An Open-Source Virtual Reality System for the Measurement of Spatial Learning in Head-Restrained Mice

Summary

Here, we present a simplified open-source hardware and software setup for investigating mouse spatial learning using virtual reality (VR). This system displays a virtual linear track to a head-restrained mouse running on a wheel by utilizing a network of microcontrollers and a single-board computer running an easy-to-use Python graphical software package.

Abstract

Head-restrained behavioral experiments in mice allow neuroscientists to observe neural circuit activity with high-resolution electrophysiological and optical imaging tools while delivering precise sensory stimuli to a behaving animal. Recently, human and rodent studies using virtual reality (VR) environments have shown VR to be an important tool for uncovering the neural mechanisms underlying spatial learning in the hippocampus and cortex, due to the extremely precise control over parameters such as spatial and contextual cues. Setting up virtual environments for rodent spatial behaviors can, however, be costly and require an extensive background in engineering and computer programming. Here, we present a simple yet powerful system based upon inexpensive, modular, open-source hardware and software that enables researchers to study spatial learning in head-restrained mice using a VR environment. This system uses coupled microcontrollers to measure locomotion and deliver behavioral stimuli while head-restrained mice run on a wheel in concert with a virtual linear track environment rendered by a graphical software package running on a single-board computer. The emphasis on distributed processing allows researchers to design flexible, modular systems to elicit and measure complex spatial behaviors in mice in order to determine the connection between neural circuit activity and spatial learning in the mammalian brain.

Introduction

Spatial navigation is an ethologically important behavior by which animals encode the features of new locations into a cognitive map, which is used for finding areas of possible reward and avoiding areas of potential danger. Inextricably linked with memory, the cognitive processes underlying spatial navigation share a neural substrate in the hippocampus¹ and cortex, where neural circuits in these areas integrate incoming information and form cognitive maps of environments and events for later recall². While the discovery of place cells in the hippocampus^3,4 and g....

Protocol

All procedures in this protocol were approved by the Institutional Animal Care and Use Committee of the New York State Psychiatric Institute.

NOTE: A single-board computer is used to display a VR visual environment coordinated with the running of a head-restrained mouse on a wheel. Movement information is received as serial input from an ESP32 microcontroller reading a rotary encoder coupled to the wheel axle. The VR environment is rendered using OpenGL hardware acceleration on the Raspberry P.......

Discussion

This open-source VR system for mice will only function if the serial connections are made properly between the rotary and behavior ESP32 microcontrollers and the single-board computer (step 2), which can be confirmed using the IDE serial monitor (step 2.4.5). For successful behavioral results from this protocol (step 4), the mice must be habituated to the apparatus and be comfortable running on the wheel for liquid rewards (steps 4.3-4.5). This requires sufficient (but not excessive) water restriction, as mice given .......

Materials

Name	Company	Catalog Number	Comments
1/4 " diam aluminum rod	McMaster-Carr	9062K26	3" in length for wheel axle
1/4"-20 cap screws, 3/4" long (x2)	Amazon.com	B09ZNMR41V	for affixing head post holders to optical posts
2"x7" T-slotted aluminum bar (x2)	8020.net	1020	wheel/animal mounting frame
6" diam, 3" wide acrylic cylinder (1/8" thick)	Canal Plastics	33210090702	Running wheel (custom width cut at canalplastics.com)
8-32 x 1/2" socket head screws	McMaster-Carr	92196A194	fastening head post holder to optical post
Adjustable arm (14")	Amazon.com	B087BZGKSL	to hold/adjust lick spout
Analysis code (MATLAB)		custom written	file at github.com/GergelyTuri/HallPassVR/software/Analysis code
Axle mounting flange, 1/4" ID	Pololu	1993	for mounting wheel to axle
Ball bearing (5/8" OD, 1/4" ID, x2)	McMaster-Carr	57155K324	for mounting wheel axle to frame
Behavior ESP32 code		custom written	file at github.com/GergelyTuri/HallPassVR/software/Arduino code/Behavior board
Black opaque matte acrylic sheets (1/4" thick)	Canal Plastics	32918353422	laser cut file at github.com/GergelyTuri/HallPassVR/hardware/VR screen assembly
Clear acrylic sheet (1/4" thick)	Canal Plastics	32920770574	laser cut file at github.com/GergelyTuri/HallPassVR/hardware/VR wheel assembly
ESP32 devKitC v4 (x2)	Amazon.com	B086YS4Z3F	microcontroller for behavior and rotary encoder
ESP32 shield	OpenMaze.org	OMwSmall	description at www.openmaze.org (https://claylacefield.wixsite.com/openmazehome/copy-of-om2shield). ZIP gerber files at: https://github.com/claylacefield/OpenMaze/tree/master/OM_PCBs
Fasteners and brackets	8020.net	4138, 3382,3280	for wheel frame mounts
goniometers	Edmund Optics	66-526, 66-527	optional for behavior. Fine tuning head for imaging
HallPassVR python code		custom written	file at github.com/GergelyTuri/HallPassVR/software/HallPassVR
Head post holder		custom design	3D design file at github.com/GergelyTuri/HallPassVR/hardware/VR head mount/Headpost Clamp
LED projector	Texas Instruments	DLPDLCR230NPEVM	or other small LED projector
Lick spout	VWR	20068-638	(or ~16 G metal hypodermic tubing)
M 2.5 x 6 set screws	McMaster-Carr	92015A097	securing head post
Matte white diffusion paper	Amazon.com		screen material
Metal headposts		custom design	3D design file at github.com/GergelyTuri/HallPassVR/hardware/VR head mount/head post designs
Miscellenous tubing and tubing adapters (1/16" ID)			for constructing the water line
Optical breadboard	Thorlabs	as per user's requirements
Optical posts, 1/2" diam (2x)	Thorlabs	TR4	for head fixation setup
Processing code		custom written	file at github.com/GergelyTuri/HallPassVR/software/Processing code
Raspberry Pi 4B	raspberry.com, adafruit.com		Single-board computer for rendering of HallPassVR envir.
Right angle clamp	Thorlabs	RA90	for head fixation setup
Rotary encoder (quadrature, 256 step)	DigiKey	ENS1J-B28-L00256L	to measure wheel rotation
Rotary encoder ESP32 code		custom written	file at github.com/GergelyTuri/HallPassVR/software/Arduino code/Rotary encoder
SCIGRIP 10315 acrylic cement	Amazon.com
Shaft coupler	McMaster-Carr	9861T426	to couple rotary encoder shaft with axle
Silver mirror acrylic sheets	Canal Plastics	32913817934	laser cut file at github.com/GergelyTuri/HallPassVR/hardware/VR screen assembly
Solenoid valve	Parker	003-0137-900	to administer water rewards