Recognition of personally familiar scenes in patients with very mild Alzheimer's disease: effects of spatial frequency and luminance

Many community-residing patients with Alzheimer's disease (AD) have way-finding problems, particularly at twilight or on rainy days. In an attempt to understand the mechanism, we prepared pictures of street scenes, including 8 personally familiar and 8 unfamiliar, divided into Low Spatial Frequency (LSF) and Low Luminance (LL) conditions to simulate foggy or rainy days and nighttime. Each picture was presented from the most difficult (level 10) to the easiest (level 1). The participants, including 20 very mild AD patients and 20 normal controls (NC) with equal basic visual acuity, were asked to judge whether a picture was familiar or not and to describe how they came to that conclusion. The accuracy of familiar scene recognition was measured by the number of pictures successfully recognized and the ability thereof by the level needed. Compared with NC, AD patients showed poorer accuracy (2.7 ± 0.2 versus 3.6 ± 0.1, mean ± SEM, p = 0.003 under LSF; 2.8 ± 0.2 versus 3.8 ± 0.1, p = 0.001 under LL) and poorer ability (2.2 ± 0.4 versus 4.3 ± 0.4 p = 0.000 under LSF; 2.9 ± 0.3 versus 5.2 ± 0.5, p = 0.000 under LL) for both conditions. The AD patients used a global element to help judge when personally familiar scenes were displayed, which was the method NC usually adopted when presented with novel scenes. In summary, this study demonstrated poorer recognition ability in very mild AD patients when personally familiar street scenes were displayed, and the underlying mechanisms may include impaired visual search performance and efficiency. The deficits also reflect their difficulty in real life situations when their familiar environments become blurred or dark.