NUTRIENTES NECESARIOS DE SUPLEMENTAR EN VACAS A PASTOREO

Several paradigms have been used to measure cognitive processing biases, either directly (e.g. eye-tracking) or indirectly (e.g. visual probe task). These tasks typically rely on measuring reaction times (RTs) to relevant stimuli to infer the amount of bias exhibited by a participant. Different subcomponents of attention and attentional bias can be

examined depending on the type of task used. For example, some tasks measure the rapid orienting of attention, while others measure the maintenance or delayed disengagement of attention (see Field & Cox, 2008, for a review). In the next section, I will outline the most commonly used tasks and focus on the procedures that are most relevant to the research studies described in Chapters 2 and 3.

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1.3.2.1 The addiction Stroop task

The modified addiction Stroop task (Cox et al., 2006b), a variant of the classic Stroop task (Stroop, 1935), has been used extensively to measure selective processing of drug-related stimuli (Drobes et al., 2006; Sharma et al., 2001; Waters et al., 2003a). In this task, participants are presented with neutral or drug-related words printed in colour and they are asked to name the colour of the words as quickly and accurately as possible while ignoring the semantic content of the words. Similarly pictorial presentations of drug- related and neutral stimuli are also used in addiction Stroop paradigms (Bruce & Jones, 2004); participants are required to name the colour of the border of pictures presented as quickly as possible while attempting to ignore the content of the pictures. A processing bias towards drug-related stimuli is indicated by slower colour naming of drug-related words/pictures in comparison to neutral words/pictures, suggesting that attention is captured by the meaning of the drug-related words/pictures and performance is thus impaired on the task.

Smokers, in comparison to non-smokers, have been found to exhibit a processing bias for smoking-related stimuli than neutral stimuli (Munafo et al., 2003). Similarly, processing biases have been found in drug-users of other substances in comparison to non-abusers, including alcohol abusers (Cox et al., 2000; Sharma et al., 2001), heroin addicts (Franken et al., 2000), cannabis users (Field et al., 2006) and cocaine addicts (Hester et al., 2006).

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There is some ambiguity over the mechanisms underlying the Stroop effect, which is why this is referred to as a ‘cognitive processing bias’ rather than an attentional bias in this discussion. This is because there are alternative interpretations for why the interference occurs and different mechanisms at play, which could give rise to the same observed interference. For example, attempts to avoid drug-related stimuli [termed cognitive avoidance in anxiety literature (Deruiter & Brosschot, 1994)] may also result in the

slower colour-naming of drug-related words. This has been demonstrated in a study where abstinent alcoholics showed increased Stroop interference when they were told to

suppress their thoughts about alcohol, in comparison to those who were not encouraged to do so (Klein, 2007). An interpretation of the Stroop effect therefore requires careful consideration; I herein refer to this effect as a processing bias throughout this thesis.

The addiction Stroop task is likely to measure the maintenance or delayed disengagement of attention rather than the rapid orienting of attention. Evidence for this comes from ‘carryover’ effects noted across addiction Stroop studies (Cane et al., 2009). Carryover effects have been found where colour naming performance is impaired on trials of neutral stimuli that have been presented immediately after trials of drug-related stimuli; smokers have been found to respond slower to words that appear after smoking-related words than after neutral words (Waters et al., 2003b). Thus, this slow down in cognitive processing which carries over into subsequent trials suggests that there might be a rumination effect on drug-related stimuli that is likely to reflect a difficulty in disengaging attention.

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1.3.2.2 The visual probe task

Another widely used measure of attentional bias is the visual probe task (MacLeod et al., 1986). In this task, participants are presented with stimuli consisting of a pair of words or pictures that are presented side by side on a computer screen. One stimulus is neutral (e.g. a picture of a man holding a pen) and the other is related to smoking (e.g. a picture of a man holding a cigarette). After a short interval, the picture pairs disappear and are replaced by a single probe stimulus (e.g. a square or triangle) that appears in the location formerly occupied by one of the pictures. The probe stimulus replaces the neutral and smoking-related pictures with equal frequency. Participants are then required to press either the up arrow or down arrow on the computer keyboard to indicate which picture has been replaced as quickly as possible in response to the probe.

Based on the principle that detection of the probe is quicker in the location in which the participant is already fixated, a participant who has an implicit bias towards smoking will presumably be looking in the direction of the smoking-related stimuli on the outset. Attentional bias towards smoking is indicated by faster RTs to the probe that appears in the location of the smoking-related picture rather than the neutral picture. Smokers, but not non-smokers have demonstrated an attentional bias for smoking-related pictorial cues (Bradley et al., 2004; Ehrman et al., 2002). Other drug users have shown an attentional bias towards drug-related stimuli of their choice; for example, cannabis users have shown faster approach responses towards cannabis-related pictorial cues than neutral cues (Field et al., 2006).

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Different stimulus presentation times are assumed to measure different components of attentional bias on the visual probe task. The stimulus onset asynchrony (SOA), which is the time in which a picture is presented on a computer screen in each trial, has been manipulated in visual probe task studies to investigate the initial orienting of attention or speeded detection of drug-related stimuli versus the maintenance or delayed

disengagement of attention. Short SOAs, typically less than 200 milliseconds (ms), are likely to reflect early processes while longer SOA durations of between 500 ms and 2000 ms reflect later slower processes. This is based on the assumption that only one shift in attention seems plausible when two stimuli are presented at an SOA duration of less than 200 ms, while multiple shifts are possible at SOAs of over 500 ms (Field & Cox, 2008). Accordingly, shorter SOAs may indicate the rapid initial orienting of attention while longer SOAs may reflect maintenance or disengagement of attention; this line of reasoning is generally accepted in the anxiety literature (Koster et al., 2004).

The differentiation between the two subcomponents has been best measured by

combining the visual probe paradigm with eye-tracking methodology. As the visual probe task is limited to measuring the allocation of attention at the time of stimulus offset, eye- tracking allows for the measurement of attention over the duration of stimulus

presentation. Thus, eye-tracking methodology provides a more direct measurement of both initial orienting and delayed disengagement. Attentional bias is inferred from eye movements, for example by measuring the direction of first eye movements or the amount of time individuals maintain their gaze on drug-related stimuli versus neutral stimuli (also known as ‘dwell time’). In a study of heavy drinkers (Schoenmakers et al., 2008) and

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cannabis users (Field et al., 2006), attentional bias RTs on the visual probe tasks were positively correlated with gaze dwell times when drug-related stimuli were presented for 2000 ms, which lends support to the notion that longer presentation times capture the maintenance of attention. Similarly, smokers have a higher proportion of first eye

movements and a longer dwell time towards smoking-related stimuli than neutral stimuli (Field et al., 2004; Mogg et al., 2003). Furthermore, smokers have been found to direct their gaze towards dynamic smoking-related cues more quickly, more often and for a longer duration in comparison to non-smokers (Lochbuehler et al., 2011).

Arguably, these two subcomponents of attention may have state-like or trait-like qualities. Faster attentional processes like the initial orienting of attention may be indicative of a trait that has developed after years of conditioning to smoking cues and accordingly, may have a consistent presence in certain drug users. On the other hand, the maintenance of attention may be regarded as a state-like construct and may be influenced more so by motivational factors such as craving (LaBerge, 1995). Biases in maintained attention may therefore be more evident in situations of high cravings, for example when smokers are nicotine deprived. Although this is somewhat speculative, these trait-like and state-like features of attentional bias may be important when examining the relationship between attentional bias and craving.

In summary, both Stroop and visual probe task paradigms have been used to demonstrate that smokers show cognitive processing biases towards smoking-related stimuli compared

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to non-smokers. It is likely that each task taps into different subcomponents of attention; the Stroop task is likely to measure the maintenance or delayed disengagement of

attention while the visual probe task appears to capture both initial orienting and delayed disengagement of attention, depending on the duration of stimulus presentations. Both of these tasks were used in the research studies described in Chapters 2 and 3.