Emotions exist as a complex constellation of psychological and physiological states. This meshwork of circuitry within the brain reflects an organism’s assessment of the meaning, relevance, and value of events of their environment. Our emotional responses guide our everyday thoughts and behavior, allowing us to respond quickly to immediate demands of our environments. These environmental events bring out emotional responses that receive preferential perceptual processing, in which emotional reactivity consists of peaks in emotional response. There has been an increasing emphasis, in neurology and psychology, on exploring the exact timeline of emotional processing. In studies examining emotional processing, researchers have discovered that early visual event-related potential (ERP) components, such as the P1, N1, and P2, are active during affective picture processing, indicating that they are responsible for emotional processing that corresponds to neutral stimuli. Two additional components that have been frequently studied in the context of emotional picture viewing are the early posterior negativity (EPN) and the late positive potential (LPP). The EPN has been linked to the early selective processing of emotional stimuli and is a temporo-occipital negativity to emotional pictures. The LPP is related to subjective ratings of emotional intensity and is related to neural activity that is occurring in the lateral occipital, inferotemporal, and parietal visual areas of the brain. Some researchers have utilized similar methods of analyzing timing of components of ERPs in emotional processing in order to study the defects of emotional processing in patients with psychiatric disorders, such as depression and anxiety; two disorders that are hard to differentiate between because they often co-occur and have overlapping symptoms. These methods of analyzing patterns of attention, by recording components such as the LPP, suggest that we could potentially find a mechanism of distinguishing between these two disorders and providing sufficient diagnoses by analyzing the differences in the neural circuits involved in emotional processing.
In the article, “Differentiating neural responses to emotional pictures: Evidence from temporal-spatial PCA,” researchers Foti et al. (2009) utilized principal component analysis (PCA) to discriminate between and identify components that are selectively sensitive to emotional stimuli compared to neutral stimuli. Foti et al. attempted to explore and solve the limitations of the earlier research that attempted to examine ERPs using PCA. PCA is a factor-analytic statistical approach that can capture variance across electrode sites and across time points and also separate latent components. Researchers utilized data from a passive affective picture viewing task, in which a large sample was shown a series of pictures representing either neutral, positive, or negative visual stimuli. Previous studies have employed PCA in this same context, however the duration at which they presented their stimuli was fairly short. Therefore, in this experiment, researchers Foti et al. increased the time of the stimulus duration in order to detect other components implemented in emotional processing. ERP data was calculated by attaching electroencephalography (EEG) sensors to participants’ scalps, actively detecting and measuring eye movement. The results of the study supported earlier findings that suggest a broad distinction between early processes and later processes of emotional processing. It was further proven that these earlier processes are involved in the initial attentional capture by emotional stimuli, while the later processes are involved in the sustained processing and encoding of these emotional stimuli. This study builds on the existing findings of the emotional modulation of ERP components. This study proved that the emotional modulation of the N1, EPN, and LPP represent effects of distinct electrocortical components. This study found eight separate temporospatial factor combinations that occurred in response to neutral pictures. Two of these corresponded to an early negativity (N1, EPN), while the remaining six components were related to later positivities (P3, LPP). The study builds on the existing literature by providing evidence that a parietal negativity is the earliest factor combination that is sensitive to emotion, providing findings similar to previous work on the N1. Subsequently, there is a more occipital negativity, which provides findings that correspond to previous studies of the EPN. Finally, it was found that these negativities occur independently of the three later positivities that occur at the occipital, parietal, and central recording sites. These signals were much larger for emotional processing related to neutral stimuli and had peaks occurring in a threshold with a wide range, consistent with the idea of a sustained LPP to emotional pictures that was previously observed in other studies. Therefore, Foti et al. further proved the participation of these factors, along with the sequence of events at which they are initiated.
Anxiety and unipolar depressive disorders are two of the most common forms of disease, or disorders, worldwide. With increasing numbers of diagnoses, it has been found that these two disorders are hard to distinguish between, as they share many common symptoms and often overlap in patients, causing them to actually suffer from both disorders. Studies have suggested that abnormal patterns of attention to threat and reward can serve as a potential mechanism of dysfunction in anxiety and unipolar depressive disorders. In the study, “Depression and Reduced Neural Response to Emotional Images: Distinction From Anxiety, and Importance of Symptom Dimensions and Age of Onset,” researchers Weinberg et al. (2015), recorded the LPP from patients with anxiety and unipolar depressive disorders. The LPP is thought to play a role as a potential index of motivated attention and therefore, the LPP activity is thought to be dramatically altered in these patients. Just like in the previous study, the participants were shown pictures of varying emotional quality during which they performed multiple tasks. Images corresponded to either rewarding-only, threatening-only, or neutral-only images. EEG recordings were collected to generate an electrooculogram (EOG) and analyze the LPPs. This study was the first to examine the magnitude of the LPP in a clinical sample of patients suffering from multiple diagnoses and the first to examine neural responses to both rewarding and threatening visual stimuli. The results of this study showed that a diagnosis of depression was associated with a weakened LPP to rewarding visual stimuli, while a diagnosis of anxiety was not. Furthermore, anxiety was not associated with variation in the LPP to threatening stimuli and a combined diagnosis of anxiety in the context of depression also did not affect the LPP. A diagnosis of depression was not significantly associated with a reduced LPP to threatening stimuli, however the association was in the same direction as the association with rewarding stimuli. The data of this study provide evidence that in general, depression may be characterized by reduced attentional engagement with motivationally salient content. Furthermore- the most interesting finding in my opinion- a diagnosis of depression was motivated by a subset of individuals with a diagnosis of early onset depression, which were characterized by a greatly reduced LPP. Individuals with early onset depression also exhibited reduced processing of rewarding and threatening information in comparison to those patients diagnosed with adult onset depression. These findings are consistent with previous studies, suggesting that early and adult onset depression may have similar clinical presentations, however early onset depression has a more hostile course that is associated with greater comorbidity with disorders like anxiety. Therefore, the findings by Weinberg et al. extend previous literature that provides evidence for impairments in reward functioning related to depression by linking a variability in the LPP to rewarding visual stimuli to the more specific and severe phenotype of early onset depression. Furthermore, the discrepancies in data between patients with anxiety and depression offers a new way to differentiate between these two disorders that is distinct from the clinical pathophysiological symptoms that are present. These findings demonstrate how diagnostic heterogeneity can cause clinicians to ignore meaningful differences in neural response.
The findings of these two studies, by Foti et al. and Weinberg et al., solidify the previous literature’s ideas on emotional processing in response to varying stimuli. Both studies utilize an affective picture viewing task, in which participants are either presented with neutral visual stimuli or stimuli associated with positive/negative events. The ERPs of these processes were analyzed extensively in both studies and the various components of the ERPs were timed and measured. The findings by Foti et al. are extremely important in our understanding of emotional processing, as it utilizes a more sophisticated technique to analyze ERPs and the timing of their peaks, allowing us to better understand when these components are firing and in response to what stimuli. These findings create the foundation for future research that can analyze the implications of manipulations done to these emotional regulation processes that can result in changes in ERP activity. Studies by Weinberg et al. analyzed the implications of these ERPs in patients with anxiety and depression and discovered new biological markers for diagnosing patients with anxiety and depression, that relies on the neuronal circuits occurring during that emotional stimulus processing, combined with the age of the patient. These findings are huge for the future of depression and anxiety treatment. Future researchers can further study the changes in the processing of rewarding and threatening information in patients diagnosed with depression and anxiety, specifically in patients suffering with early onset depression or from both disorders, and think of new ways to possibly treat these disorders that may interfere with the LPP activity, increasing it in preferential ways. The findings of both of these papers are huge as they provide the framework for future clinical work that may provide new breakthroughs in the research for the discovery of new treatment options for depression and anxiety. I am very interested to see where future research in emotional processing goes and how these techniques can be implemented to study other disorders, such as mood disorders like bipolar II disorder.
References
Foti, D., & Hajcak, G. & Dien, J. (2009). Differentiating neural responses to emotional pictures: Evidence from temporal-spatial PCA. Psychophysiology. 46. 521-30. 10.1111/j.1469-8986.2009.00796.x.
Weinberg, A., Perlman, G., Kotov, R., & Hajcak, G. (2016). Depression and reduced neural response to emotional images: Distinction from anxiety, and importance of symptom dimensions and age of onset. Journal of Abnormal Psychology, 125(1), 26–39. https://doi.org/10.1037/abn0000118
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