Surface Texture Influences Environmental Preference and Locomotion in Behaving Rats

Abstract

In rodents, the whiskers and paws are essential for somatosensory perception, yet their preference for specific textures remains unclear. Here, we examined the preference for rough versus smooth surfaces in rats using a custom open-field paradigm with a water-based motivator to ensure consistent and unbiased movement. We found that rats strongly preferred rough textures with longer interactions and increased dwell time. Notably, this preference persisted in darkness, confirming the tactile, but not visual, influence. These findings highlight the primacy of tactile input in behavioral choices and provide key insights into optimizing rodent environments.

INTRODUCTION

Tactile sensation is critical to how animals perceive and interact with their environments, shaping a wide range of behaviors such as exploration, foraging, and habitat selection. In rodents, tactile inputs—mediated primarily but not exclusively through whiskers and paws—play an essential role in both behavior and affective states. However, among tactile inputs, the differences in texture and their effects on animal behavior have rarely been investigated.

Much research to date has focused on tactile preference using head-fixed behavioral discrimination tasks.^1,2) These tasks often employ go/no-go binary paradigms, requiring active decision-making. However, this setup can constrain decision-making and provide limited insight into tactile preferences in more naturalistic settings, as selecting the correct answer takes precedence over personal preference. Additionally, it is widely believed that selective attention—necessary for accurately assessing preference—is more active when animals are engaging in free-moving tasks.³⁾ Conversely, getting rats to actively explore an open field without external motivators can be challenging, as they often remain inactive or choose to sleep—a behavioral pattern observed in our preliminary study. Moreover, rats exhibit behavioral variability in the standard open field, where the floor texture is uniform, influenced by a variety of factors such as the time of day, individual sensitivity to stress, previous housing environments, and hormone levels.^4–6)

To address these issues and assess whether rats prefer rough or smooth surfaces, we developed and conducted experiments on a novel open field paradigm. Using water as a motivator in water-deprived rats, we equalized their movement across the open field divided into equally smooth and rough areas, assuming the absence of preference. The water reward was automatically dispensed in such a way as to maximize movement, as adjacent water ports were excluded from being used next, enabling ports further away to be selected. Using our apparatus, we demonstrate that rats prefer rough surfaces over smooth ones, indicative of a shared preference. Overcoming previous paradigm limitations, this study offers new insights into tactile preferences in freely moving rats.

MATERIALS AND METHODS

Ethical Approvals

Animal experiments were performed with the approval of the Animal Experiment Ethics Committee at the University of Tokyo (Approval Nos.: P29-7 and P4-15) and according to the University of Tokyo guidelines for the care and use of laboratory animals. These experimental protocols were carried out following the Fundamental Guidelines for the Proper Conduct of Animal Experiments and Related Activities of the Academic Research Institutions (Ministry of Education, Culture, Sports, Science and Technology, Notice No. 71 of 2006), the Standards for Breeding and Housing of and Pain Alleviation for Experimental Animals (Ministry of the Environment, Notice No. 88 of 2006), and the Guidelines on the Method of Animal Disposal (Prime Minister’s Office, Notice No. 40 of 1995). While our experimental protocols mandate humane euthanasia of animals if they exhibit any signs of pain, prominent lethargy, and discomfort, such symptoms were not observed in any of the rats tested in this study. All efforts were made to minimize the animals’ suffering.

Animals

Male 8- to 10-week-old Wistar rats (Japan SLC, Shizuoka, Japan) with a preoperative weight of 300–450 g were individually housed under conditions of controlled temperature and humidity (22 ± 1°C, 55 ± 5%) and maintained on a 12/12-h light/dark cycle (lights off from 7:00 a.m. to 7:00 p.m.) with ad libitum access to food and water. Surgery for electrophysiological recording was performed as previously described,^7–10) although neural activity was not recorded during the behavioral test in this study.

Apparatus

The open-field arena was located in an underground soundproof room, measuring 90 cm in width, 90 cm in depth, and 40 cm in height, with walls made of black-painted wood. The smooth surface was made of black acrylic panels (#31861297, Monotaro, Japan), while the rough surface was made of dark gray sandpaper (#30133443, Monotaro, Japan). For each surface, the dimensions were 90 × 45 cm. All materials were odorless. Three water ports were placed on both the rough and smooth surfaces and connected with tubes under the surfaces. The tubes were hidden from the animals’ view and further routed to a custom-made regulator.

Behavioral Test

All experiments were held in a room with 50 lx lighting. Rats were acclimatized to the experimenter by daily 20 min of handling for 2 d during the dark period before the habituation period.

The water restriction protocol took 3 d in total. In accordance with animal research ethics, complete water deprivation was performed but was limited to 12 h at most before the habituation period. Notably, all rats moved during the experiment, indicating that no rats were completely inactive or unable to drink water. After the experiment, a small portion of water was provided to the rats in a controlled manner to maintain their motivation to quench their thirst. The rats were then transitioned back into a water restriction period of less than 12 h. Moreover, the rats had ad libitum access to food during water restriction. Rats weighing 250–400 g before the video recording period were used.

As for habituation, each rat was placed in the open field arena and allowed to freely explore for 4 h during the dark period (i.e., from 7:00 a.m. to 7:00 p.m.). This habituation period was repeated daily for approximately 1 week before the video recording period.

All of the video recording periods were conducted during the dark period, and lasted for up to 30 min. During the recording period, water was dispensed at regular intervals from the ports symmetrically positioned on both surfaces to motivate the water-deprived rats to move in the field (Fig. 1A). To precisely control water dispensation, a transistor-based switching circuit was employed (Fig. 1B). Using this system to ensure equal crossings across the field, water was dispensed at a randomly selected port, excluding ports adjacent to the one where water was last dispensed (Fig. 1C). During the light trials, the illumination was set to 50 lx, while in the dark trials, it was maintained at 0 lx. After every experiment, the open field was sanitized with 70% ethanol to diminish the odors of the animals as much as possible.

Fig. 1. A Novel Open Field Paradigm with Rough and Smooth Surfaces

(A) Experimental timeline (left) and an example image of the open field with a behaving rat (right). Six water ports, indicated by white circles, were positioned within the open field. (B) Schematic of the transistor-based switching circuit for automated water dispensing. A transistor-based switching circuit was used to control the activation of the solenoid valve. A TTL signal from a microcontroller was applied to the base of a 2N2219 NPN transistor through a 1kΩ resistor. The transistor functioned as a switch, allowing currents to flow through the solenoid valve when the TTL signal was high. The valve was powered by a 12V source, with a flyback diode placed in parallel to mitigate voltage spikes generated by the inductive load. (C) Illustration depicting how water was dispensed from the ports. The currently active water port is shown in yellow, adjacent ports that will not be available next are in orange, and the potential next active water ports are in blue. This setup encouraged consistent movement while minimizing side biases. (D) Example trajectory (blue) of a rat moving within the open field. Rats actively explored the environment rather than remaining stationary in a specific area. Abbreviation: TTL: transistortransistor logic.

Data Analysis

All data analyses were conducted offline using custom Python routines. Summarized data in the text are reported as the mean ± standard deviation. The significance level was set at 0.05, and the null hypothesis was rejected when p < 0.05 based on two-tailed tests unless otherwise specified.

Videos were taken at the rate of 30 frames per second (FPS). A “session” was defined as periods of more than 10 consecutive frames spent on a particular surface. A fixed number of frames (30830, approximately 17 min at 30 FPS) from the start of each session was included in the analysis. Instantaneous positions of the rats were determined utilizing DeepLabCut, a deep-learning based, markerless pose estimation method, as previously described.^7,8,11) Outliers of speeds (i.e., faster than 100 cm/s) were removed when conducting the speed analysis (Fig. 2E, Supplementary Fig. 1).

Fig. 2. Rats Prefer Rough Surfaces over Smooth Surfaces

(A) Preference ratio for the rough surface, defined as the time spent on rough surfaces divided by the total time spent on both surface types (one-sample t-test, n = 8 rats under light conditions). (B) Average duration of sessions on rough (blue) and smooth (light blue) surfaces (paired t-test n = 8 rats under light conditions). (C) Mean distance traveled per rat per session (paired t-test, n = 8 rats under light conditions). (D) Total number of sessions on rough and smooth surfaces (paired t-test, n = 8 rats under light conditions). (E) Frequency distribution of speeds on both surfaces (Kolmogorov–Smirnov test, n = 8 sessions from 8 rats). (F) Preference ratio for rough surfaces in dark (dark red) and light (salmon) conditions (Student’s t-test. n = 4 and 8 rats in the dark and light conditions, respectively). Bars indicate the mean ± standard deviation.

RESULTS

We established a novel system to ensure consistent mobility across the field in rats and reconfirmed the association between sufficient familiarization and decreased immobility in open field environments.¹²⁾ Video analysis revealed that rats moved consistently to take the water reward, rather than remaining immobile throughout the test (Fig. 1D, Supplementary Fig. 1B).

To assess whether rats exhibited a preference for one surface over the other, we quantified the amount of time spent on both surfaces. A significant preference for the rough-textured surface was observed, as evidenced by the increased preference ratio, defined as the amount of time the rat spent on the rough surface divided by the total amount of time on each surface (0.61 ± 0.13, p = 0.047, t₇ = 2.40, n = 8 rats, one-sample t-test; Fig. 2A). Temporal analysis revealed that the sessions on rough surfaces were longer than those on smooth surfaces, suggesting greater engagement with the tactile properties of rough textures (21.8 ± 11.5 s (smooth) vs. 37.9 ± 19.1 s (rough), p = 0.03, t₇ = 2.84, n = 8 rats, paired t-test; Fig. 2B). Additionally, we found a trend toward increased movement within sessions (48.2 ± 32.1 m (smooth) vs. 64.9 ± 21.9 m (rough), p = 0.18, t₇ = 1.50, n = 8 rats, paired t-test; Fig. 2C). Importantly, the number of sessions on both surfaces remained consistent in individual rats (21.8 ± 13.8 (smooth) vs. 21.8 ± 12.8 (rough), p > 0.99, t₇ = 0.00, n = 8 rats, paired t-test; Fig. 2D), confirming that the preference was not influenced by uneven water dispensation. Moreover, the cumulative distribution of speed for rough surfaces was shifted leftward, compared with that for smooth surfaces, suggesting that rats generally move slower on rough surfaces (p = 2.44 × 10⁻²⁸⁴, D = 7.54 × 10⁻², n = 8 rats, Kolmogorov–Smirnov test; Fig. 2E). In addition, the ratio of active (>1 cm/s) time to total time showed a decreasing trend on rough surfaces compared with smooth ones (Supplementary Fig. 1B). Finally, when tested under complete darkness, rats maintained their preference for rough surfaces (0.61 ± 0.17 (dark) vs. 0.61 ± 0.13 (light), p = 0.99, t₁₀ = 0.01, n = 4 (dark), 8 (light) rats, Student’s t-test; Fig. 2F), suggesting that the preference was not influenced by visual stimuli and is instead driven by tactile inputs.

DISCUSSION

In the present study, we leveraged a custom open field to continuously motivate rats to move across distant water ports, allowing us to assess their tactile preferences. The open field test has generally been used to assess locomotion and anxiety-like behaviors. Consistent with this, rats conventionally display thigmotactic responses (i.e., a tendency to stay close to walls rather than exploring open areas); that is, rats hardly enter open areas.^13–15) In this sense, our novel apparatus is crucial for addressing the challenges. While rats also show visual preferences (e.g., toward certain orientations and colors/light intensities), their preference for rough surfaces persisted even in complete darkness, highlighting the primary role of tactile sensation. Moreover, both auditory and olfactory cues were unlikely to influence behavior, given that the experiment was performed in a soundproof environment and the field was composed of odorless materials, in addition to being thoroughly cleaned with 70% ethanol to remove residue odors.

Rats may prefer rough surfaces for the following reasons. First, rough surfaces may provide more traction than smooth surfaces, making movement feel more secure. Empirically, grip and friction allow for the facilitation of steady movement. Interestingly, the speed on smooth surfaces was generally higher than on rough surfaces, possibly due to reduced friction, which could allow for faster, but less stable, movement (Fig. 2E). Moreover, given that rats needed to cross between surfaces to access the water reward and that the number of crossings remained unchanged (Fig. 2C), a greater preference ratio (Fig. 2A) and a trend toward increased movement on rough surfaces (Fig. 2C) suggest that rats preferred moving on the rough surface compared with the smooth one. Second, interacting with textured surfaces may have an anxiolytic effect. This could explain why, despite a robust overall preference for rough surfaces (Fig. 2A), the proportion of time spent active (i.e., faster than 1 cm/s) on rough surfaces relative to total time tended to be lower than that on smooth surfaces (Supplementary Fig. 1). Indeed, physical touch has been shown to decrease the effects of stress, pointing to the effectiveness of handling protocols. Additionally, physical touch has been shown to attenuate depressive-like behaviors in rats subject to chronic mild stress, coinciding with reduced corticosterone levels.¹⁶⁾ Finally, rough surfaces may provide more diverse stimuli to the whiskers and paws of rats, as the excitation of mechanoreceptors correlates with surface roughness.¹⁷⁾ Moreover, given that these factors relate to the intrinsic processing of tactile information and its associated internal states, the preference of rats for rough floors may be innate rather than learned. However, it is interesting to speculate whether prior exposure to a single surface could influence preference.

Perceived temperature may also play a role in this observed preference. Notably, surfaces of the same temperature may feel warmer or colder depending on factors such as heat capacity and thermal conduction, amplified by factors such as different surface textures. While it was difficult to isolate the effect of temperature on tactile preference in the present work, future studies may make use of substances with the same texture but distinct thermal properties and differing room temperatures to elucidate the contribution of temperature.

Additionally, surface texture likely influences both neuronal activity and plasticity. For instance, varied tactile stimuli, such as those associated with enriched environments, have been shown to enhance neuroplasticity.¹⁸⁾ Studies using optogenetics have confirmed that rough textures evoke stronger neural responses in implicated brain regions,^19,20) potentially influencing neural circuitry and plasticity. Moreover, our ongoing research suggests that smooth and rough surfaces induce distinguishable responses in the somatosensory cortex, demonstrating the saliency of the inputs themselves.

Our research has limitations to some extent. First, although all rats except one preferred the rough surface (Fig. 2A), we cannot fully exclude the effects of surgery on behavior, if any. Second, it remains unclear whether rats would exhibit similar preferences when exposed to a broader range of textures. To assess this, future research may introduce a gradient of textures to determine if rats exhibit a graded rather than binary preference. Third, it is unclear whether tactile preferences persist in a non-rewarded context, such as a free-exploration paradigm. As rats exhibit limited mobility when introduced repeatedly in an open field, making it difficult to observe preference without an external reward, future studies would examine the responses of rats introduced to the field for the first time. Moreover, while rat mobility is known to stagnate in an open field, the mobility of other rodents, such as albino mice, may remain sufficient.

Overall, our findings demonstrate a robust preference for rough surfaces, as evidenced by extended interaction durations and greater immobility on these textures. Using our novel system, behavioral neurophysiology will help elucidate what constitutes a pleasurable tactile sensation in rats and how preferences develop. Beyond contributing to sensory neuroscience, however, our findings may inform the development of enriched environments for laboratory animals, improving welfare by aligning environmental conditions with innate tactile preferences.

Acknowledgments

This work was supported by JST ERATO (JPMJER1801), JSPS Grants-in-Aid for Scientific Research (22K21353), AMED-CREST (24wm0625401h0001; 24wm0625502s0501; 24wm0625207s0101; 24gm1510002s0104), the Institute for AI and Beyond of the University of Tokyo (to Y. Ikegaya), and Grants from KOSÉ Cosmetology Research Foundation (to N. Matsumoto).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

This article contains supplementary materials.

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