When it comes to debating nature and nurture, few would doubt that our experiences affect who we are, even if that is only part of the story. In many ways, the nature/nurture debate has moved on, and scientists are now most concerned with how the two might work together, or which takes precedence over the other in which instances.
The role of experience seems most tangible when considering how experiences might affect our personality, or be essential for mastering skills, like a certain language. Yet experience also influences aspects of our biology that can remain mostly fixed throughout our lives – like the wiring of our brains that allows us to see.
Experience leaves traces visible even at the level of small groups of neurons. Its effects don’t occur in a vacuum though – experience can also affect the relationships between different brain areas, which then ultimately affect perception, cognition or behaviour.
These effects have been extensively demonstrated in some of our favourite furry friends – kittens; most notably by two researchers, David Hubel and Torsten Wiesel, working from the 1960’s onwards.
In one of their most prominent experiments, kittens and older cats were subjected to either monocular deprivation (one eye kept closed) or binocular deprivation (reared in the dark) for a few weeks. Cats older than 3 months weren’t affected by either condition, as they had had normal experiences during the most important period for developing normal vision. Closing only one eye was originally meant as a way of comparing deprivation with typical development in the same animal. However, it turned out that monocular deprivation was much more damaging than binocular deprivation for the kittens, and it started to seem that restricting some parts of the environment revealed more about the effects of experiences on cells than removing the visual environment entirely.
The cells in the cortex which merge the view from each eye into a coherent picture each receive input from both eyes. Some receive equal input from each eye, whilst some are more strongly driven by one eye. However, in monocularly deprived kittens, almost all of these cells only responded to input from the eye that had been allowed to see, and did not fire in response to information presented only to the deprived eye when it was re-opened. The cells seemed to have rebalanced to adapt to the environment, rather than retaining the capability to process input from both eyes on the off-chance that this ever happened.
Monocularly deprived kittens showed no evidence of vision in the deprived eye, yet the deprived pupil would still contract in response to light. This suggested that the eye was functioning just fine, but the parts of the brain receiving information from that eye were not. Something was amiss, either in the cortex or the path leading to it.
The researchers then looked at the lateral geniculate nucleus (LGN), a more primitive brain structure. Each layer of the LGN receives input from one of the eyes, and sends projections to visual areas of the cortex in both hemispheres. The connections from the deprived eye to the LGN were still there, and reactive, but impoverished. It seemed that these connections were innate, but had not developed properly due to lack of visual experience.
Hubel and Wiesel then started to wonder how much experience was necessary for these connections to develop – perhaps plain old light was enough, or maybe more sophisticated stimulation, like shapes, was needed. So, instead of sewing one eye closed, they placed a translucent patch over one eye, which would let in light, but no information about precise shadows or shapes. As before, the focusing cells in the cortex were unresponsive to anything presented to the deprived eye when the patch was removed. The LGN layers, however, were slightly abnormal, but much less so than before. It seemed that the LGN layers and their connections mainly needed the experience of light to develop. Cortical cells, however, required more sophisticated visual experience, which is logical, as many neurons in the first visual area of the brain, V1, respond to very precise attributes, such as the exact slant of a contour.
The need for cortical cells to receive shape information was then shown in a series of experiments in which cats were raised with limited experience of patterns. Kittens were raised in darkness, except for a few hours per day, when they were put in a cylinder decorated with either horizontal or vertical stripes, as above. In one version of the experiment, they also had one eye covered, giving them monocular deprivation for one pattern (e.g. vertical stripes) and binocular deprivation for the other (e.g. horizontal stripes). Not surprisingly, the kittens’ response to the pattern they had been exposed to was driven by projections from the seeing eye to V1, but other novel patterns, like horizontal stripes were driven by each eye, because neither pathway was better at processing these new patterns. The kittens also had many more neurons in V1 responding to the pattern they had seen than to any other kind of stipe – these cells’ pre-programmed connections had been disrupted by experience.
It seemed that it was not only the amount of visual input, but the relationships between areas receiving different amounts of input which determined the end result, by altering relationships between areas of the brain with different roles. This idea has since shed light on the exact ways in which nature and nurture work together to shape our identities and abilities, and how it is that we are so similar in some ways, and so different in others.
For a more visual description, and some really cute kittens, there is a wonderfully vintage video here.