Central perimetric sensitivity estimates are directly influenced by the fixation target

Denniss, Jonathan; Astle, Andrew T.

Abstract

Purpose: Perimetry is increasingly being used to measure sensitivity at central visual field locations. For many tasks, the central (0°, 0°) location is functionally the most important, however threshold estimates at this location may be affected by masking by the nearby spatial structure of the fixation target. We investigated this effect.
Methods: First we retrospectively analysed microperimetry (MAIA-2; CenterVue, Padova, Italy) data from 60 healthy subjects, tested on a custom grid with 1° central spacing. We compared sensitivity at (0°, 0°) to the mean sensitivity at the eight adjacent locations. We then prospectively tested 15 further healthy subjects on the same instrument using a cross-shaped test pattern with 1° spacing. Testing was carried out with and without the central fixation target, and sensitivity estimates at (0°, 0°) were compared. We also compared sensitivity at (0°, 0°) to the mean of the adjacent four locations in each condition. Three subjects undertook 10 repeated tests with the fixation target in place to assess within-subject variability of the effect.
Results: In the retrospective analysis, central sensitivity was median 2.8 dB lower (95% range 0.1–8.8 dB lower, p < 0.0001) than the mean of the adjacent locations. In the prospective study, central sensitivity was median 2.0 dB lower with the fixation target vs without (95% range 0.4–4.7 dB lower, p = 0.0011). With the fixation target in place central sensitivity was median 2.5 dB lower than mean sensitivity of adjacent locations (95% range 0.8–4.2 dB lower, p = 0.0007), whilst without the fixation target there was no difference (mean 0.4 dB lower, S.D. 0.9 dB, p = 0.15). These differences could not be explained by reduced fixation stability. Mean within subject standard deviation in the difference between central and adjacent locations' sensitivity was 1.84 dB for the repeated tests.
Conclusions: Perimetric sensitivity estimates from the central (0°, 0°) location are, on-average, reduced by 2 to 3 dB, corresponding to a 60–100% increase in stimulus luminance at threshold. This effect can be explained by masking by the nearby fixation target. The considerable within- and between-subject variability in magnitude, and the unknown effects of disease may hamper attempts to compensate threshold estimates for this effect. Clinicians should interpret central perimetric sensitivity estimates with caution, especially in patients with reduced sensitivity due to disease.

Citation

Denniss, J., & Astle, A. T. (in press). Central perimetric sensitivity estimates are directly influenced by the fixation target. Ophthalmic and Physiological Optics, 36(4), https://doi.org/10.1111/opo.12304/abstract

• Journal Article Type: Article
• Acceptance Date: Apr 18, 2016
• Online Publication Date: May 4, 2016
• Deposit Date: Apr 19, 2016
• Publicly Available Date: May 4, 2016
• Journal: Ophthalmic and Physiological Optics
• Print ISSN: 0275-5408
• Electronic ISSN: 1475-1313
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Volume: 36
• Issue: 4
• DOI: https://doi.org/10.1111/opo.12304/abstract
• Keywords: perimetry, microperimetry, visual fields, central vision loss
• Public URL: https://nottingham-repository.worktribe.com/output/791447
• Publisher URL: http://onlinelibrary.wiley.com/doi/10.1111/opo.12304/abstract
• Additional Information: This is the peer reviewed version of the following article: Denniss J & Astle AT. Central perimetric sensitivity estimates are directly influenced by the fixation target. Ophthalmic and Physiological Optics, which has been published in final form at DOI: 10.1111/opo.12304. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.