Problems of stimulus control in the measurement of peak latency of the pattern visual evoked potential

Influence of visual angle on pattern reversal visual evoked potentials

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.

Visual function in Rett syndrome

The authors examined refractive error, pattern onset visual evoked potentials, ocular posture and they performed internal and external eye examinations in 11 subjects with Rett syndrome (aged between four and 24 years) and 18 normal controls (aged between six and 20 years). Substantial refractive errors were common in the Rett syndrome group. Spectacle correction had never previously been worn and glasses were provided where appropriate. No subjects had nystagmus or optic nerve pallor and only one was strabismic. All subjects had recognisable and reproducible pattern-onset VEPs and latencies and amplitudes did not differ significantly from those of the controls. All demonstrated VEP thresholds of at least 24'. In contrast to other populations with profound disabilities, people with Rett syndrome have good function of the afferent visual pathways and, in view of their substantial refractive errors, are likely to benefit from spectacle correction.

Development of visual evoked potentials and photic driving responses in normal full term, low risk premature, and Trisomy-21 infants during the first year of life

Stroboscopic visual evoked potentials (SVEPs) and photic driving responses were recorded during the first week of life post term and at 4, 13, 26 and 52 weeks in normal full term, low risk premature, and Trisomy-21 infants. The age effect on SVEP P2 peak latency was significant (P less than 0.001). P2 latency for the Trisomy-21 group was significantly prolonged at 1 week of age only. Voltages and interhemispheric symmetry were highly variable, with no significant age or group effects. There is considerable variability in photic driving effects, even within sessions. Maximal driving frequencies are difficult to determine; they may be near 15 Hz at term. The age effect on optimal driving frequency (3-4 Hz at week 1 increasing to 5 Hz at 12 months) was significant (P less than 0.001). The between-groups effect was not significant. The results are discussed in terms of (1) our own previous results, (2) previous results reported in the literature, and (3) significance in clinical applications.

Developmental changes in the human visual system as reflected by the latency of the pattern reversal VEP

Pattern reversal visually evoked potentials (VEPs) were recorded from 439 infants and young children ranging in age from 1 month to 5 years in response to large and small checks. Qualitative analysis of the VEP wave form showed that the first major positive component, P1, is consistently present at all ages, while the frequency of occurrence of later positive components is more variable. The proportion of infants showing late positive components increases with age; by 1 year, the frequency of occurrence of late components for large checks is more adult-like than for small checks. The latency of P1 was analyzed quantitatively. Results showed that P1 latency decreases rapidly during the first year of life for both large and small checks and that the time course of the latency change differs as a function of check size. VEPs to large checks attain adult-like P1 latency values by about 1 year of age, while the P1 latency of VEPs to small checks has still not reached adult levels by 5 years of age. Data from 12 infants tested longitudinally between 1 and 7 months of age using both checkerboards and square wave gratings show no difference in P1 latency between checkerboards and gratings comprised of large (30-240 min) pattern elements, but for patterns with small (7.5 and 15 min) elements, P1 latency to checks is significantly longer than P1 latency to stripes. These results are explained on the basis of the difference in the fundamental spatial frequency between checks and stripes.

Abnormal evoked potential latencies in amblyopia

The latency of the first (P1) and second (P2) major positive waves of the pattern reversal visual evoked potential (VEP) for small checks (15 minutes of arc) was measured in 68 visually normal children and 32 amblyopic children with mild to moderate visual acuity losses. In the normal children there were no P1 and P2 interocular latency differences. The amblyopic children showed longer P1 latencies and shorter P2 latencies in their amblyopic eye than their normal fellow eye. These findings can be accounted for by a selective loss of the contrast-specific evoked potential mechanisms in amblyopia. The 'shorter' P2 latency obtained from amblyopic eyes for small checks is a reflection of the luminance responses that are normally elicited by larger (60 minute) checks.