. The refractive power mainly resides in the cornea, the  cornea,   but the overall refractive power is achieved by the actual lens changing its shape. In order to fixate on a near object, the ciliary muscle contracts around the lens to decrease its size. The suspensory The  suspensory ligaments relax and the radial tension around the lens is released. This causes the lens to form a more spherical shape achieving a higher level of refractive power. 2. What is the clinical value of determining the visual acuity for near vison?

Visual acuity (V) can be written as a Snellen fraction (m/M) in which the numerator (m) indicates the test distance and the denominator (M) indicates the distance at which the gap of the equivalent Landolt ring subtends 1 minute of arc.

NEAR POINT DISTINCTION

Results: At 7.62 cm, the volunteer was still able to identify the letters, afterwhich the letters were already undistinguished. Discussion: 1. Define near point of distinct vision - The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. 2. What ability of the eye does this experiment test? - In the test using the Jaeger chart, the person’s near visual acuity is being measured.

DEPTH PERCEPTION.

Depth perception refers to our ability to determine distances between objects and see the world in three dimensions. There are two groups of clinical tests used to measure stereopsis. These are the contour stereotests and the random-dot stereotest. An example of a contour stereotest used in the clinic is the Titmus Fly Stereotest. In the Titmus Fly Stereotest, horizontal disparity is presented via the vectographic technique. Examples of random-dot stereotests used in the clinic are the Frisby Stereotest, the Randot Stereotest, the Random-dot E Stereotest and the Lang Stereotest. The Frisby Stereotest uses real depth to determine stereoacuity. Three perspex of different thicknesses are used. Four squares of geometric shapes are painted on one side of the perspex. In one of the squares, a circle of these geometric shape is painted on the other side of the perspex. Discussion:

1. Is there a difference in depth perception using only one eye and using both eyes? Why or why not? Yes there is a difference in depth percption when using only one or both eyes. - The brain creates depth perception by comparing the images from each eye (both eyes) and uses the degree of difference to estimate distance. Thus there will be some slight changes in depth perception if one one eye is being used. 2. Is there a difference in depth perception using different backgrounds? Why or why not? - Yes there is a difference in depth perception when using different backgrounds. The visual system uses various cues from the surrounding environment to make up an image which then creates a type of visual illusion. 3. Enumerate the cues for depth perception using monocular vision. Monocular cues are the ones that are obtained from the 2D image of only one eye.

-

-

-

-

-

-

Accommodation (or focusing):  is the change in dioptric power of the interocular lens in order to see a near object more clearly. The more accommodation needed, the closer the object. Linear Perspective: the visual phenomenon in which parallel straight lines will converge in the distance. If two parallel lines are assumed to be straight, then the further separated the lines become, the closer the object must be. Interposition (or overlapping): is the visual phenomenon in which the view of one object may be blocked by another object. The object being blocked must be  behind the first object. Texture Gradient: the amount of detail in an object. The closer the object, the more detail visible. Far objects lose detail. Relative Size: is the visual phenomenon in which larger retinal image sizes are associated with objects that are closer. An object that appears larger than a similar object will be closer. Light and Shadow:  these cues can provide feedback on the elevation or recession of an object. Relative Brightness: is the visual phenomenon in which closer objects appear  brighter than further objects. Aerial Perspective:   is the visual phenomenon of distant objects appearing hazier than closer objects. This only occurs in long distances, like mountains which appear hazy due to how far away they are. Motion Parallax: is the apparent displacement of objects in space while moving that causes closer objects to move faster than distant objects. Think of looking out a car window when moving and seeing the objects in the distance move by slowly, but the objects close-up moving ver y quickly.

Reference: http://www.healthcommunities.com/vision-tests/basic-vision-tests.shtml https://en.wikipedia.org/wiki/Accommodation_reflex http://www.artificial-eyes-blog.com/artificial-eyes/depth-perception-and-monocularvision/#.Wag0Vfl96M8 http://www.icoph.org/dynamic/attachments/resources/icovisualacuity1984.pdf  http://webvision.med.utah.edu/book/part-viii-gabac-receptors/perception-of-depth/