medical or psychological conditions, expectations of disability, being female, and older age all increase the chances that someone will have PCS. Physiological and psychological factors present before, during, and after the injury are all thought to be involved in the development of PCS. Some experts believe post-concussion symptoms are caused by structural damage to the brain or disruption of neurotransmitter systems, resulting from the impact that caused the concussion. Others believe that post-concussion symptoms are related to common psychological factors. Most common symptoms like headache, dizziness, and sleep problems are similar to those often experienced by individuals diagnosed with depression, anxiety, or post traumatic stress disorder. In many cases, both physiological effects of brain trauma and emotional reactions to these events play a role in the development of symptoms. Physiological Conventional neuroimaging studies of the brain following a concussion are typically normal. However, studies have found some subtle physiological changes associated with PCS using more novel imaging modalities. Studies using positron emission tomography have linked PCS to a reduction in glucose use by the brain. Changes in cerebral blood flow have also been observed as long as three years after a concussion in studies using single photon emission computed tomography (SPECT). At least one study with functional magnetic resonance imaging (fMRI) has shown differences in brain function during tasks involving memory after mild traumatic brain injury (mTBI) although they were not examining PCS specifically. Not all people with PCS have abnormalities on imaging, however, and abnormalities found in studies such as fMRI, PET, and SPECT could result from other comorbid conditions such as depression, chronic pain, or posttraumatic stress disorder (PTSD). Proponents of the view that PCS has a physiological basis point to findings that children demonstrate deficits on standardized tests of cognitive function following a mild TBI. A few studies have shown that people with PCS score lower than controls on neuropsychological tests that measure attention, verbal learning, reasoning, and information processing, but issues related to effort and secondary gain can not be ruled out as contributing to these differences. Recovery as measured by scores on cognitive tests frequently do not correlate with resolution of symptoms; individuals diagnosed with PCS may still report subjective symptoms after their performance on tests of cognitive functioning have returned to normal. Another study found that although children with PCS had poorer scores on tests of cognitive functioning after the injury, they also had