Spatial Contrast Sensitivity

5.3.1 Introduction

The contrast sensitivity function is determined by measuring the contrast threshold for detection o f sinusoidal gratings at various spatial frequencies. In normal subjects the result is a band-pass function, with the highest contrast sensitivity in the mid-spatial frequency range - approximately 2-6 cycles/degree (Campbell & Green, 1965).

The characteristics of this curve vary with changes in luminance level and retinal location. As luminance is reduced from photopic to mesopic levels, there is a general decrease in contrast sensitivity. Maximum contrast sensitivity and the high-frequency cut-off (corresponding to visual acuity) shift to lower spatial frequencies (Patel, 1966). At low scotopic levels the low frequency attenuation in contrast sensitivity is not apparent. The same trends can be seen with increasing retinal eccentricity (Rovamo et al, 1978). This indicates the importance of stating all parameters used for contrast sensitivity measurements.

Contrast sensitivity measurement is more time-consuming than recording o f visual acuity, but provides more information. In some disorders high-spatial frequency sensitivity can remain intact whilst low spatial frequency sensitivity is significantly attenuated. In studies into visual performance in AMD, therefore, it has been important to record contrast sensitivity functions in order to determine whether it is purely high spatial frequency sensitivity which has been lost, or whether there is a more widespread deficit in spatial visual function.

Chapter 5 Visual Psychophysics

5.3.2 Contrast Sensitivity and AMD

The largest study to date was carried out by Alexander et al (1988). They discovered a reduction in the peak contrast sensitivity below the minimum of elderly controls in 80% of 100 subjects with AMD. However, the subject group employed in this study had advanced retinal changes, and visual acuity ranging from 6-30 - 6/384. Furthermore, the test parameter consisted o f an estimate o f peak contrast sensitivity based on the visibility o f 4 equally sized letters (6/405) presented at gradually reduced contrast. Changes to the contrast sensitivity function were therefore not fully characterised, and the study did not investigate the earliest changes occurring in AMD.

Sjostrand et al (1977) recorded contrast sensitivity functions to sinusoidal gratings ranging in spatial frequency from 0.7-38 cycles/deg. Eleven subjects with macular disease (3 with AMD) were investigated, and 10 healthy controls (aged 19-61). This study demonstrated that subjects with AMD show most marked reduction in contrast sensitivity for high and intermediate spatial frequencies, with contrast sensitivity almost unaffected below 2 cycles/degree. However, one subject with more advanced AMD showed a more marked impairment across the whole spatial frequency range. A further study (Sjostrand et al, 1979) on a larger subject group (n = 22) showed similar results. Sjostrand suggested that this indicates a sparing o f the peripheral part o f the test field (more sensitive to lower spatial frequencies) when an overall reduction in contrast sensitivity at all spatial frequencies might suggest the involvement o f a wider retinal area (as in those subjects with advanced AMD). However, these studies did not age-match their controls. The significance o f this is highlighted by Owsley et al (1990) who demonstrated a small but significant reduction in spatial contrast sensitivity with age in control subjects, especially at high spatial frequencies. The same pattern of loss was noted in subjects with early AMD (Owsley et al, 1990), but to a greater extent (See Figure 5.2). Once again the major locus of sensitivity loss in AMD subjects was in the high and mid spatial frequency mechanisms. Other studies have supported this finding (Brown & Lovie-Kitchin, 1987; Stcmgos et al, 1995).

Although group averaged data suggest a reduction in contrast sensitivity in individuals with AMD compared to age-matched controls, no significant difference has been found between those subjects with bilateral drusen, and those with monocular exudative AMD in the non-text eye. This indicates that this test is not sensitive to increased risk o f neovascularisation

Chapter 5 Visual Psychophysics 0.001 o . o i o 0100 1.000 SPATIAL FREQUENCY

Figure 5.2. Schematic plot illustrating three hypothetical contrast sensitivity functions from young adult (A), older adult (B), and individual with non-exudative AMD (C). The reduction in contrast sensitivity for the subject with AMD is similar to that observed with normal ageing, but more pronounced (Owsley et al, 1990).

Brown & Gamer (1983) evaluated the effect o f luminance on the contrast sensitivity function in people with AMD. Results from both control subjects, and those with AMD showed a generalised decrease in contrast sensitivity for all spatial frequencies as luminance decreased, and a shift in peak contrast sensitivity towards lower spatial frequencies. The contrast sensitivity function was found to be significantly depressed in the AMD group compared to the control group at the higher luminance levels, with only a small difference between groups at the lowest luminance level.

The implication o f these studies is that measurements of contrast sensitivity will allow a greater discrimination between AMD and control subjects at high levels of luminance and high spatial frequencies. High spatial frequencies are only detectable by the visual system at high contrast levels, where the task is very similar to that of visual acuity measurement. The recording of a full contrast sensitivity function may not, therefore, provide any additional information in the assessment o f patients with AMD than basic acuity tasks. Owsley et al

(1990) illustrated this by evaluating the visual acuity of 15 subjects with AMD (VA ranging

from 6/7.5-6/18), and 30 elderly controls using the Regan low contrast letter charts (100%, 84%, 40%, 25%, and 12% contrast). They found that the greatest acuity losses for AMD subjects were detected using the high contrast charts. Other studies however have found the lower contrast charts to be more effective at discriminating between AMD subjects and controls (Greeves et al, 1988, Kleiner et al, 1988).

Chapter 5 Visual Psychophysics