The study of verbal behavior focuses on communicative functions of the speakers/producers as they affect the behavior of listeners/observers. Effects on the listener reinforce the speaker and the listener/observer benefits (i.e., is reinforced) from the behavior of the speaker/producer. The interlocking of, and exchange of, the speaker and listener behavior between individuals and within one's own skin constitute bidirectional operants. These bidirectional operants are instances of social interactions and measures of social behavior. Evidence suggests that the act of listening, among other observing responses, is initially developmentally independent from speaker behavior. How they become joined parallels the biological phenomenon of metamorphosis. The succession of changes has been empirically identified as a succession of verbal behavior development cusps, which are described in their sequence biologically as a manifestation of functional metamorphosis. The onset of a cusp constitutes first instances of behavior and accompanying stimulus control that allows infants and children to contact parts of the environment for the first time resulting in their learning things impossible to learn before or learning faster. Cusps for the intercept of the speaker and listener lead to bidirectional operants and provide explanations for how children incidentally learn the names of things, become increasingly social, and make subsequent complex behavior possible. Many of the cusps identified in our research resulted from the missing behavior and stimulus control of children with autism. Once cusps were established, these children learned things they could not learn before, learned faster, and learned by contacting parts of the social environment they could not contact before. These findings led to a theory of verbal behavior development that point to the selection of bidirectional operants as behavioral phenotypes during functional metamorphosis, which has enhanced the survival of Homo sapiens through emergent symbolic skills for more effective

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Vacuolar myelopathy (VM) is a frequent neurological complication of the acquired immune deficiency syndrome (AIDS). A suspected connection between VM and human immunodeficiency virus (HIV) has been based only on HIV isolation from affected spinal cord tissue. We report here an AIDS patient dying after 14 months of progressive dementia, including 3 months of spinal signs and symptoms. At autopsy, the brain revealed moderate diffuse damage of the white matter compatible with HIV-induced progressive diffuse leukoencephalopathy. The spinal cord showed VM mainly in the lateral and the posterior columns. Mono- and multinucleated macrophages were localized within intramyelinic and periaxonal vacuoles. Light and electron microscopic immunocytochemistry revealed the presence of HIV antigens restricted to mono- and multinucleated macrophages within the spongy lesions. Productive HIV infection is documented for the first time within VM lesions of this case. Therefore, VM should be included among HIV-induced lesions of the central nervous system. The intimate relation of infected macrophages to vacuolar myelinopathy could suggest secretion of a myelinotoxic factor by macrophages productively infected by HIV. Immune electron microscopy appears as promising tool to detect HIV in tissue even when the density of virus may be low.

Building on Skinner's theory of verbal behavior, research over the last few decades confirmed verbal speaker operants, added the role of the listener, added the identification of speaker and listener interaction between and within individuals, and identified verbal behavior developmental cusps. Meanwhile, comparative biology focused on how and why language evolved in Homo sapiens. Findings about differences in behavior that neurotypical children demonstrated in their verbal development, and even more so in research that identified and established missing verbal behavior cusps, suggested changes analogous to metamorphosis. These striking changes in stimulus control found in the onset of cusps from the preverbal to the fully verbal child led us to an expansion of the concept of metamorphosis from morphology to the domain of behavior. The major findings of this comparative perspective are presented here as they have led us from experimental analyses of verbal development to metamorphosis as complex verbal behavior transformation and finally to a novel hypothesis about the evolution of language based on the concepts and research described here. To our knowledge, this is the first formulation of verbal development as behavioral metamorphosis in the context of evolutionary developmental biology.

A cross-sectional, dichotic listening study of 210 right-handed, middle-class children four to ten years old used thirty pairs of one-syllable words and thirty pairs of four-syllable numbers to assess the developmental course of ear asymmetry. A significant decrease in REA for both word and number pairs was found. Although right-ear and left-ear performance both increased with age, the developmental gain in left-ear performance was greater than the gain in right-ear performance, thus resulting in a decrease in REA with age. The results are discussed with respect to investigations which found no change in REA during development and a structural model based on the development of interhemispheric connectivity is proposed to explain the findings.