Abstract
Insect brains are known to express a high degree of experience-dependent structural plasticity. One brain structure in particular, the mushroom body (MB), has been attended to in numerous studies as it is implicated in complex cognitive processes such as olfactory learning and memory. It is, however, poorly understood to what extent sensory input per se affects the plasticity of the mushroom bodies. By performing unilateral blocking of olfactory input on immobilized butterflies, we were able to measure the effect of passive sensory input on the volumes of antennal lobes (ALs) and MB calyces. We showed that the primary and secondary olfactory neuropils respond in different ways to olfactory input. ALs show absolute experience-dependency and increase in volume only if receiving direct olfactory input from ipsilateral antennae, while MB calyx volumes were unaffected by the treatment and instead show absolute age-dependency in this regard. We therefore propose that cognitive processes related to behavioural expressions are needed in order for the calyx to show experience-dependent volumetric expansions. Our results indicate that such experience-dependent volumetric expansions of calyces observed in other studies may have been caused by cognitive processes rather than by sensory input, bringing some causative clarity to a complex neural phenomenon.
Highlights
Structural plasticity of the adult brain is well documented and is a prerequisite to cope with an ever-changing environment
Our main results were (i) innervation to the mushroom body (MB) calyx was primarily olfactory, (ii) antennal lobes (ALs) expansions were dependent on olfactory input, but independent of age, (iii) lack of olfactory input from one antenna prevented expansions of ipsilateral ALs and greatly reduced the expansion of contralateral ALs, and (iv) MB calyx expansions were age-dependent but independent of olfactory input
Our results show that passively received olfactory input does not affect MB calyx volume but greatly affects AL volume, indicating differential functional pathways for post-eclosion volumetric expansion of the primary and secondary olfactory neuropils in P. c-album
Summary
Structural plasticity of the adult brain is well documented and is a prerequisite to cope with an ever-changing environment. Fluctuations of the relative size of the hippocampus in birds are correlated with food storing [2] and the brains of mice living in an enriched environment differed volumetrically from the royalsocietypublishing.org/journal/rsos R. Brains of mice that lived under normal laboratory conditions [3]. Certain brain 2 structures in adult insects have demonstrated remarkable plasticity associated with learning, food search and novel tasks [4,5,6,7,8]. The direct causes of structural brain plasticity are still poorly understood
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