Biology 202

2006 Second Web Paper

On Serendip

What sparked my interest in this topic was an article stating that only babies who can crawl are afraid of heights. This is true even if babies have normal depth perception (1). It was interesting to me that 1) how we move through our environment affects how we perceive it and 2) that the fear of heights is not present immediately at birth, indicating that some "instincts" depend on experience.

Campos et al came to this conclusion using what is called a visual cliff, where a flat piece of clear plexiglass covers a ledge with a checkerboard pattern that drops off several feet with the same pattern visible below. Usually babies are placed at the edge of the cliff and their mothers call to them from the direction of the deep side to see if they will crawl across the plexiglass over the apparent drop off (2). Alternatively, in this study, babies were lowered onto the plexiglass on the deep side of the visual cliff while their heart rate was being measured. Crawling babies (or pre-crawling babies who had significant experience with a baby walker) showed a heightened heartbeat (fear response) while pre-crawling babies did not (1).

This intrigued me. Somehow, once they had learned to travel independently through their environment, babies were able to learn more about the characteristics of said environment and the possible consequences of moving through it (such as falling off of a ledge). More sophisticated and coordinated activity in their motor neurons, accompanied by sensory feedback and new proprioception signals, was altering the input received by their sensory neurons, or the interpretation of the input. In other words, a new, more informative feedback loop is put in place for crawling babies that lets them learn more about their relationship to the environment and how it changes. This feedback loop means that the Nervous System's (NS) output is affecting its input; an important example of experience affecting the brain. But it is still unclear how exactly this happens, and what causes what. And there are so many cognitive and behavioral changes occurring during this stage of development (around 6-8 months for the start of crawling) that is hard to discern which development leads to which.

Research being done at the Infant Studies Center at Berkeley has found that it is not as simple as pre and post-crawling, but that there is a delay of a few weeks between the time babies start to crawl and when they show a fear of heights. This supports the fact that it is experience crawling, not simply an immediate side effect of development at this stage that causes the fear to emerge. This also fits with anecdotal evidence of babies crawling off the edge of beds or changing tables or even down the stairs when no one is watching (3).

However, one study, which is in the minority, goes against the experience hypothesis. Richards & Rader (1991) found that it was age of crawling onset, not crawling experience, that predicted behavior on the visual cliff (4). Contrary to what would be expected, those babies with an earlier crawling-onset age (hence more crawling experience at the time of testing) performed the worst—that is, did not avoided the drop-off as much as later crawling-onset babies. Testing age did not predict performance. These researchers explained their finding by saying that early crawling onset "during the tactile phase of infancy interferes with later visual control of locomotion" (4). These findings have not been supported but they do raise a red flag at assuming experience leads to more of a fear of heights.

Regardless of whether crawling experience or age of crawling onset has more of an effect on fear of heights, all of this evidence shows that the fear is not completely innate in humans—that we have to either grow into or learn some of what we think of as our "instincts." This is obviously not adaptive for the species, since it means there is a time where babies are independently mobile yet do not yet know to avoid heights—making them vulnerable to injury or even death at a young age (5). While there is no real need to be afraid of heights before you can move significant distances independently, there is a need to once you can. But how are animals supposed to develop this fear before they have experience moving on their own (which would be necessary to eliminate the vulnerability)? The research suggests that babies cannot, and this is just another one of the many things babies are not capable of immediately after being born. Are all of these many developmental delays simply due to the fact that we are born prematurely compared to other animals—most of whom can move independently right after birth? Are they all just unfortunate evolutionary side effects that mean that we need to grow, have experiences and learn just to get to the developmental stage that so many other animals are born into?

In fact, scientists have studied animals on the visual cliff and have found that animals that rely on visual cues pass the visual cliff (meaning only walking towards the shallow side) immediately after birth (this is true of chicks that are less than 24 hours old and kids and lambs as soon as they can stand (around a day after birth)). Researchers state that "a seeing animal will be able to discriminate depth when its locomotion is adequate, even when locomotion begins at birth" (5).

This brings up the question of whether fear of heights should be considered an "instinct," and what the definition should be of "innate." Can something still be an instinct if you are not born with it? And can instincts exist on a continuum? For example, while all developmentally normal adults know that heights are dangerous, not all of them will show the same increase in anxiety symptoms when faced with them (or feel any anxiety at all). Did something go wrong in the development of adults who are not afraid of heights? Along this continuum, the fear can also become excessive and unreasonable in 2-5% of the population—becoming a diagnosable phobia called acrophobia (classified under anxiety disorders in the DSM-IV).

Other developmental examples support the idea that motor experience can lead to improvements in perception. Babies who have learned to crawl are better at finding hidden objects, and the longer a baby crawls in one session the better spatial cognition they display during that session (6). Besides just crawling, it has also been shown that as babies get better at using their hands and fingers they also get better at discriminating different properties of objects, like size, weight, texture, and temperature (7). Finally, as babies learn to control their heads, they show vast improvements at localizing a sound source. These studies show us that more complicated movement can improve perception in various different instances and modalities.

This made me wonder about the sensory experiences of individuals who never gain independent control of their movement, or those who lose the ability later in life. Do they have deficits in their perceptual systems? I also wondered whether therapies involving movement could help improve perception in those same people. There are books sold online with physical activities they claim will help keep your child from developing learning and behavioral problems (sometimes called "special physical education"). Another site claimed that 70% of kids with learning disabilities did not crawl but immediately learned to walk (8). Is it a two-way street so that damaging motion always damages sensation or the other way around? Or even, improving one improves the other? This seems unlikely since there are separate tracts in the spinal cord dedicated to sensory and motor functions.

In tackling this question of locomotion improving perception, we have seen that babies do not show a fear of heights until several weeks after they learn to crawl (or with experience moving with a baby walker) and there are also hints that crawling-onset age could be a factor. We also saw that in most cases, animals show fear of heights as soon as they gain locomotion, which is usually within one day. The concept of an instinct was discussed, and it appeared that the delay in this behavior in humans was due to how premature we are at birth relative to other animals, which results in a period of vulnerability for babies where they are able to crawl but not yet afraid of heights.

1) Berenthal, B. (1996) Origins and Early Development of Perception, Action, and Representation, Annual Review of Psychology, 47: 431-459, citing Campos et al (1992)
2)Berkeley Infant Studies video video of a baby being tested on a visual cliff
3)Berkeley Infant Studies Lab the lab's website with facts about the visual cliff
4) Richards & Rader (1991)
5)an article about the visual cliff a good article about visual cliff research at Cornell on babies and animals
6) Berenthal, B. (1996) Origins and Early Development of Perception, Action, and Representation, Annual Review of Psychology, 47: 431-459, Kermoian & Campos (1988)
7) Berenthal, B. (1996) Origins and Early Development of Perception, Action, and Representation, Annual Review of Psychology, 47: 431-459
8)Crawlies website an article on the benefits of crawling


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