For some Parkinson’s patients, brain surgery can relieve tremors, balance problems, and disease-caused stiffness. One procedure, known as pallidotomy, intentionally damages a structure deep inside the brain. After recovery, patients are generally able to complete learned, well-practiced skills, such as walking, but find it near impossible to learn new tasks. A small team of neuroscientists, presenting findings in PNAS Early Edition, believe they are closer to understanding why.
The brain area targeted in pallidotomy, the globus pallidus, helps regulate voluntary movement within the basal ganglia. Part of this area’s job is to send signals to the thalamus, which in turn relays them to the cerebral cortex–enabler of thought, perception, and voluntary movement.
Abnormal firing patterns in the cortex are the cause of disorders like Parkinson’s disease, suggest recent studies. Neuroscientists believe pallidotomy (generally reserved for patients unresponsive to medication) quiets activity in the thalamus and eliminates pathological firing patterns. Little is known, however, about how healthy basal ganglia act on the cerebral cortex, particularly when an organism is completing a task, such as a bird singing. Nor have many examined how damage in the basal ganglia affects the properties of neurons firing in the cortex.
At the University of California, San Francisco, Satoshi Kojima, Mimi Kao, and Allison Doupe used male zebra finches to examine these questions. They found basal ganglia aren’t needed for normal cortical firing, but are needed for task-specific patterns, including neuronal “bursting.” It is the bursting, patterned activations where rapid neuron spiking is followed by quiet periods, that enable motor plasticity, the authors propose.
Zebra finches are prolific singers that control song through a brain circuit homologous to the basal ganglia-thalamocortical circuit in mammals. In the study, birds were divided into four groups. Some birds received the avian equivalent of a pallidotomy, some they deafened through a local injection of 1% ibotenic acid, some were both deafened and given a pallidotomy, and some were left intact.
The intact birds, unsurprisingly, retained stable songs. Finches deprived of their hearing degenerated in their singing ability, as has been observed in previous studies. However, the two groups given the avian pallidotomy remained largely unchanged, regardless of whether they were hearing or deaf.
Finches can gradually alter their song, if they get auditory feedback. Without it, their song will change, but for the worse. “Eventually, they begin to drop some syllables and produce different syllables from one song bout to the next,” says Kao. “This degradation of song is a form of plasticity, similar to the deterioration of speech that gradually occurs when adult humans lose their hearing.”
The neuroscientists believe that in both cases plasticity comes from patterned burst firing in the prefrontal cortical analog (LMAN) and which depends upon input from the basal ganglia (Area X). Knocking out Area X didn’t change firing rate, but did change firing patterns. Without the firing “bursts” they lost the ability to be plastic. However, the basal ganglia–and the plastic ability they confer–aren’t needed for a bird to perform a well-learned song. This, the authors write, “strikingly mirror(s) clinical findings that pallidotomy does not markedly affect the execution of well-practiced skills but impairs motor plasticity.”