Extracting physiological information in experimental biology via Eulerian video magnification

Henrik Lauridsen,Selina Gonzales,Peter H Wrege,Mads F Bertelsen,Michael Pedersen,Catherine J A Williams,Jonathan T Butcher,Kathryn L Perrin,Daniela Hedwig

doi:10.1186/s12915-019-0716-7

Abstract

BackgroundVideographic material of animals can contain inapparent signals, such as color changes or motion that hold information about physiological functions, such as heart and respiration rate, pulse wave velocity, and vocalization. Eulerian video magnification allows the enhancement of such signals to enable their detection. The purpose of this study is to demonstrate how signals relevant to experimental physiology can be extracted from non-contact videographic material of animals.ResultsWe applied Eulerian video magnification to detect physiological signals in a range of experimental models and in captive and free ranging wildlife. Neotenic Mexican axolotls were studied to demonstrate the extraction of heart rate signal of non-embryonic animals from dedicated videographic material. Heart rate could be acquired both in single and multiple animal setups of leucistic and normally colored animals under different physiological conditions (resting, exercised, or anesthetized) using a wide range of video qualities. Pulse wave velocity could also be measured in the low blood pressure system of the axolotl as well as in the high-pressure system of the human being. Heart rate extraction was also possible from videos of conscious, unconstrained zebrafish and from non-dedicated videographic material of sand lizard and giraffe. This technique also allowed for heart rate detection in embryonic chickens in ovo through the eggshell and in embryonic mice in utero and could be used as a gating signal to acquire two-phase volumetric micro-CT data of the beating embryonic chicken heart. Additionally, Eulerian video magnification was used to demonstrate how vocalization-induced vibrations can be detected in infrasound-producing Asian elephants.ConclusionsEulerian video magnification provides a technique to extract inapparent temporal signals from videographic material of animals. This can be applied in experimental and comparative physiology where contact-based recordings (e.g., heart rate) cannot be acquired.

Highlights

Videographic material of animals can contain inapparent signals, such as color changes or motion that hold information about physiological functions, such as heart and respiration rate, pulse wave velocity, and vocalization
(See figure on previous page.) Fig. 1 Overview and example of Eulerian video magnification procedure. a Block diagram representing the processing steps of a Eulerian Video Magnification procedure resulting in a signal read of the signal of interest or a gating signal for another imaging procedure. b Four frames from original input video source of anesthetized axolotl (Additional file 1, left trace). c Vertical scan line from the input source over time. d Magnification of box in b showing the cardiac region of the input video
The original video recording did not display any obvious color changes or motions related to the animal’s heartbeat (Fig. 1b–d; Additional file 1, left trace), Eulerian video magnification (EVM)-based color magnification (Fig. 1e–g; Additional file 1, right trace, first part) and motion magnification (Additional file 1, right trace, second part) both revealed rhythmic color changes and motions on the skin surface directly adjacent to the position of the heart and on the external gills, changing with the same frequency as the heartbeat signal acquired with ultrasonographic imaging (Fig. 1h–i; Additional file 1, third part and sound trace for all three parts)

Summary

Introduction

Videographic material of animals can contain inapparent signals, such as color changes or motion that hold information about physiological functions, such as heart and respiration rate, pulse wave velocity, and vocalization. Lauridsen et al BMC Biology (2019) 17:103 invisible to the naked eye, from videographic material [10] In this technique, a video sequence is spatially decomposed (i.e., separated into different spatial frequency bands) and temporally filtered with an adjustable band-pass filter and the resulting signal is amplified to reveal hidden temporal variations The result of this procedure is a marked magnification of inherent local fluctuations in the video sequence, which can reveal the subtle skin color changes in the human face related to increased capillary perfusion after left ventricular ejection allowing for an easy heart rate (fH) measurement or render visible the faint motions related to breathing in infants [10]. The advantages of amplifying our sense of vision to perceive minute fluctuations with this tool should outweigh these concerns

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