Masakazu Konishi

Hormone-Induced Sexual Differentiation of Brain and Behavior in Zebra Finches

The male zebra finch sings, whereas the female does not. This behavioral dimorphism is correlated with the presence of morphological sex differences within the neural substrate that mediates this behavior, the song system. When a female chick is exposed to 17β-estradiol her song system is subsequently masculinized. Either testosterone or 5α-dihydrotestosterone may then induce such a female to sing when an adult.

Mechanisms of sound localization in the barn owl (Tyto alba)

Summary1.We investigated the mechanisms by which the barn owl (Tyto alba) determines the azimuth and elevation of a sound source. Our measure of localizing ability was the accuracy with which the owl oriented its head to a sound source.2.When localizing tonal signals, the owl committed the smallest errors at frequencies between 4 and 8 kHz. The azimuthal component of these errors was frequency independent from 1 to 8 kHz, but the elevational component increased dramatically for frequencies below 4 kHz.3.The owls mean error when localizing wide band noise was nearly three times less than its mean error when localizing the optimal frequency for tonal localization (6 kHz).4.Occluding the right ear caused the owl to orient below and to the left of the sound source; occluding the left ear caused it to orient above and to the right of the sound source.5.With ruff feathers (facial ruff) removed, the owl continued to localize sounds accurately in azimuth, but failed to localize sounds in elevation.6.We conclude from these results that the barn owl uses interaural comparisons of sound spectrum to determine the elevation of a sound source. Both interaural onset time and interaural spectrum are used to identify the azimuth of the sound source. If onset time is not available (as in a continuous sound), the owl can derive the azimuth of the source from interaural spectrum alone, but its spatial resolution is poorer.

A suboscine bird (eastern phoebe, Sayornis phoebe) develops normal song without auditory feedback

Imitative song development, its requisite auditory feedback, and the underlying neural control of learned song are becoming increasingly well known in songbirds, but the evolution of these characteristics from songbird ancestors is poorly understood. Suboscine flycatchers, which belong to the evolutionary sister group of the oscine songbirds (in the same order, Passeriformes), are thought not to imitate songs from other individuals. This study therefore examines the role of auditory feedback in song development and provides preliminary comments on neural control. Four eastern phoebes, Sayornis phoebe, were collected at 10–12 days of age and hand-reared in the laboratory; at approximately 35 days of age, before they began to sing, the birds were bilaterally deafened by removal of the cochlea. Songs of these phoebes, two males and two females, were judged to be normal when compared with songs of males recorded in nature and to songs of laboratory-reared, intact males and females. Like several non-passerines (representatives of Galliformes and Columbiformes), the eastern phoebe requires no auditory feedback for normal vocal development. Brain sections of phoebes contain no obvious cell clusters like the forebrain song nuclei of songbirds. If some of these nuclei mediate auditory feedback control of song development, the apparent absence of these nuclei in the phoebe is consistent with its ability to develop normal song without auditory feedback.

Sound localization by the barn owl (Tyto alba) measured with the search coil technique

Summary1.The dynamics and accuracy of sound localization by the barn owl (Tyto alba) were studied by exploiting the natural head-orienting response of the owl to novel sound stimuli. Head orientation and movement were measured using an adaptation of the search coil technique which provided continous high resolution azimuthal and elevational information during the behavior.2.The owls responded to sound sources with a quick, stereotyped head saccade; the median latency of the response was 100 ms, and its maximum angular velocity was 790°/s. The head saccade terminated at a fixation point which was used to quantify the owls sound localization accuracy.3.When the sound target was located frontally, the owls localization error was less than 2° in azimuth and elevation. This accuracy is superior to that of all terrestrial animals tested to date, including man.4.When the owls were performing open-loop localization (stimulus off before response begins), their localization errors increased as the angular distance to the target increased.5.Under closed-loop conditions (stimulus on throughout response), the owls again committed their smallest errors when localizing frontal targets, but their errors increased only out to target angles of 30°. At target angles greater than 30°, the owls localization errors were independent of target location.6.The owl possesses a frontal region wherein its auditory system has maximum angular acuity. This region is coincident with the owls visual axis.

Centrally synthesized maps of sensory space

When central neurons are arranged so that their spatial relationships conserve wholly or partially those in the peripheral sensory epithelium, the relevant portion or feature of the sensory epithelium is said to be mapped topographically. Retinotopic, tonotopic, and somatotopic maps are all examples of topographic projections. Here these maps will be termed ‘projectional maps’. The functional role of these maps is difficult to establish, because the coding of spatial information may not be the factor determining their topographic organisation. For example, a topographic map might also be a by-product of embryological and anatomical processes that have nothing directly to do with neural coding. A second class of brain maps exist which are not topographically related to their corresponding sensory epithelia. For example, neurons selective for sound-source locations form a map of auditory space in the external nucleus of the owls inferior colliculus (see Fig. 1). Similarly, neurons selective for echo delays are systematically distributed in the auditory cortex of the mustached bat. The distribution of orientation-selective neurons in the monkeys visual cortex also belongs to this class of maps. In these cases, primary sensory cells neither register nor extract the location, delay, or orientation of a stimulus. The selectivity for these cues is created by neuronal circuits in which the neurons forming the maps are nodal points. In this way, neuronal selectivity for stimulus orientation is not generated in the eye but ‘synthesized’ in the cortex. Similarly, neuronal selectivity for binaural disparities is obviously not present in the ear. Tuning to a particular range of echo delays is also a property of central neurons. In this review these maps will be referred to as ‘centrally synthesized’ rather than ‘computational’ maps as they were named elsewhere, because the new term is more descriptive. The study of these maps may cast light on the significance of mapping in general as a method of neural coding.

Birdsong for neurobiologists

Song is a stereotyped behavior, yet its development depends on sensory feedback and learning. Early bird fanciers recognized that young birds must have a good tutor to become a good singer. They also knew that birds could be bred for more elaborate songs, as exemplified by different breeds of canaries selected for different ways of singing. Thus, variations in song between individuals and between species contain both heritable and experiential components. The study of the experiential component has shown that the nature and timing of auditory experience play an important role in shaping the outcome of song development. The ability of a bird to modify song in adulthood has drawn special attention because it implies neural plasticity in the adult brain. The discovery of the brain areas for the control of song has brought birdsong from the purely behavioral level of analysis to the interface between behavior and neurobiology. The song control system shows al I of the developmental events that attract much current research, such as neuronal growth, death, and migration, the specificity of neuronal connections, and the waiting compartment. Furthermore, sex steroids control some of these ontogenetic events as well as the expression of song and its neural substrates in adulthood. The main aim of this article is to review current research dealing with these topics.

Deciphering the brain's codes

The two sensory systems discussed in this review use similar algorithms for the synthesis of the neuronal selectivity for the stimulus that releases a particular behavior, although the neural circuits, the brain sites involved, and even the species are different. This stimulus selectivity emerges gradually in a neural network organized according to parallel and hierarchical design principles. The parallel channels contain lower order stations with special circuits for the creation of neuronal selectivities for different features of the stimulus. Convergence of the parallel pathways brings these selectivities together at a higher order station for the eventual synthesis of the selectivity for the whole stimulus pattern. The neurons that are selective for the stimulus are at the top of the hierarchy, and they form the interface between the sensory and motor systems or between sensory systems of different modalities. The similarities of these two systems at the level of algorithms suggest the existence of rules of signal processing that transcend different sensory systems and species of animals.

The role of auditory feedback in birdsong.

Abstract: Young songbirds memorize a tutor song and use the memory trace as a template to shape their own song by auditory feedback. Major issues in birdsong research include the neural sites and mechanisms for song memory and auditory feedback. The brain song control system contains neurons with both premotor and auditory function. Yet no evidence so far shows that they respond to the birds own song during singing. Also, no neurons have been found to respond to perturbation of auditory feedback in the brain area that is thought to be involved in the feedback control of song. The phenomenon of gating in which neurons respond to playback of the birds own song only during sleep or under anesthesia is the sole known evidence for control of auditory input to the song system. It is, however, not known whether the gating is involved in switching between the premotor and auditory function of neurons in the song control system.

Dynamic control of auditory activity during sleep: Correlation between song response and EEG

The song nucleus high vocal center (HVC) sends neural signals for song production and receives auditory input. By using electroencephalography (EEG) to objectively identify wake/sleep state, we show that HVC auditory responses change with physiological states. Comparison of EEG and HVC records revealed that HVC response to auditory stimuli is greatest during slow-wave sleep. During slow-wave sleep, HVC neurons responded preferentially to the birds own song. Strikingly, both spontaneous and forced waking during sleep caused HVC auditory responses to cease within milliseconds of an EEG-measured state change. State-dependent phenomena in downstream nuclei, such as robustus archistriatalis, are likely to be derivatives of those in HVC.

Effects of Interaural Decorrelation on Neural and Behavioral Detection of Spatial Cues

The detection of interaural time differences (ITDs) for sound localization critically depends on the similarity between the left and right ear signals (interaural correlation). We show that, like humans, owls can localize phantom sound sources well until the correlation declines to a very low value, below which their performance rapidly deteriorates. Decreasing interaural correlation also causes the response of the owls tectal auditory neurons to decline nonlinearly, with a rapid drop followed by a more gradual reduction. A detection-theoretic analysis of the statistical properties of neuronal responses could account for the variance of behavioral responses as interaural correlation is decreased. Finally, cross-correlation analysis suggests that low interaural correlations cause misalignment of cross-correlation peaks across different frequencies, contributing heavily to the nonlinear decline in neural and ultimately behavioral performance.