Associational memory, as its name implies, is a type of memory that allows one to fuse multiple events in memory. If your boss constantly yells at you in his office, you might begin to form some bad memories of being in that office. While the phenomenon of associative memory is a familiar experience, the neural basis for it isn’t well understood. A prominent theory, which was formed in the mid-20th century but only tested recently is that neurons encode associations by wiring together. In the boss’s office example, the sight of the boss’s office might activate one subset of neurons, and those neurons would then “fill in” activation of neurons that code for fear or memories of yelling (obviously this is a gross oversimplification - I’m only using it to demonstrate the principle).
One interesting problem in systems neuroscience is how the nervous system’s motor output interacts with its sensory systems. Sensory inputs that result from motor commands must be either filtered out or used to guide future motor actions. In other words, the organism must distinguish between sensory inputs that are self-generated and those from the outside world. In the juvenile songbird, for example, motor commands for song generation must be sent to some internal critic (likely basal ganglia) so the bird can compare the actual song output to some internal tutor model and improve subsequent renditions.
Connectomics is the area of neuroscience that aims to collect and curate the entirety of the connections made by all neurons in a brain (the product being called a “connectome”). For the human brain, that would be a data set of 100 billion neurons, each of which is estimated to make 1000-10000 synapses with other neurons (on the order of 1017 connections). The roll-up-your-sleeves-this-will-get-really-messy way of collecting that kind of data is to slice the brain into nanometers-thick sections and to image each slice with an electron-microscope, which has resolution below the nanometer range, and can reveal the structure of cells on a fine scale. In the image from Kristen Harris's lab below you can see a part of a neuron’s dendrite making a synapse with an axon filled with neurotransmitter vesicles; EM images however cannot show individual proteins or molecules).