# Assessing Drivers' Physiological Responses Using Consumer Grade Devices
## 2 Materials and Methods
### Empatica E4  
E4 is a wearable device in the form of a wristband, equipped with a number of sensors for assessment of electrodermal and cardiovascular activity (Figure 1). The E4 wrist band can measure BVP, inter-beat interval (IBI), heart rate (HR), electrodermal activity (EDA) or GSR, skin temperature (ST), and motion with an accelerometer.  
One of the main features of the E4 device is a photoplethysmography (PPG) sensor. Photoplethysmography sensors use an optical technique to detect changes in blood volume in the microvascular bed of tissue [73]. The PPG sensor in E4 consists of two red and two green LEDs, which provide two different wavelength light sources, and two sensors for detection of the reflected light, with a sampling frequency \(f_{\mathrm{s}}\) = 64 Hz. The output is a blood volume pulse (BVP) signal with an 8-bit resolution. Based on these measurements, a proprietary algorithm from Empatica detects the heart beats (peaks) in the signal and calculates the intervals between separate beats (inter-beat intervals (IBI)) in milliseconds. Unlike the BVP signal, which has a fixed sampling frequency, the IBI signal is provided when a beat is detected. However, average heart rate (HR) signal value is provided every second, as it is calculated from the last known IBI sample.  
E4 is equipped also with an electrodermal activity (EDA) or galvanic skin response (GSR) sensor. With two 8-mm-wide silver-plated copper electrodes, located at the end of the wristband belt (see Figure 1), E4 captures skin conductance with a sampling frequency \(f_{\mathrm{s}}\) = 4 Hz.  
Additionally, E4 also features an optical infrared temperature sensor that measures skin temperature assessed at the wrist, with a sampling frequency \(f_{\mathrm{s}}\) = 4 Hz.  
E4 is equipped also with a 3-axis accelerometer, which allows monitoring of hand activity (movement). On every E4 device, the X-axis is defined as a vector from the center of device towards the USB connector, Y-axis is defined as a vector from the center of the device towards the shorter end of the wrist belt, and \(Z\)-axis is defined as a vector from the center of the device towards the bottom of the device. The dynamic range of the accelerometer is \(\pm\)2 g, the output is an 8-bit signed integer with the resolution of 1/64 g. The accelerometer's sampling frequency (\(f_{\mathrm{s}}\)) is 32 Hz.  
With the E4 device, data can be captured in two ways: In a _recording mode_ (retrieve after measurement) and in a _streaming mode_ (retrieved in real-time). E4 has an internal storage of up to 60 h of measurements. It comes prepacked with a special USB dock station and proprietary software for downloading data from the device and uploading it to a cloud platform called Empatica connect [74]. When using the recording mode, raw data can only be downloaded from this cloud platform. In the streaming mode, E4 uses Bluetooth Low Energy (BLE) to transfer data in real-time.  
Besides the official software solutions that come with the device, Empatica offers also iOS, Android and Windows development kits for researchers that want to customize and upgrade this solution to their needs. The development kits allow real-time data acquisition with custom software that gives the ability of real-time processing, feature extraction and on-demand visualization.