Making a cup of coffee and remembering to turn off the coffeemaker. Driving to the grocery store and not getting lost. Remembering anniversaries, birthdays, and where you were supposed to meet your friend for lunch. All of these activities require the seamless workings of the brain, and while we often take them for granted, there are even more activities that go undetected within the brain every single day.
In his new book, Understanding the Brain: From Cells to Behavior to Cognition, which is an updated version of his earlier book, Dowling offers a comprehensive look at how the brain functions — from how vision occurs and how neurons within the brain communicate, to the neuroscience underlying disease states, phenomena, consciousness, and even emotions.
Dowling explains, “The brain consists of hundreds of areas, each carrying out a specific task. Many areas possess neurons unique to those specific parts of the brain. And within each area, the neurons connect with one another, and some project to other areas often considerable distances away.”
But how is the brain able to maintain such a complex web of activity?
“All neurons have characteristics of sensory receptors in that they possess specialized membrane proteins that respond selectively to specific chemicals — the neurotransmitters or neuromodulators,” Dowling writes.
One feature that is unique to all receptors, however, is the process of adaptation. Over time, and with sustained stimulus, all receptor potentials decline — often distorting how we perceive the world around us.
Dowling writes, “Sensory adaptation is a distortion of the real world in the sense that receptors do not provide a faithful representation of the stimuli impinging on the organism.”
Memory is also not entirely reliable. Short-term memory, especially when describing traumatic events, is often quite fragile and subject to amnesia. Long-term memory, on the other hand, is much more stable.
Dowling writes, “One notion is that short-term memories reflect ongoing neural activity whereas long-term memories reflect structural changes in the brain — the formation of new synapses, or the structural alteration of existing synapses.”
Like the formation of memory, we are largely unaware of the sensory input coming into the brain, but when the ability to detect sensory input coming from the body is lost, the effects are dramatic.
Dowling quotes Christina, a 27-year-old woman who had lost all proprioceptive input, which allows the brain to “sense” the body: “I lose my arms. I think they are in one place and I find they are in another.”
Christina’s condition was due to a selective inflammation of the proprioceptive axons all along the spinal cord and brain, and sheds light on just how essential proprioception is to make purposeful movements and achieve bodily control.
How sensory processing is achieved, and how neurons begin to differentiate, form synapses, and ultimately wire the brain, is a process that is now known as chemoaffinity.
Dowling writes, “The proposal is that as neurons differentiate they become chemically specified; they make specific proteins on their surfaces that enable other neurons to recognize them.”
Influenced by experience and the environment, brain plasticity and the development of new neural connections is now understood to occur in some regions of the brain throughout life.
However, only during critical periods can fundamental alternations in the brain be induced. One example is children who are amblyopic because of crossed eyes. In this case, the child becomes overly reliant on the dominant eye, which leads to decreasing acuity in the opposite eye.
Similarly, cases like the young girl, Genie, who was locked in a darkened room by her psychotic father for more than ten years, or Victor, “The Wild Child of Averyron,” who was abandoned in the woods as a young child but managed to survive, highlight the importance of critical periods for the learning of language.
While both children were able to learn some words, they never developed normal language skills.
Dowling writes, “By the age of 6-12 months, neural circuits have been formed to discriminate and make all possible language sounds and to acquire grammar. If the circuitry is not used, it is rearranged to accommodate the native language(s) or perhaps even lost.”
While critical periods can influence brain development, environmental factors throughout the life cycle can influence brain functioning. One example is exposure to chronic stress, and consequent elevated levels of cortisol.
“Serious disorders have also been linked to high levels of cortisol in the bloodstream, including gastric ulcers, colitis, high blood pressure, impotency, and even excessive loss of neurons in the brain. It has even been suggested that prolonged stress can lead to premature aging of the brain and other organs,” writes Dowling.
Clearly, our rich mental life depends on the adequate development, function, and preservation of our brains. Emotional processing, visual acuity, proprioception, and even the ability to detect the intentions of others — a process that relies on mirror neurons — all depend on our dynamic and ever evolving brains.
From the source of emotions to the possible absence of mirror neurons in people with autism, Dowling’s book is a marvelous and fascinating journey through the human brain.
Understanding the Brain: From Cells to Behavior to Cognition
W.W. Norton & Company, October 2018
Hardcover, 256 pages