Dynamic components of horizontal and vertical saccades during visual search tasks

Dynamic components of saccadic eye movements were investigated following visual search in a multi-saccade paradigm. Previous research by Morgan (1999) identified faster, longer and more frequent saccades were indicative of successful performance in normal participants and that there was no perceptual disadvantage to performing saccades during the task as one might expect due to saccadic suppression. Additionally, elite players from visually demanding sports such as football, netball and soccer generally produced faster, longer and more frequent saccades than non-elite players from the same sports as well as elite athletes from non-visually demanding sports such as swimming and cycling. A group of experiments were conducted which replicated and extended Morgan’s findings and addressed a number of key methodological issues. A non-sport specific visual search strategy task (VSST) was employed which involved looking for target letters amongst distracters on a display board subtending 56° of visual angle at six limited exposure times. Saccadic eye movements were recorded by electrooculography (EOG) during the VSST. A conservative saccade detection algorithm was employed involving velocity, amplitude and duration thresholds. The limitations of this algorithm identified that small underestimates of saccade amplitude, duration and rate were apparent. The experiments were conducted in a dimly lit environment that produced a high degree of contrast between the target display luminance and the ambient illuminance level. This light level was chosen because it produced the least variation in signal amplitude over the recording period and specifically addressed concerns that amplitude variation is often greatest when participants are pre-adapted to a bright environment and are then recorded in a dark environment. Thirty-three normal participants performed the VSST in both the horizontal and vertical domain. Horizontal and vertical saccades were predominantly calibrated using a linear regression trendline however, some calibrations were improved using non-linear trendlines. Eyelid artefact was removed objectively by differentiating it from saccades using a novel method involving the low velocity periods of eye movements. From the data collected, horizontal VSST results were similar to Morgan (1999) in some variables and dissimilar in others. Higher saccade rate and greater combined amplitude and velocity were characteristics associated with successful responses at 650, 800 and 1000 ms exposure times. Saccade latency was only significant at 1000 ms exposure time with correct trials having a shorter latency as expected. The saccadic search behaviour for correct and incorrect responses during vertical VSST trials was completely unrelated to horizontal VSST trials at all exposure times. The only observable trend was that correct trials had a higher saccade rate than incorrect trials, but this was only significant to the 0.05 level for the 350 and 800 ms exposure times. One possible explanation for why the same trends were not observed may be that humans scan horizontally more than vertically during everyday tasks such as reading and this trained ability has lead to a more efficient visual search strategy. The same VSST was also performed by seven elite Australian Rules footballers and eleven non-elite footballers. Elite footballers did not exhibit any differences in dynamic components of horizontal saccades from non-elite footballers, challenging Morgan’s (1999) prior research. Remarkably, elite footballers generated smaller and slower vertical saccades at almost all stimulus exposure times. There were two major findings relating to the acquisition of visual information during visual search tasks. Firstly, at no point in time did the generation of saccades display any perceptual disadvantage during any VSST. Secondly, elite and non-elite footballers surprisingly did not reveal any horizontal search strategy differences. However, elite footballers made smaller and slower saccades on the more demanding vertical VSST, but this was not at any perceptual disadvantage.