Simple Minds: The Neurobiology of Invertebrate Learning and Memory

ALTHOUGH IT HAS BEEN FORMALLY RECOGNIZED since the end of the 19th century that invertebrates can learn (Romanes 1895), the modern neurobiological analysis of invertebrate learning did not begin until the 1960s. Starting in that decade, pioneering investigators began to use intracellular electrophysiology to probe the basic mechanisms of learning in higher invertebrates (Bruner and Tauc 1966; Kandel 1967; Krasne 1969). Initially, these studies focused on simple forms of nonassociative learning, including habituation and sensitization. However, by the early 1980s associative learning, particularly classical conditioning, had been described in several invertebrate systems that were amenable to electrophysiological and biochemical—and, in the case of Drosophila, genetic—analyses (Takeda 1961; Henderson and Strong 1972; Mpitsos and Davis 1973; Menzel et al. 1974; Dudai et al. 1976; Crow and Alkon 1978; Chang and Gelperin 1980; Hoyle 1980; Carew et al. 1981; Lukowiak and Sahley 1981). Research on learning and memory in invertebrates during the past four decades has yielded fundamental insights into our understanding of the changes that take place within an animal’s nervous system when it learns. The major advantage of invertebrate systems for cell biological analyses of learning and memory is the relative simplicity of their nervous systems. Many higher invertebrates possess only 10,000–100,000 neurons. Although still great, this sum is dwarfed by the billions of neurons in the