Classical fear conditioning paradigm was adapted for human participants in a fully immersive virtual reality setting. Using a discrimination paradigm, conditioned fear, cue and context memory retention, and extinction was measured with skin conductance response to dynamic virtual snakes and spiders (the conditioned stimuli) in two distinct virtual contexts.
Fear conditioning is a widely used paradigm in non-human animal research to investigate the neural mechanisms underlying fear and anxiety. A major challenge in conducting conditioning studies in humans is the ability to strongly manipulate or simulate the environmental contexts that are associated with conditioned emotional behaviors. In this regard, virtual reality (VR) technology is a promising tool. Yet, adapting this technology to meet experimental constraints requires special accommodations. Here we address the methodological issues involved when conducting fear conditioning in a fully immersive 6-sided VR environment and present fear conditioning data.
In the real world, traumatic events occur in complex environments that are made up of many cues, engaging all of our sensory modalities. For example, cues that form the environmental configuration include not only visual elements, but aural, olfactory, and even tactile. In rodent studies of fear conditioning animals are fully immersed in a context that is rich with novel visual, tactile and olfactory cues. However, standard laboratory tests of fear conditioning in humans are typically conducted in a nondescript room in front of a flat or 2D computer screen and do not replicate the complexity of real world experiences. On the other hand, a major limitation of clinical studies aimed at reducing (extinguishing) fear and preventing relapse in anxiety disorders is that treatment occurs after participants have acquired a fear in an uncontrolled and largely unknown context. Thus the experimenters are left without information about the duration of exposure, the true nature of the stimulus, and associated background cues in the environment1. In the absence of this information it can be difficult to truly extinguish a fear that is both cue and context-dependent. Virtual reality environments address these issues by providing the complexity of the real world, and at the same time allowing experimenters to constrain fear conditioning and extinction parameters to yield empirical data that can suggest better treatment options and/or analyze mechanistic hypotheses.
In order to test the hypothesis that fear conditioning may be richly encoded and context specific when conducted in a fully immersive environment, we developed distinct virtual reality 3-D contexts in which participants experienced fear conditioning to virtual snakes or spiders. Auditory cues co-occurred with the CS in order to further evoke orienting responses and a feeling of “presence” in subjects 2 . Skin conductance response served as the dependent measure of fear acquisition, memory retention and extinction.
1. Results
Equivalent within-session fear acquisition and extinction across groups was found (Figure 3). These data indicate that reliable and informative fear conditioning studies can be performed within the constraints and capabilities of a fully immersive environment. Moreover we also demonstrate robust contextual fear memory in the Same Context fear retention participants in the DiVE (participants who remained in the same context for Days 1 and 2, relative to those who experienced a context shift). The retention of fear is stronger in the DiVE than that observed in a conventional laboratory matched paradigm 16 (see Figure 3). With the immersive VR setup, one can also examine and manipulate rich contextual environments to probe declarative memory processes in humans, unlike in the laboratory setting where realistic multimodel context manipulations are difficult to accomplish. Finally, the VR worlds can easily be ported for use in conjunction with functional magnetic resonance imaging (fMRI) using stereoscopic VR goggles to conduct brain activation analysis during encoding or retrieval of fear acquisition, extinction, and relapse. This methodology can be utilized to bridge rodent and clinical findings in fear and anxiety.
2. Controlling context and stimulus exposure in virtual reality.
A major issue with exploiting VR for experimental use is also its strength. Specifically, fully immersive VR provides the complexity, confounds, and freedom of the real world. For example, in real life, trauma victims experience an aversive stimulus in a context for an unknown amount of time. The contextual exposure, specific features and other sensory input that were attended are also unknown, or not confirmable. By the same token, if we were to allow participants to freely explore the virtual environments we would not be able to account for context or stimulus exposure time or duration. For example, one participant may walk very quickly, and miss 3 out of 4 CS+ presentations. Another may explore only one room in the virtual apartment. Likewise, if stimulus presentation is not specified in the center of the screen, where gaze is directed prior to starting, participants will avoid or miss CS presentations. Our solution to these potential confounds was to take participants on a seated, guided tour of each environment at a rate that would allow for a specific interstimulus interval (ISI) and stimulus duration. We could then extract comparable SCR data from specific time points and specific locations across all participants (e.g., responses to the CS+, US, and CS- stimuli). Difficulties encountered after making this decision included finding a path shape, length, and movement rate that would not cause nausea or proprioceptive dissonance to the participant, and yet feel appropriate to mimic natural ambulation through a novel environment.
3. Implementing standard fear conditioning parameters to a VR system.
To simulate realistic conditioned stimuli snakes and spiders were designed after wild-life images. The snakes and spiders were first modeled in Maya, a computer graphics 3D modeling and animation software package and then imported into the VR system. We did this because Virtools is a virtual reality authoring system, not a modeling application. It is therefore best used to run a VR system and add the interaction and navigation to a scene. Specifically, in Maya four different animations for each CS type were created (e.g., a coiled snake, a spider running across the floor, a snake lunging forward with open mouth) and then imported into Virtools.
Prior to importing the dynamic snake and spider models into Virtools from Maya, a path was created in Virtools to guide the participant around the environment in a smooth circular fashion so as to allow sampling of the environment over the course of 32 conditioned stimulus presentations during fear conditioning. The shape of the path is the same for each of our three virtual worlds. The path was created to stop for four seconds for each stimulus presentation, the interstimulus interval was 11 +/- 4 seconds during which the subject was slowly moving (being guided) through the environment. This interval was determined from our previous fear conditioning experiments in the laboratory 8, 16 because it allows for recovery of the skin conductance response between stimulus presentations. Stimuli were then placed on the path at points specified by the timing parameters. This setup created specific stimulus and context conjunctions (e.g., a snake slithering on the dining room table, a spider walking around the sofa leg), which can later be probed for explicit memory. Stimuli appearances were pseudo-randomized through the scripts. All stimulus presentations appeared in the middle of the front screen to prevent the participant from having to search for the stimulus. This provided us with a controlled amount of stimulus exposure time, and a defined context location. One limitation of the forward view is that it does not take advantage of the full capabilities of the immersive system (e.g., snakes cannot enter the room from behind the participant). Additionally, stimuli were carefully placed outside of a boundary box around the participants location so that the snakes and spiders never encroached upon the participants personal space.
4. View point and head tracking.
The angle of the DiVE was set so that from a seated position the participant had a correct forward facing angle. This controlled for variations in height between participants, and minimized movement artifacts on our physiological recordings. Participants were instructed to face forward and move as little as possible, this also controlled for where participants were looking, and therefore maintained consistent stimulus and context exposure between participants. We chose to turn on a head-tracking device in the 3D goggles worn by the participants to ensure they were viewing the scene with the correct perspective. If head tracking had not been elected for, head movement to the left or right would not correctly occlude how the objects appeared in the world (e.g. objects would appear bent on screens in the DiVE as participants walked through). With head tracking elected for, we could be sure that features in the environment retained their normal proportions and were drawn correctly on each of the six walls of the DiVE for the duration of the experiment.
5. Data Collection.
In our standard laboratory version of fear conditioning 8, 16 stimulus presentation was controlled by computer script programmed in the Presentation software package. In order to maintain consistency between the lab and the virtual environment, we imported our standard fear acquisition and extinction scripts in code format to the control computer in the control room that hosts the DiVE cube (see Figure 1). The parallel port code was set to send a generated list of numeric codes to signal distinct events, such as presentations of the snake, spider, and electrical stimulation onsets. In our design, Virtools sends an Open Sound Control message (OSC 17) OSC/UDP message to a custom C++ program that sets the parallel port value. Our C++ program uses the OSCpack 18 library.
The BIOPAC’s digital input is connected to the computer parallel port. SCR data is collected on the laptop computer from BIOPAC via parallel port, then normalized, and calculated to CS+/ CS- and US onsets within specific parameters (see above for details). In addition to rendering the scene and controlling navigation, Virtools is also used to log user events (button presses). In summary, during an experiment, messages are sent from the master computer to the BIOPAC system through the parallel port. Because Virtools cannot communicate with the parallel port on the computer directly a small C++ program listens for an OSC message from Virtools and then transmits it to the parallel port.
The authors have nothing to disclose.
We thank Holton Thompson for her work in creating the Virtools 3-D worlds in Maya and Eric Monson for the schematic drawings. Research was sponsored in part by postdoctoral NIH F32 MH078471 to N.C.H, and NIDA RO1 DA027802 to K.S.L.. The DiVE was funded by NSF BCS-0420632.