Optical and neural factors contributing to age -related losses in spatial contrast sensitivity
The purpose of this dissertation was to evaluate factors involved in age-related spatial sensitivity decline. Three experiments were conducted to examine changes in the optics of the eye and neural mechanisms operating at different levels of the human visual system.
In Chapter 1, the contribution of an increase in high-order aberrations to age-related contrast sensitivity losses was measured using adaptive optics correction over a 6 mm and 3 mm pupil for 10 younger and 10 older observers. Results show that an age-related increase in high-order aberrations does contribute to the loss in spatial sensitivity, but cannot account for the complete decline. These results, in agreement with previous reports, implicate the presence of neural losses.
Chapter 2 evaluated functional changes related to the magnocellular (M) and parvocellular (P) pathways to determine if the age-related spatial vision decline is the result of a selective neural deficit in one pathway. Contrast discrimination thresholds were measured for 10 younger and 10 older observers using two paradigms thought to separate the M and P pathways based on their contrast gain signatures. Results show large age-related functional changes in both the M and P pathways. The measured shifts in contrast gain, irrespective of pathway, could translate to behavioral changes in spatial vision.
Chapter 3 evaluated if age-related physiological changes in cortical channel tuning extend to behavioral measures of contrast sensitivity. Contrast sensitivity functions were measured for 8 younger and 8 older observers following adaptation under three conditions: (1) a blank field, (2) a 1.5 c/deg Gabor, and (3) a 6 c/deg Gabor. Contrast sensitivity was reduced for older observers, but the tuning functions were similar for the two age groups.
Overall, these studies indicate that both optical and neural factors contribute to age-related losses in human spatial vision. Results from Chapters 2 and 3 suggest that neural mechanisms contributing to the decline in spatial vision are generally located at pre-cortical levels. Clear discrepancies were found between younger and older observers in Chapter 2, where pre-cortical spatial vision mechanisms were targeted, whereas no differences were found in Chapter 3 in a task thought to target cortical mechanisms.
0623: Experimental psychology