Some dyslexics show increases in reading speed and reading accuracy when they view text through coloured lenses or filters. The use of colour appears to reduce a collection of distinctive visual symptoms known as Meares-Irlen syndrome (MIS). We have reported previously that in gap saccade tasks, saccade latency is modified differently in dyslexic subjects with MIS compared to non-dyslexic control subjects. Latency in non-gap tasks was also altered when MIS subjects viewed stimuli through their precision coloured tint. This effect was significantly different from changes observed in a luminance control experiment (Northway et al., 1998, Perception, 27: 144). We have investigated further whether the type of colour filters used in the treatment of MIS or neutral density (ND) filters alter saccade latency or the gap effect on saccade latency in normal subjects.

Six non-dyslexic subjects with normal visual acuity aged between 19 and 30 were tested in a standard saccade task. Subjects' heads were stabilised and they viewed a visual display from 57 cm with their left eye. The right eye was occluded. Subjects were presented with a fixation target for a random period (0.5 - 1.5 s) after which a saccade target appeared at an eccentricity of 5 deg; targets consisted of dark 0.3 deg squares presented on a light background. In sets of four tasks presented in runs of 96 trials, normal (gap = 0 ms) and gap (gap = 200 ms) trials to the left and right were presented pseudorandomly. Left eye position was recorded using infrared oculography, digitised and stored on disc for analysis. Subjects viewed the display under 3 conditions: normal viewing, viewing through coloured lenses, and viewing through neutral density filters chosen to reduce the luminance of the background and target by the same amount as the coloured lenses. Saccade latencies were measured off-line from velocity traces. Saccades with latencies of less than 70 ms were considered to be anticipatory and not included in the analysis.

For both saccades to the left and right, there were small but statistically significant reductions in saccade latency in the colour( e.g. left mean latency 184 ±32 ms) and neutral density (185 ±38 ms) conditions compared to the normal viewing condition (196 ±39 ms). There was no significant difference between the colour and neutral density trials. In all three conditions saccade latency was reduced in gap compared to non-gap conditions (e.g. reductions for saccades to the left in colour condition: 27 ms; ND: 32 ms; normal: 28 ms). Distributions in all three conditions were unimodal; latency ranges were: colour: 74 - 349 ms; ND: 72 - 350 ms; normal: 96 - 351 ms.

Thus in normal subjects colour and neutral density filters did not appear to modify the gap effect in any clear and consistent manner but both produced small reductions in latency compared to normal viewing. This might suggest that target luminance and not hue might be the key to understanding the benefit of coloured filters and lenses. Further careful analysis of saccade performance in MIS subjects is now required to define further the benefits they receive from the changes induced in hue and luminance by coloured filters and lenses.