Brains and Birdsong

Chapter 8 Brains and birdsong ERICH D. JARVIS INTRODUCTION contrasts and their underlying principles holds promise of new and fundamental insights into The special brain structures for singing, and how brains make learning possible. The discovery learning how to sing, were discovered in 1976. of similar sets of brain structures in parrots and Since then, with still growing momentum, there hummingbirds throws new light on how brain has been a wealth of fascinating discoveries on structures for vocal learning have evolved. The the structure, function, and evolution of the shared features imply that there are strong brains of birds, especially those that engage in epigenetic constraints placed on the brain and vocal learning. Less than half of all birds possess behavior during evolution, and suggest that the ability to learn and reproduce new sounds. perhaps humans evolved somewhat similar brain These vocal learners, parrots, hummingbirds, and structures for speech and singing. Although the songbirds belong to only 3 of the 23 major bird cognitive side of song learning hardly bears groups. They have the necessary forebrain comparison with human speech, ‘bird-brained’ anatomy for producing learned vocalizations. All need no longer be a pejorative. If a parrot imitates other birds use only basal brain structures for its own and other species’ sounds so readily and vocal production and their vocalizations are a child can hardly be prevented from learning to innate, and genetically inherited from their speak, how is it that a chimpanzee does neither? parents. The answer perhaps lies in the fact that no brain Of the birds that are vocal learners, most is structures have been found in chimpanzees known about songbirds, especially the canary equivalent to those present in songbirds and and the zebra finch. These are subjects of choice humans. because they breed easily in captivity and display Other important findings about brains were opposite extremes of vocal learning behavior. In first made in the songbird vocal learning system. zebra finches, only the males sing, have vocal These include the startling discovery of large learning, and possess the appropriate forebrain sexual dimorphisms in the brain, the role of structures for the purpose; they are closed-ended hormones in regulating brain structure and vocal learners, developing one song motif as learning, and the ability of the adult songbird juveniles and singing it for life. In canaries, both brain to generate new neurons, paving the way males and females sing, and both have vocal for current brainstem cell research. This chapter learning brain structures; they are open-ended will review some of these pioneering studies, learners and, like humans, continue to learn new and how the brain controls singing behavior, songs as adults. An understanding of these which is the theme of this book. Brains and birdsong 227 GENERAL BRAIN ORGANIZATION et al. in press; Reiner et al. 2004a; Fig. 8.1B, new nomenclature). Understanding the science of birdsong ultimately The cerebrum of the bird and the mammal requires a basic understanding of the vertebrate consists of three major cell zones, the pallium, brain. Birds and mammals, including humans, the striatum, and the pallidum. The striatium are vertebrate animals. Regardless of brain and and pallidum portions combined are the basal body size, all vertebrates share a general brain ganglia, and in most animals they sit at the base organization that consists of five basic regions. of the cerebrum (Marin et al. 1998). The pallium These are the spinal cord within the vertebral zone sits above the striatum at the top of the backbone, the hindbrain in front of it, then the cerebrum, and includes the cortex in mammals midbrain, the thalamus, and the cerebrum, with and four major brain subdivisions in birds (Reiner the cerebellum (meaning ‘small brain’) on top et al. 1998; Puelles et al. 1999; Swanson 2000b). of the hindbrain (Fig. 8.1A). Among those Brain size and folding are more related to animal vertebrates that are amniotes, having embryos size than to behavioral complexity (Van Essen that develop within amniotic fluid-filled sacs, 1997). The smaller the animal, the less folding that is reptiles, birds, and mammals, all have a there is of the cerebrum. Hummingbirds with similar structural organization in these five brain one of the smallest bird brains and no folding regions, except for one, the cerebrum (Veenman have more advanced types of behavior, such as et al. 1995; Reiner et al. 1998). In reptiles and vocal learning, than many mammals with large birds the cerebrum is organized into large cell and folded brains, such as horses. clusters; in mammals only the bottom part is Neurons, the communicating cells of the brain, organized into clusters, called the basal ganglia, typically have a cell body with axons that make whereas the upper part is arranged in layers that long distance connections to other neurons (Fig. together form the cortex. Because mammals have 8.1C). Neurons also have dendrites that receive long been regarded as more intelligent than birds these axonal connections. The meeting places of or reptiles, it was assumed that one of their major the axons and dendrites are the synapses. In the anatomical differences, the mammalian cortex, basic connectivity plan of both birds and was responsible for their more intelligent behavior mammals, sensory information from the outside (Herrick 1956). Many viewed evolution as world, and from the inner body, destined for proceeding linearly with its ultimate goal the the cerebrum, first passes through sensory creation of ‘man,’ ‘his’ language, civilization, large receptor neurons that have one axon projecting brain size, and folded cortex. Accordingly, almost out into the body or the exterior, and another all subdivisions of the cerebrum of birds and into the spinal cord for the lower body or into reptiles were given names that designate the the hindbrain for the upper body. Here they mammalian basal ganglia, which were thought synapse onto their appropriate cell groups, which to be primitive (Edinger et al. 1903; Edinger in turn project up to the midbrain. The midbrain 1908; Herrick 1956; Fig. 8.1B, old neurons send projections into the thalamus. The nomenclature). We now know that the thalamic neurons process and project the organization of the bird cerebrum is much more information to the cerebrum, where they synapse similar to the mammalian cerebrum than with a network of cells involving the pallium, previously thought (Reiner et al. 1998 in press striatum, and pallidum. Neurons that control A, B; Puelles et al. 1999). In 2002, at a movement form a network that traverses in the Nomenclature Forum held at Duke University, opposite direction, from the cerebrum, to the the subdivisions of the entire bird cerebrum were midbrain, and spinal cord. Neurons of the spinal renamed to reflect more accurately the many cord, lower motor neurons, send their axons to homologies that exist between avian and muscles. The cerebrum controls complex learning mammalian brains (Jarvis et al. 2002; Medina and voluntary behaviors, and ‘conscious’ 228 Nature’s Music The Brain (A) Side view Songbird Human (B) The bird brain Old New nomenclature CDL Hyperpallium nomenclature Hp Iateral Mesopallium ventricle L2 Nidopallium Arcopallium Amygdaloid E Complex Striatum Cerebellum Cerebellum Pallidum B Thalamus Thalamus OB Midbrain Midbrain Hindbrain Hindbrain cerebrum = telencephalon pallium striatum pallidum The human brain (C) How neurons communicate Dendrites: Cortex receptors Dendrites lateral Claustrum Dendrite spines ventricle Striatum Synapse: neurotransmitters-pre-synaptic receptors-post-synaptic Thalamus Pallidum Nucleus Midbrain e- Cell body Amygdala Cytoplasm action potentials Axon OB Cell body Hindbrain Post-synaptic neuron Pre-synaptic neuron Figure 8.1 (A) Songbird and human brains compared. (B) Old and new nomenclature for the bird brain. Dashed gray lines separate regions that differ by cell density and size, such as the six layers of the cortex. The darkest gray area of the human cerebrum are all axon pathways, typically called white matter, connecting the cortex with various cerebral and non-cerebral areas. Abbreviations: E, entopallial nucleus; B, basorostral nucleus; L2, field L2 nucleus; OB, olfactory bulb; HP, hippocampus; CDL, Corticoid dorsal lateral (C) A diagram of neurons and their connections. Brains and birdsong 229 processing, whereas the midbrain is involved in nearly 200 years ago. Vocal learning is the innate behaviors and ‘non-conscious’ processing. behavioral substrate for spoken language. In the Glial cells typically provide physical support and first recognized breakthrough, in 1861, a French nourishment to neurons. physician, Paul Broca, published a paper about an autopsy of one of his patients who had a stroke (Broca 1861). The patient could only speak THE SEARCH FOR VOCAL one word, ‘tic,’ and had extensive brain damage, LEARNING BRAIN AREAS with a central location for the lesion in the now famous Broca’s Area of the cortex. From this Vocalizations are learned in some species and and several subsequent patients, Broca concluded innate in others, and this behavioral contrast is two things: (i) that language within the brain expected to reflect a brain difference. As humans, was localized, not spread out and (ii) its location however, we often have difficulty in was on the left side of the frontal lobe. Forty- understanding its true basis because, in everyday four years later in 1905, Oswald Kalischer, a language, we do not distinguish auditory learning German scientist, attempted to determine if from vocal learning. Vocal learning is the ability parrots have a Broca’s-like Area (Kalischer 1905). to imitate the sounds that you hear or improvise. He performed left and bilateral hemisphere It is a rare trait, found so far in only six animal lesions, and stimulation experiments, on Amazon groups: three birds (parrots, hummingbirds, and parrots. He found some vocal effects, but this songbirds) and three mammals (humans, bats, was preliminary work and was never followed and cetaceans, the whales and dolphins [Thorpe up.

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