Two of the pieces that I have wanted to discuss appear together in Current Biology, both on evidence of sensory integration in adults compared to their integration in children. Nature News carried a story about both articles, One sense at a time, by Matt Kaplan. As Kaplan explains, the research generally supports the idea that: ‘Adults readily integrate sight, sound, smell, taste and touch in their everyday lives without a second thought. But research is revealing that this is not the case with children. Two new studies hint that children under the age of eight only use one sense at a time to judge the world around them.‘

As I started to discuss in an earlier piece on human equilibrium (long ago — still working on parts two and three), adults learn how to weight different sensory information depending on context and the task at hand, evaluating one stream against another if they conflict. When confronted with two contradictory impressions from different senses — such as video of a person saying one thing and audio of a slightly different word — adult sensory systems figure out a way to integrate the sense world, sometimes creating ‘sensory’ compromises or syntheses. The ability to integrate sensory information is fundamental to normal human functioning, but it tends to undermine certain conceptions of brain ‘modularity,’ as I argued in the earlier post.

But with these two articles, I want to explore something a bit different, so I’m going to tackle each one individually, and then reflect on one issue that I think is important: the tendency to see child development in a teleological framework, that is, as an incomplete version of an adult system rather than as a deployment of the child’s distinctive neural resources. Before you click on ‘read more’ below though, be warned; this piece is a bit long…

Nardini et al. on spatial navigation

The first article, by Marko Nardini, Peter Jones, Rachael Bedford, and Oliver Braddick, ‘Development of Cue Integration in Human Navigation’ (abstract, pdf of full article), examined how adults and children integrated information when trying to do a spatial navigation task. Nardini and colleagues set both adults and children to work navigating a darkened room; they could rely on glowing objects as visual landmarks, on their own internal sense of movement and position, or on both together. The researchers then manipulated the subjects by either turning off the lighted landmarks or by disorienting the subjects’ physical sense of their position in space (by turning them). The researchers wanted to see how the subjects did when they had both sources of sensory information, compared to when they were deprived of one sensory channel.

Subjects were tested to see how close they could return objects to the places from which they were removed under this variety of sensory conditions. The adults were generally more accurate in placing objects back than the children, ages 4-5- and 7-8-years-old, but the real difference showed up when they compared subjects using a single sense to those with access to both visual and proprioceptive sensory information. Adults did better with two sources of information, but children were (nonsignificantly) less accurate with more sensory channels of information.

Nardini and colleagues point out that adults tend to have variations in the errors, seeming to notice when they were not on track in the navigation task, and integrating cues to reduce error. In contrast, children tended to have more ‘constant’ errors: ‘Children’s failure to reduce variance… could be explained either by suboptimal integration or by failure to integrate the cues.’

The team created statistical models for error behavior in a situation where subtle conflicts were insinuated into the task. The models predicted behavior depending upon whether subjects were integrating the two sources of information or rather alternating between the two streams. Their research showed ‘children’s behavior in the conflict condition was inconsistent with integration but consistent with alternation.’ (In contrast, the errors of the adults suggested that they were integrating the information from the two senses near to the optimum.)

Although some sensory integration may occur early in developmental time, this research suggests that children under 8 cannot integrate visual and haptic information about their position in space. The ability to combine these forms of sensory information does not develop until later.

Gori et al. on sight-touch integration

The second article from Current Biology is ‘Young Children Do Not Integrate Visual and Haptic Form Information,’ by Monica Gori, Michela Del Viva, Giulio Sandini, and David C. Burr (abstract, pdf of full article). Again, the research team compared children to adults, who generally integrate information from different senses in statistically optimal fashion. In this project, the team looked at sight and touch and found, ‘that prior to 8 years of age, integration of visual and haptic spatial information is far from optimal, with either vision or touch dominating totally, even in conditions in which the dominant sense is far less precise than the other (assessed by discrimination thresholds).’

Kaplan’s article for a general readership does a nice job of explaining the experimental design:

Burr’s group asked children between the ages of five and ten and adults to determine which of two blocks was taller than the other. While making their decisions, participants were allowed to either touch the blocks, look at the blocks, or do both. The team report in Current Biology that adults and children eight years of age and older were better at this task when they could both see and touch the blocks. Their ability fell when they were denied one of these two senses.

But children under the age of eight did not show this difference at all. They performed nearly identically in the task when given just sight, just touch, or both to work with.

In fact, with 5-year-olds, haptic information seemed to dominate when the experiment brought the two sensory systems into conflict in size judgment. Ironically, in the discrimination of object orientation, the 5-year-olds seemed to use vision rather than touch. That is, one sense was dominant — the two channels were not integrated to produce more accurate judgments — but the sense that was dominant varied depending upon the task.

The authors suggest that, during development, children constantly ‘recalibrate’ sensory information against other sensory information, slowly becoming more skillful at integrating these different sensory streams. Visuohaptic integration develops with age; by age 8 or 10, children perform much like adults, but at age 5, the children are using one sense or the other, not performing any worse when the non-dominant sense is not available. The authors provide a nice discussion of the significance of these results, in the context of other sense research:

Mammalian sensory systems are not mature at birth but become increasingly refined as the animal develops. Some basic visual and tactile properties, such as contrast sensitivity and acuity, reach near-adult levels within the first year of life, whereas other attributes, such as form, motion perception, and visual or haptic recognition of 3D objects, continue to develop through the school years until 8–14 years of age. The results of this study show that crossmodal integration of form information also develops late: Before 8 years of age, children do not integrate visual and haptic spatial information, but one or the other sense dominates, irrespective of its reliability (as assessed by discrimination thresholds), at least over the range we studied. However, there is no evidence that either vision or touch acts as a ‘‘gold standard,’’ always dominating the other.

Neurophysiological studies in animals, such as cats and monkeys, supports the argument that sensory integration develops with age, as intermodal connections do not tend to form robustly until the unimodal systems have reached maturity. But different abilities, even on related tasks, develop at different rates in the maturing organism. In this case, spatial perception skills, even if they appear to be related uses of visuohaptic information, do not take the same trajectory.

One possible explanation that the authors highlight, and that I found particularly intriguing, is that the need to recalibrate the sensory systems in relation to each other as the child is growing (and thus providing subtly different information as the size of eyes, fingers, and the like change) is more important than integrating the information, at least until the systems are mature. If the systems were too well integrated, they could not be used to recalibrate and confirm each other.

Final thoughts

This research tends to support some of the points I’ve made in previous posts, especially the doubts raised for ideas that basic abilities (like spatial perception) are ‘innate’ in the human brain, or that ‘modules’ to interpret sense information come pre-programmed. I won’t belabor these points because we’ve been over them before.

What I’m more interested in is the uneven pattern of development, the way that sensory dominance depends upon task, that integration occurs at certain thresholds, and the possibility that sensory independence actually helps a child to recalibrate one sense against the others as the body changes and grows. I’m struck by the way that the child’s brain makes the best of what it has, even if it means that the early solution to sensory problems does not resemble the mature one that they will later employ.

That is, even on the level of the individual organism’s development, we find that early stages are not determined by the ‘goal’ or ‘endpoint’ of development, but are rather configurations that make the best use of existing perceptual resources.

In other words, kids’ perceptual systems are not just forerunners of adult systems, they are adaptations to the sensory situation of the child, including basic instabilities generated by growth or organic maturation themselves.

As with evolutionary thinking, the pitfall of teleological thinking — assuming that ‘early stages are leading up to later ones’ — seems to threaten how we understand children’s sensory development. Perhaps this is one of the other dangers of not recognizing how the human nervous system (like other mammals’) is a dynamic system, achieving some sort of stability and predictability at each stage of development, rather than just an ‘adult under construction.’ Children come to different sorts of sensory solutions, not because they are becoming adults slowly, but because they are doing the best they can with the brains that they have.

References

Gori, Monica, Michela Del Viva, Giulio Sandini, and David C. Burr. 2008. Young Children Do Not Integrate Visual and Haptic Form Information. Current Biology 18 (9): 694–698. doi 10.1016/j.cub.2008.04.036 (abstract, pdf of full article)

Kaplan, Matt. 2008. One sense at a time. Nature News (1 May 2008 ) doi:10.1038/news.2008.796

Nardini, Marko, Peter Jones, Rachael Bedford, and Oliver Braddick. 2008. Development of Cue Integration in Human Navigation. Current Biology 18 (9): 689-693. doi 10.1016/j.cub.2008.04.021 (abstract, pdf of full article)