The Neural Basis of Involuntary Episodic Memories

Involuntary episodic memories are memories that come into consciousness without preceding retrieval effort. These memories are commonplace and are relevant to multiple mental disorders. However, they are vastly understudied. We use a novel paradigm to elicit involuntary memories in the laboratory so that we can study their neural basis. In session one, an encoding session, sounds are presented with picture pairs or alone. In session two, in the scanner, sounds-picture pairs and unpaired sounds are reencoded. Immediately following, participants are split into two groups: a voluntary and an involuntary group. Both groups perform a sound localization task in which they hear the sounds and indicate the side from which they are coming. The voluntary group additionally tries to remember the pictures that were paired with the sounds. Looking at neural activity, we find a main effect of condition (paired vs. unpaired sounds) showing similar activity in both groups for voluntary and involuntary memories in regions typically associated with retrieval. There is also a main effect of group (voluntary vs. involuntary) in the dorsolateral prefrontal cortex, a region typically associated with cognitive control. Turning to connectivity similarities and differences between groups again, there is a main effect of condition showing paired > unpaired sounds are associated with a recollection network. In addition, three group differences were found: (1) increased connectivity between the pulvinar nucleus of the thalamus and the recollection network for the voluntary group, (2) a higher association between the voluntary group and a network that includes regions typically found in frontoparietal and cingulo-opercular networks, and (3) shorter path length for about half of the nodes in these networks for the voluntary group. Finally, we use the same paradigm to compare involuntary memories in people with posttraumatic stress disorder (PTSD) to trauma-controls. This study also included the addition of emotional pictures. There were two main findings. (1) A similar pattern of activity was found for paired > unpaired sounds for both groups but this activity was delayed in the PTSD group. (2) A similar pattern of activity was found for high > low emotion stimuli but it occurred early in the PTSD group compared to the control group. Our results suggest that involuntary and voluntary memories share the same neural representation but that voluntary memories are associated with additional cognitive control processes. They also suggest that disorders associated with cognitive deficits, like PTSD, can affect the processing of involuntary memories.

Stimulation of GABAergic neurons of the lateral septum and its effect on movement speed

The lateral septum is associated with the regulation of innate behavior, motivation, and locomotion. Its complex interconnections with cognitive and affective regions such as the hippocampus, hypothalamus, and medial septum have made it an attractive region for studying how motivation regulates behavior in context-specific settings. This GABAergic brain region’s main output is the lateral hypothalamus, which provides downstream signaling of motor commands. Even though stimulation of lateral septum projections to the hypothalamus have shown to decrease running speed in free behaving mice, characterizing movement kinematics due to LS activation has not been studied. GABAergic medium spiny neurons of the lateral septum were selectively activated through the use of optogenetic techniques in transgenic mice. Photostimulation of the lateral septum at theta frequencies caused a non-significant decrease in head and back speed. 3D motion analysis of body movement under photostimulation was quantified, revealing a slow, linear decrease of body speed as photostimulation progressed. These results support the role of lateral septum activation in movement regulation and shed light on the specific manner in which stimulation of the LS gradually decreases movement speed.