Auditory Specializations
Kubke et al. 2004
Differential increases in brain sections can be mediated either by increases in overall brain size (passive) or selective increases in specific cell groups (active). Because there are constraints on increases in overall brain size (metabolic, developmental, etc), sensory specialization may be accompanied by an increase in cell groups associated with the specific sensory modality. Kubke et al (2004) used barn owls to investigate the regulation of auditory brain structures in birds. They found that barn owls had a greater increase in auditory structures than would be accomplished by increases in global brain size, suggesting an increase in size of specific cell groups. Changes in size of specific brain structures can be mediated by changes in cell birth, rates of mitosis, and apoptosis.
Kubke et al. 2004 discuss interspecific variation in the size and development of brain structures, but what about intraspecific variation? Specifically, I wonder how plastic is specialization in different brain structures? Can differential investment in specific brain regions be modulated during development in response to maternal and/or environmental cues?
Kubke et al. briefly discuss the importance of tropic factors in regulating the extent of cell death. What do they mean by tropic factors (ecological influences?)? Also, why are increases in brain structure size important for increases in sensory modalities? What about synaptic connections or morphological characters (ears)?
Convey (2005)
Bats also have specialized brain centers in response to echolocation and environmental/feeding guild factors. That’s about all I take from these 13 pages.
Differential increases in brain sections can be mediated either by increases in overall brain size (passive) or selective increases in specific cell groups (active). Because there are constraints on increases in overall brain size (metabolic, developmental, etc), sensory specialization may be accompanied by an increase in cell groups associated with the specific sensory modality. Kubke et al (2004) used barn owls to investigate the regulation of auditory brain structures in birds. They found that barn owls had a greater increase in auditory structures than would be accomplished by increases in global brain size, suggesting an increase in size of specific cell groups. Changes in size of specific brain structures can be mediated by changes in cell birth, rates of mitosis, and apoptosis.
Kubke et al. 2004 discuss interspecific variation in the size and development of brain structures, but what about intraspecific variation? Specifically, I wonder how plastic is specialization in different brain structures? Can differential investment in specific brain regions be modulated during development in response to maternal and/or environmental cues?
Kubke et al. briefly discuss the importance of tropic factors in regulating the extent of cell death. What do they mean by tropic factors (ecological influences?)? Also, why are increases in brain structure size important for increases in sensory modalities? What about synaptic connections or morphological characters (ears)?
Convey (2005)
Bats also have specialized brain centers in response to echolocation and environmental/feeding guild factors. That’s about all I take from these 13 pages.
posted by Ondi at 11:49 AM
4 Comments:
Kubke et al. 2004.
In general the authors explore if the size of the auditory structures in birds is due to passive (global increase in brain size) or active (enlargement of specific cells group) mode. The compare birds with “high” use of auditory system (Passerines and owls) against “non-auditory specialists”. They found that indeed the Passerines and owls have a large auditory structure compared with “non-auditory specialists”. They also found that cell dead can be similar in chicken and barn owl, from this data they suggest that cell death can regulated the degree of hyperplasia of the auditory nuclei. However the chicken data show conflicting results, for this reason more data should be collected to build strong conclusions. This section on the paper wasn’t very clear, at least for me.
I wan to clarify one comparison, when they compared the owls with Oilbird, although they are in the same order (Strigiformes) and have nocturnal habits, the Oilbird are frugivorous and use very different foraging techniques. For this reason I don’t think this comparison is very strong. On the other hand there is also a large variation in the number of auditory nuclei within the different groups (Passeriformes, owls and non-specialists; Fig 4). This variation can be due to the importance and how the auditory system is used by the different species, which is poorly address on the paper.
Covey, E. 2005.
This review paper explore the morphological similarities and differences in specific structures of the auditory system among non-flying mammals and flying mammals (bats), in their. In general different parts of the auditory system among non-flying mammals and flying mammals are very similar. However, there are notorious differences in certain pathways used by the bats. For example, the number of neurons and connections are larger in areas related with temporal and auditory information in bats. The ventral nuclei of the lateral lemniscus (VNLL) is the structure where this specialization are more pronounce.
Kubke et al. (2003) determine, using the barn owl as a model, whether the cochlear nucleus magnocellularis (NM) and its circuits to the nucleus laminaris (NL) are influenced by active-mode brain development. Additionally they do a comparative analysis of the sizes of auditory hindbrain structures across several avian orders. They find that the size of the brain stem was larger (relative to brain and body size) in owls compared to other birds and it was greater in owls with nocturnal habits and asymmetrical ear openings (most strictly noctural owls have asymmetrical ears). Owls also typically had greater numbers of cells in the NA, NL and NM regions relative to their brainstem weight, which could be directed by increased neurogenesis and reduced cell death. I was confused about their mention of trophic factors that “regulate the number of surviving neurons”—what are they talking about?
The Covey (2005) article (bleh) discusses the neural specializations found in the peripheral and central auditory structures of echolocating bats that contribute to the analysis of complex sound patterns. Many of these specializations are just “exploitations” of general structures found in mammals. My immediate reaction to the paper was that a glossary for abbreviations would have been nice, and as is written the paper seems like a grab-bag of examples of specializations, a very few of which are given with a good ecological context. Maybe my reaction would be different if I were a brain specialist…so, I’m still trying to figure out what to take away from this—general principles? Trends in specializations associated with ecological utility? Within Covey’s overview, there are interesting avenues for future research, including the ways that auditory-motor pathways have “taken over” functions of visual pathways (e.g., no LGN structure in some species).
Covey 2005,
This paper is about the neurobiological specialization in bats, specifically echolocating bats, or Microchiropterans. The nervous systems of these bats are very similar to normal mammals except for the structures involved in flight and echolocation, which have become specialized in bats. The echolocating bats have optimized the mechanisms for analyzing complex sound patterns through changes in strength and timing of synaptic input to neurons. Although I didn’t understand this paper very well, I did find it interesting that the major changes involved in developing echolocation weren’t necessarily structural.
Kubke et al. 2003
Relative size of hindbrain nuclei was compared between birds with auditory specialization (barn owl and songbirds) and birds that are auditory generalists. They found that the auditory specialists showed an active mode of evolutionary enlargement in these regions, meaning that they grew without a global increase in brain size. As would be expected, brainstem size increased with body size, but an increase in brainstem indices also seemed to correspond heavily with increase auditory specialization (nocturnal habits, asymmetrical ears, well developed sound localization). Non auditory specialists showed no such increase in size. They explained the increase in size of auditory structures as probably being a result of timing and length neuronal cell birth and death
The essential observation of the Kubke (2004) paper was that bird species that were auditory specialists (Owls) displayed increases in the size of their brain stems, and "auditory nuclei." The authors find that the reasons underlying the hyperplasia may arise from neurogenisis and cell death. To reiterate, brain size can vary either by changes in body size or encephalization. Encephalization is characterized as either passive or active. Passize growth would entail the entire brain growing proportionally, and I interpret active growth as parts of the brain increasing in size relative to the global volume.
Kubke shows that "the increased number of cells in the auditory nuclei ... could not be accounted for solely by the global increase in brainstem size. (fig. 3)" If specialized audiotry nuclei are more prevalent in specialist species, it is likely because computational areas of the brain have been selected for. Sometimes in reviewing these journals I think we overlook the difficult process of data collection. Therefore I would like to highlight how cell couting took place. In order to retrieve the nuclei "Barn owl embryos were sacrificed by decapitation" (p. 171)
The monolithic Covey (2005) review provided information that Microchiroptera have specialized auditory features in the cochlea that provide increased echolocation ability. In the Nuclei of Lateral Lemniscus, neurons respond to the "onset of sound" with one action potential that becomes adapted such that they no longer respond to a continuous sound. I think this is a rather interesting feature. To speculate, in a species that depends on auditory capability as greatly as the bat, filtering out unimportant aural sources may be critical to finding prey; frugivorous bats notwithstanding.
BBC: US "plans stealth shark spies" (by remotely controling the sharks via electrodes implanted in their brains) - http://news.bbc.co.uk/1/hi/world/americas/4767428.stm
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