The emotion of fear has captured the interest of investigators over many years. A number of definitions for fear have been proposed, most of which agree that fear is a stimulus driven, aversive strong emotional state that is elicited by the anticipation or awareness of danger (Miller, et al., 2005; Öhman, Carlsson, Lundqvist & Ingvar, 2007; Öhman & Wiens, 2003). A perspective put forward by Öhman and colleagues suggests that the emotion of fear has been predominantly shaped by the evolutionary race between predator and prey. Öhman and Mineka (2001) state that phylogenetic fear relevant stimuli tend to activate a defensive fear system, which results in changes to the physiological, behavioural and experiential state of the animal. The current thesis focused on the physiological and behavioural changes seen as a response to phylogenetic fear relevant stimuli, i.e. snakes and spiders.
The defensive fear system can be activated and fear responses can be elicited even when the fear relevant are subliminally presented, i.e. presented in such a manner that they are precluded from conscious awareness. Evidence for this comes from studies that have used paradigms like backward masking and combined it with various neuro-imagining techniques. Research studies reveal that the amygdala is central to fear processing (LeDoux, 1996). The neurobiological model of fear processing, proposes that sensory information from the environment accesses the amygdala by two routes: the high road and the low road. The high road is said to be reliant on cortical processing and is a geniculostriate pathway that conveys information at a much slower rate but with more details, hence allowing for fine grained analysis of incoming information. On the other hand, the low road is not reliant on cortical processing but allows for direct activation of the amygdala via the thalamus, without involvement of the cortex. Unlike, the high road, the low road, conveys crude information about the stimulus. It conveys rapid signals which help in quick detection of a threat, even if the animal is not aware of the threatening stimuli.
The backward masking paradigm was developed by Öhman and colleagues to assess if the defensive system could be activated even in the absence of conscious perception of fear stimuli. Research studies have presented biological fear relevant animal stimuli, along with fear irrelevant stimuli to determine whether autonomic responses (measures of fear), like skin conductance responses (SCR) would be elicited in response to the fear relevant stimuli (Öhman and Soares, 1994). Results from the studies indicated that phobic participants (snakes/spiders) showed enhanced responding when they were presented with pictures of their feared animal.
A recent study by Ruiz-Padial and Vila (2007) studied another physiological measure of fear, namely the startle reflex, in particular the eye-blink response. They presented spider and snake phobic participants with masked and non masked pictures of their feared animals, neutral and pleasant stimuli. Startle eliciting stimulus (white noise) was presented at different lead intervals. Results from the study indicated that the mask was effective in concealing the stimuli from conscious awareness. The results were also consistent with other studies that found evidence for emotional modulation of the eye blink startle (Cuthbert, Bradley & Lang, 1996; Hamm, Cuthbert, Globisch & Vaitl, 1997; Lang, 1995; Lang, Davis & Öhman, 2000) and with results of studies that found evidence for preferential processing of fear stimuli even in the absence of conscious processing (Bishop, Duncan & Lawrence, 2004; Buchanan, Tranel & Adolphs, 2004; Carlsson et al., 2004; Esteves et al., 1994; Glascher & Adolphs, 2003; Killgore & Yurgelun-Todd, 2004; Morris et al., 1998, 1999: Öhman & Soares, 1994; Vuilleumier et al., 2002; Whalen et al., 1998, 2004; Williams et al., 2004).
Experiment 1 of the present study aimed at replicating the study by Ruiz-Padial and Vila (2007), however aimed at studying non-anxious participants rather than phobic participants. The present study also extended the prepulse range, so that startle modulation at very short, short and long lead intervals could be studied. The mask used in the study was presented for 43 ms, which was informed from previous studies that have effectively masked schematic faces and animal stimuli (Lipp et al., 2006).
The results of the study suggested that the mask used in the study was not completely effective in concealing the lead stimuli, as a result, participants were partially aware of the masked stimuli. Contrary to predictions, no facilitation at very short lead intervals or inhibition of the startle response at short lead intervals were noted in response to fear-relevant stimuli. However, in line with predictions, facilitation of the startle response at long lead intervals was observed.
In order to investigate whether behavioural indices of fear could be elicited by the subliminal presentation of biological fear relevant stimuli, the affective priming paradigm was used in Experiment 2. The affective priming paradigm is a reaction time based measure which is based on the assumption that incoming stimuli is automatically evaluated as positive/pleasant or negative/ unpleasant. This evaluation takes place even when the person is not consciously aware of the stimulus in the environment. Subliminal presentation of the prime stimuli has reliably shown that reaction times are faster when the target and prime stimulus are of the same valence, while the target (Croizet et al, 1998; Greenwald et al., 1989; Hermans et al., 2003; Otten & Wentura, 1999). The evaluation of the stimuli has the potential of impacting upon subsequent mood and behaviour.
The present study aimed presenting fear relevant stimuli in an affective priming procedure. As it is well-established that fear responses can be elicited even when the fear relevant stimuli are presented outside conscious awareness and it has also been established that affective priming is a robust phenomenon that is reliant on automatic evaluation of the incoming stimuli. Hence, it is postulated that if fear relevant stimuli are presented subliminally in an affective priming study, elicitation of fear responses should be observed.
In the present study, an unselected group of participants were presented with masked and non-masked prime pictures of snakes and spiders and fear irrelevant pictures of birds and fish. The results of the present study revealed that similar to the results of Experiment 1, the mask was not completely effective in masking the prime stimuli, which resulted in partial awareness of the prime stimuli. Priming effects were seen under the non masked condition, while the results were in the predicted direction under the masked condition, however, they did not achieve a level of significance. Subsequent analysis was conducted without participants who displayed the highest rates of detection and results from this analysis indicated that previously observed priming effects were no longer present.
The results from both the experiments conducted in this study suggested that non-anxious participants did not display the predicted pattern of results, i.e. startle modulation at very short and long lead intervals, in a backward masking paradigm and faster reaction times for affectively congruent trials, especially when fear relevant primes were followed by unpleasant target words. Three explanations for the present pattern of results are discussed. Directions for future research are suggested.