Healthy aging impairs face discrimination ability

Abstract

Face images enable individual identities to be discriminated from one another. We aimed to quantify age-related changes in different aspects of face identity discrimination. Face discrimination sensitivity was measured with a memory-free "odd-one-out" task. Five age groups (N = 15) of healthy adults with normal vision were tested: 20, 50-59, 60-69, 70-79, and 80-89. Sensitivity was measured for full-face images (all features visible), external features (head-shape, hairline), internal features (nose, mouth, eyes, and eyebrows) and closed-contour shapes (control object). Sensitivity to full-faces continuously declined by approximately 13% per decade, after 50 years of age. When age-related differences in visual acuity were controlled, the effect of age on face discrimination sensitivity remained. Sensitivity to face features also deteriorated with age. Although the effect for external features was similar to full-faces, the rate of decline was considerably steeper (approximately 3.7 times) for internal, relative to external, features. In contrast, there was no effect of age on sensitivity to shapes. All age groups demonstrated the same overall pattern of sensitivity to different types of face information. Healthy aging was associated with a continuous decline in sensitivity to both full-faces and face features, although encoding of internal features was disproportionately impaired. This age-related deficit was independent of differences in low-level vision. That sensitivity to shapes was unaffected by age suggests these results cannot be explained by general cognitive decline or lower-level visual deficits. Instead, healthy aging is associated with a specific decline in the mechanisms that underlie face discrimination.

Figure 1.

Synthetic faces. Top: (a) Grayscale photograph with superimposed polar coordinate grid centered on the bridge of the nose. The head-shape was measured at 16 locations (white dots) around the external contour, angularly positioned at equal intervals of 22.5°. The polar co-ordinates of 14 of the measured points were used to define seven radial frequencies to describe the subject's head-shape. An additional nine points were used to define four radial frequencies that captured the shape of the subject's hairline. All radial frequencies were defined relative to the mean head radius of all synthetic faces of the subject's gender. The location and shape of the internal face features were also digitized. In sum, the face is described by 37 measurements. (b) Photograph filtered with a 2.0 octave bandwidth DOG filter with peak spatial frequency of 10 cycles/face width. (c) Corresponding synthetic face. Bottom: synthetic faces were adjusted by manipulating their distinctiveness (i.e., by how much they differed from the mean face) (left). Increasing face distinctiveness results in individual faces becoming progressively more dissimilar (from middle to right) to the mean face. Distinctiveness is expressed as a percentage of mean head radius and quantifies the total geometric variation between the specified face and the mean face. Typical observers can discriminate a face from the mean at about 5% distinctiveness.

Figure 2.

The Caledonian face test. Four faces were presented in a diamond configuration and participants were asked to indicate the “odd” face that differed from the others. Left: supra-threshold trial for most participants (target face differs from mean face by 10%). The target (odd one) is to the left. Right: difficult trial, approximately at threshold for a typical participant (5%). Target is to the right.

Figure 3.

Face features. The Caledonian face test was administered under the following conditions: (a) full-faces: all features varied by equivalent proportions, (b) inverted full-faces: as for (a), but presented upside-down, (c) isolated external features: only the head-shape and hairline were visible, (d) embedded external features: the same stimulus as (b), where only external features varied within an otherwise fixed face context, (e) isolated internal features: only the eyes, nose, mouth and eyebrows are visible, (f) embedded internal features: the same stimulus as (e), embedded within an otherwise fixed face context, and (g) closed-contour shapes.

Figure 4.

Discrimination thresholds for different face features. Icons illustrate the feature being tested. Here, and elsewhere, error bars represent 95% confidence intervals. Asterisks indicate significant difference in discrimination thresholds from the young adult baseline (20-year-olds) (pairwise comparisons; p < 0.05).

Figure 5.

Discrimination thresholds as a function of age for (a) full-faces (b) external features (c) internal features and (d) shapes. Solid line indicates the line of best fit; dashed lines represent 95% confidence intervals. Note that the data for external (b) and internal (c) features here are for the cases where they were presented in isolation. Thresholds for these features embedded within a fixed face context were not significantly different from those presented in isolation (see Section 3.2). Because of significant differences in sensitivity across all age groups, data for the internal features (c) are presented on a different y-axis scale. The line of best fit for full-faces (a) has been replicated within (c) (i.e., the lower line) to illustrate the difference in slope between these two conditions.

Figure 6.

Healthy aging and the face inversion effect. Data are presented as threshold elevations: inverted, relative to upright, face discrimination thresholds. Solid horizontal line indicates line of no effect. Asterisks indicate significant face inversion effect (p < 0.001).