Measuring the Behavioral Response to Spatial Audio within a Multi-Modal Virtual Reality Environment in Children with Autism Spectrum Disorder

applied
sciences
Article
Measuring the Behavioral Response to Spatial Audio
within a Multi-Modal Virtual Reality Environment in
Children with Autism Spectrum Disorder
Daniel Johnston 1,† , Hauke Egermann 2 and Gavin Kearney 1,*
1 Communications & Signal Processing Research Group, Department of Electronic Engineering,
University of York, York YO10 5DD, UK
2 York Music Psychology Group, Music Science and Technology Research Cluster, Department of Music,
University of York, York YO10 5DD, UK.
* Correspondence: dij502@york.ac.uk; Tel.: +44-(0)1904-324-229
† Current address: Communications & Signal Processing Research Group, Department of Electronic
Engineering, University of York, Heslington, York YO10 5DD, UK
Received: 30 June 2019; Accepted: 31 July 2019; Published: 2 August 2019
Abstract: Virtual Reality (VR) has been an active area of research in the development of
interactive interventions for individuals with autism spectrum disorder (ASD) for over two decades.
These immersive environments create a safe platform in which therapy can address the core symptoms
associated with this condition. Recent advancements in spatial audio rendering techniques for
VR now allow for the creation of realistic audio environments that accurately match their visual
counterparts. However, reported auditory processing impairments associated with autism may affect
how an individual interacts with their virtual therapy application. This study aims to investigate
if these difficulties in processing audio information would directly impact how individuals with
autism interact with a presented virtual spatial audio environment. Two experiments were conducted
with participants diagnosed with ASD (n = 29) that compared: (1) behavioral reaction between
spatialized and non-spatialized audio; and (2) the effect of background noise on participant interaction.
Participants listening to binaural-based spatial audio showed higher spatial attention towards target
auditory events. In addition, the amount of competing background audio was reported to influence
spatial attention and interaction. These findings suggest that despite associated sensory processing
difficulties, those with ASD can correctly decode the auditory cues simulated in current spatial audio
rendering techniques.
Keywords: autism spectrum disorders; virtual reality; auditory processing; assistive technology;
tools for therapy; multi-sensory; spatial audio; ambisonics
1. Introduction
Autism spectrum disorder (ASD) is a lifelong neurodevelopmental disorder that affects
approximately 1% of the worldwide population [1]. It is characterized through core symptoms
that include impaired development in social interaction, communication, repetitive behaviors,
restrictive interests, and sensory processing difficulties [2]. Autism is heterogeneous, and despite these
symptoms being commonly identified with the condition, the scope of which these difficulties affect
everyday life are often unique to the individual [3]. With no cure for ASD, there is an increasing need
for intervention programs that target appropriate behavioral and educational complications with the
aim to improve the quality of life for the individual and their family [4].
Appl. Sci. 2019, 9, 3152; doi:10.3390/app9153152 www.mdpi.com/journal/applsci

Appl. Sci. 2019, 9, 3152 2 of 19
1.1. Virtual Reality and ASD
Virtual Reality (VR) has been an active area of research in the development of interventions for
this population for over two decades. Empirical studies have recognized VR as an important therapy
tool for addressing the core symptoms associated with ASD such as difficulties in social interaction [5]
and communication [6]. In addition, virtual reality presents dynamic and controllable environments in
which educational and training practices such as disaster awareness [7], driving [8] and independent
functioning [9] training can take place.
Early work conducted by Strickland [10] in 1996 suggested key aspects of this technology
which can be applied to effective and engaging therapeutic solutions for both children and adults
living with autism, and despite extensive advancements in how VR is delivered these can still be
applicative to contemporary research practices. Firstly, many individuals with autism respond to
and enjoy using virtual reality technology [11]. This translates to users actively engaging with
interventions, with the enjoyment providing motivation to progress with the therapy program [12,13].
Secondly, virtual environments provide an experience with consistent and predictable interactions and
responses [14]. This can be in the form of limiting social and communication pressures that can often
cause anxiety in real world situations [15]. Furthermore, those with autism can frequently experience
complications in processing sensory information, with particular prevalence in the auditory and visual
domains. These irregularities in sensory receptiveness can often be a catalyst for a deterioration in
behavior observed as aggressive or autonomic fear responses [16]. Virtual reality can control all audio
and visual stimuli that are encountered during an intervention session, simplifying complex sensory
arrays and thus reducing the elicitation of anxiety [10,17]. The control of VE’s and interactions within
them can also construct important opportunities for individualized treatment [10]. Re-adjusting
the sensory and learning parameters of the intervention to compensate for the complex needs
of the individual [11]. Finally, virtual reality technology can replicate realistic representations
of real-world situations in which participants can learn and practice new behaviors and real-life
skills. The increased feeling of presence and ecological validity as a result of this realism can have
a positive impact on the generalization of newly acquired skills from the virtual intervention to
their real-world applications [11]. However, the visual experience of the user is often the focus of
research when investigating virtual reality interventions for ASD. Although what the user sees can
increase generalization via realistic visual similarities with the real world, the sense of realism is
reliant on the extent and fidelity of the VR technology delivering the multi-sensory stimuli to the user,
including audio.
1.2. The Importance of Spatial Audio
Larson et al. [18] suggest that the spatial properties of virtual auditory environments have been of
significant importance since the introduction of stereo sound in the 1930s. Stating that spatial sound is
used to simulate an auditory reality that gives the user the impression they are surrounded by a 3D
virtual environment. In terms of precision, spatial acuity is somewhat inferior to vision and somatic
sensory system when perceiving the environment [19]. However, it is not unimportant. Spatial hearing
is a critical way in which the user perceives space, providing information about events and objects that
are far beyond the field of view [20].
Two studies conducted by Hendrix and Barfield [21] investigated how three-dimensional
sound influences the user experience within a stereoscopic virtual environment (VE). The first
compared a silent VE to environments with spatial sound. The second was a comparison between
two environments with spatialized and non-spatialized sound sources. The results indicated that
spatialized audio significantly increased the feeling of presence, with interactive sounds to be
originating from their visual cues perceived as more realistic.
To perceive the real world, the human brain must decode a constant stream of multi-modal
information from the various sensory channels [22]. An example of which is most cases of face to
face communication. The brain must interpret visual information such as moving lips and facial

Appl. Sci. 2019, 9, 3152 3 of 19
expressions, while also listening to localized speech sound from the mouth [23]. This cross modality
synchronization can also be echoed in VR by using 3D audio, by simulating the shared spatial qualities
of both audio and visual stimuli that occupy the same time and space.
1.3. Spatial Audio Rendering
For healthy listeners, the human auditory system is adept at decoding how a sound wave diffracts
and interacts with the head, torso, and pinnae, resulting in both temporal and amplitude differences
changes as well as spectral cues [24].
In terms of localization interaural time and level differences (ITD and ILD) are the two major cues
attributed to sound localization along the horizontal plane (see Figure 1). For low frequency sound
sources (<1.5 kHz), the detection of ITD is the primary mechanism [25]. This frequency threshold is
a result of the distance a sound wave must travel to reach the furthest ear, causing the phase angle to
differ upon arrival. The corresponding angle being both the function of the wavelength and the source
positioning. With frequencies below 1.5 kHz the wavelength is greater than then maximum difference
in time, this phase difference will therefore provide accurate location cues [26]. For higher frequency
targets, ILD detection takes over from temporal differences as the primary horizontal localization
mechanism [25]. Shorter high frequency sound waves are more susceptible to the acoustic shadowing
caused by the human head. It is this increased shadowing effect that produces a greater difference in
sound levels between the two ears [27].
Figure 1. Binaural cues for horizontal localization. Plots display two time-domain representations of
Head Related Transfer Functions (HRTF) recordings (45◦ azimuth & 0◦ elevation) extracted from the
SADIE Database [28].
Localization within the vertical plane is calculated from the spectral shaping of acoustic energy by
torso and the unique and complex geometry of the pinna. Frequencies are distributed and consequently
reflected causing minute but significant delays. It is these delays that will act as a comb filter effect on
sound entering the inner part of the ear, representing a three-dimensional function of the elevation of
the sound source [29,30].
The realistic reproduction of three-dimensional auditory environments through headphones,
known as binaural-based spatial audio, is dependent upon the correct simulation of the interaural
differences and spectral shaping cues caused by the torso, head and pinnae called Head Related
Transfer Functions (HRTF) [31]. HRTF’s are the functions of both frequency and position for both

Appl. Sci. 2019, 9, 3152 4 of 19
azimuth and elevation, containing direction-dependent information on how a free field sound
source reaches the ear [32,33]. Binaural rendering can then be achieved through the convolution of
a monoaural anechoic sound source and the HRTF of both the desired direction and the corresponding
ear [32,34].
Today most VR systems rely on head-tracked headphone-based systems (see Figure 2). Head
tracking is definitive in creating a plausible and realistic virtual audio scene. Current virtual reality
head devices can provide stable multi-modal interaction through accelerometers which can detect
force, 360 orientation and measure the device’s position within a 3-D space . Once the rendering
system has received the X, Y, and Z directional information, it performs interpolation between the
closest HTRF’s resulting perceived localization of a dynamic virtual sound source [35,36]. Furthermore,
by reproducing the audio over headphones it is possible to isolate the left and right audio channels and
successfully reproduce the binaural localization cues. However, there are several auditory processing
difficulties associated with autism spectrum disorders that may affect how a user may respond to
auditory cues within a VR intervention environment.
Figure 2. Block diagram illustrating audio signal chain for headphone-based spatial audio.
1.4. Atypical Auditory Processing in Autism
Commonly, children with autism experience complications in processing sensory information
with up to 65% of individuals with ASD displaying complications in auditory domain [37–39].
There has been extensive research pertaining to auditory processing in ASD. Observations are
diverse, ranging from enhanced abilities in musical pitch discrimination and categorization [40],
to hypersensitivity of particular sound stimuli which evoke extreme negative behavioral reactions [41].
Of particular note, there have been several psychoacoustic studies which have investigated auditory
scene analysis.
It has been reported that in the presence of competing background noise, those with autism
often have difficulties in separating relevant auditory information (i.e., speech and target noises)
into discrete auditory objects connected to the different sound sources within the environment [42].
These complications in processing sound information in noise have links to a deficit in auditory
spatial attention, which is often exhibited as a failure to orient to speech and other social stimuli [43].

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Soskey et al. [44] found that compared to typically developed (TD) controls, children with ASD showed
a significant impairment in attending to sounds, in particular speech sounds.
During typical development, an infant of 6-weeks can display a significant sensitivity to social
stimuli, with particular focus to the features of the human face and speech [45,46]. Autistic children
on the other hand have been reported to display atypical cortical responses to synthetic speech-like
stimuli, culminating in reduced involuntary attention to social auditory events [47]. These early limited
reactions to social stimuli represent the core social impairments in the earliest and most basic form and
have negative impacts on future social and communication development [48].
In addition, Lin et al. [42] observed a reduced response to speech in noise in individuals with
autism, alongside reduced discrimination for ITD and ILD. These binaural cues are processed
by an area of the auditory pathway known as the medial superior olive [49]. Interestingly,
postmortem examinations of the brainstem olivary complex in those with ASDs observed neurological
malformations resulting in a reduced size of the medial superior olive [50,51]. Despite this, people
with autism spectrum disorders have been shown to perform equally well as healthy controls in noise
localization experiments across the horizontal plane [52]. Therefore, an impaired sensitivity to ITD
and ILD cues in autism have been linked to deficits in selective hearing, which can affect orientation to
target sounds within noise [42].
2. Design and Hypothesis
This study aims to examine auditory spatial attention and sound localization ability of children
and adolescents with ASD within a multi-modal virtual reality game environment. The therapeutic
outcomes of VR interventions often rely on the immersion of the participant through place
illusion [53]. This is particularly important when designing virtual interventions for individuals with
autism [11], a population who have difficulties in generalizing newly skills to real-world applications.
By conducting investigations within a VR environment similar to which participants may experience
within an intervention setting, test parameters may be precisely controlled whilst the user interactions
and behavioral models can be robustly recorded. Furthermore, the inclusion of dynamic visuals and
game mechanics maintains participant immersion, therefore examining a pattern of behavior closer
to that within a conventional virtual reality experience removed from experimental conditions [54].
Previous work by Wallace et al. [55] measured spatial presence within the ‘blue room’, an advanced
collaborative virtual environment (CVE). The room makes use of visual images projected onto four
screens located on three walls and the ceiling, with audio being delivered over a surround sound
loudspeaker system. The paper reported that children with ASD showed equal amounts of attention
to the virtual content than healthy controls and engaged with the virtual environment requirements.
However, loudspeaker-based systems are often less effective at simulating the necessary ITD and ILD
cues needed for three-dimensional spatial audio reproduction. Speaker crosstalk causes significant
signal interference between each speaker and the opposite ear of the listener, resulting in impaired
localization performance [56]. In contrast, head-track headphone-based systems are more successful at
recreating dynamic soundfields which respond to the listeners position within the virtual world [36].
This investigation followed an in-between subject design. Participants were randomly allocated
to either a 3D spatial audio, or control group. The 3D audio group would be exposed to spatial audio
reproduced binaurally over headphones. The control group would listen to a monaural representation
of the virtual auditory scene presented to both ears over headphones. Each participant group completed
a two-phase experiment which first tested spatial audio attention, followed by a sound localization
task. Both within a multi-modal virtual reality environment (see Figure 3).
It is hypothesized that subjects exposed to binaural-based spatial audio would show more
spatial attention to auditory stimuli compared with the mono sound control group, indexed through
more precise head orientation towards the presented audio targets. Furthermore, spatial audio stimuli
would influence participant positional movement with the virtual environment. Additionally, based on
previous studies of auditory scene analysis deficits in ASD, it is predicted that localization times will

Appl. Sci. 2019, 9, 3152 6 of 19
be higher for audio cues with the most amount of competing environmental background noise. Finally,
by examining spatial attention towards complex speech and non-speech sounds, it is possible to
investigate if differences in spatial auditory attention are the consequence of a reduced orientation
towards social stimuli.
Figure 3. Flow chart displaying experimental procedure.
3. Methods
3.1. Participants
A study group consisted of 29 children and adolescents (27 male and two female, mean age =
14, SD = 2.42, range of 9–19 years). Participants were recruited from two special education schools
and one charity. All participants had a formal diagnosis of autism spectrum disorder obtained from
their local national health trust, displaying social, cognitive, and motor functioning associated with
moderate to high functioning autism. Exclusion criteria were self-reported hearing problems; physical
disabilities that would limit movement around experiment space; and an inability to ﬁnish the task.
Two participants were excluded due to not being able to complete the full experiment. This study
and methods were approved by the University of York board of ethical approval, and an information
package was provided to the participants parent(s) or legal guardian(s). Participants were admitted
into the study after informed consent and assent was obtained from their parent(s) or legal guardian(s).
3.2. Equipment
All audio and visual stimuli were rendered using the Oculus Rift CV1 head mounted display
(HMD) with built-in on-ear headphones. Head tracking was also achieved using the HMD, with motion
tracking of participant position calculated by the Oculus Rift sensors [57].

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3.3. Stimuli
Visual Environment: The virtual environment used for this experiment represents an enchanted forest
setting at night (see Figure 4). It has been noted that when designing visual aspects of virtual reality for
ASD applications , the presence of visual clutter significantly affects user performance at completing
tasks [58]. Therefore, the graphical environment was designed to be engaging without presenting
visual elements that may have driven user attention away from the auditory stimuli. Furthermore,
individuals with autism that also exhibit abnormalities in processing visual information can often
attempt to avoid visual input such as bright lights [59]. The use of a darker environment reduces the
possibility of anxiety related behaviors that are a result over sensory over-stimulation [60].
Figure 4. Visual Environment—360 degree capture.
Auditory Stimuli: Figure 5a,b are overhead views of the virtual environment displaying the auditory
stimuli placement and movement area, with a summarization of each audio object in Tables 1 and 2.
All stimuli were recorded or edited using Reaper digital audio workstation and WWise [61] game
audio engine. Experimental groups were based upon audio rendering techniques. The 3D audio group
would listen to binaural-based spatial audio rendered via the Google Resonance spatial audio SDK [62].
Audio was presented at a fixed playback level of 68 dbA. This was matched to a −20dBFS RMS value
of a pink noise test signal playing at a distance of 1m from the listener.
Phase One: Eight spatial auditory events (see Table 1) were placed in the VE. All audio events were
stationary with the exception of event 7 which moved around the entire environment over the course of
30 s. Previous research conducted by the authors has noted that target stimuli among competing audio
could prompt negative effects on auditory spatial attention within VR environments for individuals
with ASD [63]. This is further supported by research that reports within competing background noise,
those with autism often display difficulties in separating relevant auditory information (i.e., speech and
target noises) into discrete auditory objects connected to the different sound sources within the
environment [42]. To minimize the effects of any complications in auditory scene analysis associated
with ASD, environmental audio was reduced to −43 LUFS while each Phase One event was rendered.
Phase Two: Eight spatial audio events representing virtual characters were placed throughout
the VE. In terms of type, audio stimuli were divided into simple and social types. Simple stimuli
consisted of a bell sound effect, with the social stimuli representing a speech-like sound. All events
were stationary except for movement towards the player once the character was found.
A spatialized composite background audio track was provided which was derived from several
discrete auditory objects placed throughout the VE, matching the visual scene. This included sounds
representing wind, water, frogs, birds, crickets and wind chimes.

Appl. Sci. 2019, 9, 3152 8 of 19
The level of the background sounds was either ≥−43 LUFS or ≥−53 LUFS. These levels were
taken from a past study that evaluated the effects of background noise on spatial attention in
VE’s with participants with ASD [63]. Using an audio object-based hierarchical model based on
a simpliﬁed technique designed by Ward et al. [64] to increase the accessibility of complex audio
content for those with hearing impairments. Background audio levels would be reduced by removing
the bird, cricket, and wind chime ambient sound objects; the aim of this process is to maintain
participant immersion.
To further investigate auditory detection in noise, target stimuli were played at differing
signal-to-noise ratio (SNR). For auditory events with +10 SNR, the stimulus level would be 10 LUFS
higher than the background noise level. For auditory events with 0 SNR, the stimulus would be set at
an equal level to that of the background noise.
Table 1. Phase One Auditory Stimuli descriptions.
Auditory Event Stimulus Visual Cue
1 MIDI-based music–Synth Pad Present
2 Bell sound effects None
3 Bubble sound effects Present
4 Explosion & Speech-like stimuli None
5 MIDI-based music–Synth Pad Present
6 Crow sound effects None
7 Explosion & Speech-like stimuli Present
8 Explosion & Speech-like stimuli None
Table 2. Phase Two Auditory Stimuli–SNR represents either the stimulus played at +10 LUFS above
the background audio level, or set to a level equal to that of the background audio.
Auditory Event Stimulus Background Sound Level (LUFS) SNR
1 Simple ≥− 53 +10
2 Social ≥− 43 +10
3 Social ≥− 43 0
4 Simple ≥− 43 0
5 Simple ≥− 53 0
6 Simple ≥− 43 +10
7 Social ≥− 53 0
8 Social ≥− 53 +10
3.4. Procedure
Participants were permitted to move freely around a pre-deﬁned tracked experimental space of
1.6 m × 1.6 m while wearing the HMD. The combination of dynamic head and positional tracking
allowed movement around the virtual environment with 6 degrees-of-freedom.
Throughout the experiment a support worker would be present to communicate with the
participant and provide assistance if they became distressed. However, they were not permitted
to deliver instructions. Prior to the start of the session, the experimenter would explain the use of the
virtual reality system, the structure of the experiment and place the HMD on the participant in order
for them to become familiarized with the device.

Appl. Sci. 2019, 9, 3152 9 of 19
4 Simple ≥ - 43 0
5 Simple ≥ - 53 0
6 Simple ≥ - 43 +10
7 Social ≥ - 53 0
8 Social ≥ - 53 +10
Table 2. Phase Two Auditory Stimuli - SNR represents either the stimulus played at +10 LUFS above
the background audio level, or set to a level equal to that of the background audio
(a) Phase one (b) Phase two
Figure 5. Top-down view of auditory stimuli positions within the virtual environment
head rotation on the azimuth axis (y) and the target position of the reproduced stimulus (n) at 100ms263
intervals.264
a = 1 −
(n − y)
180
(1)
Further data recorded the target azimuth location in degrees of each spatial auditory event with265
the respective start and end times, and virtual environment positions. Higher values of a during266
auditory stimulus playback time would represent greater spatial attention towards presented auditory267
targets.268
In order to better understand if spatial audio influences subject behaviour tracked horizontal269
plane positional data within the VE represented by x and z was also collected at 100ms intervals. A270
distance score was then calculated by measuring the distance between the participant and each of the271
presented auditory stimuli during playback time. Smaller distance values during auditory stimulus272
playback time would indicate movement towards the presented stimuli, indicating an influence of273
participant movement.274
3.4.2. Phase Two - Spatial Audio Localisation and background noise testing275
Participants took part in a localisation task which required them to locate a total of eight hidden276
characters within the virtual world based on localising sounds that they emitted and limited visual277
representations. Each would play either social or simple stimuli (see Table 2.278
(a) (b)
Figure 5. Top-down view of auditory stimuli positions within the virtual environment. (a) Phase one;
(b) Phase two.
3.4.1. Phase One—Free Exploration and Spatial Audio Attention Testing
Prior to the main experimental task in Phase One, subjects were introduced to the virtual reality
equipment and environment for a period of two minutes with minimal environmental audio and
visuals. The experimenter explains to the participant that they are free to move around and explore the
VE. This brief initial time was used to provision for any emotional excitement experienced by using
virtual reality. The participant also receives support from the experimenter to become familiar with the
virtual environment. Following this, participants were given a further five minutes free exploration
during which they would be exposed to eight auditory events played once at pre-determined times,
four of which had visual accompaniments and four had no visual accompaniments (see Table 1).
This period would be used to record participant head rotation and horizontal tracked positional data.
Once again the experimenter tells the participant that they are free to move around and explore the VE.
After this verbal communication with the participant was minimized to control any influence that may
lead to any guided exploration during the experiment.
An accuracy metric (a) was used to evaluate participant performance during the spatial audio
attention task. This was quantified by calculating the relative difference between the participant
head rotation on the azimuth axis (y) and the target position of the reproduced stimulus (n) at
100 ms intervals.
a = 1 −
(n − y)
180
(1)
Further data recorded the target azimuth location in degrees of each spatial auditory event with the
respective start and end times, and virtual environment positions. Higher values of a during auditory
stimulus playback time would represent greater spatial attention towards presented auditory targets.
To better understand if spatial audio influences subject behavior tracked horizontal plane
positional data within the VE represented by x and z was also collected at 100 ms intervals. A distance
score was then calculated by measuring the distance between the participant and each of the
presented auditory stimuli during playback time. Smaller distance values during auditory stimulus
playback time would indicate movement towards the presented stimuli, indicating an influence of
participant movement.

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3.4.2. Phase Two—Spatial Audio Localization and Background Noise Testing
Participants took part in a localization task which required them to locate a total of eight hidden
characters within the virtual world based on localizing sounds that they emitted and limited visual
representations. Each would play either social or simple stimuli (see Table 2).
The experimenter first explains the task clarifying that the participant must use their ears to
find each character. To aid the participant in becoming familiarized with the target stimuli, they are
presented with an example character in their field of view which plays both social and simple auditory
stimuli. Following this, the experimenter would activate each hidden character at a time when the
participant was comfortable to do so and repeat the explanation of the task.
Once again support worker communication was restricted so as to avoid an effect on the outcome
of the investigation. The length of this phase would be dependent on the participants ability to progress.
When each virtual character was found the participant would be given positive reinforcement in the
form of verbal praise.
Localization times of auditory stimuli within the virtual environment during Phase Two
were recorded for each participant for all eight virtual characters. Further to group comparisons,
exploratory analyses of participant reaction time also examined the relationship between the amount
of background ambient noises and spatial attention. To do so, localization times for each character
were compared based on stimulus type, background audio level and SNR.
4. Results
4.1. Phase One: Spatial Audio Attention Testing
A two-way mixed ANOVA was conducted to investigate the impact of 3D audio and auditory
stimuli on the accuracy metric during Phase One. A comparison between test groups yielded
a significant difference (F(1, 26) = 29.43, p ≤ 0.001, η2
p = 1.642) across all auditory events, with the 3D
audio test group (M = 0.661) scoring higher accuracy scores than the mono audio controls (M = 0.487).
Figure 6 shows a comparison of group mean accuracy scores with 95% confidence intervals for
each event.
1 2 3 4 5 6 7 8
Auditory Event
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
AccuracyScore()
3D Audio
Control
Figure 6. Mean accuracy (α) scores for each audio condition across all auditory events in Phase One.
The whiskers denote 95% confidence intervals.

Appl. Sci. 2019, 9, 3152 11 of 19
In addition, there is a significant effect of stimulus type on spatial attention of both groups
(F(7, 175) = 7.657, p ≤ 0.001, η2
p = 0.227), with a significant interaction between groups and stimulus
type (F(7, 175) = 2.426, p = 0.021, η2
p = 0.072). Finally, the use of visual accompaniment had no
significant effect on the accuracy of spatial attention in the spatial audio group yielded by visual
accompaniment (M = 0.627 SD = 0.187) and no visual accompaniment (M = 0.695 SD = 0.184)
paired-samples t-test; t(55) = −1.903, p = 0.062. However, there was a significant difference between
visuals (M = 0.543 SD = 0.235) and no visuals (M = 0.431 SD = 0.176) within the mono control
group; t(51) = −2.827, p = 0.007.
To better understand behavioral response to spatial audio within the virtual environment, tracked
positional data across the two groups was also analyzed. The tracked positional coordinates of all
participants on the horizontal axis collected throughout the entirety of Phase One and are visualized
as 2-dimensional density plots shown in Figure 7. In addition, the figure also displays the directions
and distance of all auditory events inside and outside of the tracked area.
(a) (b)
Figure 7. Two-dimensional density plot of positional tracking data throughout Phase One for all
participants. Arrows point toward the target audio objects outside of the plot area. Distance from
origin to audio objects—1 (6.8 m), 2 (4.57 m), 3 (5.98 m), 4 (12.43 m), 5 (5.96 m), 6 (3.86 m), 8 (5.74 m).
(a) Spatial audio condition; (b) Mono condition.
The figure shows differences in the extent of exploration depending on the auditory condition.
Despite both groups showing a condensation of exploration around the center of the tracked space,
the 3D audio group show a distribution of exploration in the direction of the majority of auditory
targets. Further analysis of the distance between each participant and the auditory target using
a two-way mixed ANOVA also yielded a significant difference between the auditory conditions
(F(7, 175) = 23.111, p ≤ 0.001, η2
p = 0.308). With participants within the 3D audio group moving
closer towards the virtual sound sources during their respective playback times.
4.2. Phase Two: Spatial Audio Localization and Background Noise Testing
A two-way mixed ANOVA showed that these differences in localization times were also
significantly different (F(7, 175) = 7.565, p ≤ 0.001, η2
p = 0.232). Figure 8 shows the mean
localization times for both experiment groups across all auditory events in Phase Two, which shows
shorter localization times in the 3D audio condition (M = 8.4 s) than in the mono control condition
(M = 17.8 s).

Appl. Sci. 2019, 9, 3152 12 of 19
Figure 8. Mean localization times for each audio condition across all auditory events in Phase two.
A hierarchical linear model was used to analyze the effects background audio, SNR, and type of
stimulus have on localization times within both the 3D audio and mono control conditions, the results
of which can be seen in Figure 9. Inferential statistics reported a significant overall effect of background
audio on the time taken to locate each auditory event for both groups: 3D audio (F(1, 54.713) = 15.243,
p < 0.001) and mono audio (F(1, 84) = 10.476, p = 0.002). Auditory stimuli within the higher
background audio level of −53 LUFS (3D audio M = 10.4 s and mono audio M = 20.9 s) displayed
higher localization times than those with −43 LUFS (3D audio M = 6.5 s, mono audio M = 15.4 s).
Further to this, the effect of SNR between background sounds and auditory stimulus was only
significant for the 3D audio group (F(1, 71.029) = 5.206, p = 0.026), with the time taken to correctly
locate auditory stimuli +10 LUFS above the background audio level (M = 7.4 s) being lower than those
with no SNR (M = 9.5 s). However, the background audio and SNR interaction was not significant for
the 3D audio group (F(1, 71.729) = 1.3586, p = 0.248). Finally, the type of auditory stimulus yielded
a significant effect on the results of localization times for both groups: 3D audio (F(1, 68.778) = 5.545,
p = 0.021) and mono audio (F(1, 84) = 4.930, p = 0.029), showing participants taking longer to locate
social stimuli (3D audio M = 9.5 s, mono controls M = 19.33 s) than simple (M = 7.3 s, mono controls
M = 16.17 s).
Simple
0 SNR
Social
0 SNR
Simple
+10 SNR
Social
+10 SNR
5
10
15
20
25
LocalisationTime(secs)
- 43 LUFS Background Level 3D Audio Group
Simple
0 SNR
Social
0 SNR
Simple
+10 SNR
Social
+10 SNR
5
10
15
20
25
- 53 LUFS Background Level 3D Audio Group
Simple
0 SNR
Social
0 SNR
Simple
+10 SNR
Social
+10 SNR
5
10
15
20
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LocalisationTime(secs)
- 43 LUFS Background Level Mono Controls
Simple
0 SNR
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0 SNR
Simple
+10 SNR
Social
+10 SNR
5
10
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20
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- 53 LUFS Background Level Mono Controls
Figure 9. Estimated means data from hierarchical linear model, comparing the effect of background
audio, signal-to-noise ratio, and stimulus type for binaural spatial audio and mono control groups.

Appl. Sci. 2019, 9, 3152 13 of 19
5. Discussion
5.1. Phase One: Spatial Audio Attention Testing
Throughout Phase One, subjects in both groups displayed overall attention towards the presented
auditory targets; however, those within the 3D audio group displayed showed higher levels of
head orientation in the direction of audio events. These results demonstrate that despite reported
impairments in interpreting ITD and ILD binaural cues associated with autism [65], spatial audio was
capable of accurately attracting participants to the areas of the virtual environment applicable to the
auditory stimulus’s location.
Individuals with autism have been recorded as having similar evoked brain responses to
speech-like stimuli than their typically developed peers, but at the same exhibit less involuntary
attention towards it [66,67]. With this in mind, it would be expected that under the test paradigms
of free exploration that measures involuntary attention, accuracy scores for speech stimuli to be the
lowest. However, the findings from Phase One of this experiment are contrary to this for both condition
groups, with the auditory event with the lowest scores being the crow (event 6). This auditory stimulus
type would appear to be similar to the ambient environmental audio track and despite the lack of
competing audio present the event was not unique enough to warrant participant attention. In the case
of attention towards speech-like sounds in Phase One, these events are a combination of explosion and
non-human speech-like stimuli, designed to create a unique audio cue that attracted attention towards
them. Furthermore, studies have observed hyperactivity in the brain with response to novel auditory
stimuli in children on the autistic spectrum [68].
Research by Lokki and Grohn [69] revealed that despite audio cues being less accurate for sound
source localization than accompanying visual cues, within a virtual environment audio-only cues
are almost equally effective as visual only cues. In addition, when objects were placed outside of
the participants field of view or subjected to occlusion of other visual objects, audio-only cues were
more successful in the preliminary phase of object localization. This could serve as justification for the
significant effect visuals accompaniments had on the overall accuracy scores for participants in the
mono audio group. The visual representations would compensate for the spatial inaccuracies of the
audio rendering. This would also account for the similar scores present in both groups for event three
(e.g., bubble sound). This incorporated visuals that spanned a large area of the VE which would attract
user attention once it arrived into the participant field of view.
Exploration of the virtual environment is also illustrated in the positional data density plot
(see Figure 7). For both conditions tracked horizontal movement was concentrated close to the starting
position of (x, y = 0, 0). Nonetheless, participants listening to spatialized audio displayed clear
movement in the direction of both the auditory only and audio-visual cues. Due to the grouping of
both audio-visual and audio-only events it cannot be confirmed which events encouraged participant
exploration in those areas. However, the reduced amount of tracked data in the direction of event
three does suggest that the larger visual stimuli negate the need for participant exploration of the VE.
5.2. Phase Two: Spatial Audio Localization and Background Noise Testing
Alongside comparing localization times between auditory test conditions, Phase Two of this study
was carried out to determine if reported impairments in decoding speech when background noise
is present would have quantifiable effects on how they would respond to similar stimuli rendered
via virtual spatial audio within a VR environment. Considering the heterogeneity of disorders in
the autism spectrum present within the sample group, it is encouraging to see that all participants
were capable of performing this task without any particular difficulties. However, the differences in
performance between participants within the experimental groups may also provide explanation for
the low effect size yielded by the two-way mixed ANOVA.
Firstly, results in Phase Two showed those in the 3D audio group performing significant better in
the localization task. These results are consistent with similar research conducted with neurologically

Appl. Sci. 2019, 9, 3152 14 of 19
healthy participants, reporting that three-dimensional auditory displays can reduce localization times
and improve audio spatial attention in VR [70]. Furthermore, these results provide more evidence that
those with ASD are capable of successfully interpreting binaural information to take advantage of the
spatial acuity provided by spatial audio rendering techniques.
In regards to the effect of background levels on selective hearing, results showed that localization
times were significantly longer when the level of background noise increased and/or the SNR between
the auditory target and ambient noise decreased. Furthermore, the type of stimuli also had a noticeable
impact on localization times, with participants taking longer to correctly locate speech-like stimuli. This
data is comparable to evidence of speech processing deficits in children with ASD in non-spatialized
audio test conditions [44,52,67]. Although the results from the mono control group would be sufficient
in evaluating the effects of background noise on the response to target audio, mono audio is rarely
used with virtual reality environments. Therefore, for this study it was important that competing
background audio had a significant effect on both experiment conditions.
Individuals with ASD tend to demonstrate elevated speech perception thresholds, poor temporal
resolution and poor frequency selectivity when compared to their typically developed peers [71].
The poor frequency selectivity alone could account for poor localization along the vertical plane [52],
as this is primarily attributed to the analysis of the spectral composition of the sound arriving at the
ear [49]. However, a combination of all three alongside a diminished ability to correctly translate ILD
and ITD sound cues would account for poor selective hearing leading to difficulties with behavioral
recognition of auditory targets in background noise [42,71].
6. Limitations and Future Work
One limitation of this study is the lack of a comparable typically developed control group.
Comparison data may have highlighted any potential differences in performance between participants
with autism and neurologically healthy counterparts. Nonetheless, participants in this study still
showed higher spatial attention towards binaurally rendered spatial audio. Demonstrating quantifiable
evidence of audio spatial interactions.
Another important consideration is the use of a non-individualized HRTF’s database within this
study. This database incorporates anechoic recordings using the Neumann KU-100 dummy head
simulator which is-based upon average dimensions of the human head and ears [28]. HRTF’s differ
greatly between individuals, due to the unique shape of the pinnae, head, and torso. Therefore, the use
of a generalized HRTF can sometimes result in localization confusion and reduced externalization
of target auditory events within a virtual environment [72,73]. In addition, recent research which
compared individualized to non-individualized HRTF’s did observe higher localization errors when
using a generic database [74]. This may provide some explanation for the varying accuracy and
localization times between participants within the 3D audio experiment group as well as the lower
effect scores yielded in the statistical analysis. However, there are significant challenges involved in
obtaining individualized recordings which warrants the use of generic dummy head recordings in the
development of virtual reality soundscapes [75].
Future investigations could also build upon the work carried out by Wallace et al. [55] and measure
self-reported presence within virtual environments that use spatial audio rendering techniques.
Changes in behavior, spatial focus, and interactions when altering the audio reproduction could be
compared. Furthermore, studies could be conducted to measure if removing aspects of the background
audio has any negative effects on the feelings of presence felt by the participants.
7. Conclusions
Alongside realistic graphical rendering and natural approaches to computer interaction,
spatialized audio can be used to significantly enhance presence and improve user interaction within
a virtual reality environment [76]. In terms of clinical applications within autism research, presenting
a more realistic experience would benefit interventions such as the treatment of phobias and vocational

Appl. Sci. 2019, 9, 3152 15 of 19
training. By matching the visual and auditory sensory experiences of the real world, users will have
a greater chance of generalizing newly acquired skills into their everyday lives, therefore increasing
the possible positive outcomes of VR-based therapy [77].
To date, this study is the first to assess how those with autism respond to virtual spatial audio
within a virtual reality game environment. The experiment involved 29 individuals diagnosed with
autism being randomly allocated to 3D audio and mono control groups. All participants completed
two experimental phases. The first compared spatial attention towards auditory stimuli by recording
head rotation accuracy and horizontal movement towards presented audio. In Phase Two, participants
were required to localize four social and four simple stimuli among varying levels of background
audio. Results collated from head rotation and tracked positional data during the experiment do
suggest that despite reported auditory processing difficulties, children with autism can make use
of auditory cues rendered by spatial audio. In addition, the amount of background audio can have
significant effects on the localization of auditory stimuli within a VE. These findings could provide
valuable insight to those designing virtual reality interventions for the autistic population. Possible
developers should make use of similar binaural-based spatial audio rendering approaches used in this
study to increase the ecological validity of the virtual environment, deliver important information via
auditory cues.
This study has shown that previously reported difficulties in auditory scene analysis associated
with autism do extend into the realms of virtual reality and binaural-based spatial audio. The amount
of competing background audio can have an effect on spatial attention towards virtual sound targets
and so therefore this should be taken into consideration when designing three-dimensional virtual
acoustic environments for ASD interventions.
Author Contributions: Conceptualization, D.J. and G.K.; Methodology, D.J.; Software, D.J.; Validation,
D.J., H.E. and G.K.; Formal Analysis, D.J., H.E.; Investigation, D.J.; Resources, D.J.; Data Curation, D.J.;
Writing—Original Draft Preparation, D.J.; Writing—Review & Editing, D.J., H.E. and G.K.; Visualization, D.J.;
Supervision, H.E. and G.K.; Project Administration, D.J.; Funding Acquisition, G.K.
Funding: Funding was provided by a UK Engineering and Physical Sciences Research Council (EPSRC) Doctoral
Training Award, via the Department of Electronic Engineering at the University of York, EPSRC Grant Number:
EP/N509802/1.
Acknowledgments: The authors would also like to thank Accessible Arts & Media (York, UK), Springwater
School (Harrogate, UK) and Riverside School (Goole, UK) for their participation and support of this research.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to
publish the results.
Abbreviations
The following abbreviations are used in this manuscript:
ASD Autism Spectrum Disorders
CVE Collaborative Virtual Environment
HMD Head Mounted Display
SNR Signal-To-Noise Ratio
TD Typically Developed
VE Virtual Environment
VR Virtual Reality
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