Experiment 3: The emergence of an external reference frame for touch in infancy

Locating a touch on the body is not a simple task; we do not simply locate touch on the skin surface but also within the external environment. In order to represent the location of a touch in external coordinates, adults dynamically remap the relation between tactile stimuli and external coordinates (Azañón & Soto-Faraco, 2008; Azañón & Soto-Faraco, 2011;

Graziano et al., 2004; Kitazawa, 2002; Overvliet et al., 2011).

As previously related, studies investigating the crossed-hands effect have shown that adults and children make errors in localizing tactile stimuli that are applied to the hands, often making errors in localization, when their hands are crossed over and in the side of space contralateral to their usual placement (Pagel et al., 2009; Overvliet, Azañón & Soto-Faraco, 2011; Groh & Sparks, 1996; Begum Ali et al., 2014 and Experiment 2 of this thesis). This has been attributed to conflict between the anatomical and the external frames of reference for locating a touch when the hands are crossed.

Research from congenitally and late blind individuals indicates that early visual experience plays an important role in the emergence of

external spatial reference for touch. Röder, Rössler and Spence (2004) investigated the “crossed-hands” deficit in a population of visually impaired individuals and found that those that were congenitally blind (i.e. blind from birth so did not have any experience of vision) did not show the “crossed-hands” deficit. However, those individuals who had acquired a visual impairment (and thus, had some visual experience in infancy and early childhood before losing their vision) performed comparably to typical, blindfolded adults without any visual impairments. In other words, the late blind participants accuracy in locating the site of the initial tactile stimulus was significantly reduced in the crossed-hands posture, compared to when the hands were uncrossed.

Röder et al. (2004) suggested that a possible explanation for this pattern of results is that congenitally blind adults may use solely an anatomical reference frame when locating touches on the body. According to this account, without an external frame of reference for touch, a conflict between the anatomical and external reference frames does not occur in the crossed hands posture and thus accuracy is comparable across postures. From these findings, the researchers concluded that early visual experience is paramount to the development of an external reference frame for locating tactile stimuli.

According to Röder et al. (2004) the comparable performance of the late-blind with control participants is the result of visual experience in early life prior to the onset of blindness. The researchers argue that even just a few years of visual experience may drive the emergence of an

external reference frame for coding touches to the body. Recent research indicates that there is a critical period in early development within which visual experience impacts on tactile spatial perception. Ley, Bottari, Shenoy, Kekunnaya and Röder (2013) report a case study of H. S., a 33- year-old male who was completely blind in the first 2 years of his life, as a result of congenital cataracts. At 2, after undergoing surgery to remove his cataracts, normal vision was restored. H. S. took part in a tactile TOJ experiment and it was found that even after his sight was restored at 2 years of age, he did not demonstrate using an external reference frame to locate touches on the body in this particular tactile localization TOJ task.

H. S.’s tactile localization accuracy did not differ across the two arm postures and was comparable to that of congenitally blind individuals in Röder et al. (2004), even though normal vision had been restored for thirty years. This supports the argument that the external frame of reference in which touches to the body are coded is influenced by visual experience in early life, with this sensitive period constrained to the first 2 years of life.

So, in regards to the sensitive period in which vision is particularly important in the emergence of an external reference frame, this raises the question of when and at what stage in the first two years does visual experience give rise to the acquisition of external spatial coding? Research with typically developing human infants suggests that the first half year of life may be important. Bremner, Mareschal, Lloyd-Fox and Spence (2008) have shown that 6.5-month-old infants manual orienting behaviour to touches was significantly worse when their hands were crossed.

Bremner et al. (2008) concluded that 6.5 months are able to use an external reference frame to code touches to the body.

In Experiment 1, the developmental trajectory (in childhood) for the use of an external reference frame when locating touches to the limbs was investigated. In this particular study, I investigated the emergence of an external reference frame for touch within the first half year of life. In order to do this, I looked at the crossed hands deficit in 4- and 6-month-old infants (Experiment 3). From previous pilot work conducted, it was found that it is difficult to persuade young infants to cross their hands over. In comparison, the legs appear to have more postural freedom. Previous research has shown that crossing the feet also elicits deficits in tactile localization in adults (Schicke & Röder, 2006), so it was decided instead to cross infants’ feet.

3.10 Methods

3.10.1 Participants

Eighteen 4-month-olds (9 males), aged between 104 and 134 days (M = 116 days; SD = 8 days) took part in this study. One female participant was excluded from the final analyses, due to equipment errors, leaving a total of 9 male and 8 female participants in this age group. Additionally, fourteen 6-month-olds (5 male), aged between 177 and 220 days (M= 196 days; SD=13 days) also participated. One male 6-month-old was excluded

prior to analyses due to his fussy behaviour in the testing session, thus leaving 4 male and 9 female participants in the 6-month-old age group.

Informed consent was obtained from the parents before testing commenced. The testing took place only if the infant was awake and appeared to be in an alert and content state. Ethical approval was gained from the Ethics Committee of the Department of Psychology, Goldsmiths University of London.

3.10.2 Apparatus and Materials

Infants were seated in a specialist baby chair (Bloom Loft high chair). The seat was reclined in a horizontal position with the back-rest parallel to the floor. Adjustable straps secured the infant in the seat. Cotton padding around the trunk and a head-rest were used to secure the posture of the infant’s head and trunk. A digital video camera located 80 cm in front of, and 60 cm above, the chair, facing the infant’s frontal midline recorded the movements of the infant’s foot. Video data were recorded for offline coding.

The vibrotactile stimuli were delivered by two voice coil tactors (that the experimenter placed on the soles of the infant’s feet, securing them in place with cohesive bandage) driven by a 220 Hz sine wave and controlled by custom software scripted in E-Prime. The E-Prime script also sent commands to a serial-controlled video titler so that the infants’ stimulus-locked behaviour could be observed and coded. Any noise emitted by the tactors was masked with grey noise played from a centrally placed

loudspeaker. This masked sound cues for both the infant and experimenter.

3.10.3 Design

Infants were presented with a maximum of three blocks of experimental trials. Each block contained 10 experimental trials in which a 1000 ms vibrotactile stimulus was presented to one of the infant’s feet in pseudorandom order (which was R, L, L, L, R, R, L, L, R, R). This 1000 ms vibrotactile stimulus was followed by an 8000 ms interval to allow sufficient time for the infant to react to the stimulus. Every 5 trials, the posture of the infants’ legs was alternated between crossed and uncrossed.

Whether crossed or uncrossed posture was adopted in the start of each block was counterbalanced between participants.

3.10.4 Procedure

On each trial, one experimenter (Experimenter A) held the infant’s legs around the ankle placing them in the assigned posture (uncrossed or crossed, with feet approximately 10 cm apart, see Figure 3.6), whilst a second experimenter (Experimenter B) controlled the delivery of stimuli within the E-Prime program. At the beginning of each trial, Experimenter A placed the infant’s legs in the required posture. A trial was then triggered by the Experimenter B. During the stimulus delivery,

Experimenter A gently held the infant’s legs in the assigned posture until the infant either moved their legs, or 8000 ms had elapsed, at which point the trial was terminated. In the 8 second period following each stimulus, Experimenter A oriented her face to the floor, in order not to distract the infant. If the infant became fussy, they were entertained with musical toys and/or bubbles until they were settled enough to continue with the study.

The study continued for as long as the infant was willing to participate, with included participants completing a minimum of one block (10 trials), and maximum of three blocks (30 trials).