Several lines of evid ence suggest that it may be worth testing for bias in select ive atten tion in clin ical patients. The fi rst of these is the bias in conscious exper i ence previ ously noted. If a patient exper i ences the world as a threat en ing place, perhaps this refl ects a tend ency to focus atten tion on threat en ing rather than harm less stimuli. At a theor et ical level, Bower’s (1981) infl u en tial network model of affect proposes that emotions are asso ci ated with the activ a tion of emotion units in a semantic network. Emotion units in turn tend to activ ate concepts and events asso ci ated with the emotion. Hence, when a person is in a state of emotion, he or she is primed to perceive and attend to stimuli congru ent with the emotion. A rather differ ent theory, Beck’s (1976) schema model of emotional dysfunc tion, makes a similar general predic tion. The model asserts that anxiety and depres sion disorders result from the activ a tion of specifi c dysfunc tional schemas, and, once activ ated, they direct atten tion towards belief- congru ent inform a tion (e.g. Beck, 1987). We consider these and other theor et ical frame works in more detail in a later section of this chapter.
A variety of exper i mental paradigms have been used to invest ig ate atten tional bias in clin ical groups. For conveni ence, we loosely group these paradigms into three main types, termed the encod ing, fi lter ing and Stroop test paradigms, although the fi rst two categor ies subsume a variety of specifi c tasks. In encod ing paradigms, the person simply has to recog nise or make a simple decision about a single stim- u lus, so that there is only one overt channel of atten tion, and no external source of distrac tion. An example would be percep tion of words of differ ing affect ive content presen ted briefl y in a tachis to scope. The general mood- congru ence hypo thesis would predict that depress ives should have a lower threshold for perceiv ing depres sion- related words, and a higher threshold for words related to happi ness. Despite the lack of overt selec tion, such tasks may be theor et ic ally inform at ive, because select ive atten tion bias may be a specifi c mani fest a tion of a
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54 Attentional bias in emotional disorders
more general bias in processing. For example, within a network model, a bias in the strengths of connec tions between units, or a bias in their tonic activ a tion levels, might infl u ence both processing of a single input channel and selec tion between chan nels (e.g. Bower, 1981). Likewise, both single- channel encod ing and selec tion may be controlled by the same upper- level schema or strategy.
Studies of percep tual thresholds are partic u larly suited to invest ig at ing early, pre- attent ive encod ing processes, over which the person has little, if any, volun tary control, but other encod ing tasks may engage more upper- level infl u ence over processing.
In typical fi lter ing paradigms, there are two or more sources of input or atten- tional chan nels, discrim in ated by a simple phys ical cue. One channel is to be atten ded, and the other ignored, which is fairly easy to do on this kind of task.
Typically, emotional stimuli are presen ted on the unat ten ded channel, and the inter fer ence of this mater ial with focal atten tion is assessed. For example, in a dichotic listen ing task, with separ ate spoken messages presen ted to both ears, we are inter ested in whether patients pick up more inform a tion concern ing emotional stimuli from the ignored ear. We also discuss tasks requir ing switch ing of atten- tion between discrete chan nels under the heading of “fi lter ing”, although other atten tional processes will also be involved. A popular task devised by MacLeod, Mathews and Tata (1986) initially requires the subject to attend to one spatial loca tion, and ignore a second. An emotional stim u lus may be presen ted in either of the two chan nels. Response to a subsequent probe indic ates whether the subject has main tained the initial fi lter ing of chan nels, or whether atten tion has switched to the initially unat ten ded channel. It is expec ted that the choice of loca tion is infl u enced by the emotional content of the stimuli fi rst presen ted. Use of these tasks is some times motiv ated by the early selec tion theor ies described in the previ ous chapter. If patients show greater aware ness of unat ten ded affect ive mater ial, perhaps their atten u at ing fi lter (Treisman, 1964) is “tuned” to let such mater ial into conscious ness, to some extent.
The original Stroop test, requir ing naming of the ink colour of colour words such as RED, was described in the previ ous chapter. In clin ical versions, patients are required to name the colour of words related to their psycho path o logy: a spider phobic might be required to respond to words such as COBWEB and TARANTULA (Watts et al., 1986a). As in fi lter ing tasks, we may assess the extent to which the to- be- ignored affect ive stim u lus, typic ally a threat word, intrudes into ongoing processing and conscious ness. The Stroop differs from encod ing paradigms in that the subject must ignore some of the inform a tion encoded from the atten ded channel. The key distinc tion between Stroop and fi lter ing paradigms is that in the former the subject is forced by task instruc tions to encode the emotional stim u lus, whereas in the latter the subject has a choice of input chan nels. Often, the emotional stim u lus need not be encoded at all to meet task instruc tions. We have seen in Chapter 2 that multiple prop er ties of single percep tual “objects”, such as the colour and verbal content of a Stroop word, tend to be selec ted or rejec ted together, as a single package. Hence, in the Stroop test,
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Depression 55
it is partic u larly diffi cult to ignore the distract ing emotional inform a tion, which may disrupt perform ance of the primary colour- naming task. Some theor ists would attrib ute Stroop test inter fer ence to an atten tional mech an ism oper at ing after early selec tion, since no percep tual fi lter ing is possible; late- selec tion theor ists might see Stroop and fi lter ing tests as depend ent on similar under ly ing mech an isms.
In the fi rst part of this chapter, we review exper i mental evid ence on effects within these three atten tional paradigms of (1) depres sion, (2) general anxiety and (3) other anxiety disorders such as phobias. Studies of bias on other tasks, not expli citly atten tional, are also briefl y described, because there may be common causes of bias across appar ently dissim ilar tasks. We focus primar ily on clin ical studies, but exper i ments on mood and atten tion in normal subjects will also be described. The main point of interest is whether patients show mood- congru ent atten tion—heightened sens it iv ity to stimuli related to their clin ical condi tion. It should be noted that bias of this kind is not the only feature of atten tion in clin ical groups, which may also be impaired in overall effi ciency of atten tion. Such effects are considered in Chapter 6. In this chapter, we aim to provide a fairly descript ive review of the main empir ical fi nd ings. Theoretical implic a tions of the atten tional data are discussed at greater length in the next chapter.
Depression Encoding tasks
Two main tasks have been used to study early percep tual and attent ive processes in depressed subjects: accur acy of recog ni tion of briefl y presen ted words, and speed of lexical decision (recog nising a letter string as a valid English word).
Powell and Hemsley (1984) found only a non- signi fi c ant trend towards depress- ives recog nising a higher ratio of unpleas ant to neutral words in a percep tual defence study. In normal subjects, Gerrig and Bower (1982) found no effect of induced mood on visual recog ni tion threshold, but Small (1985) showed that, with care fully controlled stim u lus mater i als, negat ive mood induc tion facil it ated recog ni tion of dysphoric content words. Small and Robins (1988) replic ated the effect, and also found that induced depres sion lowered recog ni tion threshold for elation content words. Matthews and Southall (1991) collec ted lexical decision data as part of a priming study, in which unprimed words were preceded by the non- inform at ive word BLANK, provid ing subjects with a prepar at ory inter val or foreperiod between this initial stim u lus and the subsequent target letter string.
With a brief foreperiod (240 msec), depressed patients were faster at making lexical decisions on both pleas ant and unpleas ant words, relat ive to neutral words, but matched controls were faster at neutral words. However, the effect was elim- in ated by extend ing the foreperiod to 1500 msec, imply ing that it was rather weak, and capable of being over rid den if subjects were given suffi cient warning.
Mild depres sion in under gradu ates appears to gener ate a some what similar pattern
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56 Attentional bias in emotional disorders
of results as clin ical depres sion: faster recog ni tion of depres sion- related words, but no differ en tial priming effects (Spielman & Bargh, 1990). Failure to allow for the role of foreperiod dura tion may account for prior fail ures to fi nd mood- congru ence in lexical decision, in studies of mood induc tion (Clark, Teasdale, Broadbent, & Martin, 1983) and of clin ical depres sion (MacLeod, Tata, &
Mathews, 1987). Challis and Krane (1988) found a mood- congru ent effect of induced elation, but not of induced depres sion. Matthews and Southall (1991) also tested depres sion effects on priming of asso ci ated word pairs, varying the prime- target inter val or SOA to discrim in ate auto matic priming (short SOA) and controlled, expect ancy priming (long SOA). Depressed patients showed enhanced auto matic priming of neutral words, and reduced priming of affect ive words, imply ing that this group may be impaired in the auto matic asso ci ation of emotional concepts. Overall, affect ive bias in percep tion and encod ing is weak in depress ives and in depressed moods. When bias is found, it often relates to the contrast between affect ive and neutral mater ial, as much as to the contrast between posit ive and negat ive mater ial, imply ing an effect depend ent on general emotion al ity.
Filtering tasks
There is a surpris ing lack of studies of depres sion and fi lter ing, partic u larly in clin ical patients. Gotlib, McLachlan and Katz (1988) presen ted mildly depressed and non- depressed under gradu ates with pairs of words varying in content:
depressed, neutral or manic. Depression was meas ured with the Beck Depression Inventory (BDI; Beck et al., 1961). Pairs were always of differ ing content.
Selective atten tion to the two words was assessed by simul tan eously repla cing the two words with two colour bars, and requir ing subjects to state which colour bar appeared to be presen ted fi rst. The expect a tion was that depressed students would select pref er en tially the input channel in which the depres sion words were presen ted, and so would perceive the colour bar follow ing these words as occur- ring earlier than the other colour bar. In fact, depress ives showed no atten tional bias at all, but non- depress ives showed enhanced atten tion to the manic words, compared with the other two types. Gotlib et al. (1988) suggest that depress ives are actu ally “even- handed” in their atten tion, whereas normals show a bias towards posit ive mater ial. It is diffi cult to infer the locus of selec tion in this study, however. Word pairs were presen ted for 730 msec, allow ing time for strategy and conscious atten tion to over ride any auto matic bias in atten tion. Gotlib and co- workers’ task was a modi fi c a tion of one used by MacLeod et al. (1986) to invest ig ate anxiety effects, which we shall discuss in detail in a subsequent section.
MacLeod et al. (1986) found that anxious but not depressed patients showed a bias towards threat words. Hill and Dutton (1989) also failed to fi nd any bias in select ive atten tion in students selec ted for depres sion. Bower (1987) describes an unpub lished dichotic shad ow ing study in which mood induc tion failed to infl u ence distrac tion by happy and sad messages on the unat ten ded channel.
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Depression 57
Stroop test
Gotlib and McCann (1984) used a modi fi ed Stroop task, comprised of neutral, depress ive or posit ive- manic content words, to explore select ive atten tion in depres sion. They showed that while the colour- naming perform ance of non- depressed students was unaf fected by word type, mildly depressed college students showed signi fi c antly slower colour- naming responses for negat ive words. In other words, depressed students appear to be distrac ted from the task at hand by the depres sion- related content of the words. Similar results were obtained by Klieger and Cordner (1990). In a further study of depressed patients, Gotlib and Cane (1987) demon strated greater Stroop inter fer ence in this group for depres- sion- related but not neutral or manic words, relat ive to controls. After recov ery, the same subject groups were retested, and no signi fi c ant biasing effects were found, although the recovered depress ives were still signi fi c antly higher in state depres sion than controls.
Three studies have combined the emotional Stroop with priming manip u la- tions. Gotlib and Cane found that a priming proced ure, involving prior expos ure to negat ive words not used in the subsequent Stroop test, had no effect on inter- fer ence. Bargh (1992) describes an unpub lished replic a tion of the Gotlib and McCann (1984) studies in which BDI depres sion was asso ci ated with the predicted pattern of inter fer ence only when the BDI was admin istered before the Stroop test rather than after it. Bargh (1992) suggests that prior expos ure to the ques tion- naire is neces sary to activ ate depress ive cogni tions in vulner able subjects. Possibly Gotlib and Cane (1987) failed to obtain a priming effect of this kind because complet ing the BDI is more likely to gener ate self- refer ent negat ive cogni tions than simply reading negat ive words. Segal and Vella (1990) report an inter est ing study which invest ig ated the role of self- aware ness directly. There were three groups of subjects: depressed patients, normal controls and a group of normal subjects exposed to a self- focus manip u la tion (watch ing them selves perform in a mirror and listen ing to a tape- record ing of their voice). The self- focus manip u- la tion infl u enced self- aware ness but not mood. The task used was rather differ ent from the stand ard emotional Stroop in that subjects were required to colour- name adject ives previ ously clas si fi ed as either self- descript ive or not. Each word was preceded by a prime, whose self- relev ance was also varied. Related and unre- lated pairs of ordin ary nouns were used as control word pairs. All three subject groups tended to select more posit ive than negat ive words as self- descript ive, although the depress ives selec ted a higher propor tion of negat ive words (29%) than the other two groups (3–5%). The key result concerns inter fer ence on the self- descript ive adject ives. In depressed and self- aware groups, inter fer ence was increased by prior present a tion of a self- descript ive prime, but prime type had no effect on inter fer ence in the controls. A further analysis of the depressed group showed similar effects were obtained for both posit ive and negat ive self- descript ive words. An import ant feature of the method is that the prime–target SOA was long in dura tion (1200 msec), imply ing that priming effects depended on
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58 Attentional bias in emotional disorders
expect an cies and controlled processing (Neely, 1991). (Segal and Vella do not actu ally discuss the relev ance of SOA.) Hence, among depress ives and the self- aware, controlled processing of self- relev ant prime words biases subsequent atten tion towards selec tion of self- relev ant Stroop words. Since depress ives tend to be self- focused (see Chapter 9), it may be self- aware ness rather than negat ive affect per se which was respons ible for the inter fer ence effects. A diffi culty with this conclu sion is that depressed patients, but not self- aware normals, also showed increased inter fer ence on semantic ally related ordin ary nouns. The role of self- focus in depres sion- related Stroop effects requires further research.
Both Gotlib and McCann (1984) and Clore and Bower (Bower, 1987) failed to fi nd Stroop inter fer ence congru ent with mood induced in normal subjects, although an earlier study by Clore (Bower, 1981) found that both angry and happy subjects were more prone to inter fer ence from emotional words in general.
Williams and Nulty (1986) also used the modi fi ed Stroop paradigm to assess the extent to which disrup tion in perform ance among depress ives is related to mood state or under ly ing stable trait char ac ter ist ics. In this study, groups of depressed and non- depressed subjects were compared; the groups were composed of subjects who had shown stable depres sion scores over a one- year period or subjects who were stable non- depress ives. Stability of depres sion was assessed by two admin is- tra tions of the BDI. The depressed subjects showed greater disrup tion in perform- ance on the emotional Stroop task than non- depressed subjects. The fi rst BDI score was a better predictor of disrup ted Stroop perform ance than the BDI score at time of Stroop testing.
Depression: Key fi nd ings
The data reviewed show that atten tional bias in depres sion is most evident on the Stroop test, in both patient and student groups. However, induced mood states do not seem reli ably to gener ate mood- congru ent inter fer ence on the Stroop. The Stroop studies have not, in general, attemp ted to test specifi c processing mech an- isms as sources of bias, although the Segal and Vella (1990) study implies that inter fer ence is enhanced by self- refer ent processing, and by prior controlled processing of prime words. There is some evid ence that depressed indi vidu als are more sens it ive to both pleas ant and unpleas ant words in encod ing paradigms, although the effect is rather unre li able. Studies of depres sion and fi lter ing have gener ally repor ted non- signi fi c ant results, although there are too few studies, partic u larly of patient groups, for any strong conclu sions to be drawn.
Generalised anxiety Encoding tasks
Evidence for anxiety- induced affect ive bias on simple encod ing tasks is limited.
Watson and Clark (1984) reviewed studies of percep tual defence and person al ity