Further insights into letter crowding: the role of contour interaction, contrast and gaze fixations

Visual acuity is reduced when optotypes are viewed in the presence of surrounding contours. This reduction in acuity is known as the crowding effect and is thought to be caused by a varying combination of contour interaction, gaze instability and attention. Traditional studies have used single optotypes surrounded by flanking bars to investigate crowding. Such targets may not realistically replicate the crowding effect inherent in clinical vision charts. The aim of this thesis was to systematically investigate the effect of crowding on visual thresholds in subjects with normal vision and in subjects with amblyopia, using specially designed charts. In the 1st and 2nd experiment, contour interaction was assessed using a high (80 %) and low contrast (5.8%) Sheridan Gardiner repeat letter (SGRL) chart in subjects with normal vision. The effect of contour interaction was investigated by varying the inter-letter separation in the SGRL chart. Significant contour interaction was obtained at the abutting condition for both the contrast conditions. In the 3rd experiment the same protocol was repeated but in amblyopes. Significant contour interaction was obtained at 0.2 letter separation and the abutting condition for both the contrast conditions. The effect of contour interaction appears to be less for low contrast than for high contrast letters in normal, non-amblyopic and amblyopic eyes. Finally, in the 4th experiment a Sheridan Gardiner Complex Interaction (SGCI) chart that requires imposed gaze fixations was constructed to measure visual acuity in normal’s and amblyopes. The effect of any gaze instability on crowding was investigated by comparing SGRL thresholds to SGCI thresholds. The SGCI thresholds were higher than the SGRL thresholds at all the separations measured, suggesting an important effect of gaze instability on crowding. In conclusion, this research has shown that gaze instability is an important component of the crowding effect for letter chart acuity measurements. Visual acuity especially when screening for amblyopia should be measured using a whole optotype chart that requires optotype to optotype fixation.