IMedical
Hypotheses
Visual Perception -
A Hypothesis
S. A. R. DOI Ahmadu Be/lo University, CILS, PMB 1013, Zaria, Nigeria
Abstract - Based on the results of after-image experiments, it can be concluded that the retinal perception of image position is linked to head position information, since this affects after-image positioning. Why actual perception of object position is not affected is hypothesised as a resetting of the retinal horizon, from information received on head position. Tilting of the after-image is thus explained as superimposition of the former image on a reset (tilted) horizon (axis).
Globe position
1). This vertical meridii plane (3).
As a primary position or starting point we may consider the position in which the eye is looking straight ahead when the head is erect. After any purely horizontal or vertical movement of the fixation axis from the primary position, the eye is said to have reached a secondary position. If the eye executes a movement which causes both a horizontal and vertical displacement of the fixation axis (the oblique position of gaze), it is said to adopt a tertiary position. AU movements of the globe around the hypothetical center of rotation can be analysed in terms of a coordinate system with three axes peqndicular to each other and intersecting at the center of rotation. These three axes were descirbed by Fick (2) and are the X, Y, and Z axes. However, we can think of the eye in the primary position and talk in terms of compass directions. The plane that divides the eyeglobe into a northern and southern hemisphere, we can call the retinal horizon (Y-axis of Fick lies in this plane) and the plane that divides the eyeglobe into an eastern and western hemisphere the vertical meridian (Fig.
Retinal orientation
Date received 19 October 1987 Date accepted 21 June 1990
is perpendicular to Listing’s
The appreciation of direction (of a line, for example) depends in large measure on the directions the images of external objects make on the retina, and if the orientation of the retina is upset estimates of direction can become erroneous (4). So long as this plane keeps a definite orientation in space, the images of horizontal lines in a frontal plane fall on the horizontal meridian or parallel to it (4). Similarly, the images of vertical lines in a frontal plane fall on the vertical meridian or parallel to it (4). The normal position of the retinal horizon can be defined in relation to the median plane of the body. So long as the retinal horizon remains perpendicular to this, its position is normal and there is no torsion. When the eye moves into a secondary position a horizontal line still falls on the meridian because there has been no torsion. Tertiary positions of the eye are achieved with reference to Fick’s axes by a simultaneous rotation around the horizontal and vertical axes, a movement
148
149
VISUAL PERCEI’ITON
once, I propose that this is due to the pro%tion of the horizontal and vertical parallels (Figs 2 and 3) of the horizontal and vertical meridians on the frontal plane (Fig. 4). This is quite similar to the drawings of the lines of latitude of the Earth seen from the side. The similarity between this diagram and the diagram of the inclination of the projection of the after-images drawn up by Helmholtz (1) is apparent. Due to torsion, the vertical and horizontal meridians are tilted over to the left in the case of the eye looking up and to the right. Owing to the characteristics of the frontal plane projections of the meridians on a frontal plane in space, the after-image of a vertical line, fixated in the primary position appears tilted over to the right whereas that of a horizontal line to the left. Fig. 1
that can be considered to occur around an oblique axis lying in Listing’s plane (5). As the eye rotates obliquely out of the primary position, the vertical axis of the globe is seen to tilt with respect to the X and 2 axes of Fick. This tilt is termed false torsion (6) for there is no real rotation around the Y-axis, only an apparent movement.
Fig. 3
Head tilt and after-image positioning
Fig. 2
Tertiary eye position and after-image positioning The phenomenon of the appearance of torsion can most accurately be studied physiologically by the observation of after-images (7). With the eyes in the primary position if a cross is fixated for a few setonds and the eye is moved about, the limbs of the after-image of the cross remain vertical and horizontal in the secondary position of the eye. However, in the tertiary position of the eye, the limbs of the cross slope in opposite directions to each other (6). Since the eye cannot exhibit torsion in two ways at
True torsion (or wheel rotation) of the eye around the Y-axis is not under voluntary control and does not occur except for compensatory torsional movements of the eye in response to labyrinthine and tonic neck reflexes (8). Torsional movements of the eye compensate only for small degrees of lateral head tilt from the erect posture and tend to keep the vertical meridians of the retina perpendicular to the horizon (8). Head tilts of greater magnitude cannot be compensated by eye torsion (8). From after-image experiments, it can be observed that if a vertical line is fixated in the primary position of the eye with the head erect, its after-image projected in the frontal plane tilts in the same direction as the head tilt. The tilt of the after-image is probably due to a resetting of the vertical and horizontaI meridians to the pull of gravity, since this tilt of the after-image
150
MEDICAL HYPOTHJSES
horizon. From the other observations mentioned it can be proposed that a specific receptor of head tilt is present which relays information to the ‘space rep resentation center’. This then brings about a resetting of the retinal horizon, so that although the after-image tilts, objects are not viewed as tilted. The utricle is most likely to be this receptor. It would be interesting to see the results of after-image experiments in conditions of zero gravity. References Fig. 4
1. Hemholtz H Von. Physiological optics. Vol III. Rd. Southall. Optical Sot. Am, 1925. Fick A. Die Bewegungett des menschlichen Augapfels, Henle ” und pfeufer, Zeitschrift IV: 101, 1854. 3. Listing. See Ruete’s Lhb. d. Gpthahnologie. 2nd ed., Braunchweig, 1: 37, 1855. 4. Davson H. ‘lhe physiology of the eye. Churchii Livingstone. London, 3rd ed. 1972: 435-436. 5. Listing JB. Moleschott’s Untenuch. Vol 193: 1854. 6. Duke-Elder S. Wybar S. Ocular motility and strabismus ~101 in System of ophthalmology Vol VI, London, Henry Kimptat, 1973. 7 Ruete. Das Gpthalmotrop. Gottingen, 1846. 8: Moses RA (ed.). Adlers physiology of the eye. 7th ed. C.V. Mosby Co. London, 1981: p. 91. L
takes place even when the neck is kept rigid and the whole body tilted. Implications for visual perception The planes of the retina can be proposed to be sim-
ilar to (i.e. when projected on the frontal plane) the lines of latitude of the earth projected on a flat paper. The after-image shifts with head tilt because it may be recognized in relation to the formerly set retinal
Hypotheses
Visual Perception -
A Hypothesis
S. A. R. DOI Ahmadu Be/lo University, CILS, PMB 1013, Zaria, Nigeria
Abstract - Based on the results of after-image experiments, it can be concluded that the retinal perception of image position is linked to head position information, since this affects after-image positioning. Why actual perception of object position is not affected is hypothesised as a resetting of the retinal horizon, from information received on head position. Tilting of the after-image is thus explained as superimposition of the former image on a reset (tilted) horizon (axis).
Globe position
1). This vertical meridii plane (3).
As a primary position or starting point we may consider the position in which the eye is looking straight ahead when the head is erect. After any purely horizontal or vertical movement of the fixation axis from the primary position, the eye is said to have reached a secondary position. If the eye executes a movement which causes both a horizontal and vertical displacement of the fixation axis (the oblique position of gaze), it is said to adopt a tertiary position. AU movements of the globe around the hypothetical center of rotation can be analysed in terms of a coordinate system with three axes peqndicular to each other and intersecting at the center of rotation. These three axes were descirbed by Fick (2) and are the X, Y, and Z axes. However, we can think of the eye in the primary position and talk in terms of compass directions. The plane that divides the eyeglobe into a northern and southern hemisphere, we can call the retinal horizon (Y-axis of Fick lies in this plane) and the plane that divides the eyeglobe into an eastern and western hemisphere the vertical meridian (Fig.
Retinal orientation
Date received 19 October 1987 Date accepted 21 June 1990
is perpendicular to Listing’s
The appreciation of direction (of a line, for example) depends in large measure on the directions the images of external objects make on the retina, and if the orientation of the retina is upset estimates of direction can become erroneous (4). So long as this plane keeps a definite orientation in space, the images of horizontal lines in a frontal plane fall on the horizontal meridian or parallel to it (4). Similarly, the images of vertical lines in a frontal plane fall on the vertical meridian or parallel to it (4). The normal position of the retinal horizon can be defined in relation to the median plane of the body. So long as the retinal horizon remains perpendicular to this, its position is normal and there is no torsion. When the eye moves into a secondary position a horizontal line still falls on the meridian because there has been no torsion. Tertiary positions of the eye are achieved with reference to Fick’s axes by a simultaneous rotation around the horizontal and vertical axes, a movement
148
149
VISUAL PERCEI’ITON
once, I propose that this is due to the pro%tion of the horizontal and vertical parallels (Figs 2 and 3) of the horizontal and vertical meridians on the frontal plane (Fig. 4). This is quite similar to the drawings of the lines of latitude of the Earth seen from the side. The similarity between this diagram and the diagram of the inclination of the projection of the after-images drawn up by Helmholtz (1) is apparent. Due to torsion, the vertical and horizontal meridians are tilted over to the left in the case of the eye looking up and to the right. Owing to the characteristics of the frontal plane projections of the meridians on a frontal plane in space, the after-image of a vertical line, fixated in the primary position appears tilted over to the right whereas that of a horizontal line to the left. Fig. 1
that can be considered to occur around an oblique axis lying in Listing’s plane (5). As the eye rotates obliquely out of the primary position, the vertical axis of the globe is seen to tilt with respect to the X and 2 axes of Fick. This tilt is termed false torsion (6) for there is no real rotation around the Y-axis, only an apparent movement.
Fig. 3
Head tilt and after-image positioning
Fig. 2
Tertiary eye position and after-image positioning The phenomenon of the appearance of torsion can most accurately be studied physiologically by the observation of after-images (7). With the eyes in the primary position if a cross is fixated for a few setonds and the eye is moved about, the limbs of the after-image of the cross remain vertical and horizontal in the secondary position of the eye. However, in the tertiary position of the eye, the limbs of the cross slope in opposite directions to each other (6). Since the eye cannot exhibit torsion in two ways at
True torsion (or wheel rotation) of the eye around the Y-axis is not under voluntary control and does not occur except for compensatory torsional movements of the eye in response to labyrinthine and tonic neck reflexes (8). Torsional movements of the eye compensate only for small degrees of lateral head tilt from the erect posture and tend to keep the vertical meridians of the retina perpendicular to the horizon (8). Head tilts of greater magnitude cannot be compensated by eye torsion (8). From after-image experiments, it can be observed that if a vertical line is fixated in the primary position of the eye with the head erect, its after-image projected in the frontal plane tilts in the same direction as the head tilt. The tilt of the after-image is probably due to a resetting of the vertical and horizontaI meridians to the pull of gravity, since this tilt of the after-image
150
MEDICAL HYPOTHJSES
horizon. From the other observations mentioned it can be proposed that a specific receptor of head tilt is present which relays information to the ‘space rep resentation center’. This then brings about a resetting of the retinal horizon, so that although the after-image tilts, objects are not viewed as tilted. The utricle is most likely to be this receptor. It would be interesting to see the results of after-image experiments in conditions of zero gravity. References Fig. 4
1. Hemholtz H Von. Physiological optics. Vol III. Rd. Southall. Optical Sot. Am, 1925. Fick A. Die Bewegungett des menschlichen Augapfels, Henle ” und pfeufer, Zeitschrift IV: 101, 1854. 3. Listing. See Ruete’s Lhb. d. Gpthahnologie. 2nd ed., Braunchweig, 1: 37, 1855. 4. Davson H. ‘lhe physiology of the eye. Churchii Livingstone. London, 3rd ed. 1972: 435-436. 5. Listing JB. Moleschott’s Untenuch. Vol 193: 1854. 6. Duke-Elder S. Wybar S. Ocular motility and strabismus ~101 in System of ophthalmology Vol VI, London, Henry Kimptat, 1973. 7 Ruete. Das Gpthalmotrop. Gottingen, 1846. 8: Moses RA (ed.). Adlers physiology of the eye. 7th ed. C.V. Mosby Co. London, 1981: p. 91. L
takes place even when the neck is kept rigid and the whole body tilted. Implications for visual perception The planes of the retina can be proposed to be sim-
ilar to (i.e. when projected on the frontal plane) the lines of latitude of the earth projected on a flat paper. The after-image shifts with head tilt because it may be recognized in relation to the formerly set retinal
