Materials and Methods
Participants
We tested 48 participants (29 Female, 19 Male, Age M=22.1, SD= 3.9, IQR
= {19,23}) recruited by opportunity sampling at Bournemouth
University. Participants received £5 for their time. The study was
approved by the Research Ethics Committee of Bournemouth University and
was carried out in accordance with the principles of the Declaration of
Helsinki. Participant numbers could not be determined by standard power
calculation. Similar eye-tracking tasks require 6 to 10 participants
(e.g. Meso, Montagnini, Bell, & Masson, 2016; Meso, Gekas, Mamassian,
& Masson, 2022), so trait inventory requirements determined numbers.
Previous power estimates suggested 45 participants (Meso, De Vai,
Mahabeer, & Hills, 2020), as did a recent reliability and replicability
study on SPEM and traits (Schröder, Baumert, & Ettinger, 2021).
Stimulus and Materials
Stimuli were generated on a Windows 7 PC running bespoke Matlab
(Mathworks) routines in Psychtoolbox (Brainard, 1997; Pelli, 1997).
Presentation was on a Cambridge Research Systems 32’ Display++ Monitor
with 1920 x 1080-pixel resolution and 100Hz refresh rate. The monitor
was placed 80cm from participants. Eye movements were recorded from the
right eye using an SR Eyelink Video eye tracker operating at 1000Hz with
movement restricted by a head/chinrest. The stimulus was based on Meso
et al., (2020), with sizes scaled to an on-screen virtual square with
sides of 900pix containing the stimulus presentation area of 23.4
degrees of visual angle (°). The black ball had 0.21° diameter with
motion characterised by Equations (1) to (4).
Vx(t ) = d ∙Sx (1)
Px(t ) = X0 +d ∙Sxt (2)
Vx in Equation (1) is the constant horizontal component
of the speed with Sx set from {4, 16} °/s for
fast/slow and direction d set from {-1,1} for left/right. The
time-varying horizontal position Px in Equation (2)
depended on Sx starting at the centre of the screen,
X0.
Vy(t ) = Sy + ε +g ∙t (3)
Py (t) = Y0 + (Sy +
ε)t + (g ∙t 2)/2 (4)
Vy in Equation (3) is the vertical speed component
initiated as Sy = 2°/s and ε is a number from a flat
continuous distribution of ± {0 to 0.5}°/s away from the direction of
acceleration g , which is ±9.81°/s2 for the
gravity (+) and antigravity (-) conditions. The position
Py in Equation (4) incorporates the initial position at
the centre of the screen Y0 and the integration of
Equation (3) for position. The resulting motion is that expected for a
ball just smaller than a professional soccer ball.
Procedure
Participants were screened for normal or corrected-to-normal vision with
a visual acuity letter chart. Bespoke Matlab programs were used for
trait inventories with mouse clicks to record responses. The 74-item
Schizotypal Personality Questionnaire – SPQ (Raine, 1991) and 24-item,
5-point Subthreshold Autism Trait Questionnaire – SATQ (Kanne, Wang, &
Christ, 2012) were used. The tracking task was separated into three
blocks of Gravity, Antigravity and Control. In the control condition,
stimulus orientation was rotated by 90 degrees from the gravity
condition so that vertical motion was defined by Equations (1) and (2)
and horizontal by (3) and (4) and gravity acted rightwards. Each block
had 160 trials of 1.25s duration with participant-initiated button
presses to proceed. Trials started with a 500ms central dark grey
fixation circle which disappeared at trial onset and the stimulus was
followed by a grey screen. Participants were instructed to fixate on the
central spot and track the ball as well as they could. Blocks contained
80 fast and 80 slow trials and lasted approximately 10 minutes. The task
interleaved the inventories with the conditions fixed in the same order
i.e. SPQ, gravity, SATQ, antigravity and control, with breaks in between
so that it lasted about 40 minutes. Changes from the procedure of Meso
et al., (2020), were that participants always started on the gravity
condition, trials were shorter (1.25s not 2s), tasks included the
control condition and we used a higher precision psychophysics screen,
the CRS Display++.
Design and data analysis
We used a multivariate within-participants design. The Independent
Variables were Gravity direction with three levels: Gravity (G) –
downwards acceleration, Antigravity (AG) – upwards and Control (C) -
rightwards; ball Speed with two levels: Slow (4°/s) and Fast (16°/s).
The key measures were the two inventories: the SPQ and SATQ, RMSE (Root
Mean Square Error between dynamic eye position and ball position), and
Saccades (rates and sizes). We also recorded participant age and sex.
Data pre-processing to extract the RMSE and Saccades is detailed in Meso
et al., (2020). We reduced the RMSE responses to 25 samples, each
covering a 20ms window from onset at 0ms to 500ms. RMSE was measured for
the x-direction capturing responses to the motion component at aconstant speed for conditions G and AG, and for the y-direction
capturing responses subject to acceleration due to simulated
rightward gravity . We quantified learning as performance improvement
during a block of 80 trials, by subtracting averaged RMSE value for the
last 20 trials from that of the first 20 trials under the same speed
condition. Learning was positive if there was improvement. We analysed
the trait responses and a restricted subset of eye movement measures
focusing on (i) the relationship between SPQ and SATQ, (ii) the
relationship between the inventories and RMSE measures, (iii) the
relationship between the inventories and the saccades, and (iv) dynamic
changes in the relationships between both RMSE and saccades under the G
and AG conditions. For each of the four, we ran correlations with alpha
adjusted for the number of comparisons undertaken. (v) We also ran mean
comparisons between gravity conditions for RMSE, Saccades and
RMSE-learning. These were done using a Wilcoxon signed rank test because
of the deviations from normality. Finally, we ran a correlation type
Principal Components Analysis (Joliffe, 2002) to unpack the relationship
between a restricted set of 21 measures. These variables were selected
to cover sub-traits of the SPQ and SATQ, Saccade rates and amplitudes
and RMSE measures, both at stimulus onset (0ms) and during the open loop
of response (~160ms). Meaningful components were
identified using parallels analysis of the variance (Joliffe and Cadima,
2016). For simplicity, we focused analysis on the slow speed condition
which showed similar patterns to the faster condition but was an easier
tracking task.