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.;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.;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.