Abstract
Purpose:
Vernier and grating acuity can be measured with swept-parameter visual evoked potentials (sVEP). However, whether sVEP Vernier and grating acuities are comparable in predicting letter acuity has not been systematically evaluated. This study evaluated the validity and reliability of sVEP Vernier and grating acuity for the detection of amblyopia in adults.
Methods:
Three types of acuity were measured in 36 adults with amblyopia and 36 age-matched normal-vision controls. Letter acuity was measured with a logMAR chart. Both Vernier and grating acuity were estimated by sVEP and psychophysics for the same stimuli. Regression analyses were performed between the perceptual and electrophysiologic acuity measurements.
Results:
SVEP Vernier and grating acuities were significantly correlated with their corresponding psychophysical acuities (P < 0.001). Both the sVEP Vernier (P < 0.0001) and grating (P < 0.01) acuities were also significantly correlated with letter acuity. However, Vernier acuity more precisely reflected the magnitude of the letter acuity loss than did grating acuity for both sVEP and psychophysical measures. Repeating sVEP grating acuity tests with different temporal frequencies and modulation types indicated good reliability of sVEP acuity measures.
Conclusions:
SVEP Vernier acuity has a 1:1 relationship with letter acuity, but sVEP grating acuity does not. SVEP Vernier acuity thus provides a better characterization of the magnitude of the amblyopic acuity loss than does sVEP grating acuity. Nonetheless, each of the sVEP measurements can be used to predict letter acuity and because they can be made without a behavioral response, they may be useful measures of visual function in pre- and nonverbal patients.
Accurate visual acuity estimation is an essential tool for clinical practice, but is problematic in pre-, and nonverbal patients. The most common application of visual acuity assessment in pediatric patients is detection of amblyopia, the most frequent cause of monocular visual acuity loss in early childhood,
1 affecting about 3% of the population.
2 Importantly, early detection and management of amblyopia improves treatment efficacy.
3 However, early detection of amblyopia can be difficult, due to an inability to perform standard clinical visual acuity tests (discrimination of high contrast letters, referred to as letter acuity) in infants and nonverbal children. Therefore, it is routine clinical practice to use risk factors that are commonly associated with amblyopia to estimate the likelihood and severity of amblyopia. These factors include strabismus (turned eye), visual fixation responses
4; anisometropia (unequal refractive errors); and occlusion of the visual axis (e.g., congenital cataract, ptosis, etc.). A quantitative and objective technique for visual acuity assessment in pre- or nonverbal patients is needed. Potential objective techniques for measuring visual acuity include the swept-parameter visual evoked potential (sVEP) and forced-choice preferential looking (FPL)
5,6 techniques. In this study, we focused on sVEP acuity measures.
There are two types of acuity other than letter acuity that have been measured psychophysically (behavioral response dependent on perception) in adults with amblyopia.
7,8 The first is grating acuity, the high spatial frequency visibility limit, and the other is Vernier acuity (the smallest perceptible misalignment).
7 These two types of acuity have both been measured with the sVEP in infants,
9,10 and in children with cortical visual impairment who are often unable to provide reliable behavioral measures of visual acuity.
11,12 Measurement of sVEP Vernier acuity also offers a sensitive visual assessment in amblyopia. For example, an amblyopia-like effect on sVEP Vernier acuity was recorded in infants with a history of unilateral periocular vascular birthmarks that caused intermittent occlusion of one eye despite a normal clinical assessment of acuity.
13 SVEP Vernier acuity measurements also reflect visual acuity loss in adults with amblyopia.
14
The sVEP does not require behavioral responses or an ability to respond to instructions, making it well-suited for objective measurement of visual acuity in pre- and nonverbal patients. However, whether Vernier and grating acuities with sVEP measure are comparable in predicting letter acuity has not been systematically evaluated. The reliability (test-retest) of sVEP acuity measures also needs evaluation. A necessary prerequisite for any proposed nonverbal test of visual acuity is that it should produce valid measures in adults. In this study, we evaluated the validity and reliability of sVEP acuity measures in predicting letter acuity in adult amblyopia with a wide range of visual acuity losses in age-matched normal-vision controls. We made clinical letter acuity measurements, along with sVEP and psychophysical acuity measurements. Validity was assessed in two ways: whether sVEP acuities measured in verbal subjects who can report their perceptual experience are accurate reflections of their psychophysical Vernier and grating acuity for the same stimuli; and whether sVEP Vernier and grating acuity measurements are comparable in predicting letter acuity, the gold standard visual acuity measurement in clinical practice. Reliability was assessed by repeating sVEP grating acuity measures with different temporal frequencies and modulation types. A portion of the data has been reported previously.
14
Figure 1 illustrates the Vernier displacement sweep paradigm (left column) and the spatial frequency (grating) sweep paradigm (right column, shown as grating on/off at 3.75 Hz as an example). Prior pilot testing had optimized the temporal frequency for the Vernier sVEP, but no comparable data were available from our laboratory for pattern reversal versus pattern appearance presentation modes or for stimulus temporal frequency, so these measurements were obtained here. The 7.5 Hz pattern reversal condition was included because this temporal frequency is commonly used for steady-state VEP spatial frequency measurements with pattern reversal.
15 The 15 Hz on/off condition was included because the dominant response component (the first harmonic) has the same frequency as the dominant response component of the 7.5 Hz pattern reversal response (e.g., the second harmonic at 15 Hz).
16,17
A frequency of 3.75 Hz was used for the pattern On/Off sweep to equate the temporal frequency of the first harmonic of the grating response to the first harmonic of the Vernier response. The repeated sVEP measurements also served as a conservative estimate the test and retest reliability of sVEP grating acuity measurements. The measure is conservative in that it includes both intrinsic repeatability differences and any (small) stimulus-related differences.
Stimulus generation and signal analyses were performed by in-house software running on separate computers (both Power Macintosh G3; Apple Computer, Cupertino, CA, USA). The stimuli were generated on a multisync video monitor (1600 × 1200 pixels; 60 Hz vertical refresh, video bandwidth, 150 MHz; MRHB2000; Richardson Electronics, Inc., LaFox, IL, USA) at a space average luminance of 110 cd/m2 and a Michelson contrast of 80%. Viewing distance was 150 cm, which generated a display size of 12° × 9°. A small fixation point in the center of the stimuli was given during the experiments.
The authors thank Lisa Young, Vanitha Sampath, and Margaret Q. McGovern for the assistance in recruiting participants.
Supported in part by funds from the West China Hospital Foundation, National Institutes of Health Grant R01-EY025018 and the Children's Eye Foundation of the American Association for Pediatric Ophthalmology and Strabismus. Funding organizations had no role in the design or conduct of this research.
Disclosure: C. Hou, None; W.V. Good, None; A.M. Norcia, None