Neural activity was recorded from neurons in the antennal lobes and mushroom bodies of locusts, honeybees, and fruitflies, using patch-clamp, intracellular, and extracellular electrodes. The antennal lobe is a glomerular structure, analogous to the vertebrate olfactory bulb. The mushroom body is a structure involved in the formation of odor memories (Menzel 1987). Its functional analog in vertebrates might be the piriform cortex, although the structural similarities between mushroom bodies and piriform cortex are much less obvious than those between antennal lobes and olfactory bulbs. The responses of the principal neurons of the antennal lobe (projection neurons, direct targets of the olfactory receptor neurons) and of the mushroom body (Kenyon cells, direct targets of the projection neurons) are compared here (Figure 10.1).
Odor representations are distributed and dynamic (on several timescales) in the antennal lobes but are sparse, brief, and synthetic in the mushroom bodies. Later figures will examine the mechanisms thought to underlie the transformation from the first to the second.
Neural Activity Is Distributed across the Population of Antennal Lobe Principal Neurons (Figure 10.1a)
These spike rasters represent the activity of 110 different projection neurons in the locust antennal lobe in response to the delivery of the same odor (hexanol, gray shadow). The projection neurons were recorded in identical conditions using tetrodes, in groups of about 10-20 projection neurons each, from different preparations. The locust antennal lobe contains about 830 projection neurons per antennal lobe. Note that many (about 50% on average) projection neurons respond to this odor, but that response patterns differ from projection neuron to projection neuron. Many responses are multiphasic and each response phase can last several hundred milliseconds. This type of response is typical in this system and suggests a distributed, dynamic representation. Similar patterns are observed in the antennal lobe of Drosophila (Wilson et al. 2004).
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