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 Table of Contents  
REVIEW ARTICLE
Year : 2014  |  Volume : 2  |  Issue : 1  |  Page : 3-6

Refractive errors and their effects on visual evoked potentials


1 Department of Physiology, MGIMS, Sevagram, Wardha, Maharashtra, 3Department of Physiology, AIIMS, Patna, Bihar, India
2 Department of Anatomy, MGIMS, Sevagram, Wardha, Maharashtra, 3Department of Physiology, AIIMS, Patna, Bihar, India
3 Department of Ophthalmology, MGIMS, Sevagram, Wardha, Maharashtra, India
4 Department of Physiology, AIIMS, Patna, Bihar, India

Date of Submission15-May-2013
Date of Acceptance25-Jun-2013
Date of Web Publication3-Dec-2013

Correspondence Address:
Ruchi Kothari
Department of Physiology, MGIMS, Sevagram, Wardha - 442 102, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2320-3897.122625

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  Abstract 

In view of the increasing use of visual evoked potentials (VEP) technique in neuro-ophthalmological diagnosis, it was thought pertinent to appraise the changes brought about in VEPs in the presence of refractive error (RE) as studied by the vision researchers and neurophysiologists. The purpose of this review was to provide a comprehensive quintessence of the work carried out in this fi eld with an attempt to summarize the previous concepts, recent perspective and current notion about the value of RE in electrophysiologic testing particularly the VEP technique.

Keywords: Defocus, refractive errors, visual evoked potential


How to cite this article:
Kothari R, Bokariya P, Singh S, Narang P, Singh R. Refractive errors and their effects on visual evoked potentials. J Clin Ophthalmol Res 2014;2:3-6

How to cite this URL:
Kothari R, Bokariya P, Singh S, Narang P, Singh R. Refractive errors and their effects on visual evoked potentials. J Clin Ophthalmol Res [serial online] 2014 [cited 2022 Jun 28];2:3-6. Available from: https://www.jcor.in/text.asp?2014/2/1/3/122625

Visually evoked cortical response testing has been one of the most exciting clinical tools to be developed from neurophysiologic research and has provided us with an objective method of identifying abnormalities of the afferent visual pathways.

Visual evoked potentials (VEPs) reflect electrical phenomena occurring during the visual processing and are a graphic illustration of the cerebral electrical potentials generated by the occipital cortex evoked by a defined visual stimulus. [1] Therefore, VEPs can be used both in research and in clinical practice to elucidate the function of the visual system.

It is known that the technical and physiological factors such as pupil diameter, refractive errors (REs), type of stimulus, age and sex, electrode position and anatomical variations may affect VEP. [2] It is assumed that RE cause defocus. Defocusing may affect the VEP, which if allowed to persist, can result in corresponding neurological changes.

It was perceived that there was no clear cut presumption with regards to how actually does the RE alters the visually evoked response of the brain, whether the VEPs are more affected by myopia or hypermetropia and to what extent the degree of RE would affect the VEPs. Therefore, we made an attempt to review the various aspects related to the effects of RE on VEPs.

Beginning with background reading about the topic, we traced the specific resources, extensively searched the literature, analyzed the shortcomings and strengths of various workers, tabulated their methodologies and findings in chronological manner and tried to derive at an inference incorporating the previous, recent and finally our own perspective regarding the relationship of VEPs with REs.

Multiple database searches using MedLine, Google scholar, EMBASE and PubMed were conducted to identify all the previous as well as the recent studies and publications pertinent to this issue. All identified documents were examined and those that were relevant were retrieved for inclusion in the review. The relevant reports were skimmed, retrieved; compiled and important conclusions from the studies were laid down in proper chronology. The methodologies and findings of various authors were tabulated for a quick glance and to make easy comparisons between them. Reference lists of retrieved documents were hand searched to identify the additional publications. Then a critical analysis of the relationship among different works was performed and finally this research was coupled to our own work.


  Perception Decades Ago Top


Minute neural discharges that occur in visual cortex upon brief exposure of the eye to patterned stimuli after monitoring by topical scalp electrodes, amplification and summation produce characteristic waveform that exhibits a relationship to retinal image clarity. A systematic relationship was first established long back. [3] They observed that the amplitude for both a negative wave appearing at 80-100 ms after a flash of patterned target and a positive wave following this flash by 180-200 ms was greater with retinal image clarity and lessened with its degradation over a wide range of dioptic values.

The amplitude of the response in a pattern reversal VEP is dependent on the visual system's ability to resolve the pattern and on the degree of retinal image focus. Small errors of refraction tend to reduce the average amplitude of the waves of VEPs. A quantified documentation was provided [4] who found the VEP amplitude to be decreased 25% per diopter (D) of defocus and the effect was appreciable for 0.25 D. They employed a rotating polaroid in conjunction with a checker pattern made of polaroid strips, in which the intensity for each neighboring check varied sinusoidally in time. The overall intensity of light transmitted through the pattern therefore remained constant. The subject's eye was situated 85 cm from the plane of the pattern.

The VEP is more sensitive to small refractive changes than electroretinogram (ERG), perhaps because the VEP heavily emphasizes the foveal region while the ERG is more broadly representative of the entire stimulus field.

Considerable attention was given in the past to the use of checkerboard-pattern stimuli in the study of the VEPs in subjects as a technique for determining REs. Duffy and Rengstorff. [5] electronically subtracted the response to light from the response to pattern plus light that yielded a residual contour response. Working at a 20 feet refraction distance, they performed an initial scan over a wide range of spherical lens values with large increments on relatively large checker squares, 10 min of arc. They finalized the spherical lens measurement by use of small lens increments on a finer checker square pattern (2.5´). They claimed precision of spherical refraction of 0.25 D. The value determined was found to be more myopic or less hyperopic than that established by the conventional refractive techniques.

Reduction in amplitude of the VEP with RE was also reported. [6] They showed that there is consistently greater reduction in VEP amplitude for small amounts of plus lens defocus than for minus and it showed that subjects partially accommodated for minus lens. They found that decrease in amplitude in non-cycloplegic refraction measurements seem to occur more rapidly for plus lens than for minus and it is thought to be due to the partial correction of defocus brought about by accommodative effort of the subject.

The REs blur the stimulus and blurred vision also has been shown to decrease the amplitude of the conventional pattern reversal VEP. [7] Large REs, introduced by the use of ophthalmic lenses, cause the waves to approach zero amplitude. Collins et al. [8] studied the effect of introduced REs on the VEP on five women and eight men aged 19-45 years.

REs were created by introducing the following combined standard lenses: (+2.00 DS + 2.00 DC × 90°), (+1.00 DS + 1.00 DC × 90°), (−1.00 DS −1.00 DC × 90°) and (−2.00 DS −2.00 DC × 90°) diopters. Using 3° radius field stimulation and 12-min checks, monocular pattern-reversal VEPs were recorded without and then with each introduced RE by a computer-based data collection system and compared with previously established normal values. There was a pronounced effect on the P100 component of the VEP with these introduced REs.

The P100 latency was abnormally prolonged in 31% (5/16) of recordings with the (−2.00 DS −2.00 DC × 90°) diopter lens and in 87.5% (14/16) with the (+2.00 DS + 2.00 DC × 90°) diopter lens. The maximum P100 latency was 126 ms. For all other recordings, the P100 latency was less than 113 ms, but there was considerable temporal dispersion and reduction in amplitude of the P100 component, especially for the convex lenses. Indeed, the VEP was almost abolished in some of the recordings when a (+2.00 DS + 2.00 DC × 90°) diopter lens was used. Their study, using the pattern-reversal method, illustrated the significant changes in absolute and relative latency of the P100 component when Res, which approximated to those found in the population at large, were introduced to defocus a small stimulus field and high spatial frequency pattern. This effect was seen to be greatest for REs of + 2.00 DS + 2.00 DC × 90° diopters.

Pattern defocusing has been used to evaluate the contribution of different spatial frequency components in checks to VEP latency. Latency shifts with increasing blur (−2.50 to +2.50) were determined [9] for sinusoidal grating and check patterns. The effect of blur was found to be more for the higher spatial frequencies. Zislina et al. [10] in their studies with congenital myopia have shown significant deviations from reference of component P100.

REs were induced [11] in normal subjects by means of positive D lenses to reduce visual acuity (VA) from an initial level of 20/20 to 20/100 and then to 20/200. Pattern visual evoked potentials (PVEPs) were recorded at each of these three levels of VA using high contrast checkerboard stimuli subtending 11´ and 42´ of visual arc. Their findings confirmed the need to take REs into account because latencies fell outside normal limits with decreased VA.

Reduction of VA or of the contrast of the stimulus induces a prolongation of the pattern reversal visual evoked potential (PR-VEP) latencies, perhaps because these conditions cause deterioration of the visual capacity to recognize objects and may preferentially activate the slower central retina channel. The PR-VEP was obtained [12] with a video stimulator and three kinds of stimuli: Total video field, video with a central scotoma and a restricted central stimulus. The subjects were tested under conditions of normal (20/20) and reduced VA (20/200) with 14´ and 56´ checks and 60% contrast and under conditions of normal VA (20/20) with 14´ checks and with stimulus contrast of 60% and 25%. Blurring increased latencies and decreased amplitudes only with the 14´ checks stimulus but not with 56´ checks and the amplitudes obtained with the central stimulus became greater than those obtained with a central scotoma. Reducing contrast increased only latency and there was no difference between amplitudes obtained with a central stimulus or a central scotoma. They deduced that blurring small checks induces a preferential stimulation of receptors in the central retina, but the same effect was not observed when stimulus contrast was reduced. Thus, prolongation of latency and decreased amplitude with a reduction in VA was established in their study.


  Recent Perspective Top


In their study, Perlman et al.[13] investigated the correlation between reduced VA and VEP in volunteers with normal corrected VA and in patients suffering from inherited macular degeneration or from age related macular degeneration. In both groups of patients suffering from macular dysfunction, pattern reversal VEP was very subnormal and was characterized by prolonged implicit time. These findings indicated that the PVEP directly correlates with foveal function. Therefore, they suggested that recordings of PVEP can be used to differentiate between RE and macular disorders as causing reduction in VA when other clinical signs are missing or not available.

Lee et al.[14] tried to evaluate the P100 latency of VEP according to refraction. They studied 28 patients (12 males, 16 females) with myopia. Subjects were divided into three groups (mild, moderate, severe myopia) according to refraction and they evaluated the results of VEP studies. The Mean values of refraction and latency (P100) of naked eyes were -4.27 DS, 103.95 ms and those of corrected eyes (in glasses) were -0.25 DS, 100.59 ms. Respectively, in mild, moderate, and severe myopia, the P100 latency of naked eyes were 101.27 ms, 102.59 ms, 107.99 ms and those of corrected eyes were 98.33 ms, 100.58 ms, 102.19 ms respectively (P < 0.05). There was significant negative correlation between refraction and P100 latency in myopia.

Marr et al. [15] undertook a retrospective case review and recruited 112 children with age less than 10 years presenting over 3 years who were found to have high myopia (defined as one or both eyes demonstrating six diopters spherical equivalent or more of myopic RE on retinoscopy). They concluded that high myopia in early childhood is strongly associated with systemic and ocular problems. In 54%, there was an underlying systemic association with or without further ocular problems (e.g., developmental delay, pre-maturity, Marfan, Stickler, Noonan, Down syndrome) and in the remaining 38% there were further ocular problems associated with the high myopia (e.g., lens subluxation, coloboma, retinal dystrophy, anisometropic amblyopia).

Marr et al. [16] reviewed 114 consecutive children under 10 years of age with high hypermetropia (greater than + 5.00 DS) during a 5-year period and reported that high hyperopia has a similar incidence of associated ocular abnormalities as high myopia.

To understand how REs affect multifocal visual evoked potential (mfVEP) responses, monocular mfVEP responses were obtained using a pattern reversal dartboard display . [17] The right eye was tested under simulated RE. For the simulated RE condition, significant centrally located abnormalities were seen for all subjects. They concluded that factors such as uncorrected REs can produce apparent field defects on the mf VEP.

A very recent study conducted by Anand et al. [18] examined effects of uncorrected REs in a short-duration transient visual evoked potential system and investigated their role for objective measurement of RE. REs were induced by means of trial lenses in 35 emmetropic subjects. A synchronized single-channel electro encephalogram was recorded for emmetropia and each simulated refractive state to generate 21 VEP responses for each subject. P100 amplitude (N75 trough to P100 peak) and latency were identified by an automated post-signal processing algorithm. They construed that induced hypermetropia and myopia correlated strongly with both P100 amplitude and latency.

Most of these studies have been reported in western populations and no such comparative study is available in Indian population. Since, there are differences as regards to the age of detection, accuracy of correction and regularity of usage of correcting glasses, a recent study [19] was conducted to estimate the effect of RE on VEP recordings in Indian population. To test the hypothesis that the changes in VEP due to REs in Indian population are different from western population, pattern reversal VEP recordings were performed in a total of 50 hypermetropics and 50 myopics having age in the range of 18-40 years. The subjects having astigmatism (>0.5 DC) and RE of more than 5 D were excluded in the study. They were investigated for VEP recordings with and without glasses. Their results were compared with those of 50 age and sex matched controls. P100 latency was increased and amplitude decreased with and without correction of RE. The statistical analysis revealed a significant difference (P < 0.05) in latency of P100 and amplitude of P100 between controls and myopics with glasses and highly significant difference (P < 0.001) between controls and myopics without glasses; so, VEPs were affected in Indian subjects with RE irrespective of correction given, but more so without correction.

The difference in P100 latency and amplitude between controls and hyperopics with glasses and those without glasses were found to be non-significant in this study.

A RE develops when there is lack of coordination among the factors required for the growth of the eye and for the eye to become emmetropic like changes in refractive components and in eye size. A myopic eye is generally larger than emmetropic or hyperopic eyes and changes in scleral tissue may be the factor when emmetropization does not occur. Animal studies have shown that poor image quality on the retina can elicit a signal to sclera tissue components to strengthen or weaken in an attempt to move the retina to the best location for a clear image.

Scleral remodeling causes axial lengthening that occurs in myopia; the scleral tissue is weakened and thins. In progressive myopia existing collagen is degraded, the production of new collagen is reduced and matrix proteoglycans are lost. [20],[21] So in this way, if we recollect the physiological changes occurring in the development of myopia, substantial explanation to the above finding unfurls.


  Conclusion Top


To conclude, the observations of the studies [Table 1] in the past as well as the recent research in this field suggest that the RE blur the stimulus and cause defocus, which lead to significant changes in VEP (P100 latency and amplitude) in the presence of RE. As per our perspective, among the REs, VEPs seem to be more affected by myopia than hypermetropia. However, the principal cause in both is the degree of defocusing of the image produced by the RE.

Since no clear inference could be drawn with regards to whether the VEPs seem to be more affected by myopia or hypermetropia as higher degrees of the REs were excluded; so, further longitudinal studies on a larger sample of ametropic population are required to confirm the contributions of the retinal defocus associated with particular RE to the alterations in pattern reversal VEPs.
Table 1: Comparative account of the studies on relationship between VEP and refractive error

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Implication

If pattern stimuli are to be used in VEP investigation, it is important that a patient be tested with RE corrected. REs tend to affect the interpretation of the VEP results. Therefore in performing the VEP study, one should consider the refraction and VA.

 
  References Top

1.Goldie WD. Visual evoked potentials in paediatrics - Normal. In: Holmes GL, Moshe SL, Jones HR Jr., editors. Clinical Neurophysiology of Infancy Childhood and Adolescence. Elsevier Philadelphia: 2006. p. 206-15.  Back to cited text no. 1
    
2.Walsh TJ. Electrodiagnosis - Visual evoked potential. In: Walsh TJ, editor. Neuro-Ophthalmology: Clinical Signs and Symptoms. 2 nd ed. Philadelphia: Lea & Febiger; 1985. p. 303-40.  Back to cited text no. 2
    
3.Harter MR, White CT. Effects of contour sharpness and check-size on visually evoked cortical potentials. Vision Res 1968;8:701-11.  Back to cited text no. 3
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4.Millodot M, Riggs LA. Refraction determined electrophysiologically. Responses to alternation of visual contours. Arch Ophthalmol 1970;84:272-8.  Back to cited text no. 4
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5.Duffy FH, Rengstorff RH. Ametropia measurements from the visual evoked response. Am J Optom Arch Am Acad Optom 1971;48:717-28.  Back to cited text no. 5
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6.Ludlam WM, Meyers RR. The use of visual evoked responses in objective refraction. Trans N Y Acad Sci 1972;34:154-70.  Back to cited text no. 6
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7.Sherman J. Visual evoked potential (VEP): Basic concepts and clinical applications. J Am Optom Assoc 1979;50:19-30.  Back to cited text no. 7
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8.Collins DW, Carroll WM, Black JL, Walsh M. Effect of refractive error on the visual evoked response. Br Med J 1979;1:231-2.  Back to cited text no. 8
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9.Bobak P, Bodis-Wollner I, Guillory S. The effect of blur and contrast on VEP latency: Comparison between check and sinusoidal and grating patterns. Electroencephalogr Clin Neurophysiol 1987;68:247-55.  Back to cited text no. 9
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10.Zislina NN, Sorokina RS. Possibilities of the use of visual evoked potentials in the evaluation of visual acuity in congenital myopia in children. Vestn Oftalmol 1992;108:35-7.  Back to cited text no. 10
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11.Bartel PR, Vos A. Induced refractive errors and pattern electroretinograms and pattern visual evoked potentials: Implications for clinical assessments. Electroencephalogr Clin Neurophysiol 1994;92:78-81.  Back to cited text no. 11
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12.Tumas V, Sakamoto C. Comparison of the mechanisms of latency shift in pattern reversal visual evoked potential induced by blurring and contrast reduction. Electroencephalogr Clin Neurophysiol 1997;104:96-100.  Back to cited text no. 12
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13.Perlman I, Segev E, Mazawi N, Merhav-Armon T, Lei B, Leibu R. Visual evoked cortical potential can be used to differentiate between uncorrected refractive error and macular disorders. Doc Ophthalmol 2001;102:41-62.  Back to cited text no. 13
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14.Lee SM, Kim C, Ahn JK. The change of visual evoked potentials in patients with myopia in correction of refraction. J Korean Acad Rehabil Med 2002;26:734-8.  Back to cited text no. 14
    
15.Marr JE, Halliwell-Ewen J, Fisher B, Soler L, Ainsworth JR. Associations of high myopia in childhood. Eye (Lond) 2001;15:70-4.  Back to cited text no. 15
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16.Marr JE, Harvey R, Ainsworth JR. Associations of high hypermetropia in childhood. Eye (Lond) 2003;17:436-7.  Back to cited text no. 16
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17.Winn BJ, Shin E, Odel JG, Greenstein VC, Hood DC. Interpreting the multifocal visual evoked potential: The effects of refractive errors, cataracts, and fixation errors. Br J Ophthalmol 2005;89:340-4.  Back to cited text no. 17
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18.Anand A, De Moraes CG, Teng CC, Liebmann JM, Ritch R, Tello C. Short-duration transient visual evoked potential for objective measurement of refractive errors. Doc Ophthalmol 2011;123:141-7.  Back to cited text no. 18
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19.Kothari R, Bokariya P, Singh R, Singh S. Influence of refractory error on the pattern reversal VEPs of myopes and hypermetropes. Int J Physiol 2013;1:57-61.  Back to cited text no. 19
    
20.McBrien NA, Cornell LM, Gentle A. Structural and ultrastructural changes to the sclera in a mammalian model of high myopia. Invest Ophthalmol Vis Sci 2001;42:2179-87.  Back to cited text no. 20
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21.McBrien NA, Gentle A. The role of visual information in the control of scleral matrix biology in myopia. Curr Eye Res 2001;23:313-9.  Back to cited text no. 21
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    Tables

  [Table 1]


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