Tuning and pruning
A note for Lesson no. 3, "Little Brains Wire Themselves to Their World," in Lisa Feldman Barrett.
Some context from page 50 is:
...causing gradual brain changes that we’ve called plasticity. These changes nudge the infant’s brain toward higher complexity via two processes we’ll call tuning and pruning.
Plasticity is the ability of a brain to change its own wiring as a result of experience. In Seven and a Half Lessons About the Brain, I refer to these changes as tuning and pruning. Scientists sometimes call these changes "experience-dependent" changes. The word "experience" is a bit misleading here, because really the issue is exposure to events that send sense data to the brain. These can be events in the world and events that occur in the body.
Pruning occurs by removing synapses that are not heavily used. For example, a type of glial cell in the brain, called microglia, prunes the brain by killing neurons. This is a normal process and is responsible for normal brain development, where neural connections are pruned during childhood and adolescence. As a consequence, infants begin life perceiving some things that they lose the capacity to perceive later on. Without such pruning, the brain does not function normally. (But glial cells also contribute to disease in the nervous system: in the service of metabolic efficiency — they can kill neurons to try to keep your body budget in balance when things get out of hand.) With chronic inflammation, they help cause some of the brain atrophy that is associated with illness.
Tuning occurs by various mechanisms, such as changing synaptic strength and myelin thickening (e.g., ). Tuning can mean learning to perceive something with more precision, called perceptual narrowing (e.g., infants learn to detect small differences in the faces of people that they are exposed to frequently, usually members of their own ethnicity). Tuning can also be perceptual broadening (e.g., during development the neurons within the prefrontal cortex gradually become responsive to a broader range of events).
In Seven and a Half Lessons About the Brain, I give an example of tuning that makes infants more perceptually sensitive to faces that are from the same ethnicity as their caregivers (which is usually the same as the infant's ethnicity). The visual system becomes tuned to faces that are more relevant to the infant's well-being (who feeds the baby, who cares for her, and so on). Rarely-seen faces don't get that advantage; hence the "other-race face effect" I mentioned in the book. We know this effect is due to tuning, rather than pruning, because experiments can quickly reverse the other-race effect by cuing infants to pay attention to other-race faces. In fact, scientists can reverse the effect so that infants attend more, and better remember, faces of other races rather than their own race.
See this brief discussion of plasticity (from the webnotes for How Emotions are Made: The Secret Life of the Brain), (mainly age-related tuning, rather than pruning), with references therein.
- Dubois, Jessica, Ghislaine Dehaene-Lambertz, Sofya Kulikova, Cyril Poupon, Petra S. Hüppi, and Lucie Hertz-Pannier. 2014. "The Early Development of Brain White Matter: A Review of Imaging Studies in Fetuses, Newborns and Infants." Neuroscience 276: 48–71.
- E.g., Kelly, David J., Paul C. Quinn, Alan M. Slater, Kang Lee, Liezhong Ge, and Olivier Pascalis. 2007. "The Other-Race Effect Develops During Infancy: Evidence of Perceptual Narrowing." Psychological Science 18 (12): 1084–1089.
- E.g., Werchan, Denise M., and Dima Amso. 2017. "A Novel Ecological Account of Prefrontal Cortex Functional Development." Psychological Review 124 (6): 720–739.
- Sugden, Nicole A., and Alexandra R. Marquis. 2017. "Meta-Analytic Review of the Development of Face Discrimination in Infancy: Face Race, Face Gender, Infant Age, and Methodology Moderate Face Discrimination." Psychological Bulletin 143 (11): 1201–1244.
- Markant, Julie, Lisa M. Oakes, Dima Amso, and Dima Amso. 2015. "An Attentional but Not Racial Bias Underlies the Other-Race Effect In Infancy." Developmental Psychobiology 58 (3): 355–365.