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 Table of Contents    
ORIGINAL ARTICLE
Year : 2014  |  Volume : 7  |  Issue : 3  |  Page : 120-125  

Influence of visual angle on pattern reversal visual evoked potentials


1 Department of Physiology, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha, Maharashtra, India
2 Department of Ophthalmology, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha, Maharashtra, India
3 Department of Physiology, All India Institute of Medical Sciences, Patna, Bihar, India
4 Department of Anatomy, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha, Maharashtra, India

Date of Web Publication11-Oct-2014

Correspondence Address:
Ruchi Kothari
Department of Physiology, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Wardha - 442 102, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0974-620X.142593

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   Abstract 

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


How to cite this article:
Kothari R, Singh S, Singh R, Shukla A K, Bokariya P. Influence of visual angle on pattern reversal visual evoked potentials . Oman J Ophthalmol 2014;7:120-5

How to cite this URL:
Kothari R, Singh S, Singh R, Shukla A K, Bokariya P. Influence of visual angle on pattern reversal visual evoked potentials . Oman J Ophthalmol [serial online] 2014 [cited 2020 Feb 20];7:120-5. Available from: http://www.ojoonline.org/text.asp?2014/7/3/120/142593


   Introduction Top


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 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 Top


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.

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 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 typical VEP elicited by a pattern reversal is a negative- positive-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 <0.05 was taken to represent a significant difference.


   Results Top


A total of 80 eyes of 40 subjects were investigated for full field pattern reversal VEPs with visual stimuli as checkerboard patterns with checks of 5 visual angles-15, 30, 90, 120, and 180 minutes of arc. The mean age of subjects in Group I was 46.5 ± 6.77 years (range 25-57 years). The mean age of subjects in Group II was 47.05 ± 9.01(range 36-60 years).

The effect of visual angle on P100 waveform of the two groups under study has been represented in [Table 1] and [Table 2].
Table 1: Influence of visual angle on P100 waveform of Group I subjects (n=40 eyes)

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Table 2: Influence of visual angle on P100 waveform of Group II subjects (n=40 eyes)

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In Group I, the maximum mean P100 latency of 98.8 ± 4.7 (with a range of 90.30-109.30 msec) was obtained with checks of 90 minutes and the minimum mean P100 latency of 97.51 ± 4.26 msec (with a range of 89.10-107.85 msec) was observed with checks of 120 minutes. Further, the maximum mean P100 amplitude of 10.05 ± 3.1 μV (with a range of 3.37-14.32 μV) was obtained with checks of 90 minutes and the minimum mean P100 amplitude of 6.38 ± 2.52 μV (with a range of 3.10 − 12.86 μV) was obtained with checks of 180 minutes.

In Group II , the maximum mean P100 latency of 105.19 ± 4.75 msec (with a range of 98.40-117.50 msec) was observed with checks of 15 minutes and the minimum mean P100 latency of 97.64 ± 3.25 msec (with a range of 90.3-103.8 msec) was observed with checks of 120 minutes. The minimum mean P100 amplitude of 7.12 ± 2.57 μV (with a range of 3.9-15.46 μV) was obtained with checks of 180 minutes and the maximum mean P100 amplitude of 8.23 ± 3.30 μV (with a range of 3.69-15.55μV) was obtained with checks of 15 minutes.

The effects of visual angle on N75 and N145 have been tabulated in [Table 3] and [Table 4]. In Group I, it was observed that the mean N75 latency was shortest with 67.66 ± 5.15 msec (with a range of 58.75-78.10) for 180 minutes and the most prolonged duration of 70.42 ± 4.26 msec (with a range of 60.90-78.10) for the visual angle 90 minutes. On the other hand, the mean N145 latency was found to be the longest with 138.62 ± 9.60 msec (with a range of 125.30-159.40) for 180 minutes checks and was the shortest with 134.27 ± 10.88 msec (with a range of 116.30-159.60) for 90 minutes checks.
Table 3: Influence of visual angle on N75 and N145 of Group I subjects (n=40 eyes)

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Table 4: Influence of Visual angle on N75 and N145 of Group II subjects (n=40 eyes)

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In Group II, it was found that mean N75 latency was shortest with 68.02 ± 4.53 msec (with a range of 60.00-79.40) for 120 minutes and the most prolonged duration of 76.58 ± 5.50 msec. (with a range of 75.00-88.80) for the visual angle 15 minutes. Likewise, mean N145 latency was also found to be the longest with 143.34 ± 7.61 msec (with a range of130.00-159.10) for 15 minutes checks and was the shortest with 134.13 ± 7.03 msec (with a range of 124.40-150.3) for 120 minutes checks.

On statistical analysis the difference between means of P100 latency for the visual angles 15 and 30 minutes with reference to its value for 120 minutes (8 × 8) was statistically highly significant (P < 0.001). When the mean value of P100 amplitude for 120 minutes (8 × 8) was compared with that of checks of 90 and 180 minutes, the difference of means was found to be statistically highly significant (P = 0.003). For the N75 latency, statistical significance (P = 0.02) was obtained for the difference in means of all the visual angles (15, 30, 90, 120, and 180 min. of arc). For N145 latency, the difference in means between 120, 15, and 30 minutes checks was statistically significant (P < 0.05).


   Discussion Top


The present work was conducted to study the effect of altered visual angle of the checkerboard pattern on the various components of PRVEPs.

Considerable evidence appears to support the notion that visual system processes information along multiple parallel channels. The optic tract starts from optic chiasma and terminates in the lateral geniculate body (LGB). From LGB, visual information is transmitted to striate area 17 via two principal pathways- Magnocellular or M pathway which is sensitive to low spatial frequency (large checks) and Parvocellular or P pathway more sensitive to high spatial frequency (small checks). Thus the specific range and degree of operation of each channel is a function of the size of the visual stimulus presented.

PRVEP is a very important non-invasive and highly objective tool in detecting abnormalities of visual system. It is useful not only for clinical neurophysiologist or ophthalmologist but also for neurologists and neurosurgeons, since many of the neurological disorders present with visual abnormalities. They may detect those abnormalities of the optic nerves which are poorly visualized by magnetic resonance imaging (MRI) and reflect subclinical involvement of the CNS even before the disease clinically manifests. The test is relatively inexpensive and can be repeated numerous times with high reliability. With proper understanding of its limitations and appreciation for its qualities, it will always remain one of the simple, harmless and invaluable tests to diagnosis abnormalities of the visual pathway.

Clinical utilities of pattern reversal VEPs include the following

  • Preferable in optic nerve lesions
  • More sensitive than MRI or physical examination in prechiasmatic lesions and very useful in detecting an anterior pre-chiasmatic visual conduction disturbance
  • Objective and reproducible test for optic nerve function
  • Abnormality persists over long periods of time
  • Inexpensive as compared with to MRI
  • Under certain circumstances, may be helpful to positively establish optic nerve function in patients with subjective complaint of visual loss; normal VEP excludes significant optic nerve disorder.


PRVEPs to full-field stimulation are best suited to evaluate anterior visual pathways. The transient pattern VEP is a straightforward investigation which could be performed within 10-15 minutes in total and requires the patient to fixate for only about 30-60 seconds at any one time. It thus entails less cooperation than conventional kinetic or computerised static perimetry considered as a gold standard in detecting and monitoring diseases like glaucoma and therefore has distinct advantages with regard to that group of patients who have difficulty in performing a field investigation. It has therefore been suggested that VEP technique is a potentially useful tool in the early detection of functional deficits in diseases like glaucoma and its longitudinal assessment.

It has been well documented that VEP is primarily a reflection of activity originating in the central 2-6 degrees of visual field. [6] It clearly implies that the conventional visual evoked potential response is dominated by the central macular response which is in agreement with the principal finding of our study that is the checks of 2° (120 minutes) produced the shortest P100 latency with substantially sound P100 amplitude in both the groups of our study. This observation is further indicative of optimal evoked response to a visual angle of 120 minutes.

Previous researchers [7],[8],[9] using different stimulation techniques have described an optimum check size for humans between

10-30 minutes of arc because maximum amplitude VEP was seen with checks which subtend visual angles in this range. [10] According to them, when the check size becomes smaller or larger than this; the VEP shows a corresponding drop in amplitude.

In our present study also, for the visual angle of 15 minutes, a high value of mean P100 amplitude of 8.23 ± 3.30 μV (with a range of 3.69-15.55) was observed. When the angle increased to 30 minutes from this value, a drop in P100 amplitude was obtained with the exception of an increase seen with the pattern of 90 minutes which showed maximum amplitude.

On the other hand it has also been reported that in some subjects the P100 amplitude continues to increase as check size increases and finally levels off. [11] This is presumably thought to be due to "switchover" from contrast specific VEPs with small checks to luminance specific VEPs to large checks. Large checks may evoke small signals in some individuals. This appears to have some relationship to the inter-subject variability in luminance contributions to large checks.

Studies of adult pattern VEP have shown that the latency of major components varies as a function of pattern element size. The results have been quite variable and contradictory at times. VEPs with longer latencies using small checks have been observed by some [12],[13] whereas some have suggested that as check size increases above 30 minutes, the latency of P100 also increases. [10] Contrary to this we have noticed a slight P100 delay as visual angle decreased from 120 upto 15 minutes.

However, our data is not in agreement with a study conducted to determine the optimum stimulus conditions for the detection of optic nerve damage using two check sizes (12 and 48 minutes). They found that the largest number of VEP abnormalities in terms of P100 delay and reduction in amplitude, were with the large checks. [14]

But our observations are in consonance with a recently conducted study by Nakamura et al. [13] on pattern reversal visual evoked magnetic field and potential. In that study, 7 healthy subjects were exposed to half-field stimuli with or without central occlusion with check sizes of 15, 30, 60, 90 and, 180 minutes of visual arc and simultaneously their pattern reversal visual evoked magnetic fields (VEF) and VEP were recorded. They have also reported that the latencies for the smaller checks were significantly longer than those for the larger checks.

The effect of visual angle using checks of 17, 35, 70, 144, and 288 minutes has been studied in the past where the values of

P100 latency showed a U-shaped function with check size rather than a perfect linear relationship. [15]

In case of N75 latency, the present study reveals a linear rise (as per Statistical analysis by ANOVA) with corresponding decrease of visual angle from 180-15 minutes. A reduction in check size was encountered with prolongation of N75 and P100 latency by some earlier workers [16] but the relationship was not a linear one. The N75 latency was found by others [17],[18] to have a significant relationship with a decrease in latency with increment of check size as we also have noted.

Modifications of the components of PRVEPs with changes in check size of the stimulating pattern had been studied in

11 healthy subjects by some previous workers where they have used 8 different check sizes ranging between 10 and 90 minutes of arc. They reported significant inverse linear relationship of latency and amplitude of N75 with the logarithm of the check size, while the P100 and N145 latencies showed significant curvilinear relationships, with minimal latencies at check sizes around 35 minutes. [10]

Another study [19] on the effect of check size on PRVEPs in healthy adult volunteers revealed that as the check size increased, P100 amplitude together with N75 latency had a linear relationship. No significant relationships were found for P100 and N145 latencies. They concluded that N75 wave originated mainly from the activity of the foveola, whereas the more eccentric regions contribute more to the formation of P100, and the interaction of both regions elicited the N145 wave.

As to the N145 latency, one previous study [18] revealed significant exponential decrement with an increase in check size again differing from our results. N145 latencies in our study displayed a parabolic relationship with visual angle. Maximum latency was observed for the pattern of 15 minutes and the second highest value was for 180&#. The U shaped modification of N145 latency implies presence of its spatial tuning and suggests that it originates in the cortex since visual cortical neurons possess this kind of feature.

Since the minimum mean P100 latency in both the groups of our study was observed with checks of 120 minutes and also considerable P100 amplitude was obtained with this check size, we consider this visual angle as the most suitable for optimal foveal stimulation. It further fosters our selection of this angle as the reference at our neurophysiology unit.


   Conclusion Top


It is evident from our study that the variation in visual angle subtended by the checks of the checkerboard pattern significantly influences the latency and amplitude of the PRVEPs. Investigation of the effect of altering the size of stimulus (visual angle) indicates that the best visual evoked responses are obtained when the central macular area of retina is stimulated. This can be achieved by employing an appropriate and optimal check size which we in our study have proposed to be of 120 minutes of arc. It would help in accurate interpretation of PRVEPs and better assessment of the optic nerve function and integrity of anterior visual pathways.

 
   References Top

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Blakemore C, Campbell FW. On the existence of neurons in the human visual system selectively sensitive to the orientation and size of retinal images. J Physiol 1969;203:237-60.  Back to cited text no. 1
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Birch EE, Bosworth RG. Visual evoked potentials in infants and children: In: Aminoff MJ, ediotrs. Electrodiagnosis in Clinical Neurology, 5 th ed. New York: Churchill-Livingstone; 2004. p. 439-50.  Back to cited text no. 3
    
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Chen WT, Ko YC, Liao KK, Hsieh JC, Yeh TC, Wu ZA, et al. Optimal check size and reversal rate to elicit pattern-reversal MEG responses. Can J Neurol Sci 2005;32:218-24.  Back to cited text no. 5
    
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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