In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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12 From Chemotaxis to the Cognitive Map: The Function of Olfaction--Lucia F. Jacobs
12 From Chemotaxis to the Cognitive Map: The Function of Olfaction--Lucia F. Jacobs
Pages 211-228
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From page 211... ...
This could have served as a scaffold for the evolution of the parallel map structure of the mammalian hippocampus, and possibly the arthropod mushroom body, and offers an explanation for similar flex ible spatial navigation strategies in arthropods and vertebrates. Department of Psychology, University of California, Berkeley, CA 94720.
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From page 212... ...
hypothesis offers a unique explanation for the independent scaling of the vertebrate OB: that the scaling reflects directional selection on animals to decode and map patterns of odorants for the purpose of spatial navigation. CONVERGENCE IN OLFACTORY SYSTEM STRUCTURE AND FUNCTION The need to orient in space to maximize fitness by acquiring resources and avoiding competition and predation is universal.
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However, perhaps these paradoxes arise from the assumption that the primary function is discrimination. If instead the OS hypothesis is correct, the structural similarities may be explained by convergent cognitive processes for spatial navigation.
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A navigator could use this pattern to confirm its direction or speed of movement along the gradient. If two demarcated FIGURE 12.1 Schematic predictions of the spatial olfaction hypothesis.
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If wrong, the navigator could recalibrate its position by searching for FIGURE 12.2 Schematic predictions of the spatial olfaction hypothesis. The distributions of synthetic odor objects are landmarks in a dynamic olfactory space.
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From page 216... ...
The synthetic object map would have higher spatial resolution but would also be slower to construct, with the navigator having to learn the location of unique synthetic objects. However, by encoding an odorant ratio in two ways, a navigator could use this information to shortcut between synthetic object locations along elemental gradients (Fig.
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From page 217... ...
(A) BE: arrows indicate the vector information extracted from two directional cues, a distant mountain and the polarized shape of an oblong body of water.
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From page 218... ...
PREDICTIONS OF THE OS HYPOTHESIS If the function of olfaction is navigation, perhaps using a parallel map geometry, olfactory structure size should scale with navigational demand. At the same time, the impairment of olfactory structures should impair olfactory discrimination and olfactory navigation.
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From page 219... ...
Even in vertebrates, scaling of the vomeronasal and accessory olfactory systems, or the question of patterns in OR gene number, cannot be assessed here, although an OS-based analysis of these structures and gene families is under way. If the olfactory system encodes spatial maps of odorants, the absolute size of the OB should covary with the need to make maps of high spatial resolution.
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From page 220... ...
. It illustrates a basic tenet of the OS hypothesis: that the function of the OB is spatial navigation, not simply odorant discrimination, as the lesion of the olfactory epithelium impaired discrimination but not navigation.
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. The pinniped loss of olfaction, combined with low predictability in prey movements, would decrease selection for spatial tracking (Stephens, 1991)
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, in which wing size is a proxy for navigational ability, increasing in cluttered environments. Wing size increased with relative hippocampal size in microbats, but was unrelated to relative OB size.
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From page 223... ...
. The alternative OS explanation is that this is evidence of mammals evolving more sophisticated spatial cognitive abilities, with increases in OB size accompanied by increases in hippocampal size and olfactory cortex size with eventual increases in IS.
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REPTILES Chemical stimuli play a pivotal role in the behavior of reptiles, but we lack studies addressing the covariation of absolute OB size and navigational ability. There is a correlation, however, between relative medial cortex (medial pallium homologue)
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FISH Chemical stimuli are a primary source of information for spatial orientation in fish, from short reorientations to long-distance homing of salmon. Across all spatial scales, fish orient to odorants by calibrating odor sampling to their lateral line perception of hydrodynamic trails (DeBose and Nevitt, 2008)
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From page 226... ...
. Antennal lobe size should covary with the use of olfaction in navigation, whereas the multisensory mushroom body, encoding visual, mechanosensory, and olfactory information, should covary with antennal lobe size when navigation is primarily in relation to odorants.
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From page 227... ...
to the Cambrian bloodbath of predator eating predator that probably supplied the selective force necessary for the evolution of the first brains. In a highly competitive regime, active prey demand active predators.
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From page 228... ...
First, the primary function of olfaction would be navigation and its organization explained not by its ability to discriminate but to map odorants in space. Second, the OS system would represent the first and primary driving force in the evolution of associative learning, instantiated by the hippocampus in vertebrates and the mushroom body in arthropods and other protostomes.
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