Low-level features in mental images

Martarelli, Corinna S.; Mast, Fred W. (10 June 2015). Low-level features in mental images (Unpublished). In: 21st annual Toward a Science of Consciousness (TCS) conference. Helsinki, Finland. 09.06.-13.06.2015.

Visual perception includes bottom-up and top-down processes whereas mental imagery does not require a sensory input but still draws on some of the same mechanisms that are also involved in the process of visual perception. If mental images have picture-like qualities, they are supposed to contain low-level features. It is known that salient low-level features drive the first saccades when people inspect a new picture (e.g., Parkhurst, Law, & Niebur, 2002). We investigate whether low-level properties of mental images elicit similar eye movements. Twenty-five participants performed a visual imagery task with high vs. low spatial frequency and high vs. low contrast gratings. The procedure was adapted from Kosslyn, Sukel, and Bly (1999). The perceptual encoding phase was composed of forty-eight arrays of gratings presented for 6 seconds. Each array contained four gratings (two contrast levels and two spatial frequencies). After the presentation of each array, participants had to imagine the gratings they just saw (image generation) and then answer to a specific question (image inspection). We determined visual saliency of the gratings using the algorithm of Harel, Koch, and Perona (2006). The Graph-Based Visual Saliency map suggests that the low spatial frequency – high contrast grating contains the most salient information. We found that both during visual perception and during mental imagery the first fixation was more often allocated to the low spatial frequency – high contrast grating, thus showing that eye movements are influenced not only by physical properties of visual stimuli but also by its imagined counterpart. Inclusion of the first three fixations instead of only including the first fixation revealed similar results during perception and imagery (no change in rejecting or accepting the null hypothesis). The last fixations during the preceding perception condition were not predictive for the first fixations during subsequent image generation. The results from this study suggest that low-level features elicited specific eye movements not only during the process of perception but also during mental imagery. The findings support the account of imagery as perception-like; pictorial low-level features are preserved in mental imagery.

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