Posts Tagged ‘eye movements’

Eye Movements as an Objective Measurement of Mind Wandering

26/03/2012 3 comments

We spent a lot of time mind wandering. Cognitive neuroscience has recently started investigating this phenomenon. However, the subjective nature of mind wandering makes capturing and measuring it exceptionally difficult. As a result, there is still no way to objectively measure mind wandering. In the majority of published studies researchers ask participants at random intervals how focused they are on a given task. Uzzaman and Joordens in a recently published paper explored the use of eye movements as an objective measure of mind wandering while participants performed a reading task.

Eye movements are thought to reflect (to some degree) cognitive processes (for a brief overview of eye movement research, see the Scholarpedia entry). Uzzaman et al. study was based on an earlier paper by Reichle, Reineberg, and Schooler (2010) who suggested that eye movements may provide an objective measure of mind wandering. Reichle et al. investigated this hypothesis by comparing the fixation-duration during mind wandering and normal reading episodes. The results were very encouraging and suggested that the participants’ eye movements became progressively decoupled from the ongoing task (i.e., text processing) during mind wandering episodes.

Uzzaman et al. used a reading task coupled with a self-classified probe-caught mind wandering paradigm to obtain a subjective account of mind wandering episodes. They recruited 30 participants who were explicitly informed of the definition of mind wandering episodes prior to the start of the experiment and were instructed that they would be asked to report their mind state at random intervals. The authors defined explicitly mind wandering “as reading without text comprehension, or thinking about anything other than the text on hand”. They also provided several examples to make sure the participants fully understood the concept.

The participants read sixteen pages of “War and Peace” by Tolstoy on a computer screen while their eye movements were tracked and recorded. Randomly every 2–3 min, a probe would appear on top of the text asking what was the mind state of the participants at this specific point. Participants would have to answer to continue the experiment. On average participants received 10 probes in total, in which mind wandering was reported on 49% of them.

The eye movement behaviours of the participants were categorised into mind wandering or reading conditions, based on their self-reports. This analysis was conducted for the 5 s time interval preceding the probe for reading and wandering conditions within each participant. Nine pairs of eye movement variables were analysed (e.g., count of blinks, fixations, saccades, fixation duration, within-word regression count), which displayed different degrees of sensitivity to mind wandering.

Statistical differences were found in two of the eye movement variables, run count and  within-word regression count. Run count was defined as the “the total number of runs, where a run is two consecutive fixations within the same interest-area” and  within-word regression count as “the sum of all fixation durations from when the word was first fixated upon, till the last fixation”.  

Specifically, there were fewer within-word regressions for periods before mind wandering episodes compared to periods before reading reports (z = −2.305, p = 0.021). Also, the total run count was also lower during mind wandering episodes (z = −1.997, p = 0.046). In addition, fixation count, saccade count and total number of saccades within the interest-area were lower during mind wandering reports, although these variables fell slightly short of the conventional significance criterion (all z < −1.755,p > 0.079).

During comprehensive reading all the words were being cognitively processed deeply and effort was put forth. On the contrary, a different pattern was observed during mind wandering episodes, as it was suggested by the lower number and duration of within-word regressions that shows that the  text was not being processed deeply, and as a result limited lexical information was being extracted. As a result, reading became less effortful and more automatic.

The current study revealed a correlation between subjective reports of mind wandering, and objective ocular behaviour. These findings could be further exploited in future studies and lead to the development of algorithms that would mathematically predict the likelihood of mind wandering based on eye movements. Such a development might provide valuable insights into the neural correlates of mind wandering.
ResearchBlogging.orgUzzaman, S., & Joordens, S. (2011). The eyes know what you are thinking: Eye movements as an objective measure of mind wandering Consciousness and Cognition, 20 (4), 1882-1886 DOI: 10.1016/j.concog.2011.09.010

Reichle ED, Reineberg AE, & Schooler JW (2010). Eye movements during mindless reading. Psychological science, 21 (9), 1300-10 PMID: 20679524

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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