Despite the knowledge that synaptic plasticity has a significant influence on cognitive performance, the relationship between actual synaptic (nerve connections) arrangements and individual differences in learning and memory, remains inadequately understood. Researchers recently reported on the use of a visual discrimination task to examine the correlation between the density of synaptic complexes (microglomeruli) within the bumblebee brain, with individual visual experience, visual learning and memory performance.
The team found that bumblebees with higher microglomeruli density performed best in visual discrimination tasks and likewise had a better memory of their learned colour-related task two days after training.
Linking synaptic density to memory and speed of learning
Drawing on work from the SPACERADARPOLLINATOR project, the team report in Proceedings of the Royal Society B
how, in one experiment, bumblebees were trained to differentiate between ten artificial flowers, all differently coloured. Five of the artificial flowers contained desirable sugar water, whereas the other five had an undesirable bitter quinine solution. After two days the bees were tested for how well they retained the memory of which colours offered the reward.
The team found that the bees which had a higher collar region (for visual association) microglomeruli density, seemed to be faster learners as evidenced by fewer landings in search of the rewarding artificial flowers. They also displayed a better memory of the colour task two days after training, with the team suggesting that for the first time it had been demonstrated that changes in microglomeruli density could be induced through the acquisition of visual memory.
Incorporation of their 10-colour learning paradigm allowed the researchers to build on previous studies which had typically only used two-colour visual discrimination, resulting in limited performance variation across individuals. The team did indeed find that bumblebees had greater microglomeruli density when exposed to many coloured artificial flowers as opposed to no colour differentiation or only two colours.
The researchers used a technique called whole-brain immunolabelling to measure microglomeruli density in the collar region of the bumblebee brain, along with confocal microscopy for a close look at the brain itself. They hope that the findings will help shed light on the neural basis of cognition in all animals, including humans.
The role of environmental enrichment
Previous studies have demonstrated that structural and functional neural plasticity, alongside improved learning and memory, correlates with an increase in the range of environmental stimuli with which an animal interacts. The researchers speculate that their controlled laboratory 10-colour experiments, may have offered just such an enriched environment inducing structural reorganisation in the visual regions of the brain.
While individual differences in microglomeruli density across bee colonies could be used as a predictor of foraging performance and exploratory behaviour, the researchers also suggest that natural variation in performance may exist as an adaptation to fluctuations in food sources. Rather than some bees simply being less cognitively capable overall, those bees may actually be prioritising alternative survival solutions of use to the colony, not currently understood.
The SPACERADARPOLLINATOR project is developing radar tracking technology to better understand the rules governing bee movements, including those critical for natural resource exploration and exploitation. The project hopes to ultimately contribute to better conservation management efforts.
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