1.3 Identidad Visual
1.3.2 Elementos de la Identidad Visual
T he previous chapters have established that the un attended stim uli can be processed under certain circum stances and evoke specific patterns o f cerebral activity related not sim p ly to sensory pro cessing o f the stim uli b u t also rela ted to th e b eh av io ural consequences o f such processing such as negative prim ing. In this chapter an attem pt is m ade to characterise in a m ore ex p licit fashion the d eterm inants o f p ro cessin g o f unattended stimuli. In particular, the role o f task dem ands in m odifying the processing o f V irrelevant and ignored stimuli are exam ined. A ttention is often characterised as som e kind o f m onolithic force or process that acts on sensory processing. H ere, an alternative hypothesis is sketched out: that selective processing in the visual system occurs as an autom atic result o f the availability o f processing resources, and is not necessarily under direct control by the subject.
5.1 Introduction
A classic issue in the study of attention is w hether identity is extracted from stim uli when they are ignored. The previous chapter show ed that, in the context o f a relatively easy counting task, the identity of ignored visually presented num bers could interfere with task perform ance. As this negative prim ing only occurs w hen the ignored num bers predict the su bseq uent counting response, the identity o f the ignored num bers m ust have been extracted by the visual system in order to generate this interference. This type of study show s that ignored stimuli are processed to the level o f identity. H ow ever other studies show th at ignored stim uli receive little processing beyond a rudim entary analysis o f sensory attributes (Broadbent, 1958; Treism an and G elade, 1980). Subjects perform ing dichotic listening tasks, for exam ple, appear to have little or no access to inform ation about the identity o f stim uli presented to the ignored ear. M ore recently experim ents w here selection is made between com peting stim uli on the basis of spatial location show that m odulations of sensory evoked responses occur both early in tim e and early in the visual processing pathway. Two contrasting positions can be distinguished. A ccording to th e first, selective processing begins early in the visual system and unattended stim uli receive little or no processing beyond a rudim entary analysis o f their sensory attributes. H o w ev er the second position suggests on the basis o f negativ e prim ing and other
Neural correlates o f selective attention
p h e n o m e n a that unattended stimuli are processed beyond a m ere analysis o f physical attributes to the level of identity. Any selective process such as a ttention m u st be operating post-perceptually. How can these two contrasting positions be reconciled, and what are the determinants of selective perception? In this chapter, two e xperim ents are presented that com bine functional imaging and behavioural m easures to test a new theory that may resolve this longstanding controversy.
5.1.1.
A direct test of the perceptual load hypothesis
L a v ie ’s theory (Lavie, 1995; Lavie and Tsai, 1994) proposes that capacity for perception is limited, but within those limits perception proceeds automatically. Thus, while we may not be able to perceive everything, we are unable to stop perceiving w hatever we can. The extent to which a target task exhausts available capacity thus determ ines the degree to which irrelevant distracting stimuli will be processed. If the processing load of the target task exhausts available capacity, irrelevant stimuli will not be ^perceived. How ever, if the target processing load is low attention will inevitably spill o v e r to the p ro ce ssin g o f irrelevant distractors. The extent to which irrelevant stimuli are excluded from perception does not thus depend simply on subjects’ intentions to ignore them. Irrelevant stimuli are excluded from perception
o n ly
when the processing load o f a task engages full attention u nde r conditions of high load. The concept of perceptual load involves increasing the n u m b er of items i ^ a display, or increasing the processing req u irem e n t for the same n u m b er of items. < ^ v i e w of previous behavioural studies o f attention sup p o rte d the c la im s of perceptual load theory (Lavie, 1995). R esults c o m p a tib le with se le c tiv e p erception were typically obtained in situations of high attentional load, and results com patible with nonselective perception were typically obtained in situations of low load. For exam ple, the negative priming experiment presented in the previous chapter would be considered a low perceptual load task as t^ief^little processing o f attended stimuli was required to generate the counting response. Consistent with this, the results indicate nonselective processing with the identity of ignored numbers producing negative prim ingo f the attended items. C vrcLxl
This theory was investigated by studying the perception o f irrelevant visual m otion during performance of a task involving linguistic judgem ents on single words. A lthough both these tasks involve visual input, they are thought to rely on c om pletely different psychological processes. Yet if both depend on a com m on source of attention, as the load
Neural correlates of selective attention
theory suggests, then they should be strongly interdependent. Specifically, the prediction was that subjects will fail to ignore irrelevant visual m otion as long as processing load in the linguistic task is low, but that higher load in the linguistic task will prevent perception o f the irrelevant m otion. D espite subjects’ wishes to ignore the m otion distractors in all conditions, they will only succeed in doing so under conditions o f high load in another task that exhausts their attentional capacity.
Visual m otion was used as a distracting stimulus as it is know n to activate a distinct area o f the brain, V5, w hose location has been reliably identified in p revious fun ctio nal im aging studies (Sereno et aL, 1995; Tootell et a l , 1995; T ootell and T aylor, 1995; W atson et a l , 1993; Zeki et a l , 1991). A ctivation of V5 by a m oving stim ulus should therefore allow determ ination o f w hether processing o f irrelevant visual m otion has o c c u rre d . P r e v io u s fu n c tio n a l im a g in g , p s y c h o p h y s ic a l a n d s in g le c e ll electrophysiological studies have all suggested that m otion perception m ay depend to som e extent on attention (Buchel et aL, 1998a; Chaudhuri, 1991; C orbetta et aL, 1991b; O 'C raven et aL, 1997; Treue and M aunsell, 1996). H ow ever, none o f these studies have provided a critical test for the claim made here, that the crucial factor determ ining when subjects can ignore motion distractors is the attentional load in an unrelated task. Every previous study has com pared explicit attention to motion with explicit ignoring o f m otion. A ny difference in m otion related brain activity betw een these tw o conditions can be attrib uted to an enhancem ent of perception with delib erate attention to the m oving stim ulus, rather than successful ignoring o f m otion in the unattended condition. These previous studies therefore cannot provide any clear answ er to the principal issue in attention theory: w hether irrelevant distractors can be excluded from perception. H ere the load theory is tested by characterising changes in selective p rocessin g o f a m otion stim ulus that is always irrelevant, while varying the attentional load o f an unrelated target task.
5.2 Methods
5.2.1.
Subjects and stimuli
F or the irrelevant m otion stimulus an optic flow field was used w ith a full field o f dots m oving radially tow ards the screen (Figure 5-1). This type o f m otion may be particularly d ifficult to ignore due to its biological relevance. O ptic flow is continuously used in
Neural correlates o f selective attention
judgem ents about m otion direction in the environm ent (for exam ple, an expanding field indicates approaching to the source o f the expansion) (G ibson, 1950; G ibson, 1977). Irrelevant m otion processing was characterised by m easuring brain activity in m otion- related areas during changes in the dem ands of an im posed (and unrelated) linguistic task.
Subjects view ed a display with tw o different com ponents (Figure 5-1). In the periphery o f the display w ere placed scattered w hite dots. M otion perception w as assessed by com paring conditions where these dots m oved with conditions w here the dots w ere static This com ponent of the display com prised four hundred single w hite dots each subtending 0.1 degrees scattered randomly outside a central ellipse in a display 17 degrees across. W hen the display was moving, each dot m oved radially outw ards at a constant velocity of approxim ately 5 degrees per second. As dots left the edge o f the display new dots w ere added at the periphery o f the central ellipse surrounding the w ords to ensure the dot density o f the display rem ained constant. Subjects w ere asked to ignore the w hite dots throughout and w ere told that they w ere always irrelevant to the experim ent, and m ight produce un pleasant m otion after-effects if they w ere not ignored. In the centre o f the display single words w ere presented successively in a blank ellipse that separated them from the dots. H igh frequency five letter nouns w ere presented in the cen tre o f the display, surrounded by a blank ellipse to distinguish the w ords from the periph eral display. Each w ord subtended approxim ately 1.4 degrees and the ellipse 2.7 (vertical) by 4.1 (horizontal) degrees. Each w ord w as presented for 750m s, follow ed by a blank interval o f 250 ms. D uring each experim ental epoch, there w ere eight targets and tw enty- four foils. The words were identical for each subject in high load and low load conditions, though their order w ithin an epoch was random ised. The position o f targets w ithin an epoch w as co nstrain ed to be m atching in both high and low load con ditio ns. The condition order and stimulus materials w ere counterbalanced across subjects.
Neural correlates o f selective attention