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Effects of Individual Differences and Culture on Eye Movements

Can fixation durations and saccade lengths in one task predict eye movements in other tasks for a given viewer? This was one of the questions posed by a Rayner and colleagues in an interesting 2007 study.

Surprisingly, only a few studies to date have addressed this issue. The first one was by Andrews and Coppola (1999), who recorded eye movements of 15 viewers while they were performing five different tasks.: (1) natural occurring eye movements in darkness, (2) viewing simple textured patterns, (3) scene perception, (4) visual search, and (5) reading. The authors concluded that apart from the visual environment, idiosyncracies also have a significant effect on eye movements.

In addition to examining the stability of eye movements across different tasks, Rayner et al also examined the possibe cultural differences influence on eye movements across the different tasks. Previous studies have provided some preliminary evidence of cultural differences. More specifically, Chua, Boland, and Nisbett (2005) examined the eye movement patterns of Chinese and native English speaking participants when looking at scenes. Cua et al found that Chinese were more likely to look at the background information in a scene, while the Americans looked at the foreground objects longer and sooner than the Chinese.

Rayner et al recruited seventy-four participants, which were divided into three different groups based on their knowledge and fluency of English and Chinese. The first group, the American group, consisted of native English speakers, the second , the native Chinese group consisted of 3 native speaking Chinese participants (who learnt to speak english later in their life), and finally, the Bilingual group consisted of Chinese participants who were either born in the USA or moved there before the age of 5. The right eye movements of each participant were tracked. All participants performed the following six tasks: (1) English reading, (2) face processing, (3) scene perception, (4) visual search, (5) Chinese character count, and (6) Chinese character search. In the English reading task, 40 English sentences were read . In the face processing and scene perception tasks, participants were shown 16 pictures of female faces and 24 pictures of scenes and asked to remember them for a later memory test. In the visual search task, participants were asked to find a brown square that was part of an array of brown circles and pink squares. In the Chinese character count task, participants counted the occurrences of a Chinese character in a paragraph of Chinese text. Finally, the participants who could read Chinese read 36 Chinese sentences.

Fixation duration and saccade size were used as primary indices of temporal and spatial processing in the tasks.

It is generally assumed (see Rayner, 1998) that (1) fixation duration reflects the time needed to process the informa-tion around fixation and the time needed to plan the next saccade and (2) saccade size is related to how much information can be processed on a fixation and how the next saccade target is selected.

The results of the present study suggest that fixation durations for a given individual tend to be fairly stable across different tasks. For the American and Bilingual groups, fixation durations in English reading did not correlate especially well with fixation durations in the other tasks, while for the Chinese group fixation durations in English reading did tend to correlate highly with the other tasks. Fixation durations in scene perception and face processing were highly correlated across all three participant groups. Saccade length did not correlate as well across tasks as did fixation duration. The correlations, however, tended to be positive when the reading tasks were eliminated from the analysis.

No correlation was found between fixation duration and saccade length across all of the tasks. According to Rayner et al this could be evidence that the mechanism responsible for determining when to move the eyes is largely independent of the mechanism responsible for determining where to move the eyes next.

Reading visual tasks are in general more understood than other visual tasks. The correlations in fixation durations for these different tasks does, however, suggest that there might be common aspects of processing between them. The nature of these mechanisms is not very clear. Rayner et al. suggest that:

Perhaps in non-reading tasks, some kind of timing mechanism determines when the eyes move; such a common timing mechanism would be expected to lead to correlations across tasks, particularly in fixation durations (and to a lesser extend, saccade length). Another possibility is that something like visual saliency.

Another interesting finding of this study was that the more experience participants had with either English or Chinese, the shorter the fixations and the longer the saccades they made. This effect did not depend on cultural characteristics of the participants. As far as the effect of culture on eye movements is concerned, only a few differences were identified. Chinese participants had systematically shorter fixations in the scene perception, face processing, and Chinese count tasks than the Americans. Their performance was similar on the two search tasks. The difference in the duration of fixation observed between Chinese and American participants is due to the Chinese trading off number of fixations with fixation duration. In other words, while they made somewhat shorter fixations, they also made slightly more fixations.

ResearchBlogging.orgRayner K, Li X, Williams CC, Cave KR, & Well AD (2007). Eye movements during information processing tasks: individual differences and cultural effects. Vision research, 47 (21), 2714-26 PMID: 17614113

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