In last month's post on conscious awareness I mentioned Kathy Mullen's work using an fMRI machine in Australia. Although that machine is one of the finest available, it is still a blunt instrument, but that it is all we can do with humans, where we are limited to non-invasive procedures.
With other animals we can use intracranial electrical stimulation, but those needles are still huge in relation to synapses. What we would really like to do is to activate specific spatial patterns of neurons, so we can for example mix imaginary stimuli (recollections) with physical stimuli. With this, we can study the interactions in cognition that occur between representations of the physical world and our internal thoughts.
Aleena Garner et al. have invented a mouse genetically engineered so that neurons with activated c-fos gene (which responds rapidly to different stimuli) express a receptor called hM3Dq in brain cells, thereby labeling the cells with the receptor. Later, a drug can be injected to activate this receptor, and these activated receptors will reflect the patterns of c-fos activation that occurred earlier in those neurons expressing hM3Dq receptors. These patterns will, in some sense, reflect a memory of past experience as the mouse goes about its behavior in a test chamber where this part of the study is conducted.
Garner calls it the the hM3Dq DREADD receptor, for Designer Receptor Exclusively Activated by Designer Drug. In the experiment, the mouse should imagine that it is somewhere other than where it is currently located. Using fear conditioning, the researchers were able to show that the mice remembered the current and the imaginary stimuli.
Read the paper in Science 23 March 2012: Vol. 335 no. 6075 pp. 1513-1516 DOI: 10.1126/science.1214985 at this link.