Equipment and Stimuli
Steady-state VEPs were recorded using a Diagnosys Espion E3 System (Lowell, MA, USA). Checkerboard stimuli were presented in brief onset mode, two frames (33.3 ms) on at 100 cd/m2 and six frames (100 ms) off, corresponding to 7.5 Hz with a stimulus distance of 180 cm and a contrast of 40%. Three sets of check sizes were used, one for the highest VA range (‘‘Range A’’), one for medium VA range (“Range B”) and one for the lowest VA range (‘‘Range C’’). For Range A, the check sizes were 0.37° to 0.05°, for Range B they were 1.19°–0.17° and for Range C they were 4.0°–0.57°. Six check sizes were used in each Range, and cumulatively across all three Ranges there were twelve unique check sizes (i.e., there is overlap of checks sizes between the Ranges). In this study for the three acuity conditions of each participant, Range A and Range B were always used, and depending on the adjusted acuity of the participant with the strongest Bangerter foil, either Range B or Range C was used for the lowest acuity recording. The benefit of having three check size ranges available is that in clinical use this approach will usually keep the test time shorter for a patient. Typically, the clinic has a general understanding of the acuity range the patient is likely to fall within thereby enabling it to choose one of the three shorter protocols. In cases where the clinic does not have that knowledge, they can run the full set of twelve check sizes on a patient.
Freiburg Acuity Test (FrACT)  measurements were taken on a standard PC with a screen size of 58.5 cm (diagonal) using the same Bangerter foils used for the steady-state VEP recordings, also at a distance of 180 cm from the computer.
The VEP was recorded using gold cup electrodes at Oz, O1 and O2, referenced to Fz. In accordance with the ISCEV VEP standard , Oz was placed on the midline at 10% above the inion. O1/O2 were placed laterally to Oz at a distance of 10% the ½ head circumference on either side of the Oz electrode. Signals were amplified by a factor of 8, digitized at a rate of 1 kHz with 32-bit resolution and digitally filtered in the range of 5–50 Hz. Averaging was arranged to capture exactly eight on-/offset periods in 1066-ms epochs. fourty sweeps were taken for each step within an artifact rejection window of ± 100 µv. For each step, the Laplace transform is calculated from the signals obtained at the Oz, O1 and O2 electrode locations (VEPLaplace = 2Oz–(O1 + O2)). The software then calculates the Fourier transform of each resulting signal at each step and plots the resulting six amplitudes by log spatial frequency (dominant of the check size ). Finally, the software calculates a visual acuity estimate based on methods described below. A simplified recording setup is depicted in Fig. 1. We define a set of six traces (along the chosen checkerboard set) to be one “recording.”
There were twenty-four participants in the study (14 male and 10 female), with an age range of 19 to 74 years old (mean age was 46.5), and in each case both eyes were tested. Participants were given the choice of either using their habitual eyeglasses during the test or not, and that same condition was used for all tests for that participant. Under these conditions the participants’ visual acuity ranged from approximately 0.6 to better than − 0.15 LogMAR. Each participant completed one set of tests with full vision conditions (defined as participants with their chosen correction, and no Bangerter foil), which was typically 0.30 LogMAR or better. An additional set of tests were then conducted using a Bangerter foil that was intended to reduce the participants’ acuity to approximately 0.4 LogMAR, and a final step with a foil intended to reduce acuity to approximately 1.0 LogMAR. Since every participant was recorded under several conditions, one might think that the “eyes or patients” problem might arise . Given that we are using descriptive, not inferential statistics, this is not a problem.
Response traces were de-trended and subjected to a discrete Fourier transform (DFT). Because care was taken to choose the analysis interval (1066 ms) to be an integer multiple of the stimulation period, there is no overspill in the spectrum  and the noise can be estimated by averaging the magnitudes recorded at the two neighboring frequencies (6.5 and 8.5 Hz). The ‘true’ response magnitude at 7.5 Hz was calculated by non-linearly subtracting the noise from the magnitude measured at 7.5 Hz, and finally a significance for the response at 7.5 Hz was also calculated .
Responses are recorded over six check sizes. Ideally, the stimulation of the various check sizes would be interleaved, but this was not yet implemented on the system in this study. The six check sizes were selected from a range of 0.05° to 4.0°, as appropriate for the expected VA. The responses were processed as described above, resulting in 6 values for the response magnitude plus the associated significances. From these, the heuristic algorithm, starting at small check sizes, selects as many points as possible up to peak response and avoiding a notch  if present. The resulting points are regressed to zero magnitude on a log(spatial frequency) scale, resulting in the value SF0. SF0 is divided by 17.6 (calibration factor, ), yielding a decimal acuity estimate VAdec(VEP). This is converted to LogMAR using the standard formula: VALogMAR = − log10(VAdec). When insufficient points are found or other irregularities occur, a “no result” outcome is flagged, reducing “testability.”
The relationship between behavioral acuity and the VEP-based acuity estimate is quantified in terms of the Bland-Altman limits of agreement (LoA) . Frequently for such a task the correlation coefficient is computed; that is, however, an inappropriate measure because it is normalized by range [12, 15].