![]() Second, the gaze location in the visual world is actually contingent not only on the eye orientation with respect to the head but also on the head orientation with respect to the visual world. The image of the pupil and/or of the pupil corneal reflection (normally the first Purkinje image) is then used to calculate the orientation of the eye in its orbit. First, to correctly measure the direction of the foveal vision, an infrared illuminator (normally with the wavelength around 780-880 nm) is normally laid along or off the optical axis of the camera, making the image of the pupil distinguishably brighter or darker than the surrounding iris. To understand how a contemporary commercial eye-tracking system works, the following points should be considered. ![]() Thus, a contemporary commercial eye tracking system normally uses optical sensors measuring the orientation of the eye in its orbit 7, 8, 9. However, the sampling rate of such an eye-tracker is relatively low, and the coding procedure is time consuming. Eye movements are then manually coded through frame-by-frame examination of the video recording. The simplest, least expensive, and most portable system is just a normal video camera, which records an image of the participant's eyes. In a comprehension study using the visual world paradigm, participants' eye movements on the visual display are monitored as they listen to the spoken utterances talking about the visual display.ĭifferent eye tracking systems have been designed in history. This methodological article will focus on comprehension studies. The visual world paradigm can be used to study both the spoken language comprehension 1, 2 and production 5, 6. ![]() When and where saccadic eye movements are launched in relation to the speech signal are then used to deduce the online language processing. With this paradigm, researchers intend to determine at what temporal point, with respect to some acoustic landmark in the speech signal, a shift in the participant's visual attention occurs, as measured by a saccadic eye movement to an object or a picture in the visual world. This attention shift will have a high probability to initiate a saccadic eye movement to bring the attended area into the foveal vision. The basic logic, or the linking hypothesis, behind this paradigm is that comprehending or planning an utterance will (overtly or covertly) shift participants' visual attention to a certain object in the visual world. In a typical eye tracking study using the visual world paradigm, participants' eye movements to pictures in a display or to real objects in a visual workspace are monitored as they listen to, or produce, spoken language depicting the contents of the visual world 1, 2, 3, 4. Eye movements recorded in the visual world paradigm can be used to overcome this challenge. It is a challenge to experimentally study this temporal, rapidly change speech signal. Spoken language is a fast, ongoing information flow, which disappears right away. The protocol described in this article illustrates how a typical visual world eye tracking study is conducted, with an example showing how the online processing of some semantically complex statements can be explored with the visual world paradigm. More importantly, the paradigm is extremely sensitive to fine grained manipulations of the speech signal, and it can be used to study the online processing of most topics in language comprehension at multiple levels, such as the fine grained acoustic phonetic features, the properties of words, and the linguistic structures. This paradigm has high versatility, as it can be used in a wide range of populations, including those who cannot read and/or who cannot overtly give their behavioral responses, such as preliterate children, elderly adults, and patients. In a typical eye tracking study using the visual world paradigm, participants' eye movements to objects or pictures in the visual workspace are recorded via an eye tracker as the participant produces or comprehends a spoken language describing the concurrent visual world.
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