If I fits I sits: A citizen science investigation into illusory contour susceptibility in domestic cats (Felis silvestris catus)
Introduction
In 2017, cat fans took to Twitter to document their cats’ attraction to tight spaces by taping complete shape outlines on their floors and observing their cats sit inside, spurring over eighty-two-thousand retweets and trending hashtag #CatSquare (Fig. 1). Affectionately termed “if I fits I sits,” the urge to inhabit enclosed spaces is well-known to cat owners and has been documented to decrease stress in laboratory cats (Carlstead et al., 1993) and shelter cats given boxes in which to hide (Hawkins, 2005; Kry and Casey, 2007; Vinke et al., 2014). In fact, cats deprived of shelter resources like boxes will attempt to manufacture their own by hiding behind or underneath box-like objects like litter pans (Gourkow and Fraser, 2006). The reason for this behavior is still unknown but is clearly highly desirable.
Regardless of the reason for their attraction to enclosed spaces, this behavioral phenomenon proves to be an excellent tool to study the visual perception of shapes and contours in domestic cats. Neurological study of this phenomenon began in cats and found that these nonhuman animals’ (hereafter animals) retinal receptive fields are sensitive to contours along a luminance gradient (Redies et al., 1986). Contour comprehension is theorized to be evolutionarily critical in the understanding of physical objects and boundary interpolation (Kellman, 2003), and the study of visuo-cognitive phenomena such as illusion susceptibility offers a fascinating perspective into the effects of environmental pressures and life experience on vision (Kelley and Kelley, 2014).
The ability to perceive visual illusions is remarkable. One’s susceptibility to an illusion derives from the visual system processing an image’s features as the most likely physical reality based on learnt probabilities, a top-down incorporation of one’s preconceptions and past experience—even if it is not a true representation of reality (Gregory, 1997; Haber and Hershenson, 1973). In the real world, this processing is usually veridical or close to veridical (Palmer, 1999). Graphically rendered illusions are special cases where the same processing will result in a perception that deviates considerably from what is physically real.
The type of visual illusion considered here is subjective illusory contours, in which one mentally perceives fictitious contours connecting a shape’s inducers (modal completion) due to luminance contrast (Kanizsa, 1955). Study of the effects of age on susceptibility to the Kanizsa contour illusion in humans finds that illusory-contour perception may develop around 3–4 months and strengthens with age (Otsuka et al., 2004). Susceptibility to illusory contours has also been studied in a wide range of animal species in almost exclusively train-and-transfer testing paradigms comprising dogs (Byosiere et al., 2017), chimpanzees (Fagot and Tomonaga, 2001), bamboo sharks (Fuss et al., 2014), honeybees (Horridge et al., 1992), mice (Kanizsa et al., 1993), barn owls (Nieder and Wagner, 1999), redtail splitfin fish (Sovrano and Bisazza, 2009), and goldfish (Wyzisk and Neumeyer, 2007) (for a full reviews, see Byosiere et al., 2020; Feng et al., 2017; Kelley and Kelley, 2014; Nieder, 2002).
Previous research reveals that cats are, indeed, susceptible to certain visual illusions. De Weerd et al. (1990) found that domestic cats could discriminate illusory contour orientation via contour-inducing semicircles. In 2019, Szenczi et al. revealed that cats are susceptible to the size distorting Delboeuf illusion. Further, two studies found that both lions (Panthera leo) (Regaiolli et al., 2019) and domestic cats (Bååth et al., 2014) are susceptible to the Rotating Snake illusion, comprising a “moving” image caused by peripheral drift eliciting hunting-related behavior.
Perhaps most relevant, a study by Bravo et al. (1988) examined domestic cats’ susceptibility to subjective contours via operant response to the Kanizsa square illusion. Two young, female cats were trained to indicate where they viewed a subjective contour on an array of sectored disks in various orientations. The researchers controlled for other potential cues like luminance, temporal changes, and local patterns by introducing and modifying variables like motion and duration of stimuli exposure. They found that the cats demonstrated susceptibility to the Kanizsa illusion, indicating that cats likely perceive subjective contours as humans do (see Table 1 for a summary for illusion studies in cat species).
The present study supplements the results of Bravo et al.’s (1988) experiment with the addition of an increased sample size and a more inclusive sex and age range, in pet, rather than laboratory, cats. Moreover, rather than using standard operant conditioning procedures, the current study utilizes a more ecologically valid, real-world setting in which to evaluate spontaneous behavior. As cats transferred to novel environments can exhibit stress-related behaviors and thus not behave naturally (Amat et al., 2015), this study also offers an at-home environment to explore domestic cats’ susceptibility to Kanizsa square contours in a natural setting. Specifically, we evaluate whether cats will sit or stand within the contours of an illusory Kanizsa square more often than a control stimulus in a spontaneous choice task. Importantly, to date, cats’ attraction to enclosed spaces has been limited to 3D spaces (Carlstead et al., 1993; Gourkow and Fraser, 2006; Hawkins, 2005; Kry and Casey, 2007; Vinke et al., 2014), and thus this study also aims to formally examine the extension of this behavior to 2D shapes (such as that which was seen in the #CatSquare challenge). This study was conducted entirely remotely through citizen science engagement during the COVID-19 pandemic, and to the best of our knowledge, is the first published citizen science experiment to examine cat cognition.
Section snippets
Rationale and approval
The Kanizsa square illusion, rather than the classic Kanizsa triangle (Kanizsa, 1955, 1974), was chosen for consistency with the Bravo et al. (1988) study. In order to avoid priming subjects to the experimental square stimuli (for effects of exposure to visual tasks on visual perception in cats, see Hua et al., 2010; Sasaki et al., 2010), as well as the evidence for cats’ high motivation to sit in enclosed spaces (Gourkow and Fraser, 2006; Hawkins, 2005; Kry and Casey, 2007; Vinke et al., 2014
Descriptive data
The preliminary survey received 561 enrollments. Over the course of the experiment, 121 cat-owner pairs completed Trial 1 (∼22 % of total); 53 completed Trial 2 (∼9% of total); 43 completed Trial 3 (∼8% of total); 38 completed Trial 4 (∼7% of total); 34 completed Trial 5 (∼6% of total); and 30 completed Trial 6 (5% of total). Of the 30 that completed the experiment, the nine subjects that made at least one stimulus selection came to 16 total stimulus selections: the square was chosen on eight
Discussion
The cats in this study stood or sat in the Kanizsa and square stimuli more often than the Kanizsa control, revealing susceptibility to illusory contours and supporting our hypothesis that cats treat an illusory square as they do a real square. These findings confirm preexisting research of cats’ susceptibility to illusory contours (De Weerd et al., 1990), and to the Kanizsa square illusion specifically (Bravo et al., 1988). To the best of our knowledge, this study is the first of its kind in
Data statement
All data and materials are accessible within this manuscript. All data and materials will also be made available by contacting the corresponding author.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
We wish to extend our gratitude to the cats, the citizen scientists, and the reviewers/editors for their instrumental feedback in helping to improve the manuscript.
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