Validating the Stimulus Generator

To test that the circuit functioned as designed, the board was examined using an oscilloscope to confirm that the square wave the circuit was applying to the motors for modulation was consistent, as can be seen in Figure 5.8.

Fig. 5.8: The modulation wave recorded by the oscilloscope from the Arduino circuit. The voltage oscillates between 0 and just under 5 V at precisely the pre-selected test frequency of 18 Hz.

In order to record the motors’ frequency ranges and corresponding Arduino current values they were connected to the accelerometer shown in Figure 5.9 and their oscillations at different settings were recorded.

Fig. 5.9: The accelerometer encased in a block of resin. Each motor was attached to the resin and a range of current values were applied. The resulting signal recorded by the accelerometer was analysed to determine the motor’s speed and power.

The current values assigned to the motors in the Arduino program, from 0 – 255, were decreased by intervals of 50 and their subsequent vibrations recorded. FFTs were then calculated to find the frequency and PSD of the vibrations in question. An example of this can be seen with Motor 2 in Figures 5.10, 5.11 and 5.12.

Fig. 5.10: The recorded vibrations of the second motor at settings 255/255, 200/255, 150/255 and 100/255 with the oscillations visibly slowing between steps.

Fig. 5.12: The recorded vibrations of the second motor at setting 50/255 and the FFT showing its frequency and PSD.

The results of all four motors showed that the frequency and oscillatory amplitudes in response to changing current levels differed between individual motors, as shown in Table 5.3. The different responses between the same models of motor were speculated to be caused by minor variations in the manufacturing process. This model of motor was not designed to be run at highly specific frequencies and if repeated, different motors would be used in the design of the stimulus generator.

TABLE 5.3. SOFTWARE SETTINGS’ CORRESPONDING VIBRATIONAL FREQUENCY AND ITS PSD FOR EACH MOTOR

Motor Code Value Frequency of Vibration (Hz) PSD of Vibration (cm2/Hz)

1 255 200 0.0020 200 200 0.0022 150 158 0.0011 100 112 0.0004 50 71 0.0001 2 255 212 0.0027 200 175 0.0012 150 153 0.0013 100 120 0.0006 50 63 0.0001 3 255 248 0.0023

200 224 0.0017 150 196 0.0012 100 139 0.0006 50 78 0.0001 4 255 222 0.0034 200 188 0.0025 150 154 0.0009 100 116 0.0003 50 63 0.0001

These data points were then plotted and used to extrapolate a graph predicting the frequency and amplitude of all the current values for each of the four motors. An example of this with Motor 1 can be seen in Figures 5.13 and 5.14.

Fig. 5.13: The resulting frequencies of the motor’s vibrations from a range of different current values. In this case the relationship extrapolated from the datapoints indicates a positive linear relationship that plateaus once the current value

Fig. 5.14: The resulting PSD of the motor’s vibrations from a range of different current values. The relationship extrapolated from the datapoints is similar to that of the frequencies but not as strong or well-defined. It was speculated that the decrease

in the PSD at setting 255 might be due to heat causing the casing to swell slightly and increase friction between it and the motor.

This allowed the selection of software settings during calibration of the system that caused all four motors to vibrate at the same frequency and amplitude. As the

frequency/amplitude vs current relationship differed for each motor, having the motors vibrate at the same amplitude was prioritised over having the same carrier frequency. This was because one motor being of greater amplitude than another could unbalance the stimuli in the “standard” experimental protocol. The carrier frequency was considered too high to impact the EEG readings and the modulation frequency would be unchanged.

Verillo’s study in [123] of vibrotactile perception, mentioned previously, found that users were most sensitive to frequencies between 150 – 200 Hz. The motors’ values are all well within that range meaning that a carrier frequency that Verillo found the nerve endings in the fingers to be highly sensitive to can be used, combined with a modulation frequency that was found by Snyder 1992 [118] to produce the largest SSSEPs.

When it came to implementing the stimulus using the motors the equipment set-up was as follows:

 Standard stimulus – Two motors were placed on each hand on the index and

middle fingertips. Motors 1 and 2 on one hand used a 25 Hz modulation frequency, and motors 3 and 4 on the other hand used a 16 Hz modulation frequency.

 Novel stimulus – Motor 1 was placed on one hand’s index finger at the value

deemed to be below the participant’s ATT by the pre-test. Motors 2, 3 and 4 were placed on the index, middle and ring fingers of the other hand. Both had a

modulation frequency of 25 Hz.

 Neutral stimulus – Motors 1 and 2 were secured to one hand but were not

activated. Motors 3 and 4 were placed on the other hand’s index and middle fingers with a modulation frequency of 25 Hz.

The details of the 16 participants recruited are listed below:

TABLE 5.4. PERTINENT DETAILS OF THE PARTICIPANTS FOR NOVEL SSSEP STUDY

Participant Age Sex Dominant Hand BCI Aware

P1 20 Male Right No P2 34 Male Right Yes P3 27 Female Right No P4 28 Female Right Yes P5 22 Male Right No P6 25 Female Right No P7 19 Male Right No P8 25 Male Right No P9 63 Male Right Yes P10 34 Male Right No P11 25 Female Right Yes P12 33 Female Right Yes P13 23 Male Right No P14 27 Male Right Yes P15 25 Female Right No P16 37 Male Right No

No participant suffered from any pre-existing neurological condition and all were ably- sighted.