The effect of senescence on orientation discrimination and mechanism tuning

Abstract

Accurately processing orientation information is a fundamental component of visual performance. Single-unit recordings have shown that the orientation tuning of individual neurons in macaque cortical areas V1 and V2 is reduced dramatically with age (M. T. Schmolesky, Y. Wang, M. Pu, & A. G. Leventhal, 2000; S. Yu, Y. Wang, X. Li, Y. Zhou, & A. G. Leventhal, 2006). These researchers suggest that losses in single-unit orientation selectivity result in declines in orientation discrimination and object recognition in older humans. Three experiments were conducted to determine whether human performance is affected by putative age-related changes in tuning of cortical neurons. Ten younger and ten older observers participated in this study. Experiment 1 demonstrated significant differences in the contrast sensitivity of the two age groups. Experiment 2 showed significant differences in orientation discrimination thresholds. However, when thresholds were plotted in terms of multiples of detection threshold, age-related differences were not observed. In Experiment 3, perceptual orientation tuning curves did not significantly differ in shape for younger and older subjects. As in Experiment 2, at any given contrast, there is a large difference in sensitivity between younger and older adults. This implies a model of orientation processing that allows the adult visual system to maintain consistent and reliable orientation information at the network and ultimately the perceptual level.

Figure 1

Contrast thresholds for older participants (red squares) and younger participants (blue circles) for 1 and 4 cpd Gabor patterns. Error bars represent ±1 SEM across 10 participants.

Figure 2

Orientation discrimination thresholds for older participants (red squares) and younger participants (blue circles) for 1 cpd (top panel) and 4 cpd (bottom panel). Error bars represent ±1 SEM across 10 participants.

Figure 3

Orientation discrimination thresholds re-plotted as multiples of thresholds obtained in Experiment 1 for older participants (red squares) and younger participants (blue circles) for 1 cpd (top panel) and 4 cpd (bottom panel). Error bars represent ±1 SEM across 10 participants.

Figure 4

Orientation discrimination thresholds for Gabor patterns at the maximum contrast (same physical contrast for both groups) for older participants (red squares) and younger participants (blue circles) for 1 and 4 cpd. Error bars represent ±1 SEM across 10 participants.

Figure 5

Illustration of the masker stimulus. Detection thresholds were measured for a vertically oriented Gabor (left panel) superimposed on a sine wave masker (middle panel) to produce a combined stimulus (right panel). Various masker orientations were used in the experiment. In this example, the masker is 10 degrees from vertical.

Figure 6

Orientation tuning curves for older participants (red squares) and younger participants (blue circles) for 1 cpd (top panel) and 4 cpd (bottom panel). Error bars represent ±1 SEM across 10 participants.

Figure 7

Orientation tuning curves re-plotted using multiples of detection thresholds obtained in Experiment 1 for older participants (red squares) and younger participants (blue circles) for 1 cpd (top panel) and 4 cpd (bottom panel). Error bars represent ±1 SEM across 10 participants. The black horizontal line indicates no threshold elevation.

Figure 8

The multiples of threshold masking data from Experiment 3 were fitted with exponential curves for both older (red squares) and younger (blue circles) participants. The bandwidths were calculated at half height and are illustrated with dashed lines. The black horizontal line indicates no threshold elevation.