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The method of studying frontal asymmetrical activity was initially used to measure “traits” (e.g. depressive traits). However, as will be discussed below, it is now argued to be a measure of emotional processing, emotional state, and motivational tendency (Davidson, 1992, 1998). The theoretical models used to account for frontal asymmetry will be examined.

4.4.1 Dispositional model of frontal asymmetry

According to a critical review of frontal asymmetry by Allen and Kline (2004), Davidson et al. (1979) were among the first to use asymmetric frontal cortical activity to make inferences about asymmetrical brain activity and emotion. They argued that the left hemisphere is dominant for processing positive emotions whereas the right hemisphere is dominant for processing negative emotions. This means that if individuals have greater electrocortical activity in the right frontal region, they will have a disposition towards focusing on negative emotions and information. Supporting evidence for this came from Davidson and Fox (1982) who suggested that patterns of lateralized brain activity can be identified as early as infancy and to test this hypothesis they measured activity of 10- month old infants while they viewed videotapes consisting of happy or sad facial expressions. Activity in frontal and parietal regions was recorded and it was found that viewing happy faces corresponded to increased activation in the left cortical region (relative to the right) whilst viewing sad faces corresponded to increased relative

this differential pattern of activity was only found in the frontal regions, not the parietal regions.

Followed from this, Davidson and Fox (1989) further examined whether

temperament might be related to individual differences in asymmetry of frontal cortical activation. Again, EEG recordings were taken from the left and right frontal and parietal scalp regions of 10-month-old infants (n=13) and behaviour was observed during a brief period of maternal separation. It was found that infants who cried in response to maternal separation showed greater rightward frontal activation compared with infants who did not cry. Both studies by Davidson and Fox (1982, 1989) concluded that frontal asymmetry may serve to indicate the emotional reactivity or the emotional state of an individual (collectively known as the affective style). Based on the findings of frontal asymmetry, Davidson et al. (1979, 1984) developed the ‘dispositional model’. The model holds a valence hypothesis that predicts frontal hemispheric asymmetry for different types of emotional processing (Davidson, 1992; Davidson, Ekman, Saron, Senulis, & Frieson, 1990; Tomarken, Davidson, & Henriques, 1990). Specifically, the right hemisphere is responsible for the processing of negative affect while the left hemisphere is responsible for the processing of positive affect. To support this argument, Tomarken et al. (1990) compared the EEG recordings from 32 female participants under varying emotional states. Recordings were taken before and after watching video clips that aimed to elicit amusement and happiness, or fear and disgust. Consistent with the dispositional model, participants induced into negative mood (fear and disgust) showed greater activity in the right prefrontal region, however there was no significant association between positive mood (amusement and happiness) and relative leftward frontal activation. On the basis of

this Tomarken et al. (1992) suggested that frontal asymmetric indices not only reflect an individual’s emotional state, but they may also be an indication of how an individual might respond to different emotional stimuli (also referred to as affective responsivity).

There are ample findings on asymmetric frontal cortical activation to support the argument that the use of frontal asymmetric index can serve as an indicator of emotional state and response, yet in some instances it has proven difficult to reconcile findings (Kemp et al., 2010; Ranganath & Rosenfeld, 1998; Tomarken et al., 1990). Compared with the stronger evidence regarding depressive traits, there is less evidence to show that frontal asymmetrical activation may be related to suicidality. This has initiated interest to investigate to what extent suicidality may impact frontal asymmetry.

4.4.2 Frontal asymmetry and suicidality

To date, the literature has focused on depression and emotional responsivity and these are well substantiated by the related theoretical models. Building from this, Graae et al. (1996) have attempted to investigate frontal lateralization in relation to suicidality. They recorded EEG from 16 adolescents with a history of suicide attempts and compared this with 22 healthy controls. The EEG recordings were taken during eyes open and eyes closed resting state. Results showed that individuals with a past suicide attempt differed from healthy controls in their levels of frontal brain activity. Specifically, the healthy control group had relatively less activation in the right hemisphere, whereas suicidal adolescents showed a trend in the opposite direction (exhibiting greater rightward frontal activation). Moreover, compared to controls, past suicide attempters also displayed lesser brain activation in the left hemisphere than the right in the posterior regions (temporal

and parietal lobes). This shows that brain lateralization between the left and the right hemisphere may help to distinguish between individuals with or without suicide behaviour.

Using event-related fMRI, Jollant et al. (2008) compared the neural activity of previously depressed men with past suicide attempts, previously depressed men with no suicide attempts, and healthy male controls. All participants were shown a series of faces expressing happiness, anger or neutral emotions followed by a short recognition memory test for faces. This test intends to confirm participant’s ability to process facial

information. Across the three groups of participants, only those with a past history of suicide attempts showed frontal asymmetrical differences in response to emotional faces (angry, happy, and neutral). Specifically, they showed increased neural activation in the right lateral orbitofrontal cortex in response to angry faces relative to neutral faces. Jollant et al. explained that increased sensitivity to another person’s disapproval (e.g. in the form of an angry facial expression) and a higher propensity to process and act on negative emotions may exacerbate suicide behaviour. Notably, for the other two groups of participants who had no past history of suicide behaviour, activation in the frontal regions did not differ in relation to emotion. This brain laterality may represent

vulnerability markers of suicide behaviour in those with a history of depression. As the study specifically recruited male participants mostly middle-aged, and with a history of major depressive disorder, this has questioned the replicability of the findings to those who may have milder forms of suicidal behaviour but may not be categorized as having a clinical disorder (e.g. a non-clinical student population in the current research).

age, education and did not assess the duration of symptom-free periods in patients. These are all potential confounding factors (especially when taking medication at the time of scanning) in neurological studies and will need to be considered for the investigations of this thesis.

There is additional empirical evidence from neuroimaging studies that pinpoint specific deficits in executive functions may be related to frontal brain activity and degrees of suicidality. To investigate the links between inhibitory control deficits and suicidality, Pan et al. (2011) measured response inhibition using the Go/No-go task in adolescents with various degrees of suicidality. The sample included 15 depressed adolescents with a history of suicide attempts, 15 depressed adolescents with no history of suicidality, and 14 healthy controls. The Go/No-go task requires participants to press a button in response to a target stimulus (Go), but to inhibit the button press and do nothing in response to a non-target stimulus (No-go). Go trials (when a target is presented)

usually make up 75% of trials within a Go/No-go task to create a prepotent response that must then be inhibited on the less frequent No-Go trials (Garavan, Ross, Murphy, Roche, & Stein, 2002). The number of errors on no-go trials across the whole task for happy and sad faces served as the measures of behavioural inhibition and emotional modulation of this inhibition, respectively. There was no significant difference in task performance (both accuracy and response times) between the three groups of participants. However, in the healthy controls fMRI recordings showed increased activation of the prefrontal, anterior cingulate, and parietal cortical regions. The anterior cingulate in particular is considered crucial for inhibitory control (Løvstad et al., 2016) and whilst the depressed individuals with no past history of suicidality did not differ from the controls with regard

to activity in this area, the depressed adolescents with a history of suicide attempts showed significantly reduced activity. This indicates impairments in neural activation during inhibitory control for suicidal individuals and also shows the relationship between cognitive processing and cortical activity. The findings suggest that this association may constitute a neurobiological basis for predisposition to suicide behaviour. It is proposed that suicidality corresponds to lateralized frontal cortical activity and therefore

neurological measures may be used to predict suicidality of individuals beyond the currently used self-report measures.

4.5 Study 3: Exploring the neurological markers and testing the cognitive markers as