It depends on how the memories got encoded and the level of protection the brain assumes the individual needs.
Flashbacks are distressing, sensory-based (predominantly visual images) involuntary memories of traumatic events that intrude into our consciousness. These intrusive, involuntary memories range from fleeting sensory impressions of traumatic events, to (rarely) full-blown flashbacks which are so intense the client dissociates and feels as if they are back at the time of the traumatization.
A ‘flashback’ is ‘a mental vision of a past experience’ (Stein et al. 2007).
Not all memories of trauma become flashbacks. Flashback memories tend to be just an instant of distinct moments within an event, rather than of the whole traumatic event. During one, a sufferer might see, hear, and smell things they saw, heard, and smelled during the traumatic moment. Therefore, in cases where there are no images, we may not even notice we are having “flashbacks” even if we are. We could smell like burnt rubber and think that is somewhere near us even when it’s just part of a memory.
The brain behaves differently during the time of experiencing emotional events that will subsequently become involuntary memories – flashbacks.
Encoding of scenes that later caused flashbacks is associated with widespread increases in activation, including: those associated with emotional processing (amygdala), higher-level visual processing and mental imagery (ventral occipital cortex), and threat.
Memory encoding involves the amygdala and hippocampus.
The amygdala is associated with emotional memory — especially the formation of fear-related memories. It evolved to ensure our survival by encoding memories of past dangers we’ve experienced so that we recognize and respond to those threats if we see them again.
The hippocampus acts like the “brain’s historian.” It catalogs all the different details of an experience like who was there, where it happened, and what time of day it was into one cohesive event we can consciously recall as a memory. Apparently, our amygdala and hippocampus work together to turn our experiences into distinct long-term memories constantly.
However, during a traumatic event this system works a bit differently. Because there is danger, the fight-or-flight mechanism takes over and our amygdala is overactivated while the hippocampus is suppressed. Evolutionary, the processes involved in building a cohesive memory are de-prioritized in favor of paying attention to the immediate danger. As a result, our memories become disarrayed.
A cognitive science perspective suggests that intrusive sensory memories of emotional events may indeed be considered as occurring on a continuum with PTSD flashbacks at an extreme (Rubin et al. 2008). In this case, we could be having small reliving of memories all the time and some people may call them flashbacks and the report would be that they experience them very often.
In general, the more we get exposed to situations similar to the traumatic event that caused PTSD, the more often we’ll experience flashbacks since the brain will assume that we need to be constantly reminded of the risks. Also, the level of fear that the person falls into could increase the frequency of flashbacks. The more afraid we are, the more active the amygdala, the more frequent the PTSD symptoms.