**Purpose**:
The purpose of this study was to establish a normal reference database for fixation stability measured with the bivariate contour ellipse area (BCEA) in the Macular Integrity Assessment (MAIA) microperimeter.

**Methods**:
Subjects were 358 healthy volunteers who had the MAIA examination. Fixation stability was assessed using two BCEA fixation indices (63% and 95% proportional values) and the percentage of fixation points within 1° and 2° from the fovea (P1 and P2). Statistical analysis was performed with linear regression and Pearson's product moment correlation coefficient.

**Results**:
Average areas of 0.80 deg^{2} (min = 0.03, max = 3.90, SD = 0.68) for the index BCEA@63% and 2.40 deg^{2} (min = 0.20, max = 11.70, SD = 2.04) for the index BCEA@95% were found. The average values of P1 and P2 were 95% (min = 76, max = 100, SD = 5.31) and 99% (min = 91, max = 100, SD = 1.42), respectively. The Pearson's product moment test showed an almost perfect correlation index, *r* = 0.999, between BCEA@63% and BCEA@95%. Index P1 showed a very strong correlation with BCEA@63%, *r* = −0.924, as well as with BCEA@95%, *r* = −0.925. Index P2 demonstrated a slightly lower correlation with both BCEA@63% and BCEA@95%, *r* = −0.874 and −0.875, respectively.

**Conclusions**:
The single parameter of the BCEA@95% may be taken as accurately reporting fixation stability and serves as a reference database of normal subjects with a cutoff area of 2.40 ± 2.04 deg^{2} in MAIA microperimeter.

**Translational Relevance**:
Fixation stability can be measured with different indices. This study originates reference fixation values for the MAIA using a single fixation index.

^{1–12}Furthermore, the analysis of the location and quality of fixation loci has been of particular importance to different authors because of its relevance to reading abilities in patients with central vision loss.

^{13}Such assessments are independent of the quality of fixation and eye movements. This is possible due to the accurate test-retest of the same retinal point, which is monitored and the projection controlled by eye-tracking technology.

^{6–8}Additionally, fixation locations may be mapped across the retina in order to characterize, evaluate, and compare sequential data in cases where the PRL may vary over time.

^{14}

^{15}The main advantage of this method is the clinical classification of FS as suggested by Fuji et al.,

^{16}where eyes with P1 greater than 75% are classified as having stable fixation. If P1 is less than 75% and P2 is more than 75%, fixation is classified as relatively unstable. If both P1 and P2 are less than 75%, the pattern is described as unstable fixation.

^{16}This methodology has been criticized in the literature due to the arbitrarily selected distance of 1° and 2° used to establish such stability index. The second FS measurement method suggested by Crossland et al.,

^{17}is known as the bivariate contour ellipse area (BCEA). It calculates the area and orientation of an ellipse encompassing a given proportion (

*ρ*) of the fixation points' dataset. This is a two-dimensional elliptical representation that describes the limits of the retinal surface area used during a fixation attempt where lower BCEA values define better fixation stability.

^{17–19}The advantage of the BCEA calculation over the PFP is that it is based on a mathematical model used in statistics to describe movement of variables; however, it is not related to any clinical classification in MP.

^{20}where

*BCEA*is the bivariate contour ellipse area,

*σ*is the standard deviation of point location over the horizontal meridian,

_{H}*σ*the standard deviation of point location over the vertical meridian, and

_{V}*ρ*the product moment correlation of these two position components. The value

*k*defines the limit for the ellipse and is dependent upon the probability area chosen given by: where

*e*is the base of the natural logarithm, from where,

*P*) values,

^{19}such as 0.63,

^{20,21}0.68,

^{17–19}or 0.95.

^{22}The clinical and statistical significance of such

*P*values have never been reported for MAIA MP.

^{5,9,23}The purpose of this study is to establish a reference database for FS measured with the BCEA metric in a group of normal subjects tested with the MAIA microperimeter. It does not directly compare the MAIA BCEA values with other technologies.

*MAIA Operator's Manual*). A total of 326 eyes were included in the analysis after the exclusions.

*i*); subsequently, the projection of the preselected stimuli grid map begins with the grid centered on such PRL

*i*. On completion of the test, the center of the mass (barycenter) of the total fixation points is calculated and defined as PRL_final (PRL

*f*),

^{15}which corresponds to the foveal area in normal subjects. The projection strategy was the standard 4-2, and the examination was performed without dilation in a darkened room (mesopic conditions). Patients were instructed to stare at the center of the fixation target during the examination. The fixation target consists of a 1° diameter red circle with light intensity of 10 ± 3 apostilb (asb).

*i*and PRL

*f*(ΔPRL),

^{15}the MAIA examination time, and the BCEA orientation, which corresponds to the angle between the ellipse major axis (EMA) and the horizontal axis (HA) of the visual meridian, where values between 0° and +90° correspond to an angle measured counterclockwise between the HA and the EMA; values between 0° and −90° are measured clockwise between the HA and the EMA; 0° corresponds to a horizontal orientation and 90° to a vertical one.

*P*value less than 0.05 was considered statistically significant. Linear regression, best-fit values, 95% confidence and prediction intervals, correlation coefficient of determination, and analysis of variance (ANOVA) with 95% CI were calculated and plotted using GraphPad Prism (version 6; GraphPad Software, Inc., La Jolla, CA).

*n*= 326) were analyzed (Table 1), average areas of 0.80 deg

^{2}(min = 0.03, max = 3.90, SD = 0.68) for the parameter BCEA@63% (Fig. 2A) and 2.40 deg

^{2}(min = 0.20, max = 11.70, SD = 2.04) for BCEA@95% (Fig. 2B) were found. The average values of P1 (Fig. 3A) and P2 (Fig. 3B) were 95% (min = 76, max = 100, SD = 5.31) and 99% (min = 91, max = 100, SD = 1.42), respectively. The maximum fixation value (100%) was recorded in 65 (20%) cases of P1 and 199 (61%) cases of P2. Macular Integrity Assessment P1 and P2 linear regression analysis showed a more sensitive P1 index than P2 (Fig. 3A and 3B). The average sensitivity was 29 dB (min = 18, max = 34, SD = 2.60). The mean ΔPRL was 0.2° (min = 0.0, max = 1.0, SD = 0.16) (Fig. 4). The angular orientation of the BCEA was variable with a mean angle of 3° (min = −90, max = 90, SD = 53.83). The mean examination time for each eye was 5 minutes, 50 seconds (min = 4 minutes, 2 seconds, max = 10 minutes, 40 seconds, SD = 59 seconds).

*r*= 0.999) between BCEA@63% and BCEA@95%. The P1 index showed a very strong correlation with BCEA@63% (

*r*= −0.924), as well as with BCEA@95% (

*r*= −0.925). Index P2 demonstrated a slightly lower correlation with both BCEA@63% and BCEA@95%, with

*r*values of −0.874 and −0.875, respectively. The correlation between P1 and P2 (

*r*= 0.792) was lower than that found with the BCEA indices. Finally, low correlations (

*r*< 0.28) between any of the fixation indices and the patient's age were found.

^{16}as well as two BCEA indices (63% and 95%) as proposed by Crossland.

^{19}The PFP indices allow the classification of fixation as stable or not stable, but the BCEA indices are not related to any clinical stability standard at the present. This study reports a normative reference database for BCEA FS indices for the MAIA instrument.

^{19}suggested the analysis of FS by means of BCEA instead of PFP after demonstrating a strong relationship between FS measured with BCEA and many parameters of reading ability, as well as a very poor correlation between PFP indices with any of the standard parameters of reading ability in low vision patients.

^{24}have stated that the BCEA description of fixation is scientifically more acceptable than the Fuji classification due to the analysis being based on a well-known mathematical model versus the quantification of fixation points falling into an arbitrarily selected fixed circular area of 1° and 2° in radius. However, the main advantage of the Fuji metrics is that it has established a differentiation between normal and non-normal fixation with cutoff values of 75% within 1° radius from the centroid of the fixation dataset.

^{16}is worthy of comment. Our studied population demonstrated higher P1 values (fixation points within 1° radius) than those proposed by Fuji

^{16}(95% ± 5% vs. 75%), while the P2 index (fixation points within 2° radius) reached a ceiling effect in a high number (61%) of the studied subjects. In contrast, our BCEA values showed only one participant who reached the floor effect in the BCEA@63%, while none of the subjects reached the lowest limiting value in the BCEA@95%.

^{24–27}It is noteworthy that some of these previously reported results are based on a fixation-only test lasting between 10 and 30 seconds, and the amount of data collected is less than 800 points. In contrast, the MAIA-based BCEA ellipse is derived from a CFP with more than 8000 data points, a greater than 10-fold difference in data sampling. Furthermore, because the complete MAIA MP examination has a median duration of 5.5 minutes, fixation fatigue may play a major role in creating a larger BCEA ellipse.

*N*values of the study groups, the motivation and inclusion criteria for selecting study participants, differences in the fixation target dimensions and appearance, and other protocol differences that are beyond the scope of this report.

^{24}reported in a study with the Nidek MP-1 (Nidek, Gamagori, Japan), 10 experienced healthy controls (age 41 ± 18.1 years) with very small mean BCEA@68% values (0.053 ± 0.022 deg

^{2}).This is different from the results obtained by Dunbar et al.,

^{25}who reported the BCEA@68% values of 16 normally sighted volunteers with the Rodenstock SLO and the Nidek MP-1, showing different values among both of their studied instruments that are even larger than our findings (3.3 deg

^{2}with the Rodenstock and 5.0 deg

^{2}with the Nidek).

^{26}reported BCEA data in subjects with and without maculopathies with two different systems, the MP-1 and optical coherence tomography (OCT)/SLO. The technologies behind the two instruments are different. The MP-1 uses an infrared camera to image the retina controlled by a 25-Hz eye tracker, which is the same eye-tracker frequency found in the MAIA. The second is an SLO imaging device with a much slower eye tracker (8 Hz). Such a study was based on a fixation-only test of 20 seconds, which is largely different from the examination time (mean 350 seconds) studied in this article. Their results suggest that the patient's pattern of FS may be the same, although calculated with different technologies. Furthermore, Liu et al.

^{26}normalized their data with a log transformation of the BCEA units (minutes of arc

^{2}) to undertake statistical comparison of the two technologies. In our study, the whole dataset was analyzed using the MAIA output layout with the purpose of establishing reference values for the MAIA that is useful to clinicians.

^{27}who evaluated BCEA indices in the MP-1 microperimeter (Nidek), our study demonstrates a weak, but statistically significant, correlation between the fixation indices and the patient's age.

^{2}in MAIA MP. This study now establishes an important normative standard for future fixation stability studies of diseased and disease-suspect eyes analyzed with BCEA in the MAIA MP (Fig. 5A and 5B).

**M.U. Morales**, (C);

**S. Saker**, None;

**C. Wilde**, None;

**C. Pellizzari**, (E);

**A. Pallikaris**, None;

**N. Notaroberto**, None;

**M. Rubinstein**, None;

**C. Rui**, (E);

**P. Limoli**, None;

**M.K. Smolek**, None; W

**.M. Amoaku**, None

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