In the past, studies of similarity between eye movements during mental imagery and perception have considered mainly spatial aspects. For example, Spivey and Geng (
2001) and Johansson, Holsanova, and Holmqvist (
2006) examined saccade directions to see if they corresponded to directions expressed in voice recordings. Other authors compared the proportion of fixations spent in predefined areas of interest (AOI) during perception and mental imagery (Laeng & Teodorescu,
2002; Martarelli & Mast,
2013; Richardson & Spivey,
2000). Brandt and Stark (
1997) split their screen into a finer grained grid than, for example, Laeng and Teodorescu (
2002) and compared fixation sequences in these grids. All these methods analyze the spatial dimension of fixations or saccades, but do not describe eye movements during mental imagery in terms of their temporal regularities and dependencies. Methods addressing the temporal patterns of eye movements, however, can reveal the way mental images evolve over time, how they are generated and maintained in the first place (Kosslyn,
1994). For example, mental images could be constructed serially, where one piece at a time is generated from memory. Such a serial construction would be reflected in a gaze that remains at the same location for several consecutive fixations before moving on to the next area to be constructed. Refixations would thus happen soon or immediately after the initial fixation of an area. Alternatively, mental images could be constructed in a more holistic fashion: Several parts of the mental image could be quickly generated from memory and, once in place, they would only need to be maintained. Such maintenance would be reflected in alternating eye movements between different areas. Thus, in a holistic construction process, we expect more time to pass between refixations of the same area than in a serial construction, where refixations happen
without intermediate visits to other areas. The serial and the holistic process can nevertheless lead to the same overall distribution of fixations over the screen: In the serial process, the areas are visited one after the other while in the holistic process the gaze alternates between these areas. Therefore, spatial information does not discriminate between the two processes and they can only be distinguished by considering the temporal patterns of eye movements. It is known that mental images fade over time (De Beni, Pazzaglia, & Gardini,
2007; Farah,
1989; Kosslyn,
1994), and therefore it is more plausible that eye fixations jump back and forth between parts of the image to counteract the fading of the mental image. Although the serial and holistic construction are not the only conceivable ways a mental image could be generated and maintained, they illustrate that the analysis of temporal characteristics can add to the understanding of the mental imagery process.