Physiology of Vision The Eye as a Sense Organ. Rodolfo T. Rafael,M.D. Topics

Transcription

1 Physiology of Vision The Eye as a Sense Organ Rodolfo T. Rafael,M.D. 1 Topics Perception of Light Perception of Color Visual Fields Perception of Movements of an Object Perception of Distance of an Object Visual Pathways 2 1

2 PERCEPTION OF LIGHT 3 PERCEPTION OF LIGHT Receptor The Photochemical Basis of Vision Dark and Light Adaptation 4 2

3 Receptor The retina performs a dual function involving the two different receptors rods cones

4 Cones 6 million Concentrated centrally Found in the fovea centralis higher threshold Daylight vision (photopic vision) Adapts faster Concerned with color vision The photochemical substance is iodopsin. 7 RODS 120 millions Absent in the fovea centralis Increasing in number towards the periphery Lower threshold Night vision (scotopic vision) Adapts slowly The photochemical substance is rhodopsin

5 Stimulus The eye is sensitive to a narrow band of wavelengths The visible spectrum ( 723 nm to 397 nm) The wavelength the intensity of light necessary to elicit a sensation determines the hue of chroma

6 Photochemical Basis of Vision 11 Dark and Light Adaptation Periphery of the retina sensitive to dim light Dark adaptation 60% accomplished in the first 5 minutes (12%/min) and completed 20 minutes (2.66%/min). Light adaptation Loss of sensitivity attained through dark adaptation occurs upon reexposure of the eyes to light. Less time to lose dark adaptation than to acquire it

7 Changes taking place during Dark Adaptation Resynthesis of rhodopsin Dilatation of the pupil Purkinje shift change in sensitivity to different wavelength blind to red increased sensitivity to the blue-green Change over from cone vision to rod vision 13 Changes involved in light adaptation Bleaching of rhodopsin Constriction of the pupil Purkinje shift to the yellow side Change over from rod vision to cone vision

8 PERCEPTION OF COLOR 15 Attributes of Color Tone or Hue wavelength of the light stimulus Brightness or luminosity Amount of light (intensity of radiation) Saturation or purity White sensation

9 Primary colors Red Green Blue 17 Young-Helmholtz Theory of Color Visions This was proposed by Young in 1801 and later modified by Helmholtz. Red, Green, Violet

10 EVIDENCES IN FAVOR OF THIS THEORY: it is in accord with the doctrine of specific nerve energies it explains the positive and negative after images blindness to one color is explained by the absence of one photochemical substance 19 EVIDENCES AGAINST THIS THEORY: It cannot explain the sensation of gray and white arising in the peripheral portions of the retina not sensitive to color. Total color blindness with persistence of the vision of form. Sensation of yellow given by stimulation of the area outside of retina. Sensitive to red and green (red + green = yellow) Color visual fields: green (smallest)-yellow-redblue-white (biggest)

11 Color Blindness John Dalton in % of otherwise normal and healthy men 2% of women are color blind to a certain degree. According to Young-Helmholtz theory, color blindness would be due to abnormality in one or more of the three types of cones 21 Von Kries Classification of Color Blindness Based on the Young-Helmholtz Theory Trichromats Dichromats Monochromats Normal Protanomalous Deuteranomalous Protanopia (most) Deuteranopia Tritanopia (less) Achromat (total color blindness)

12 Normal vision is considered Trichromatic. Trichromats who perceive red and green differently from normal subject Protanomalous individuals are deficient in vision of red Deuteranomalous ones are deficient in vision of green Dichromats In protanopia, the most common anomaly there is blindness to red green is deficiently perceived blue is seen normally. Deuteranopia blindness to green red is deficiently perceived blue is seen normally. Tritanopia, there is blindness to blue, but green and red are perceived normally. is much less frequent

13 Dichromats are individuals who do not see one of the colors, supposedly because the corresponding receptor is missing Tests for Color Visions Holmgren Test. The subject is given a skein of colored wool and told to choose from skeins of assorted colors, those of similar hues. Isihara s Test (Modification of Stilling s test). Consists of a series of plates in which a digit formed of spots of one color is hidden in field of other colored spots. Matching of spectral colors The subject is asked to match yellow by mixing red and green. The proportions of each color used are compared with those used by normal subjects. This is the most accurate method

14 l x w x h; 27 x 12 x 4 cm markings on standards: green "A"; purple "B"; red "C". Each wool skein has a brass plate with a number marking "1", "2" "3" etc., with a moveable brass disc to cover the number. Place the 40 small skeins together. Keep the tags covered. Select the 10 skeins that best match the light green master A. Next, from the remaining thirty, select the 5 skeins that best match the red master, C. Finally, from the remaining twenty-five, select the 5 skeins that belong with the rose master, B. Record the tag numbers of each selection set, arranging them in order of closest match to the respective master skeins

15 Visual Fields 29 The Visual Field is the extent of the external world which can be seen without a change in the fixation of the eye. The visual field is inverted upon the retina because of the lens images of the objects on the right side of each eye fall upon the left side of the retina images of the objects on left side of each eye fall upon the right side of the retina

16 The visual field of each eye is limited by: nose on the nasal side, eyebrows and the cheeks above and below The visual field of each eye can be mapped out separately with an instrument called the perimeter 31 Perimetry (Confrontation test) One eye is covered while the other is fixed upon a central point. A small object is moved towards this central point along the various meridians and the location in degrees of arch away from the central point that it first becomes visible is plotted

17 33 For an object to be seen as one, the two images must fall on corresponding points on the retinas such as: the two fovea right halves of both eyes left halves of both eyes upper halves of both eyes lower halves of both eyes Non corresponding points in the two retinas are points when stimulated give rise to two visual sensations (diplopia). When visual images chronically fall on noncorresponding points in the two retinas, one is eventually suppressed and diplopia disappears. This suppression is a cortical phenomenon, the suppressed eye eventually shows a reduction or loss of visual acuity, even blindness, when tested separately, a condition called amblyopia

18 Advantages of Binocular Vision over Monocular Vision Optical defects of one eye is masked by well defined images of the other eye. Defective vision in parts of the visual fields of both eyes are hidden as long as defects do not affect the same spot or part of the fields. Example: the blind spot Combined fields of the two eyes is larger than either alone Provides a very accurate perception of depth, size and distance, called stereoscopic vision PERCEPTION OF MOVEMENT OF AN OBJECT Perception of moving object is maximal Light Distance of 10 to 15 degree from the fovea. Lateral vision - better idea of its movements than central or foveal vision. Smallest movement perceived, if there are stationary objects in the neighborhood. No other objects, movements must be 10 to 20 times greater to be perceived. The perception of the movement of an object maybe due to stimulation of groups of receptors as the image moves over the retina

19 PERCEPTION OF DISTANCE (OR DEPTH) OF AN OBJECT: Determination of distance by relative sizes of objects Determination of distance by moving parallax Determination of distance by stereopsis Accomodation of the eye The apparent change in the color of an object with distance Linear perspective 37 Determination of distance by relative sizes of objects. The greater the distance, the smaller the image The less the distance, the larger the image

20 Determination of distance by moving parallax When you move your head from side to side, objects that are close to you move rapidly across your retina. However, objects that are far away move very little. In this way, your brain can tell roughly how far something is from you Determination of Distance by Stereopsis Binocular vision one eye is a little more than two inches to one side of the other eye the image on the two retinas are different an object that is one inch in front of the bridge of the nose will form an image on the temporal portion of the retina of each eye, whereas a small object 20 ft. away in front of the nose will have its image at closely corresponding points in the middle of the eye. This type of parallax is present all the time when both eyes are being used. Persons with two eyes can judge better relative distance when objects are nearby than a person who has only one eye. Useless for depth perception beyond 20 feet

21 Accomodation of the Eye Near objects require a greater effort of accommodation than does a more distant object for its image to be focused upon the retina, some cue may be possibly given as to the relative distances of two objects from the eye The apparent change in the color of an object with distance Three-clad hills, which we know to be green, appear bluish in the distance The colors of many objects appear to fade in the distance

22 The blocking out of parts of a distant view by objects in between it and the eyes, gives a sensation of depth, for the overlapping of parts of farther objects by nearer ones gives an indication of their relative distances from the eyes Linear perspective Straight lines moving into the distance which are actually parallel are convergent in the retinal image

23 VISUAL PATHWAYS 45 VISUAL PATHWAYS

24 At the optic chiasm, fibers from the temporal hemiretina pass into the optic tract of the same side those from the nasal hemiretina decussate and pass into the optic tract of the opposite side The optic tract ends in the lateral geniculate body where functional regrouping of the fibers occur. Visual fibers, that is, those carrying impulses that provoke visual sensations enter the lateral geniculate body, while the afferent fibers of the papillary light reflexes end in the pretectal region. Some of the macular fibers are bilateral

25 The axons of the geniculate neurons form the geniculostriate bundle which end in a fan like manner in the striate area, in the depth and lower upper lips of the calcarine fissure of the occipital cortex LESIONS OF THE VISUAL PATHWAY

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27 Thank You For Not Listening