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With the right hand he pointed to a shovel because, he reasoned, you need it to clean up the chicken droppings. What did this finding mean? Such split-brain research was a revelation. The brain was not, as had once been thought, homogeneous in its capacity to process information. Rather it was organized as modules and circuits for specialized actions. The research also veered into philosophical territory. Gazzaniga writes.
Split-brain work began at a time when psychologists were skeptical that the content of the mind could be studied scientifically. Behaviorism was in the ascendancy, with its view of action being shaped solely by a calculation of reward and punishment and its practitionersregarding the mind as a mysterious black box.
The work of Mr.
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Gazzaniga, who is now regarded as the father of cognitive neuroscience, and of others have helped illuminate the black box, making way for many of the neuroscientific wonders we are familiar with today. Those who have read Mr. This means that it cannot be explained fully at the neural or molecular level.
There is the story of how Mr. Gazzaniga met William F. Using this technique, Wang and colleagues implanted electrodes in HVC of each hemisphere and found that stimulating HVC in one hemisphere caused song disruptions only when these short stimuli were delivered during certain portions of the motif.www.hiphopenation.com/mu-plugins/tobias/a-to-z-of.php
Both Sides of the Brain Are Active During One-Sided Arm Movement
Astonishingly, the authors observed that stimulation of the contralateral HVC interrupted song at precisely the time when stimulation in the other HVC had no effect on song. In other words, left hemisphere stimulation disrupted song during periods when the right hemisphere stimulation was ineffective and vice versa. When the authors increased the amount of current, this effect disappeared and stimuli were effective at perturbing song temporal structure throughout the whole motif, suggesting that the threshold for perturbing syllable sequencing within a motif is what switches rapidly between hemispheres.
Multi-unit neural recordings from HVC during singing in the same species [ 27 ] show clear premotor activity in each hemisphere during the entirety of the motif. The ability of electrical stimulation to perturb activity in each hemisphere in rapid alternation therefore reflects changes that are subtler than simple all-or-none changes in premotor activity. Given the evidence for alternation between the left and right half of the syrinx, a critical question is whether the timing of hemispheric switching is synchronized to the switching observed in the syrinx.
The authors did not directly measure syringeal dynamics, but they did compare switching patterns with the major acoustic transitions in the song. They did not find any obvious relationship between hemispheric switching and transitions in the song e. While this approach provides a reasonable initial characterization, extrapolating syringeal switching patterns from the acoustic signal is likely to be inaccurate.
Future experiments combining HVC stimulation with syringeal muscle recordings or airflow measurements, parameters that are typically used to measure syringeal switching [ 28 ], are therefore necessary to unequivocally reveal any relationship between central hemisphere and peripheral syrinx switching. An additional direction that might be explored to link interhemispheric switching with song output is to compare interhemispheric switching patterns in birds that have been tutored with, and therefore learned, the same exact song, a process that is possible under specific tutoring paradigms [ 29 ].
Having birds sing identical songs would allow analysis across birds of the general types of song transitions that lead to interhemispheric switching. When hemispheres contribute differentially to behaviors that recruit muscles bilaterally, such as for bimanual movement or vocal production, it is sometimes assumed that hemispheric coordination is achieved exclusively by the corpus callosum, a massive fiber bundle that connects the left and right cerebral hemispheres.
This commissural system is evolutionarily recent and is only observed in placental mammals, yet hemispheric switching is observed in animals such as fish and birds that do not have a corpus callosum.
A possible neural substrate for hemispheric coordination and switching might therefore include brainstem neuromodulatory systems. These are interconnected across the midline and project diffusely throughout the hemispheres, and could therefore differentially influence each hemisphere. Consistent with this view, a recent study has shown that cortical release of the neurotransmitter acetylcholine is lateralized during asymmetric sleep in fur seals [ 30 ].
While this mechanism might be well suited for hemispheric switching during sleep, the generally slow time course of action of neuromodulators makes this mechanism somewhat less attractive for rapid hemispheric switching. This is plausible in the song system given known projections from the brainstem back up to the forebrain via the intermediary of the thalamus, and the direct influence that these brainstem areas have on neural activity in forebrain song control nuclei [ 31 ].
Interestingly, a similar relationship between the brainstem and forebrain has been demonstrated in other motor control systems such as the primate oculomotor system [ 32 ]. Further supportive evidence for a role of the brainstem in hemispheric switching comes from findings that switching during binocular rivalry [ 17 ] can occur in the absence of a corpus callosum [ 33 ] and is modulated by serotonin receptor subtypes that are located largely in the brainstem [ 34 ].
Book Review: ‘Tales From Both Sides of the Brain’ by Michael S. Gazzaniga - WSJ
For song production, the brainstem VRN is well placed to play a central role in hemispheric switching, but it is unlikely that it acts as a simple oscillating circuit, given that Wang and colleagues never observed any periodicity in the pattern of hemispheric switching. In fact, they often observed a wide range of switching intervals from 4 ms to ms within a single song motif. An important direction for future work will be to identify the signal s responsible for the transitions that underlie hemispheric switching.
In the song system, the VRN and the inputs it receives from RA are well placed to show spiking patterns that might be predictive of switching times. One of the primary unanswered questions is why brain systems have evolved to switch rapidly between hemispheres. One possibility is that interhemispheric switching is part of a strategy to optimize processing power within each hemisphere while keeping both sides synchronized. Similar strategies are certainly used computationally in multicore processors. In songbirds, it is certainly clear from lesion studies that normal adult birds require both hemispheres to produce song [ 35 ].
The question of how the elements of song production are divided between hemispheres, however, is unclear. Wang and colleagues failed to find any relationship between switching times and features of song acoustics such as syllable onsets, yet they observed a wide range of switching time intervals.
Presumably these intervals are not arbitrarily determined but reflect some aspect of song production, such as switching between sides of the syrinx. It is also important to note that the bilateral nature of the song system appears to have a learned component because unilateral lesions of RA in juvenile birds causes these birds to sing approximately normal songs as adults [ 35 ].
This suggests that the song system can compensate for whatever functional advantages are conferred by bilateral processing. For example, do unilateral birds also show switching at the level of the syrinx?
If so, how is such switching achieved? More generally, the purpose of interhemispheric switching may become clearer as more data are collected through the paradigm developed by the Wang lab, and similar experiments are performed in other motor systems.
Functional specialization is found throughout the nervous system but requires communication among many different areas of the brain to result in cohesive activity. Interhemispheric switching provides a compelling example, and may very well elucidate more general principles of brain function and the production of temporally complex behaviors. I wish to thank many members of the Schmidt lab, in particular Chris Glaze and Jon Raksin, for reading various versions of the manuscript.
Marc F. E-mail: ude. National Center for Biotechnology Information , U. PLoS Biol. Published online Oct Marc F Schmidt. Copyright and License information Disclaimer. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.
This article has been cited by other articles in PMC. Hemispheric Specialization as a General Feature of Brain Systems Hemispheric specialization, or lateralization as it is often referred to, was originally thought to be a unique human characteristic but appears to be a general property among vertebrate brains. Song Production in Birds: A Highly Asymmetric Behavior Song production in passerine birds is one of the most striking and best characterized asymmetric behaviors at the peripheral level [ 20 ].
Open in a separate window. Figure 1. Based on recordings performed in the cowbird by Rod Suthers. Figure 2. Bilateral Organization of the Song Motor Control System This schematic is a simplified representation of the motor portion of the avian song system emphasizing its bilateral organization. Rapid Interhemispheric Switching Occurs during Precise Transitions in the Song One approach to test whether a behavior is being driven by the rapid alternation of motor commands sent by each hemisphere is to briefly stimulate the circuits responsible for producing the behavior and ask whether the ability of such perturbation to alter the behavior is dependent on the specific hemisphere that is stimulated.
The Switching Mechanism: The Brainstem as a Possible Key Player When hemispheres contribute differentially to behaviors that recruit muscles bilaterally, such as for bimanual movement or vocal production, it is sometimes assumed that hemispheric coordination is achieved exclusively by the corpus callosum, a massive fiber bundle that connects the left and right cerebral hemispheres. Functional Implications One of the primary unanswered questions is why brain systems have evolved to switch rapidly between hemispheres. Acknowledgments I wish to thank many members of the Schmidt lab, in particular Chris Glaze and Jon Raksin, for reading various versions of the manuscript.
Footnotes Marc F. Karl Wernicke — found that people who struggled with language processing tended to have damage to specific areas on the left sides of their brains. The researchers concluded that those two areas were important to language processing, announcing to the world that language lives in the left brain.
That idea is probably at the root of this notion that the left brain is more creative and the right brain is more analytical. Jekyll and Mr. Hyde" helped spread the idea of left and right brains in popular culture, according to Anne Stiles , a professor of English at Saint Louis University.