Free to roam: the open field test is a behavioral staple

Few behavioral tests get used for as many different reasons as the open field. Depending on the question a lab is asking, the same arena and the same footage can serve as a general health and locomotor screen or a dedicated anxiety-like behavior test. It sounds simple, just an arena and a camera. However, it is the question that makes the open field test interesting and varied. That adaptability is why it has remained an important part of behavioral neuroscience for nearly a century.

What is the open field test?

The open field test is a behavioral assay that measures locomotor activity, exploration, and anxiety-like behavior by placing an animal in an enclosed arena and recording how much it moves and how much of the space it uses. It consists of an enclosed arena, usually round or square. An animal is placed in it and filmed from above for a fixed period, typically five to ten minutes. Unlike many other behavioral tests, there is no reward or task for the animal to learn: the whole test rests on the arena being novel. Arena size and material vary with the species and the question being asked, but the basic logic is the same everywhere: give an animal room to move and make sure there is enough contrast between the material and the animal.

The inception of the open field dates back to the 1930s. Calvin Hall used it to study individual differences in 'emotionality' in rats, scoring defecation and urination as crude proxies for fear [1]. Hall and later researchers noticed that rats didn't move through the arena randomly. They stayed close to the walls, venturing into the open center only gradually, if at all. This wall-hugging tendency has a name: thigmotaxis. And this is still the single most-reported measure in open field studies today.

What is Thigmotaxis?

Thigmotaxis is generally read as a sign of anxiety-like behavior. The center of an exposed arena is aversive. So, animals that spend more time there or enter the space more often are typically interpreted as less anxious [2, 3]. That interpretation isn't universally accepted. Critics argue that the open field really captures a fear-driven avoidance response rather than anxiety in a more general sense, and that a proper anxiety test needs both an aversive and an appetitive pull to be meaningful [2]. Most labs sidestep this debate. They treat the open field as a first-pass screen — a quick check for gross changes in locomotion and exploration before moving on to more targeted tests.

Applications and recent research: pairing the open field with EthoVision

The open field is used for a great variety of applications. Across the studies below, researchers extracted the same handful of measures (distance traveled, time in the center versus the periphery, velocity, entries into defined zones) using EthoVision video tracking.

Toxicology and chemical exposure

Ko and colleagues provided the first experimental evidence that polyhexamethylene guanidine phosphate (PHMG-p), a humidifier disinfectant chemical tied to South Korea's mass lung-injury cases, is neurotoxic in mice. EthoVision tracked open field distance and center-zone time alongside elevated plus maze and forced-swim behavior. Exposed mice showed hypoactivity and anxiety-like behavior that tracked with cortical neuron and synapse loss [4]. In zebrafish, Pompermaier and colleagues used EthoVision to quantify distance, turn angle, and center/periphery time in larvae exposed to a diquat-based herbicide. The herbicide altered exploratory behavior alongside survival and heart-rate effects, even at environmentally relevant concentrations [5].

Stress, immunity, and the gut-brain axis

Yao and colleagues linked long-term social isolation during adolescence to worse outcomes after a simulated heart attack in mice. Open field center time and distance, alongside elevated plus maze and forced-swim measures, established an anxiety and depression phenotype that correlated with reduced cardiac function [6]. Mo and colleagues took the opposite approach. They tested whether a probiotic, Lactobacillus reuteri, could reverse LPS-induced depression- and anxiety-like behavior via the gut-brain axis. EthoVision-tracked open field distance and center/periphery time showed the probiotic restored locomotion and reduced center-avoidance behavior [7]. And Yuan and colleagues found that knocking out acid sphingomyelinase, an enzyme linked to toll-like receptor immune signaling, left mice with restricted open field activity and less time in the center. That's a heightened-anxiety profile, consistent with an immune contribution to anxiety-like behavior [8].

Neurodegeneration and neurodevelopmental models

The open field is a fixture in disease-model phenotyping. Shetty and colleagues included it among several assays showing altered exploratory behavior in mice lacking a neuronal RNase complex, alongside impaired long-term spatial memory [9]. Jin and colleagues used open field testing to support a PI3K/AKT pathway analysis of a fruit-juice concentrate's neuroprotective effects in a Parkinson's disease mouse model [10]. Buglinina and colleagues tracked open field distance, speed, and center-zone entries in mice after spinal cord injury. Knocking out the receptor TAAR5 sped up motor recovery, but it did not reduce anxiety-like behavior, which stayed elevated regardless of genotype [11]. In a maternal immune activation model of autism, Zhang and colleagues found that neutralizing IL-6 largely reversed the anxiety-like behavior seen in open field and elevated plus maze testing. It left the model's core social-deficit and repetitive-behavior measures untouched [12].

Sex differences and specificity controls

Several recent studies used the open field to check whether an effect held up, or differed, across sexes. Gellman and colleagues compared an endocannabinoid-like lipid and a synthetic CB2 agonist in a genetic rat model of depression. Open field tracking revealed a clear sex split: the endocannabinoid-like compound was anxiolytic in males, but anxiolytic and antidepressant in females [13]. Sacko and colleagues used open field distance and center-zone time as a specificity control when testing a BET inhibitor's effect on cocaine-seeking in rats. Those measures were unchanged by the drug, so the researchers could rule out general sedation as an explanation for the reduced drug-seeking they saw elsewhere [14]. In a pilot study of paclitaxel-induced peripheral neuropathy, Kim and colleagues recorded open field behavior in a PhenoTyper home-cage observation arena running EthoVision. Chemotherapy reduced locomotion and rearing in male rats, with only milder locomotor effects in females [15].

Early-life adversity

Bonauto and colleagues combined the open field with playback of aversive, rat-specific alarm calls to probe how early-life adversity shapes affective processing in juvenile rats. EthoVision-quantified locomotion and center-zone exploration, during baseline and playback, revealed sex-specific responses to the threat cue. The open field can capture not just a resting anxiety state, but how an animal responds to a stressor introduced mid-test [16].

Beyond rodents

The open field translates well to other model species. Herrera-Castillo and colleagues used EthoVision to track thigmotaxis and center/periphery time in goldfish, testing whether food-anticipatory activity reflects a ghrelin-driven anxiety state. Ghrelin-receptor antagonists reduced the anxiety-like behavior seen before scheduled feeding. That ties a metabolic hormone directly to an exploration-based measure [17].

Tips for using the open field test with EthoVision

• The open field is forgiving. But a few habits can make the difference between clean data and a noisy dataset with unreliable results.
• Habituate the room, not just the arena. Move animals to the testing room and let them settle before the trial starts. A stressful transport or an unfamiliar room can confound the behavior you're trying to measure.
• Clean between animals. Residual scent cues from a previous occupant can bias exploration in the next one. A thorough wipe-down with a low-odor disinfectant between trials matters more than researchers assume.
• Keep lighting and time of day constant. Both directly influence behavior. A light gradient across the arena, or testing animals at different times of day, can introduce variability that looks like a treatment effect.
• Define your center zone deliberately. Center/periphery boundaries are usually set as a fixed proportion of arena area. However, that proportion should be scaled with the species and arena size you're using. A boundary that works for a rat arena won't automatically suit a zebrafish tank.
• Keep the experimenter out of view. Animals track human presence. An experimenter lingering in the room during recording can suppress exploration, independent of anything you're testing.
• Consider whether you need acute or long-term data. A single five- to ten-minute trial captures a novelty response. However, some questions (general activity patterns, circadian effects, drug time-courses) call for repeated or continuous monitoring. That's why systems like PhenoTyper exist: an instrumented home-cage-style arena with integrated video and EthoVision tracking, built for longer-run phenotyping.
• Track more than distance if you can. Rearing, grooming, and freezing all carry information that raw distance traveled doesn't. Automated pose-based tracking can pick these up alongside standard locomotion measures.

For more on adapting classic behavioral assays with automated tracking, have a look at our earlier blogs on the Y-maze and the radial arm maze.

In summary, some frequently asked questions

References

1. Hall, C.S. Emotional behavior in the rat. I. Defecation and urination as measures of individual differences in emotionality. J. Comp. Psychol. 1934, 18, 385–403.
2. Prut, L.; Belzung, C. The open field as a paradigm to measure the effects of drugs on anxiety-like behaviors: a review. Eur. J. Pharmacol. 2003, 463, 3–33.
3. Seibenhener, M.L.; Wooten, M.C. Use of the open field maze to measure locomotor and anxiety-like behavior in mice. J. Vis. Exp. 2015, 96, e52434.
4. Walsh, R.N.; Cummins, R.A. The open-field test: A critical review. Psychol. Bull. 1976, 83, 482–504.
5. Ko, M.Y.; Choi, J.; Min, E.; et al. Polyhexamethylene guanidine phosphate exposure induces abnormal behaviors by disrupting synaptic formation and activity in the cerebral cortex. J. Neuroinflammation 2026, 23, 190.
6. Pompermaier, A.; Chagas, F.B.; Tamagno, W.A.; Freeman, J.L.; Hartmann, P.A.; Hartmann, M. Diquat based herbicide impair the development and behavior of zebrafish embryos and larvae. Sci. Rep. 2025, 15, 42194.
7. Yao, Y.; Wang, A.; Di, C.; Guo, D.; Zhang, S.; Zong, R.; Qi, R.; Han, Y. Long-term social isolation during adolescence exacerbated cardiac dysfunction after myocardial infarction. Transl. Psychiatry 2026, 16, 193.
8. Mo, X.; Guo, S.; He, D.; Cheng, Q.; Yang, Y.; Wang, H.; Ren, Y.; Liu, L.; Xie, P. Lactobacillus reuteri DSM 17,938 ameliorates LPS-induced depression-like and anxiety-like behaviors by modulating gut microbiota and brain metabolic function. Gut Pathog. 2025, 17, 65.
9. Yuan, H.; Xu, Y.; Jiang, H.; Jiang, M.; Zhang, L.; Wei, S.; Li, C.; Zhao, Z. Acid sphingomyelinase modulates anxiety-like behavior likely through toll-like receptor signaling pathway. Mol. Brain 2025, 18, 8.
10. Shetty, M.S.; Kasuya, J.; Fu, X.; et al. Genetic inactivation of Translin/Trax RNase disrupts miRNAs, hippocampal synaptic plasticity, and memory. iScience 2026, 29, 116188.
11. Jin, T.; Liu, L.; Kuang, F.; Chen, M.; Chen, H.; Deng, J.; Yang, Y.; Sun, B.; Luo, H. Neuroprotective effects of Rosa roxburghii Tratt juice concentrate powder in Parkinson's disease mice via the PI3K/AKT signaling pathway. Pharmaceuticals 2026, 19, 711.
12. Buglinina, A.D.; Romanyuk, E.A.; Chesnokov, A.A.; et al. TAAR5 modulates sensorimotor recovery after spinal cord injury. Biomedicines 2026, 14, 796.
13. Zhang, X.; Luo, W.; He, K.; Li, Y.; Chen, Y.; Xu, Z.; Zhou, Z.-K. IL-6 inhibition partially ameliorates maternal immune activation-induced autism-like behavioral abnormalities in mice. Curr. Issues Mol. Biol. 2025, 47, 852.
14. Gellman, J.; Zemliana, N.; Loterstein, Y.; et al. Differential effects of oleoyl serine and HU-910 on anxiety-like and depression-like behaviors in male and female WKY rats. Int. J. Mol. Sci. 2026, 27, 3177.
15. Sacko, T.J.; Seyednejad, A.; Engelhardt, J.; Sartor, G.C. The selective bromodomain and extra-terminal domain (BET) inhibitor RVX-208 reduces cocaine-seeking behaviour and alters proteomic pathways in the nucleus accumbens. Addict. Biol. 2026.
16. Kim, S.J.; Cao, K.; Dougherty, P.M.; et al. Gender-specific behavior reductions in spontaneous locomotive and rearing in a rat model of paclitaxel-induced peripheral neuropathy: a pilot study. J. Pain Res. 2026, 19.
17. Bonauto, S.M.; Wilson, P.L.; Honeycutt, J.A. Aversive 22-kHz ultrasonic vocalization playback reveals differences in affective (dys)function following early life adversity in male and female juvenile rats. Behav. Brain Res. 2026, 511, 116272.
18. Herrera-Castillo, L.; Saiz, N.; de Pedro, N.; Isorna, E. Food reward entrainment increases mealtime anxiety in goldfish via a ghrelin-dependent mechanism. Sci. Rep. 2025, 15, 27768.