The effect of age and fixation instability on retinotopic mapping of primary visual cortex.

PURPOSE

Functional magnetic resonance imaging (fMRI) experiments determining the retinotopic structure of visual cortex have commonly been performed on young adults, who are assumed to be able to maintain steady fixation throughout the trial duration. The authors quantified the effects of age and fixation stability on the quality of retinotopic maps of primary visual cortex.

METHODS

With the use of a 3T fMRI scanner, the authors measured cortical activity in six older and six younger normally sighted participants observing an expanding flickering checkerboard stimulus of 30 degrees diameter. The area of flattened primary visual cortex (V1) showing any blood oxygen level-dependent (BOLD) activity to the visual stimulus and the area responding to the central 3.75 degrees of the stimulus (relating to the central ring of our target) were recorded. Fixation stability was measured while participants observed the same stimuli outside the scanner using an infrared gazetracker.

RESULTS

There were no age-related changes in the area of V1. However, the proportion of V1 active to our visual stimulus was lower for the older observers than for the younger observers (overall activity: 89.8% of V1 area for older observers, 98.6% for younger observers; P < 0.05). This effect was more pronounced for the central 3.75 degrees of the target (older subjects, 26.4%; younger subjects, 40.7%; P < 0.02). No significant relationship existed between fixation stability and age or the magnitude of activity in the primary visual cortex.

CONCLUSIONS

Although the cortical area remains unchanged, healthy older persons show less BOLD activity in V1 than do younger persons. Normal variations in fixation stability do not have a significant effect on the accuracy of experiments to determine the retinotopic structure of the visual cortex.