A Single Sound Can Convince Your Brain That Touching Objects Never Made Contact

In a deceptively simple laboratory demonstration, two identical squares glide smoothly toward each other on a screen, their leading edges meet, and then both vanish. Asked what they saw, most observers insist the squares never actually touched before disappearing. This phenomenon, known as the non-contact illusion, reveals something profound about how the brain reconstructs visual events after the fact. Now new research shows that a brief, entirely task-irrelevant sound delivered at the exact moment the squares touch can dramatically strengthen this illusion, persuading even more viewers that the objects remained spatially separate when they were, in fact, in contact. The finding, published in Attention, Perception, & Psychophysics, establishes a novel cross-modal perceptual effect and offers a fresh window into the architecture of multisensory processing in the human brain.

The study was inspired by an earlier hypothesis in the field sometimes called sound-induced visual segmentation. When two two-dimensional objects approach one another and reach their point of closest approach, a coincident sound appears to lead observers to overestimate the distance between them, as though the auditory event slices the visual scene into distinct objects. The research team, led by Wenxuan Song, Xiaoying Li, Yuan Guo, and Song Zhao of Soochow University in China, reasoned that if a sound can segment approaching objects in this way, it might also amplify the non-contact illusion that arises when two moving squares disappear precisely at the instant their edges meet. Across two experiments, they tested whether a sound presented at contact would increase the probability that observers report the squares as having never touched.

The results were clear and consistent. In both Experiment 1 and Experiment 2, participants were significantly more likely to give a non-contact response when a sound accompanied the moment of visual contact than when the squares collided in silence. This sound-induced increase in the illusion occurred even though the sound carried no information about the visual stimulus and participants were explicitly instructed to ignore it. The effect is therefore a genuine cross-modal influence: an auditory signal, meaningless on its own, reshapes the conscious perception of a visual event in a systematic and reproducible way.

What makes the finding particularly striking is that the effect runs in the opposite direction from what a simple collision interpretation would predict. The classic stream-bounce literature, dating back to Sekuler and colleagues’ 1997 demonstration that a sound can make two crossing discs appear to bounce off one another, has often been interpreted through the lens of causal inference: the sound suggests a collision, so the brain infers a collision. But in the new study, the sound produced more non-contact judgments, not more contact judgments. If the sound had merely signaled that two objects struck one another, it should have biased observers toward reporting contact. Instead, it deepened the impression that the objects remained apart, ruling out this response-bias account.

The researchers also tested and rejected an attentional explanation. One possibility is that the abrupt sound simply distracts observers, impairing their ability to encode the visual event and thereby pushing them toward a default or guess response. If that were true, non-contact responses in the sound-present condition should have been slower than contact responses, reflecting a disruption of processing. The reaction-time data told a different story: the non-contact response was never slower than the contact response when the sound was present. In fact, supplementary analyses showed that reaction times for non-contact responses were significantly shorter in the sound-present condition than in the sound-absent condition, while contact responses did not differ between sound conditions. This pattern contradicts any account based on attentional distraction or slowed processing.

To probe the neural mechanisms underlying the effect, the team recorded high time-resolution event-related potentials, or ERPs, during Experiment 2. The critical comparison focused on an early cross-modal component known as PD170, a positivity peaking between roughly 125 and 175 milliseconds after sound onset that has previously been associated with low-level audiovisual interactions in early sensory cortex. The results showed that the PD170 was significantly larger on sound-present trials in which participants ultimately reported the non-contact percept than on sound-present trials in which they reported contact. This dissociation, emerging within the first fifth of a second after the sound, indicates that the sound-induced enhancement of the illusion is rooted in early, low-level cross-modal interactions rather than in later cognitive evaluation or decision processes.

The PD170 finding carries substantial theoretical weight. Early cross-modal components of this kind have been linked to interactions between auditory and visual cortex that occur automatically, before attention and higher-order cognition can shape the percept. By tying the strength of the behavioral illusion to the amplitude of this early component, the study substantiates the perceptual nature of the effect: the sound does not merely change what people say about the display but genuinely alters what they see. This aligns the new phenomenon with a family of well-documented sound-induced visual illusions, including the sound-induced flash illusion, in which a single flash accompanied by two beeps is perceived as two flashes, and the audiovisual bounce-inducing effect, in which a sound at the crossing point of two moving discs promotes a bouncing percept.

At the same time, the non-contact illusion and its sound-induced enhancement differ in an important way from the stream-bounce paradigm. The non-contact illusion does not require a sound to emerge; it arises purely from the visual statistics of the display, specifically the sudden disappearance of two squares at the moment their edges make contact. The sound merely strengthens an already-existing visual illusion rather than resolving an ambiguous motion event. This makes the paradigm a particularly clean tool for studying how auditory signals modulate visual spatial perception, because baseline and sound-modulated conditions can be compared within the same unambiguous geometric event. The authors note, following recent work by Zeljko and Grove, that there is no inherent bias toward any particular percept in the sound-absent condition unless it is intermixed with sound-present trials, further underscoring the importance of careful experimental design in this domain.

The methodological rigor of the study strengthens its conclusions. The researchers used mixed-effects logistic modeling with maximal random-effects structures, in line with contemporary best practices for categorical data analysis, and complemented frequentist tests with Bayesian analyses to evaluate null results. All trial-level data, subject-level data, analysis scripts, and experimental files for both experiments are openly available on the Open Science Framework, allowing independent verification and reuse. The study was approved by the Institutional Review Board of Soochow University, informed consent was obtained from all participants, and the work was supported by the National Natural Science Foundation of China and an undergraduate innovation training program at Soochow University.

Looking forward, the authors suggest that the sound-induced enhancement of the non-contact illusion constitutes a promising new paradigm for investigating multisensory processing. Because the effect is behaviorally robust, mechanistically traceable to an identifiable early ERP component, and free from the confounds that complicate stream-bounce designs, it offers researchers a versatile instrument for dissecting when, where, and how the brain integrates sound and sight. Some limitations remain: the design’s contact-duration constraints restricted reaction-time analyses to a single duration condition, and ERP requirements limited the number of participants meeting trial-count criteria in some conditions, pointing toward richer future experiments. Even so, the demonstration that a meaningless beep can tip the brain’s reconstruction of a visual collision toward the conviction that no contact ever occurred is a vivid reminder that perception is not a passive recording of the world. It is an active, multisensory construction, one that sound can quietly rewrite within a fraction of a second.

Subject of Research: Sound-induced enhancement of the non-contact illusion, a cross-modal audiovisual perceptual effect studied with behavioral and ERP measures.

Article Title: A novel cross-modal perceptual effect: Sound-induced increase of the non-contact illusion

Article References: Song, W., Li, X., Guo, Y., & Zhao, S. (2026). A novel cross-modal perceptual effect: Sound-induced increase of the non-contact illusion. Attention, Perception, & Psychophysics, 88(7), Article 183. https://doi.org/10.3758/s13414-026-03332-6

Image Credits: AI Generated

DOI: 10.3758/s13414-026-03332-6

Keywords: non-contact illusion, cross-modal interaction, audiovisual perception, multisensory integration, event-related potential, PD170, sound-induced illusion, visual motion perception, stream-bounce effect, perceptual psychology, ERP, attention perception psychophysics