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A Simple Model of Acuity Loss Due to Photoreceptor Dropout
Olpyright @ IF AC Modelina met Control in BiomcItical Systems, GalveIUJD, Teua, USA, 1994
centered in the fovea removes central rings leaving the outer ones unchanged. A lesion centered away from the fovea does not remove entire rings. but causes some to be shortened. An estimate of VEP amplitude is calculated by sununing the modified set of rings. Figures 3 through 5 show a raw image of a damaged eye. the same image with the damaged areas and the optic disk highlighted and separated. and the damaged area superimposed on the rings.
A Simple Model of Acuity Loss due to Photoreceptor Dropout Thomas L. Amow, Harold G. Longbotham Conceptual MindWorks, Inc. 4318 Woodcock, Suite 210 San Antonio, Texas, 78228 e.m. [email protected] ph. (210) 736-0777 ABSTRACT The effects of laser damage in the eye have been studied with an electrophysiological measure, the Visual Evoked Potentials (VEP). The VEP is generated by placing electrodes at the back of a subject's head, and having the subject watch a pattern of alternating light and dark bars. The spatial frequency of the bar pattern. measured in cycles per degree. increases as the bars become finer and finer. The bar pattern changes in time. as the light bars become dark. and visa versa. The rate of this change is the temporal frequency. measured in Hertz. The VEP amplitudes referred to in this paper are a sum of even harmonics of the temporal frequency (since the pattern change two times for every temporal cycle.) after filtering the raw data. As spatial frequency increases. a point will be reached where the subject will no longer be able to resolve the bars. and the VEP amplitude will become indistinguishable from noise. This spatial frequency is a measure of visual acuity.
CSF
CPD
Figure 1 .~".I F,.,quertOy' Model
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A computer model has been developed to simulate vision loss from laser bums to the retina. It has been checked against experimental data from Rhesus monkeys with bums made under laboratory conditions. Vision was checked before and after the bums with VEP at a temporal frequency of six Hertz. This model divides the visual field into concentric rings centered about the visual axis. Each ring contributes to the VEP by an amount that depends on its eccentricity. or radius in degrees. as well as the spatial frequency. We use contrast sensitivity functions (CSF). available from the vision literature to calculate the contribution of each ring. A set is shown in figure I. from Rovamo[l]. Estimated VEP amplitudes are weighted sums of the contrast sensitivity for the sets of rings. at the appropriate spatial frequency. Weights for each ring are calculated mathematically by multiplying the square of the cortical magnification factor[2.3] by the area of each ring. An overall schematic of the model is shown in figure 2. To simulate eye damage, the shape of the lesion is used to remove or shorten some of the rings. A lesion
Figure 2
Figure 3
525
effective in measuring loss of acuity due to a laser lesion. Figure 6shows raw data points and the curve generated by the model after training to the points. Figure 7 shows points after laser exposure and the curve predicted by the model.
[3] Wilson, H.R. et al., The Perception of Form Retina to Striate Cortex. In Spillmann, Lothar and Wemer, John, Editors. Visual Perception: The Neuropbysiological Foundations. San Diego: Academic Press, Inc. 1990 pp. 231
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ACKNOWLEDGEMENT Supported by the Armstrong Lab, Brooks AFB, San Antonio, Texas, contract no F41624-93-C-9015.
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Figure 7
Circular Model 557c Figure 5 REFERENCES [1] Revamp et Al. Nature 271 pp54-56 1978 [2] Cortical magnification factor predicts the photopic contrast sensitivity of peripheral vision. Nature (London) 271, 54-56. (232-233 1978»
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centered in the fovea removes central rings leaving the outer ones unchanged. A lesion centered away from the fovea does not remove entire rings. but causes some to be shortened. An estimate of VEP amplitude is calculated by sununing the modified set of rings. Figures 3 through 5 show a raw image of a damaged eye. the same image with the damaged areas and the optic disk highlighted and separated. and the damaged area superimposed on the rings.
A Simple Model of Acuity Loss due to Photoreceptor Dropout Thomas L. Amow, Harold G. Longbotham Conceptual MindWorks, Inc. 4318 Woodcock, Suite 210 San Antonio, Texas, 78228 e.m. [email protected] ph. (210) 736-0777 ABSTRACT The effects of laser damage in the eye have been studied with an electrophysiological measure, the Visual Evoked Potentials (VEP). The VEP is generated by placing electrodes at the back of a subject's head, and having the subject watch a pattern of alternating light and dark bars. The spatial frequency of the bar pattern. measured in cycles per degree. increases as the bars become finer and finer. The bar pattern changes in time. as the light bars become dark. and visa versa. The rate of this change is the temporal frequency. measured in Hertz. The VEP amplitudes referred to in this paper are a sum of even harmonics of the temporal frequency (since the pattern change two times for every temporal cycle.) after filtering the raw data. As spatial frequency increases. a point will be reached where the subject will no longer be able to resolve the bars. and the VEP amplitude will become indistinguishable from noise. This spatial frequency is a measure of visual acuity.
CSF
CPD
Figure 1 .~".I F,.,quertOy' Model
-----:...-
'--,-="",...J - -
A computer model has been developed to simulate vision loss from laser bums to the retina. It has been checked against experimental data from Rhesus monkeys with bums made under laboratory conditions. Vision was checked before and after the bums with VEP at a temporal frequency of six Hertz. This model divides the visual field into concentric rings centered about the visual axis. Each ring contributes to the VEP by an amount that depends on its eccentricity. or radius in degrees. as well as the spatial frequency. We use contrast sensitivity functions (CSF). available from the vision literature to calculate the contribution of each ring. A set is shown in figure I. from Rovamo[l]. Estimated VEP amplitudes are weighted sums of the contrast sensitivity for the sets of rings. at the appropriate spatial frequency. Weights for each ring are calculated mathematically by multiplying the square of the cortical magnification factor[2.3] by the area of each ring. An overall schematic of the model is shown in figure 2. To simulate eye damage, the shape of the lesion is used to remove or shorten some of the rings. A lesion
Figure 2
Figure 3
525
effective in measuring loss of acuity due to a laser lesion. Figure 6shows raw data points and the curve generated by the model after training to the points. Figure 7 shows points after laser exposure and the curve predicted by the model.
[3] Wilson, H.R. et al., The Perception of Form Retina to Striate Cortex. In Spillmann, Lothar and Wemer, John, Editors. Visual Perception: The Neuropbysiological Foundations. San Diego: Academic Press, Inc. 1990 pp. 231
-r-----------------------~
I-·-:.~-~I
10
10
ot-________-=====~;=~J •
•
10
12
..
..
•
ID
VEP",
Figure 6
Figure 4 _r-----------------------~
ACKNOWLEDGEMENT Supported by the Armstrong Lab, Brooks AFB, San Antonio, Texas, contract no F41624-93-C-9015.
ID
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.~--~~----~----~~--~ o J ~ Q W • • ~ VEP..,
Figure 7
Circular Model 557c Figure 5 REFERENCES [1] Revamp et Al. Nature 271 pp54-56 1978 [2] Cortical magnification factor predicts the photopic contrast sensitivity of peripheral vision. Nature (London) 271, 54-56. (232-233 1978»
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