Tonic accommodation, tonic vergence and surround propinquity.

Previous studies have typically assessed tonic accommodation (TA) and tonic vergence (TV) by placing subjects in total darkness. However, a recent investigation demonstrated that even under such apparently stimulus-free conditions, subjects' awareness of the proximity of the adjacent surroundings and overall room topography (i.e. surround propinquity) may still influence the accommodative response. Accordingly, the aim of the present study was to measure TA and TV while subjects (n = 34) viewed an illuminated distant target (viewing distance = 6 m) to minimize this propinquity effect. TA was measured using an objective, open-field, infrared optometer under two conditions: first, while viewing the distant target monocularly through a 0.5 mm pinhole, and second in total darkness. Additionally, TV was assessed by determining the heterophoria subjectively while subjects viewed the distant target biocularly through 0.5 mm pinholes. The mean level of TA using the pinhole technique (mean = 0.28 D) was significantly lower than that observed in total darkness (mean = 0.60 D). Furthermore, a significant positive correlation (r = 0.86) was found when comparing the difference between the two TA measurements with the level obtained in total darkness. Those subjects having apparently high levels of TA in darkness exhibited the largest propinquity effect. Thus measurements of accommodation recorded in total darkness are likely to have overestimated both the magnitude and normal range of TA. The mean level of TV was 0.58 metre angle. No significant correlation was observed between TV and either of the two TA measurements.(ABSTRACT TRUNCATED AT 250 WORDS)