Original Article
Influence of visual angle on pattern reversal visual evoked potentials Ruchi Kothari, Smita Singh1, Ramji Singh2, A. K. Shukla1, Pradeep Bokariya3 Departments of Physiology, 1Ophthalmology, 3Anatomy, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha, Maharashtra, 2 Department of Physiology, All India Institute of Medical Sciences, Patna, Bihar, India
Purpose: The aim of this study was to find whether the visual evoked potential (VEP) latencies and amplitude are altered with different visual angles in healthy adult volunteers or not and to determine the visual angle which is the optimum and most appropriate among a wide range of check sizes for the reliable interpretation of pattern reversal VEPs (PRVEPs). Materials and Methods: The present study was conducted on 40 healthy volunteers. The subjects were divided into two groups. One group consisted of 20 individuals (nine males and 11 females) in the age range of 25-57 years and they were exposed to checks subtending a visual angle of 90, 120, and 180 minutes of arc. Another group comprised of 20 individuals (10 males and 10 females) in the age range of 36-60 years and they were subjected to checks subtending a visual angle of 15, 30, and 120 minutes of arc. The stimulus configuration comprised of the transient pattern reversal method in which a black and white checker board is generated (full field) on a VEP Monitor by an Evoked Potential Recorder (RMS EMG. EPMARK II). The statistical analysis was done by One Way Analysis of Variance (ANOVA) using EPI INFO 6.
Results: In Group I, the maximum (max.) P100 latency of 98.8 ± 4.7 and the max. P100 amplitude of 10.05 ± 3.1 μV was obtained with checks of 90 minutes. In Group II, the max. P100 latency of 105.19 ± 4.75 msec as well as the max. P100 amplitude of 8.23 ± 3.30 μV was obtained with 15 minutes. The min. P100 latency in both the groups was obtained with checks of 120 minutes while the min. P100 amplitude was obtained with 180 minutes. A statistically significant difference was derived between means of P100 latency for 15 and 30 minutes with reference to its value for 120 minutes and between the mean value of P100 amplitude for 120 minutes and that of 90 and 180 minutes. Conclusion: Altering the size of stimulus (visual angle) has an effect on the PRVEP parameters. Our study found that the 120 is the appropriate (and optimal) check size that can be used for accurate interpretation of PRVEPs. This will help in better assessment of the optic nerve function and integrity of anterior visual pathways. Keywords: Pattern reversal, P100 latency, P100 amplitude, VEP, visual angle
Introduction Access this article online Quick Response Code: Website: www.ojoonline.org DOI: 10.4103/0974-620X.142593
Visual system is well adapted to the processing and coding of pattern information. Human visual cortical activity as measured by visual evoked potentials (VEP) is highly sensitive to the sharpness and density of contours of the evoking stimulus. The preferred stimulus for VEP is a checkerboard pattern of black and white squares. The patterned stimuli are widely
Copyright: © 2014 Kothari R. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Correspondence: Prof. Ruchi Kothari, Department of Physiology, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha - 442 102, Maharashtra, India. E-mail: [email protected]
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Kothari, et al.: Influence of visual angle on PRVEP preferred because response to a pattern is much larger and bears a closer relationship to the act of seeing. Checks help to explore the function of striate cortex (area 17) because local spatial frequency analyzers are presumably present there.[1,2] Since one of the primary functions of human visual system is to analyze contours and edges, the use of patterned stimuli seems to have an added advantage in providing more information in this regard. The size of the individual checks is usually reported in terms of visual angle in minutes of arc (β) which is expressed as β = tan-1(W/2D) × 120 where W is width of checks in millimeters and D is distance of the pattern from the corneal surface in mm.[3] It is already known that retina can be divided into central foveal, para-foveal, and peripheral region. The fovea subtends 5° of visual angle while para-foveal area subtends 8°. Smaller size of pattern elements is thought to be optimal for foveal stimulation and larger sized patterns stimulate both fovea and extra-foveal region. Thus, by selecting the appropriate pattern element size one can predominantly stimulate fovea or peripheral retina. So when a checkerboard pattern is used, by altering one of its most important attribute that is the visual angle, one can better understand the mechanisms of visual processing. The influence of altered visual angle on the latency and amplitude of major positive component of pattern reversal visual evoked potentials (PRVEP) and other components namely, N75 and N145 is not well-understood and there is scanty data on how and to what extent they are modified so we made a systematic study of effects of different check sizes in terms of visual angle on the latencies and amplitude of the parameters of PRVEPs. For this we performed an evaluation of visual evoked potential to pattern reversal stimulation (VEP-PR) in two groups of healthy subjects with normal visual acuity at baseline. The purpose of this study was to determine the visual angle which is the optimum and most appropriate among a wide range of check sizes for the reliable interpretation of PRVEPs and accurate assessment of optic nerve function and integrity of anterior visual pathways.
Materials and Methods The study population consisted of 40 healthy volunteers consisting of 19 males and 21 females. They were divided into two groups. One group consisted of 20 individuals ( nine males and 11 females) in the age range of 25-57 years and they were exposed to checks subtending a visual angle of 90, 120, and 180 minutes of arc. Another group comprised of 20 individuals (10 males and 10 females) in the age range of 36-60 years and they were subjected to checks subtending a visual angle of 15, 30, and 120 minutes of arc. Each of the subjects was given thorough eye examination as a preliminary measure to exclude any ocular pathology. No subject had a history of relevant neurological or heart disease or of drug abuse. Oman Journal of Ophthalmology, Vol. 7, No. 3, 2014
Inclusion Criteria • • • •
Visual acuity at least 6/6 (with or without corrective glasses) Normal pupillary size (2-4 mm) and reactions Normal Fundus and optic disc Intra-Ocular pressure < 21 mm Hg (as measured by the Non contact tonometer).
This project was approved by the Institutional Ethics committee and written informed consent was taken from the volunteers before the study.
Stimulus The stimulus configuration comprised of the transient pattern reversal method in which a black and white checker board is generated (full field) on a VEP Monitor (colour 14”) by an electronic pattern regenerator inbuilt in an Evoked Potential Recorder (RMS EMG.EP MARK II). The rate of pattern reversal (1.7 Hz), the luminance (59 cd/sqm) and contrast level (80%) was kept constant for all the recordings in all the cases. The different sizes of the checks used were 15, 30, 90, 120, and 180 minutes of arc. Checks of 8 × 8 subtending a visual angle of 120 minutes are the most commonly used in the neurophysiological laboratories, so it was kept as a reference for evaluation of the effects on VEP parameters. To determine the impact of check size on neuromagnetic visual cortical responses, visual evoked fields to pattern-reversal stimulation with central occlusion in ten subjects were recorded in the past.[4] It was reported that magnitude of cortical activation during visual contrast processing is check size-dependent and the 120 minutes checks are optimum for studies on neuromagnetic visual cortical functions using central-occluded stimulation. The corresponding neuronal activation demonstrated a short refractory period less than 0.16 s. This also supports our contention of keeping 120 minutes visual angle as a reference. Standard disc electro encepgalogram (EEG) electrodes were placed on the scalp areas after preparing the skin by degreasing and abrading with a conducting jelly or electrode paste (RMS recording paste) rubbed lightly into the area with a cotton swab. The standardized methodology as recommended by the International Federation of Clinical Neurophysiology (IFCN) Committee and International Society for Clinical Electrophysiology of Vision (ISCEV)[5] was followed. As per 10-20 International System of EEG placements, the reference electrode (Fz) was placed 12 cm above the nasion, the ground electrode (Cz) at the vertex and the active electrode (Oz) at approximately 2 cm above the inion. The electrode impedance was kept below 5 KΩ. The recording was done in a quiet, darkened room with a constant temperature (27-30°C) in the Neurophysiology unit. The subjects were seated comfortably at a distance of one meter away from the screen of the VEP monitor. They were exposed 121
Kothari, et al.: Influence of visual angle on PRVEP to full-field monocular stimulation for the left and right eyes separately with the subjects wearing corrective glasses, if any during the test. The signals recorded were filtered (low cut and high cut frequency filter) through a band spread of 2 − 100 Hz. The sensitivity was kept at 2 μV. The sweep duration was maintained at 300 ms. Responses to 200 stimuli were amplified and averaged for each eye, which were then analyzed by inline computer having automatic artifact rejection mechanism. An average of two trials with well-defined PRVEPs were obtained for all check sizes in all subjects.
The effects of visual angle on N75 and N145 have been tabulated in [Tables 3 and 4]. In Group I, it was observed that the mean N75 latency was shortest with 67.66 ± 5.15 msec (with a range of
Table 1: Influence of visual angle on P100 waveform of Group I subjects (n=40 eyes) Check size 180' (5×5) 120’ (8×8)
The typical VEP elicited by a pattern reversal is a negativepositive-negative complex that is recorded maximally in the mid-occipital region with reference to mid-frontal region. The components of VEP are termed as N75, P100, and N145 to indicate their polarity and approximate latency (in msec.). The absolute latencies of the peaks of positive wave P100 and the negative waves N75 and N145 were recorded along with the peak to peak amplitude of N75-P100. The influence of visual angle on latencies and amplitude of PRVEP components was assessed by One Way Analysis of Variance (ANOVA) using the statistical software EPI INFO 6. P
Influence of visual angle on pattern reversal visual evoked potentials Ruchi Kothari, Smita Singh1, Ramji Singh2, A. K. Shukla1, Pradeep Bokariya3 Departments of Physiology, 1Ophthalmology, 3Anatomy, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha, Maharashtra, 2 Department of Physiology, All India Institute of Medical Sciences, Patna, Bihar, India
Purpose: The aim of this study was to find whether the visual evoked potential (VEP) latencies and amplitude are altered with different visual angles in healthy adult volunteers or not and to determine the visual angle which is the optimum and most appropriate among a wide range of check sizes for the reliable interpretation of pattern reversal VEPs (PRVEPs). Materials and Methods: The present study was conducted on 40 healthy volunteers. The subjects were divided into two groups. One group consisted of 20 individuals (nine males and 11 females) in the age range of 25-57 years and they were exposed to checks subtending a visual angle of 90, 120, and 180 minutes of arc. Another group comprised of 20 individuals (10 males and 10 females) in the age range of 36-60 years and they were subjected to checks subtending a visual angle of 15, 30, and 120 minutes of arc. The stimulus configuration comprised of the transient pattern reversal method in which a black and white checker board is generated (full field) on a VEP Monitor by an Evoked Potential Recorder (RMS EMG. EPMARK II). The statistical analysis was done by One Way Analysis of Variance (ANOVA) using EPI INFO 6.
Results: In Group I, the maximum (max.) P100 latency of 98.8 ± 4.7 and the max. P100 amplitude of 10.05 ± 3.1 μV was obtained with checks of 90 minutes. In Group II, the max. P100 latency of 105.19 ± 4.75 msec as well as the max. P100 amplitude of 8.23 ± 3.30 μV was obtained with 15 minutes. The min. P100 latency in both the groups was obtained with checks of 120 minutes while the min. P100 amplitude was obtained with 180 minutes. A statistically significant difference was derived between means of P100 latency for 15 and 30 minutes with reference to its value for 120 minutes and between the mean value of P100 amplitude for 120 minutes and that of 90 and 180 minutes. Conclusion: Altering the size of stimulus (visual angle) has an effect on the PRVEP parameters. Our study found that the 120 is the appropriate (and optimal) check size that can be used for accurate interpretation of PRVEPs. This will help in better assessment of the optic nerve function and integrity of anterior visual pathways. Keywords: Pattern reversal, P100 latency, P100 amplitude, VEP, visual angle
Introduction Access this article online Quick Response Code: Website: www.ojoonline.org DOI: 10.4103/0974-620X.142593
Visual system is well adapted to the processing and coding of pattern information. Human visual cortical activity as measured by visual evoked potentials (VEP) is highly sensitive to the sharpness and density of contours of the evoking stimulus. The preferred stimulus for VEP is a checkerboard pattern of black and white squares. The patterned stimuli are widely
Copyright: © 2014 Kothari R. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Correspondence: Prof. Ruchi Kothari, Department of Physiology, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha - 442 102, Maharashtra, India. E-mail: [email protected]
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Kothari, et al.: Influence of visual angle on PRVEP preferred because response to a pattern is much larger and bears a closer relationship to the act of seeing. Checks help to explore the function of striate cortex (area 17) because local spatial frequency analyzers are presumably present there.[1,2] Since one of the primary functions of human visual system is to analyze contours and edges, the use of patterned stimuli seems to have an added advantage in providing more information in this regard. The size of the individual checks is usually reported in terms of visual angle in minutes of arc (β) which is expressed as β = tan-1(W/2D) × 120 where W is width of checks in millimeters and D is distance of the pattern from the corneal surface in mm.[3] It is already known that retina can be divided into central foveal, para-foveal, and peripheral region. The fovea subtends 5° of visual angle while para-foveal area subtends 8°. Smaller size of pattern elements is thought to be optimal for foveal stimulation and larger sized patterns stimulate both fovea and extra-foveal region. Thus, by selecting the appropriate pattern element size one can predominantly stimulate fovea or peripheral retina. So when a checkerboard pattern is used, by altering one of its most important attribute that is the visual angle, one can better understand the mechanisms of visual processing. The influence of altered visual angle on the latency and amplitude of major positive component of pattern reversal visual evoked potentials (PRVEP) and other components namely, N75 and N145 is not well-understood and there is scanty data on how and to what extent they are modified so we made a systematic study of effects of different check sizes in terms of visual angle on the latencies and amplitude of the parameters of PRVEPs. For this we performed an evaluation of visual evoked potential to pattern reversal stimulation (VEP-PR) in two groups of healthy subjects with normal visual acuity at baseline. The purpose of this study was to determine the visual angle which is the optimum and most appropriate among a wide range of check sizes for the reliable interpretation of PRVEPs and accurate assessment of optic nerve function and integrity of anterior visual pathways.
Materials and Methods The study population consisted of 40 healthy volunteers consisting of 19 males and 21 females. They were divided into two groups. One group consisted of 20 individuals ( nine males and 11 females) in the age range of 25-57 years and they were exposed to checks subtending a visual angle of 90, 120, and 180 minutes of arc. Another group comprised of 20 individuals (10 males and 10 females) in the age range of 36-60 years and they were subjected to checks subtending a visual angle of 15, 30, and 120 minutes of arc. Each of the subjects was given thorough eye examination as a preliminary measure to exclude any ocular pathology. No subject had a history of relevant neurological or heart disease or of drug abuse. Oman Journal of Ophthalmology, Vol. 7, No. 3, 2014
Inclusion Criteria • • • •
Visual acuity at least 6/6 (with or without corrective glasses) Normal pupillary size (2-4 mm) and reactions Normal Fundus and optic disc Intra-Ocular pressure < 21 mm Hg (as measured by the Non contact tonometer).
This project was approved by the Institutional Ethics committee and written informed consent was taken from the volunteers before the study.
Stimulus The stimulus configuration comprised of the transient pattern reversal method in which a black and white checker board is generated (full field) on a VEP Monitor (colour 14”) by an electronic pattern regenerator inbuilt in an Evoked Potential Recorder (RMS EMG.EP MARK II). The rate of pattern reversal (1.7 Hz), the luminance (59 cd/sqm) and contrast level (80%) was kept constant for all the recordings in all the cases. The different sizes of the checks used were 15, 30, 90, 120, and 180 minutes of arc. Checks of 8 × 8 subtending a visual angle of 120 minutes are the most commonly used in the neurophysiological laboratories, so it was kept as a reference for evaluation of the effects on VEP parameters. To determine the impact of check size on neuromagnetic visual cortical responses, visual evoked fields to pattern-reversal stimulation with central occlusion in ten subjects were recorded in the past.[4] It was reported that magnitude of cortical activation during visual contrast processing is check size-dependent and the 120 minutes checks are optimum for studies on neuromagnetic visual cortical functions using central-occluded stimulation. The corresponding neuronal activation demonstrated a short refractory period less than 0.16 s. This also supports our contention of keeping 120 minutes visual angle as a reference. Standard disc electro encepgalogram (EEG) electrodes were placed on the scalp areas after preparing the skin by degreasing and abrading with a conducting jelly or electrode paste (RMS recording paste) rubbed lightly into the area with a cotton swab. The standardized methodology as recommended by the International Federation of Clinical Neurophysiology (IFCN) Committee and International Society for Clinical Electrophysiology of Vision (ISCEV)[5] was followed. As per 10-20 International System of EEG placements, the reference electrode (Fz) was placed 12 cm above the nasion, the ground electrode (Cz) at the vertex and the active electrode (Oz) at approximately 2 cm above the inion. The electrode impedance was kept below 5 KΩ. The recording was done in a quiet, darkened room with a constant temperature (27-30°C) in the Neurophysiology unit. The subjects were seated comfortably at a distance of one meter away from the screen of the VEP monitor. They were exposed 121
Kothari, et al.: Influence of visual angle on PRVEP to full-field monocular stimulation for the left and right eyes separately with the subjects wearing corrective glasses, if any during the test. The signals recorded were filtered (low cut and high cut frequency filter) through a band spread of 2 − 100 Hz. The sensitivity was kept at 2 μV. The sweep duration was maintained at 300 ms. Responses to 200 stimuli were amplified and averaged for each eye, which were then analyzed by inline computer having automatic artifact rejection mechanism. An average of two trials with well-defined PRVEPs were obtained for all check sizes in all subjects.
The effects of visual angle on N75 and N145 have been tabulated in [Tables 3 and 4]. In Group I, it was observed that the mean N75 latency was shortest with 67.66 ± 5.15 msec (with a range of
Table 1: Influence of visual angle on P100 waveform of Group I subjects (n=40 eyes) Check size 180' (5×5) 120’ (8×8)
The typical VEP elicited by a pattern reversal is a negativepositive-negative complex that is recorded maximally in the mid-occipital region with reference to mid-frontal region. The components of VEP are termed as N75, P100, and N145 to indicate their polarity and approximate latency (in msec.). The absolute latencies of the peaks of positive wave P100 and the negative waves N75 and N145 were recorded along with the peak to peak amplitude of N75-P100. The influence of visual angle on latencies and amplitude of PRVEP components was assessed by One Way Analysis of Variance (ANOVA) using the statistical software EPI INFO 6. P
