[Paleopsych] SW: On the Sizes of Bird Brains

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Neuroscience: On the Sizes of Bird Brains

    The following points are made by Fahad Sultan (Current Biology 2005
    1) How does brain size and design influence the survival chances of a
    species? A large brain may contribute to an individual's success
    irrespective of its detailed composition. The author has studied the
    size and shape of cerebella in birds and looked for links between the
    bird's cerebellar design, brain size, and behavior. Results indicate
    that the cerebellum in large-brained birds does not scale uniformly,
    but occurs in two designs. Crows, parrots and woodpeckers show an
    enlargement of the cerebellar trigeminal and visual parts, while owls
    show an enlargement of vestibular and tail somatosensory cerebellar
    regions, likely related to their specialization as nocturnal raptors.
    The enlargement of specific cerebellar regions in crows, parrots and
    woodpeckers may be related to their repertoire of visually guided
    goal-directed beak behavior. This specialization may lead to an
    increased active exploration and perception of the physical world,
    much as primates use of their hands to explore their environment. The
    parallel specialization seen in some birds and primates may point to
    the influence of a similar neuronal machine in shaping selection
    during phylogeny.
    2) The cerebellum is a highly conserved part of the brain present in
    most vertebrates[1], well suited for a comparative study of size and
    design. The cerebellum in birds, as in mammals, consists of a strongly
    folded thin sheet of gray matter located dorsally to the brainstem. In
    birds, it largely consists of a single narrow strip that varies in
    different species in the antero-posterior extension, which corresponds
    to the cerebellar length. The cerebellum of birds is commonly
    subdivided into ten groups of folds termed lobuli[2]. Both variability
    and regularity are evident in the lobular pattern of the bird
    cerebella. To quantify these structural varieties and relate them to
    functional or phylogenetic differences, a principal component analysis
    was performed on the residuals of the lobuli length, obtained from a
    double-logarithmic regression of lobuli length against body size.
    3) What could be the behavioral denominator common to crows, parrots
    and woodpeckers that is not developed in owls? All of these birds also
    have large brains; however, their cerebellar designs differ arguing
    against a simple co-enlargement model. The enlargement of specific
    visual and beak-related cerebellar parts in crows, parrots and
    woodpeckers fits well with their marked adeptness in using their beaks
    and/or tongues to manipulate and explore external objects. Their
    skills are even comparable to those of primates in using their hands.
    The tight temporal coupling between motor command, expected sensory
    consequences and resulting afferents during visually guided hand and
    beak usage may be the reason why these animals need large cerebella.
    The comparative analysis of the birds cerebella reveals that some
    brains may have enlarged to solve similar problems by similar means
    during phylogeny. Furthermore it shows that large brains have a
    specific architecture with dedicated building blocks.[3-5]
    References (abridged):
    1. Braitenberg, V., Heck, D., and Sultan, F. (1997). The detection and
    generation of sequences as a key to cerebellar function: experiments
    and theory. Behav. Brain Sci. 20, 229-245
    2. Larsell, O. (1948). The development and subdivisions of the
    cerebellum of birds. J. Comp. Neurol. 89, 123-189
    3. Whitlock, D.G. (1952). A neurohistological and neurophysiological
    study of afferent fiber tracts and receptive areas of the avian
    cerebellum. J. Comp. Neurol. 97, 567-635
    4. Arends, J.J. and Zeigler, H.P. (1989). Cerebellar connections of
    the trigeminal system in the pigeon (Columba livia). Brain Res. 487,
    5. Clarke, P.G. (1974). The organization of visual processing in the
    pigeon cerebellum. J. Physiol. 243, 267-285
    Current Biology http://www.current-biology.com
    Related Material:
    The following points are made by J.A. Kleim et al (Proc. Nat. Acad.
    Sci. 2002 99:13228):
    1) "For every act of memory, every exercise of bodily aptitude, every
    habit, recollection, train of ideas, there is a specific neural
    grouping, or co-ordination, of sensations and movement, by virtue of
    specific growths in cell junctions." (1)[Bain, A. (1873) Mind and
    Body: The Theories of Their Relation (Henry King, London).
    2) The neural circuits critical for the acquisition and performance of
    the conditioned eyeblink response are localized to the cerebellum (2).
    Information regarding the unconditioned stimulus (US) and conditioned
    stimulus (CS) converge within both the cerebellar cortex and the
    interpositus nucleus. CS information is relayed via ponto-cerebellar
    projections, whereas US information is relayed via the
    olivo-cerebellar pathway (2,3). Although the cerebellar cortex is
    involved in modulating some aspects of the conditioned response (CR)
    (4,5), the interpositus nucleus is the critical brain structure
    supporting long-term retention of the CR (2). Neuronal activity within
    the interpositus nucleus is highly correlated with development of the
    CR (5), and inactivation of the interpositus prevents both CR
    acquisition and performance.
    3) Although the locus of the memory trace is clear, the cellular
    mechanisms underlying the formation of the CS/US association are
    poorly understood. Several mechanisms have been proposed, including
    increases in the intrinsic excitability of interpositus neurons and
    reduced inhibition via depression of Purkinje cell activity. The fact
    that inhibition of specific synaptic enzymes and neurotransmitter
    receptors within the interpositus nucleus impair learning suggests
    that changes in synaptic function are involved. Transient changes in
    enzyme or receptor activity, however, would seem incapable of
    supporting the long-term encoding of the CS/US association. Recent
    work has shown that microinjections of a protein synthesis inhibitor
    into the interpositus nucleus impairs the acquisition but not the
    expression of the CR. This finding suggests that strengthening of the
    CS pathway may involve more permanent changes in cell structure.
    4) In summary: The idea that memory is encoded by means of synaptic
    growth is not new. However, this idea has been difficult to
    demonstrate in the mammalian brain because of both the complexity of
    mammalian behavior and the neural circuitry by which it is supported.
    The authors examine how eyeblink classical conditioning affects
    synapse number within the cerebellum; the brain region essential for
    long-term retention of the conditioned response. Results show
    eyeblink-conditioned rats to have significantly more synapses per
    neuron within the cerebellar interpositus nucleus than both explicitly
    unpaired and untrained controls. Further analysis demonstrates that
    the increase was caused by the addition of excitatory rather than
    inhibitory synapses. Thus, development of the conditioned eyeblink
    response is associated with a strengthening of inputs from
    precerebellar nuclei rather than from cerebellar cortex. The authors
    suggest these results demonstrate that the modifications of specific
    neural pathways by means of synaptogenesis contributes to formation of
    a specific memory within the mammalian brain.
    References (abridged):
    1. Bain, A. (1873) Mind and Body: The Theories of Their Relation
    (Henry King, London).
    2. Thompson, R. F. (1986) Science 223, 941-947.
    3. Steinmetz, J. E. (2000) Behav. Brain Res. 110, 13-24.
    4. Lavond, D. G. & Steinmetz, J. E. (1989) Behav. Brain Res. 33,
    5. Perrett, S. P. , Ruiz, B. P. & Mauk, M. D. (1993) J. Neurosci. 13,
    Proc. Nat. Acad. Sci. http://www.pnas.org
    Related Material:
    The following points are made by R.D. Seidler et al (Science 2002
    1) Despite extensive research, the role of the cerebellum in learning
    motor skills remains controversial (1,2). The concept of the
    cerebellum as a learning machine comes from the theoretical work of
    Marr (3) and Albus (4) and has been supported by data showing that it
    is essential for adaptive modification of reflex behavior (5) and is
    activated during motor learning. However, learning invariably leads to
    changes in motor performance, which in itself can activate the
    cerebellum. Efforts to deal with the issue of learning versus
    performance have required complex behavioral manipulations, such as
    subtracting an estimate of the performance effect.
    2) The authors present a learning paradigm in which learning and
    performance change are effectively dissociated, using a modification
    of the serial reaction time task. Typically, participants learn the
    sequence embedded in the serial reaction time task within a few
    hundred trials. However, when asked to perform the task concomitantly
    with certain distractor tasks, they show no evidence of sequence
    learning. When retested upon removal of this distractor, it is evident
    that participants did actually learn the sequence during the initial
    training. Therefore, the distractor task served only to suppress
    performance change but did not prevent learning, allowing the
    determination of the underlying neural substrates for sequence
    learning separately from performance.
    3) The authors report they performed a functional magnetic resonance
    imaging investigation during an implicit, motor sequence-learning task
    that was designed to separate the two processes, the effects of motor
    learning and changes in performance. During the sequence-encoding
    phase, human participants performed a concurrent distractor task that
    served to suppress the performance changes associated with learning.
    Upon removal of the distractor, participants showed evidence of having
    learned. No cerebellar activation was associated with the learning
    phase, despite extensive involvement of other cortical and subcortical
    regions. There was, however, significant cerebellar activation during
    the expression of learning. The authors conclude that the cerebellum
    does not contribute to learning of the motor skill itself but is
    engaged primarily in the modification of performance.
    References (abridged):
    1. J. R. Bloedel and V. Bracha, Behav. Brain Res. 68, 1 (1995)
    2. J. P. Welsh and J. A. Harvey, J. Neurosci. 9, 299 (1989)
    3. D. A. Marr, J. Physiol. 202, 437 (1969)
    4. J. S. Albus, Math. Biosci. 10, 25 (1971)
    5. M. Ito, Annu. Rev. Neurosci. 5, 275 (1982)
    Science http://www.sciencemag.org

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