Human speech and birdsong have numerous parallels. Both humans and songbirds learn their complex vocalizations early in life, exhibiting a strong dependence on hearing the adults they will imitate, as well as themselves as they practice, and a waning of this dependence as they mature. Innate predispositions for perceiving and learning the correct sounds exist in both groups, although more evidence of innate descriptions of species-specific signals exists in songbirds, where numerous species of vocal learners have been compared. Humans also share with songbirds an early phase of learning that is primarily perceptual, which then serves to guide later vocal production. Both humans and songbirds have evolved a complex hierarchy of specialized forebrain areas in which motor and auditory centers interact closely, and which control the lower vocal motor areas also found in nonlearners. In both these vocal learners, however, how auditory feedback of self is processed in these brain areas is surprisingly unclear. Finally, humans and songbirds have similar critical periods for vocal learning, with a much greater ability to learn early in life. In both groups, the capacity for late vocal learning may be decreased by the act of learning itself, as well as by biological factors such as the hormones of puberty. Although some features of birdsong and speech are clearly not analogous, such as the capacity of language for meaning, abstraction, and flexible associations, there are striking similarities in how sensory experience is internalized and used to shape vocal outputs, and how learning is enhanced during a critical period of development. Similar neural mechanisms may therefore be involved.

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BIRDSONG AND HUMAN SPEECH: Common Themes and Mechanisms

Visual recognition, navigation, tracking, and imagery are posited to share certain high-level processing subsystems. In the first part of this article, a theory of some of these subsystems is formulated. This theory is developed in light of an analysis of problems that must be solved by the visual system and the constraints on the solutions to these problems; computational, neurological, and behavioral constraints are considered. In the second part, inferences about perceptual subsystems are used to develop a theory of how mental images are generated. Support for this theory is adduced from studies of split-brain patients and a review of relevant neuropsychological findings. In the third part, a computational mechanism is developed to account for how visual function becomes lateralized in the brain; this mechanism is used to predict how the hypothesized processing subsystems become lateralized. In the fourth part, some critical tests of the theory of lateralization of perceptual processing subsystems are reported, and in the fifth part the theory is extended to account for the lateralization of image-transformation subsystems. In the sixth part, the theory is used to account for the almost ubiquitous variability (both between subjects and within subjects) evident in the neuropsychological literature on lateralization. Finally, in the concluding part of the article, the computational-neuropsychological approach is discussed and evaluated.

Seeing and imagining in the cerebral hemispheres: a computational approach.

Unlike older children and adults, children of less than about 10 years of age remember photographs of faces presented upside down almost as well as those shown upright and are easily fooled by simple disguises. The development at age 10 of the ability to encode orientation-specific configurational aspects of a face may reflect completion of certain maturational changes in the right cerebral hemisphere.

https://science.sciencemag.org/content/sci/195/4275/312.full.pdf

From piecemeal to configurational representation of faces

The traditional verbal/nonverbal dichotomy is inadequate for completely describing cerebral lateralization. Musical functions are not necessarily mediated by the right hemisphere; evidence for a specialist left-hemisphere mechanism dedicated to the encoded speech signal is weakening, and the right hemisphere possesses considerable comprehensional powers. Right-hemisphere processing is often said to be characterized by holistic or gestalt apprehension, and face recognition may be mediated by this hemisphere partly because of these powers, partly because of the right hemisphere's involvement in emotional affect, and possibly through the hypothesized existence of a specialist face processor or processors in the right. The latter hypothesis may, however, suffer the same fate as the one relating to a specialist encodedness processor for speech in the left. Verbal processing is largely the province of the left because of this hemisphere's possession of sequential, analytic, time-dependent mechanisms. Other distinctions (e.g., focal/diffuse and serial/parallel) are special cases of an analytic/holistic dichotomy. More fundamentally, however, the left hemisphere is characterized by its mediation of discriminations involving duration, temporal order, sequencing, and rhythm, at the sensory (tactual, visual, and, above all, auditory) level, and especially at the motor level (for fingers, limbs, and, above all, the speech apparatus). Spatial aspects characterize the right, the mapping of exteroceptive body space, and the positions of fingers, limbs, and perhaps articulators, with respect to actual and target positions. Thus there is a continuum of function between the hemispheres, rather than a rigid dichotomy, the differences being quantitative rather than qualitative, of degree rather than of kind.

The nature of hemispheric specialization in man

• Three retrospective studies were conducted to examine functional brain asymmetry in the regulation of emotion. In the first study, reports of 119 cases were collected of pathological laughing and crying associated with destructive lesions. Pathological laughing was associated with predominantly right-sided damage, whereas pathological crying was associated with predominantly left-sided lesions. In the second study, 19 reports detailing mood following hemispherectomy were collected; right hemispherectomy was associated with euphoric mood change. In the third study, lateralization of epileptic foci was assessed in reports of 91 patients with ictal outbursts of laughing (gelastic epilepsy). Foci were most likely to be predominantly left-sided. The findings are congruent with studies of the effects of unilateral brain insult on mood, and a general model of hemispheric asynmetry in the regulation of emotion is presented.

Bruno Kohn

Papers

Comprehension of syntax in infantile hemiplegics after cerebral hemidecortication: left-hemisphere superiority.

Selective impairments of visuo-spatial abilities in infantile hemiplegics after right cerebral hemidecortication

Observation and discrimination learning in the rat: Effects of stimulus substitution ☆

Observation and discrimination learning in the rat: specific and nonspecific effects.

Right-hemisphere speech representation and comprehension of syntax after left cerebral injury ☆