John P Frisby | PubMed Health

PubMed2004

The effects of ageing on stereopsis. A VEP study.

Differences in brain activation between young (n = 11, ages 21-35) and elderly (n = 8, ages 70-84) healthy participants were studied using visual evoked potentials (VEPs) to different kinds of computer generated random-dot patterns. The main stimulus of interest was a 2 x 2 array of rectangles whose rectangles moved to and fro in depth. Control conditions were similar 2 x 2 arrays, one with side-to-side lateral motion (LM) of the rectangles, and one a stationary baseline condition. The third non-stereo stimulus was an expanding field of small dots in radial motion (RM). Significant stereo related activation was found in both age groups. The stereo VEPs showed a longer latency of depth reversal triggered VEP peaks compared to control LM VEPs. The amplitudes of LM VEPs were larger than the baseline VEPs. Age-related differences were found not only in stereo but also in the other conditions. Thus the latency of early VEP peaks was shorter and their amplitude higher in the elderly for LM and baseline conditions, and the younger participants showed significantly higher activation in the later VEP peaks of all conditions. Our main finding is no evidence for age related stereo specific effects in brain activation, but instead more general and subtle changes that affect widely different visual stimulus conditions.

Adult↗

PubMed2002

Accommodation to large disparity stereograms.

PURPOSE: The purpose of this study was to test whether observers would show accommodation changes to depth depicted in large disparity stereograms and whether the amount of accommodation was related to the amount of depth seen in the stereograms. Previous studies have shown small accommodation changes to depth depicted in stereograms. We tested a larger range of depths than previously investigated and also asked observers to judge the depth depicted in the stereogram, because a previous study had suggested that this might be an important factor. METHODS: All our observers had emmetropia. Accommodation was measured with a Canon Autoref R-1autorefractor as an observer viewed a depthful 4-by-4 cm planar target set against a 20-by-20 cm planar background 57 cm from the observer. The target was presented at a range of depths, -8 to +8 cm, either in stereogram stimuli or real stimuli. Each observer also made depth judgments of stereogram targets, after training with real targets. We also recorded the time taken for each observer to discern the depicted depths in stereograms. RESULTS: The accommodation shown by all observers (n = 9) to stereogram targets was significant but was on average about 60% of that predicted. There were individual differences in the accommodation an observer showed to stereogram targets, but this was not related to the amount of depth the observer saw in the stereogram. CONCLUSIONS: Accommodation changes did occur to stereogram targets, but these were generally less than predicted and also less than those shown for real targets. Possible reasons for these findings and the implications for the use of stereograms in clinical tests are discussed.

Accommodation, Ocular↗