Deciphering the neural basis of subjective experience remains one of the great challenges in the natural sciences. The structural complexity and the limitations around invasive experimental manipulations of the human brain have impeded progress towards this goal. While animals cannot directly report first-person subjective experiences, their ability to exhibit flexible behaviours such as motivational trade-offs are generally considered evidence of sentience. The worm _Caenorhabditis elegans_ affords the unique opportunity to describe the circuitry underlying subjective experience at a single cell level (...) as its whole neural connectome is known and moreover, these animals exhibit motivational trade-offs. We started with the premise that these worms were sentient and then sought to understand the neurons that were both necessary and sufficient for a motivational trade-off involving the rewarding experience of food and the negative experience of an aversive odour. A simple hierarchical network consisting of two chemosensory neurons and three interneurons was found to produce an output to motoneurons that enabled worms to respond in a contextually appropriate manner to an aversive odour according to the worm's hunger state. Given that this circuitry is like that found in the human spinal cord, retina, and primary visual cortex, three regions which are neither necessary nor sufficient for subjective experience, we conclude that motivational trade-offs are not a criterion for subjective experience in worms. Furthermore, once the neural substrate for a behaviour is described, we question the explanatory role of subjective experience in behaviour. (shrink)

What is the function of new neurons entering the olfactory bulb? Many insights regarding the molecular control of adult neurogenesis have been uncovered, but the purpose of new neurons entering the olfactory bulb has been difficult to ascertain. Here, studies investigating the role of adult neurogenesis in olfactory discrimination in mice are reviewed. Studies in which adult neurogenesis is affected are highlighted, with a focus on the role of environment enrichment and what happens during ageing. There is evidence for a (...) role of adult neurogenesis in fine discrimination tasks, as underscored by studies that enhance adult neurogenesis. This is also observed in ageing studies, where older mice with reduced levels of adult neurogenesis perform poorly in olfactory discrimination. Differences in methodology that could account for alternative conclusions, and the importance of specificity in methods being used to investigate the effect of adult neurogenesis in olfactory performance are emphasized. (shrink)