IEEE Consumer Electronics Magazine - October 2016 - 50

Table 3. A comparison of GSR systems.
gsr system

software

real time

Channels

Connectivity

Portable

additional Features

Procomp infinitia
[61]

BioGraph infiniti
(Suites)
● eeG
● Physiology
● rehab
● dynaMap

yes

Wired

●

integrates with various sensor
types (eeG, ecG, eMG, GSr, Hr,
BvP, respiration, force)

Biopac MP150
+ Bionomadix
PPGed-r tX/rX
pair [44]

acqknowledge

yes

1 (GSr)
+ 1 (PPG)

Wireless
(2.4 GHz bidirectional
digital rF)

●
●

additional PPG sensing
can be used simultaneously
with other Biopac modules

yes

2 (GSr)
+ 1 (PPG)

Wireless
(Bluetooth)

●
●

Shimmer3 GSr+ [62] consensys

additional PPG sensing
Multiconfiguration PPG
(finger or earlobe)
● 9 doF sensor
● Lightweight
● onboard 8-Gb memory

athe

Procomp infiniti is considered to be the GSr gold standard.
BvP: blood volume pressure, Hr: heart rate, PPG: photoplethsymography, rF: radio frequency.

necessary, especially if the subject wearing the electrodes has
moist skin. Dry GSR electrodes can be placed directly on skin
and measure EDA [56]. These conveniences have made wearable GSR sensors, such as the Shimmer3 GSR unit and the
Empatica E3 wristband, possible.
Similar to the ECG and EEG units available as modules to
the Biopac MP150 system, Biopac also offers a GSR unit
(GSR100C) that allows for the measurement of EDA. This unit
uses the same AcqKnowledge software suite as all of the other
amplifier modules for the MP150 system. As an alternative to
the normal wired interface, Biopac also offers a wireless version included in its Bionomadix PPGED-R module that allows
for tether-free transmission of measured data to the MP150
acquisition unit. The Bionomadix PPGED-R module offers
both PPG and GSR sensing capability.
Thought Technology, Inc., like Biopac, offers an array of
modules and sensors that can be used for affective computing. Specific to their GSR capabilities, Thought Technology
offers a skin conductance sensor (SA9309M) that interfaces
with either of its ProComp or FlexComp data acquisition systems. These systems are (optical) wire-based systems that
have been used in several studies for GSR measurement [57],
[58]. Each acquisition system comes with the Biograph Inifiti
software suite that allows real-time display of raw data.
Shimmer Sensing offers a GSR unit that has many of the
same benefits as its ECG unit; the form factor is small, wireless, and simplistic. Like the ECG module, it too contains
9 DoF sensing. Additionally, the GSR module has an expansion port for PPG sensing. This module is used with the same
Consensys base and software suite as the ECG module,
which allows for real-time acquisition and analysis.
Empatica Inc. is a company that rose out of MIT
research, focusing on effective affective computing systems
for research. Empatica's flagship device is the E4 wristband,
50 IEEE Consumer Electronics Magazine

OCTOBER 2016

which houses PPG, EDA, and temperature sensors, along
with a three-axis accelerometer within a compact wristband.
Data collected is logged in real time, whether natively in the
onboard flash or streamed wirelessly to its RealTime app. A
study comparing Empatica's GSR measurements to those of
Thought Technology's ProComp Infiniti system [60]
revealed a 93% correlation in data. See Table 3 for a comparison of GSR systems. The E4 wristband is revisited in the
following section.

Blood activity and PPG
BLood activity
Inherent in the change of HR and HRV is a change in blood
volume and blood pressure. Blood volume pulse (BVP) is the
phasic change in blood volume that corresponds to each heartbeat interval [23]. Furthermore, BVP is also used to determine
changes in blood pressure in correspondence with blood volume. Variability in blood pressure is due to vasodialation and
vasoconstriction of arteries, capillaries, and other vasculature,
which can be discerned by BVP measurements [23]. These
fluctuations of blood pressure and blood volume are direct
products of heart activity. Therefore, another means of stress
measurement is made available by way of BVP.

PPG
PPG is a low-cost, noninvasive optical technique that is commonly used to detect blood volume changes in microvasculature [65]. A PPG sensor simply uses an optical pulse generated
by a red or near-infrared (NIR) light source (typically a lightemitting diode), with a closely placed photodetector acting as
the receiver of reflected light. The amount of light received
back at the photodetector provides insight into the amount of
blood volume in the area illuminated. Less light received by

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - October 2016