The Effect of Systematic Variance in Action Capabilities on Grasp Ability Perception.

The Effect of Systematic Variance in Action Capabilities on Grasp Ability Perception.

Megan Rose Readman

2018

The ecological approach to visual perception asserts that individuals perceive environments relative to the possibility of action within their environment. Hence, to successfully interact with one’s environment, individuals must be able to accurately perceive the extent over which actions can be performed, widely referred to as action boundaries. Furthermore, as the world in which we inhabit is continually changing and subsequently placing various constraints upon ones action boundaries, it is necessary for individuals to be able to update their perceived action boundaries to accommodate for such variance. While research has displayed that individuals can update their perceptions to accommodate for variance, what is unclear in these circumstances is which action boundary does the perceptual system calibrate to. This study investigated this by analysing the effect of systematic variance on perceived grasp ability in virtual reality. Participants provided estimates of grasp ability following motor experience grasping with either a small, normal, large or a varied size hand. In the variance condition, participants experienced the small hand 25% of the time, the normal hand 25% of the time, and the large hand 50% of the time. The results indicated that participants’ perception of grasp ability reflected the artificial manipulation such that grasp ability was largest in the large hand condition. In addition, regarding the variable condition participants took all visual information into consideration however erred on the side of caution. However, it may be that factors such as age and personality influenced the results.

Embodied perception
Grasp ability
Affordance perception
Virtual Reality

Open Science Framework (OSF)
This study has been pre-registered with the OSF; See https://osf.io/zkjdt/ for the main OSF project page. The following study deviated from the pre-registration in that data collection occurred for 12 days longer than initially intended as participant uptake was not as high as initially assumed it would be.
Participants
30 Lancaster University Students (5 males and 25 females) aged between 18-26 (Mage = 21.07, SDage = 1.17), naïve to the purpose of this study, participated. All participants were recruited via opportunity sampling, utilising the Lancaster University Sona research participation system, advertisements and the researcher’s social network, and were paid £5 for their participation. Of these participants 29 were right-handed, and one was mixed-handed. The one mixed-handed participant elected to complete the study with their right hand, therefore, the following conclusions and data should be treated as all right-handed participants. In addition, all participants had normal or corrected-to-normal vision and had no known medical history of visual atypicalties, beyond being long or short-sighted, motoric or rheumatologic difficulties. All participants provided informed consent. Lancaster University Research Ethics Committee granted ethical approval for this study.
Stimuli and apparatus
A virtual environment was developed in Unity 3D© Gaming Engine with the Leap Motion plugin. The 3D VR colour display comprised a 3D model of a room in which a table was located in the centre. Upon this table were either two grey dots (in the calibration trials; See Panel A of Figure 2) or a grey block (block size manipulation trials; See Panel B Figure 2). The participant’s viewed the VR from a first-person perspective reflecting their natural eye-height. The environment was presented to participants through an Oculus Rift CV1 HMD, which displayed the stereoscopic reality at 2160×1200 at 90Hz split over both displays (Binstock, 2015).
The movement of the head was tracked by the head mounted display (HMD) and updated in real-time as the participant looked around the environment. Furthermore, the location of the hand was tracked in real-time, using the Leap Motion hand-tracking sensor mounted on to the Oculus Rift CV1 HMD, and was mapped onto the virtual hand thereby causing the virtual hand to move in correspondence with the natural hand.

Procedure
Each participant was required to attend one testing session, which lasted approximately 30 minutes in duration. Prior to the commencement of the study, full information regarding the requirements of the study was provided by means of a written information sheet. This information sheet was supplemented with a verbal explanation and an opportunity to ask questions. Once full understanding of the study requirements was established, participants provided informed consent and were reminded of their right to withdraw. Following the attainment of consent, participants were required to complete a simple demographic questionnaire notably detailing the participant’s age, sex, hand dominance, and the presence of ocular atypicalities and motoric or rheumatologic difficulties. Critically, at this time the grasp that the participants are required to visualise employing during the perceptual task was defined and demonstrated. This grasp was defined as the ability to place their thumb on one edge of the block and extend their hand over the surface of the block and place one of their fingers on the parallel edge of the block.
Following this participants were required to don the oculus rift HMD with attached Leap Motion Sensor and complete four experimental conditions, the order of completion was randomly counterbalanced across participants. The four experimental conditions were the constricted grasp condition, the normal grasp condition, the extended grasp condition and the systematically varied grasp condition. In the constricted grasp condition participants gained motor experience with a virtual hand that was 50% of the size of their actual hand, therefore constricting the grasp to 50% of the normal grasp ability. In the normal grasp condition participants gained motor experience with a virtual hand reflecting the true size of their actual hand, therefore grasp ability was 100% of their normal grasp ability. In the extended grasp condition participants gained motor experience with a hand that was 150 % of the size of their actual hand thereby extending their grasp ability 50% beyond normal grasp ability. Whilst in the systematically varied grasp ability condition the participants experienced the constricted hand size 25% of the time, the normal hand size 25% of the time and the extended hand size 50% of the time.
Each experiential condition consisted of two phases: the calibration phase and the block size manipulation phase. The calibration phase consisted of 30 trials in which participants viewed the virtual display comprising of a table upon which two grey dots, one to the left and one to the right, were located (See Panel A Figure 2). The inclusion of a calibration phase occurred to provide the participants with the necessary amount of synchronous visual motor information to subsequently induce the illusion that the virtual hand is the participant’s hand (Kilteni et al., 2012). The engagement of this illusion is critical because if the participants do not employ this illusion, the subsequent results will not accurately reflect the study manipulations. In addition, the calibration phase provided participants with visual and motor experience regarding the action boundary associated with the virtual hand.
Completion of the calibration phase required participants to touch the leftmost dot with the leftmost digit of their dominant hand and the rightmost dot with the rightmost digit again of their dominant hand. Participants were informed that it was ok if they could not reach the dot so long as they performed the action. After the participants had performed the action touching both dots, the two dots disappeared and reappeared in a different location on the table. The location of the dots and the distance between the dots was randomly varied across all 30 trials. However, the distance away from the participants that the dots appeared was maintained throughout as dictated by the Z coordinate in the study script.
On completion of the calibration phase participants were instructed to place both their hands on their lap, this occurred so that the hand was out of range of the Leap Motion Sensor and hence the virtual hand was not visible in the virtual reality. At this time the virtual reality display was altered so that that the participant viewed the display of the table upon which there was a white block located (See Panel B Figure 2). Once the new display was presented the researcher placed the participant’s hand, they had just completed the calibration phase with on the right and left arrow keys of a standardised QWERTY keyboard. Participants were then instructed to imagine that they were going to grasp the block, employing the previously demonstrated grasp, and manipulate the size of the block to reflect the maximum size they believe they would be able to grasp with their dominant hand using the right and left keys. Each button press altered the size of the block by 1cm. Once the participant was happy that the block reflected the maximum size they could grasp with their dominant hand the researcher saved the final size and presented another block. This phase consisted of eight trials, in four of which the block started small at 3cm and the remaining four the block started large at 20cm. This occurred in order to control for the potential influence previous perception has on later judgements, a phenomenon commonly known as hysteresis (Poltoratski & Tong, 2014)
On completion of both the calibration and block size manipulation phases for each four conditions participants were given a short verbal debrief regarding the true aims and theoretical underpinning of the study and an opportunity to ask any questions. To supplement this verbal debrief participants were also provided with a written debrief again documenting the aims and theory of the study and contact details for the lead researcher.
The subsequent raw data obtained included eight maximum grasp block size estimates, four relating to the block that started at 3cm and four relating to the block that started at 20cm, for each experimental condition; small hand size, normal hand size, large hand size and variable hand size. Therefore 32 estimates were obtained from each participant.

Lancaster University

data/SPSS.sav

Readman2018

Ellie Ball

Open

None

English

Data

LA1 4YF

Dr Sally A. Linkenauger

MSc

Cognitive Psychology

30 Lancaster University Student (5 males and 25 females)

ANOVA