Spatial frames of reference for touch in children and infants
3.11 Data coding
The direction and latency of infants’ first foot responses to the tactile
(A) (B)
Figure 3.6: Experimental set up showing infant in the reclined baby seat. Panel A shows an infant in the uncrossed-feet posture and Panel B shows the infant in the crossed-feet posture.
side of stimulus presentation, but were provided with stimulus onset and offset information. The initial 133 ms following stimulus presentation on each trial were not coded as any movement during this period was considered to be anticipatory. After this period, the first foot to move independently (of the other) was accepted as a directional (orienting) foot response to the tactile stimulus. Additionally, the latency of the first directional foot response to the tactile stimulus was also noted. A second rater coded a proportion of the total trials across all participants, with trial-by-trial agreement at 85% for both the 4- and 6-month-old age groups.
3.12 Results
The proportion of foot orienting responses which were made to the side which had received the tactile stimulus (i.e., correct directional responses/total number of responses) were computed for both crossed and uncrossed postures (see Figure 3.7). One-sample t-tests of the proportion tactile localization accuracy scores in each age group and condition showed that 4-month-olds were performing reliably above chance (0.5) in both posture conditions, whereas the 6-month-olds were only performing above chance in the uncrossed feet posture condition (see Table 3.4)
Figure 3.7: Mean tactile localization accuracy (proportion of correct first directional foot movements to vibrotactile stimulus) of 4- and 6-month-olds in the crossed-feet and uncrossed-feet posture.
To investigate developmental changes in the use of an external reference frame for touch, a 2 x 2 mixed measures ANOVA of tactile localization accuracy with the within-participants factors of Posture (Uncrossed-feet / Crossed-feet) and the between-participants factor of Age (4-month-olds / 6-month-olds) was conducted. This revealed a main effect of Age [F(1, 28) = 4.504, p = .043, !p2 = .14], and a main effect of Posture [F(1, 28) = 8.604, p
=.007, !p2 = .24] (Uncrossed posture: M = .7, SD = .13; Crossed posture: M
= .62, SD = .19), which was qualified by the significant interaction of Posture x Age [F(1, 28) = 7.92, p = .009, !p2 = .22]. I investigated this interaction with four post-hoc comparisons (alpha was set at p = .0125).
First, we conducted a comparison looking at the effect of posture on tactile localization in each of the age groups. This revealed a significant effect of posture in the 6-month-olds [t(12) = 3.31, p = .006, d = 1.28], but not the 4-month-olds, [t(16) = .104, n.s.]. Next, I examined the effect of Age within each of the posture conditions using paired sample t-tests. There was no difference between the 4- and 6-month-olds in the uncrossed posture [t(28) Table 3.4: Results from one sample t-tests comparing infant’s
tactile localization accuracy with 0.50 (chance performance) across age groups and experimental conditions
Uncrossed feet Crossed feet
Age group n df t p t p
4-month-olds 17 16 5.28 <.001 6.15 <.001 6-month-olds 13 12 7.36 <.001 .1 .92
= .01, n.s.]. However, the 4-month-olds significantly outperformed the 6-month-olds in the crossed posture [t(28) = 3.07, p = .002, d = 1.12].
I also examined the latencies of infants’ foot orienting responses.
Figure 3.9 plots the cumulative timings (following tactile stimulation) of directional responses to tactile stimuli across trials. The 4-month-olds responded more quickly, and more often to tactile stimuli on their feet.
The mean latencies of the infants’ foot responses (Figure 3.8) were entered into a mixed 2 x 2 ANOVA with Posture (Uncrossed-feet / Crossed-feet) and Age (4-month-olds / 6-month-olds) as independent variables. This revealed a main effect of Age [F(1, 28) = 24.62, p < 001, !p2 = .47]. No other main effects or interactions were significant (all Fs < 2).
There is the possibility that the tactile stimuli activated more of a protective withdrawal system in the 4-month-old infants. To investigate this I compared the frequency of the two types of foot orienting responses produced by the two age groups. I found that the withdrawal response only contributed to a small proportion of the overall foot responses (M = .12, SD = .11 and M = .12, SD = .09 for the 4- and 6-month-olds respectively) in comparison to the exploratory foot “wriggle” (M = .88, SD = .11 and M = .88, SD = .09 respectively). I found no significant differences between the groups in the extent to which they responded with either an exploratory foot “wriggle” [t(28) = .01, n.s.] or a withdrawal foot response [t(28) = .1, n.s.].
Figure 3.8: Mean latency of first directional foot response to vibrotactile stimuli of 4- and 6-month-olds in the crossed-feet and uncrossed-feet posture.
3.13 Discussion
The youngest infants (4-month-olds) in Experiment 3 demonstrated the ability to correctly locate and respond to a tactile stimulus presented to one of their feet, regardless of the posture (i.e. they did not demonstrate a
“crossed-feet deficit”). This extends previous findings on tactile localization in early life which have shown that infants as young as 6 months of age make manual orienting responses to tactile stimuli on the hand (Bremner et al., 2008). However, I found no effect of posture; the 4-month-olds whom I tested were equally accurate at orienting a foot motor response to a tactile stimulus whether their feet were placed in an uncrossed-feet posture, or a crossed-legs posture.
Figure 3.9: Cumulative frequency of first response latency to vibrotactile stimuli of 4- and 6-month-olds in the crossed-feet and uncrossed-feet posture.
There could be two potential explanations of this finding. Firstly, it may be that infants may locate touches to the feet in external spatial coordinates, but they may be more competent at doing this across changes in posture of the legs than they are for the hands (Bremner et al., 2008).
Therefore, one would expect a general trend towards external coding in early infancy, and expect both 4- and 6-month-olds to show an effect of posture on tactile localization to the feet (as has already been established with the hands; Bremner et al., 2008). However, this explanation is unlikely given that this pattern of results was not found – the 6-month-old infants in this study showed poorer tactile localization with crossed feet (compared to uncrossed feet), thus demonstrating the crossed feet deficit.
It is not that infants are better at localizing tactile stimuli to the feet (in anatomical space), but there are age related changes in the way in which infants in the first half year of life locate touches to the body.
Indeed, a second and more likely explanation for this pattern of results is that, at 4 months of age, infants have not yet acquired the ability to code tactile locations in an external spatial frame of reference, irrespective of the locus of tactile stimuli on the body. In the absence of external coding, a reliance on anatomical coordinates to make their response would explain the lack of a posture effect, as anatomical spatial coding is unaffected by posture (e.g., a touch on the right foot is always considered as a touch on the right foot, irrespective of where that foot lies in space).
Further to this, when scrutinizing data concerning the latency of foot responses to the touch stimulus, I found that the younger infants were much quicker to respond to the stimuli compared to the older age group across posture conditions. There could be several potential explanations for this. One possible explanation could be that due to the fact that the younger infants had shorter limbs. It is possible that the signals sent from the skin receptors to the brain and then effector muscles had a shorter distance to travel, thus resulting in faster responses. However, considering the small differences in the length of these limbs between 4 and 6 months of age and the large differences between response latencies in these age groups, this explanation seems unlikely.
Conversely, a much more interesting and plausible explanation could be related to the way in which infants at different stages in the first half year of life process touches to the body, with longer processing and response times reflecting more complex processes. This theory is in accordance with Kitazawa (2002), who argued that, when using an external reference frame to locate touches to the body, individuals first map the touch in space before mapping the touch on the body.
Alternatively, others have suggested that this process is reversed, with individuals mapping touches in somatosensory co-ordinates and then in external space (Azañón & Soto-Faraco, 2008). Regardless of the exact order of these events, this is a more complex process than simply mapping touch directly onto the skin surface, and the increase in response time of the 6-month-olds (compared to the 4-month-olds) illustrates this.
This study has shown that influences of external spatial coordinates on tactile localization emerge in human infancy between 4 and 6 months of age. It is at 6 months that infants show a marked decrease in accurate orienting to a stimulated foot in the crossed-feet posture (in comparison to the uncrossed-feet posture), that is they demonstrate the “crossed-feet effect” and the use of an external reference frame to code touches to the body. In comparison, the youngest age group tested showed no such effect, performing to a high level of orienting accuracy regardless of their leg posture.
From this, it seems reasonable to conclude that it is during the two month period between the ages of 4 and 6 months that infants begin to code touches on their bodies with respect to external space. As proposed by Röder et al. (2004) and Ley et al. (2013), I consider it very likely that this reference frame emerges as a consequence of visual experience in the first months of life. I have been able to support this argument and also demonstrate that early visual-tactile experience is particularly important in the narrow time frame in which infants are aged between 4 and 6 months. If infants are deprived of visual experience during this time (as in the case of congenitally blind infants or those with cataracts), I would expect that these infants would not locate touches to their bodies with respect to the external world, simply locating touches on the skin surface.