The following is a literature review I wrote in 2017 on the subject of Top-Down and Bottom-up cognitive processing and PTSD.
Our perception of the world around us is affected by our attention processing styles. Attention processing styles can be divided into top-down and bottom-up styles. (Avery, Dutt & Krichmar 2014) Bottom-up processing refers to the processing of sensory stimulus whereas top-down processing refers to the processing of goal directed thought (Goldstein 2015, Avery, Dutt & Krichmar 2014). Posttraumatic Stress Disorder (PTSD) symptoms, effect nearly 10% of the population, include disturbances in the processing of environmental sensory experiences. (Fagelson 2007, Stewart & White 2008). Because of this it is worth examining altered attention processing and its role in PTSD. First, we will examine the nature of PTSD and its symptoms. Secondly, we will examine Attention Processing theories, with an emphasis on Gibson’s bottom-up processing theory. Then we look at cognitive factors such as bottom-up processing and their relationship to PTSD. Lastly, we will consider the brain regions that are implicated in both bottom-up processing and top down processing as they relate to PTSD.
What is Post Traumatic Stress Disorder?
PTSD is a type of anxiety disorder and is often seen as the result of wars, natural disasters, and domestic violence. These sorts of trauma usually involve exposure to real or perceived threats of serious injury or death. This results in the symptoms that are associated with PTSD such as reexperiencing the distressing event, avoidance of stimuli that are reminders of the event, exaggerated startle response, increased general arousal and hypervigilance (Victor et al 2005, Stewart & White 2008, Calhoun et al 2011). Of interest to processing styles is the inability to manage and filter stimuli and hypervigilance. (Lane, Chua & Dolan 1999, Stewart & White 2008)
The inability to filter sensory stimuli is one major factor in PTSD symptoms. Disruptions to sensory filtering are described as a failure to filter responses to environmental stimuli that are not relevant. (Stewart & White 2008) Although there seems to be a connection between sensory filtering and PTSD symptoms such as re-experiencing traumatic events or hypervigilance the exact relationship is not clearly defined but prior research has established a relationship between stress and pain and reduced sensory filtering (Stewart & White 2008).
Theories of Attention Processing
Bottom-Up, Direct Processing Theory. James Gibson’s theory of direct perception discusses bottom-up style processing. In his view evolution drove humans to develop a bottom-up approach to attention processing due to the need to survive in a hostile world. It is thought that selective pressures caused the development of receptors that were sensitive to meaningful stimuli in the environment (Démuth 2013).
Gibson goes on to theorize that our perception is caused by optical flows. This optical flows can be understood as patterns of light in the environment. Light from the environment enters the sensory organs, the eyes, and transmit information about an object in the environment. There are billions of of light waves entering the eyes at any given time and many details such as shape, size, and texture can be observed in this way. Although he discussed light rays, Gibson’s theory equally applies to any sensory input including touch or sound (Démuth 2013).
Gibson postulated that our position and physical proximity to objects in the environment affect our field of view. Changing position and proximity changes our visual field and allows us to map our environment from different angles. This allows us to gather further information such as texture gradients and the relative size of objects in our field of view. Gibson thought that the main feature in our perception was the stimulus from the environment, ie light waves, sound waves etc and that these stimuli were mainly just processed by the receptor organs. He postulated that the perception of these stimuli is consistent, objective and unchanging (Démuth 2013).
Top-down, Indirect Perception Theory. Unlike Gibson, Richard Gregory’s theory indicates that although we perceive sensory stimuli, the stimuli themselves are not important. Instead Gregory says it’s our interpretation of them that is important. He goes on to explain that our interpretation of stimulus is subjective and the interpretation of them is subject to prior experience (Démuth 2013).
Gregory believed that our subjective interpretation of stimulus was processed in higher corext centers of the brain. Unlike Gibson, he felt that both our receptors (eyes, ears etc) and our brain are important in perception. According to Gregory the sensory receptors receive ambiguous data that must be then interpreted by the brain itself. He states that the processes of attention is an active process where an individual extracts environmental stimuli, evaluates it, and interprets. He states that perception itself is the end result of this complex multistep process. Gregory also states that an individuals prior knowledge, motivation, and emotions play an integral role in an individual’s interpretation of the environmental stimuli (Démuth 2013).
Due to all the factors that are involved in top down processing there are a variety of way that an individual may interpret any particular set of stimuli. This may sometimes lead to inaccurate interpretations of stimuli (Démuth 2013).
Attention Processing and PTSD
PTSD is often associated with wartime trauma and prior research has suggested that exposure to loud noises of combat may contribute to sensory filtering disturbances (Victor et al 2005, Stewart & White 2008). However current research suggests that sensory filtering disturbances and hypervigilance can occur due to a variety trauma and is not singularly the result of wartime trauma or exposure to related noise. Researchers used self-report measures to assess undergraduate students with PTSD, students with trauma but no PTSD diagnosis, and students with minor trauma history. (Stewart & White 2008)
They found that persons with both high trauma and PTSD had altered sensory filtering but individuals with minor trauma did not. These finding suggest that trauma may cause altered sensory filtering and therefore changes to bottom-up processing. However, one limitation to these findings is that participants were not assessed for sensory processing before and after the trauma and therefore it is hard to discern if increased trauma causes changes to processing resulting in PTSD symptoms or if these individuals had pre-existing sensory processing issues that when combined with high levels of trauma predispose them to PTSD symptoms. It’s important to note that persons who experience PTSD may also comorbidly experience depression and other anxiety disorders. These disorders can also impact attention and processing and may affect the result of research (Dalgleish et al 2003).
Additional studies have shown that pain, stress and anxiety are associated with reduced ability to filter environmental stimuli, all of these are associated with PTSD. (Stewart & White 2008, Lane, Chua & Dolan 1999). The changes to processing of environmental stimulus, as in the case PTSD, directly relates to the cognitive factors that are described by Gibson’s Bottom up processing theory (Démuth 2013).
Bottom-up processing is implicated in emotions, perception, learning, and memory. Interestingly emotion, perception, learning, and memory are all areas that have been shown to be altered in persons diagnosed with PTSD causing individuals to have an increased reactions to environmental stimuli. (Lane, Chua & Dolan 1999, Oschner et al. 2009). Bottom up processing is also associated with experiencing pain and chronic pain is associated with PTSD (Asmundson & Taylor 2006) Additionally the PTSD symptom of hypervigilance relates to bottom up processing in that this processing style is associated with stimulus salience, responding to environmental stimulants like color, contrast or movement (Goldstein 2015, Stewart & White 2008, Démuth 2013).
Attention, PTSD, and the Brain
The physical brain and all related activities are affected by attention processing. (Goldstein 2015 p. 112) The prefrontal cortex and the amygdala specifically are implicated in Gibson’s Bottom-up theory, Gregory’s top-down theory, and in PTSD symptoms. (Victor et al 2005, Koch et al. 2013, Démuth 2013) The relationship between various brain regions is complex and and research differentiating between the role of top-down and bottom-up processes in the brain has been limited. This is because prior studies were behavioral in nature and only measured behavioral results as opposed to the results produced by modern fMRI studies. Of the existing neuroscience studies many have focused on bottom-up processes and their role in brain systems such as the amygdala. Few studies have focused on top-down processes (Oschner et al. 2009). The prefrontal cortex is usually associated with top-down processing and reduced ability to control fear response. There is a large body of research that associates the amygdala with bottom-up processing and increased sensitivity and decreased ability to filter environmental stimulus (Avery, Dutt & Krichmar 2014, Oschner et al. 2009, Reynaud et al. 2015, Scheider et al. 2015, Koch et al 2013).
Additionally, in individuals who experience PTSD, blood flow to the prefrontal cortex is inversely correlated with the blood flow to the amygdala (Reynaud et al. 2015). This suggests that increased blood flow to the amygdala would produce an increase in emotional reactions to environmental stimuli in harmy with Gibson’s bottom-up theory. The corresponding decreasing in blood flow to the prefrontal cortex would produce a decrease in cognitive ability to assess and environmental stimuli appropriately and this is in harmony with Gregory’s top-down theory.
Another interesting relationship between the prefrontal cortex and amygdala is found in research regarding animals that were selectively bred for reduced fear. These animals had decreased volume in the amygdala and increased volume in the prefrontal cortex. This again suggest that the amygdala is important in Gibson’s Bottom-up theory of attention processing and the prefrontal cortex is important in Gregory’s top-down processing theory. (Brusini et al. 2018, Démuth 2013). This supports the idea that the larger prefrontal cortex is giving the animals greater top-down ability to process and filter irrelevant stimuli that would normally be interpreted as fear inducing by the amygdala.
A study by Oschner et al. (2009) they found specifically that only the left amygdala was activated in top-down processing but both the left and right were activated by bottom-up processing. In this study involving 20 females participants were shown an image and told to respond naturally to the image or to generate a negative emotional response. Researchers were trying to asses the brain regions responsible for the different types of emotion generation. The natural response to the image was considered the bottom-up response and the deliberate generation of a negative response was considered the top-down response. There findings suggest that the left amygdala is responsible for both types of emotion processing. They found that the only the bilateral amygdala, occipitotemporal cortex, the right parietal cortex and the lateral prefrontal cortex were activated during bottom up processing specifically. The left ventral and dorsal lateral prefrontal cortex, the bilateral dorsal medial prefrontal and anterior cingulate cortex, and the bilateral temporal cortex and putamen were only activated during top-down processing (Oschner et al. 2009).
This suggests that the left amygdala may be affected more by top-down processing that relates to emotion and anxiety and interestingly some studies have found that individuals with PTSD have consistently smaller left amygdalas (Koch et al 2014). The left amygdala specifically may be implicated Gregory’s top-down theory of higher processing whereas and the bilateral amygdala, lateral prefrontal cortex, and parietal cortex are related to Gibson’s bottom-up pressing. Additional studies found that the amygdala was activated in individuals with PTSD when they were shown trauma-related stimuli, this further stresses the role of these brain regions in PTSD and altered attention processing (Oschner et al. 2009, Koch et al 2014).
We have looked at the symptoms and causes of PTSD as they relate to attention processing. We examined Gibson’s theory of bottom-up processing and how it may be augmented individuals with PTSD. We also reviewed Gregory’s theory and explored how top-down processing might be reduced in individuals with PTSD. Lastly we looked at brain regions that may play a role in both attention processing and PTSD. We saw how the amygdala plays an important role in bottom-up processing and the prefrontal cortex plays a role in top-down processing. We also noted that in individuals with PTSD activation and blood flow to these these areas is inversely correlated.
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