Pseudocódigo CLM

3.1 Introduction

In the first exp erim ental chap ter, the rate o f p resen tatio n o f visual and auditory inform ation was varied to give an index of stim ulus processing. The prim ary focus o f this w ork addressed the nature o f attentional selection. In particular this w ork w as used to in v e stig ate the d ifferen ces b etw een the brain activity p ro d u ced by atten d ed and unattended stimuli. A lthough the m aterial review ed in C hapter 1 suggested that attention operates by changing the sensory evoked responses to visual stim uli, here the particular m echanism is exam ined in m ore detail. E vidence from neurophysiological studies is review ed that leads to the suggestion that there are tw o different types o f attentional Y signal that are involved in the selection o f visual inform ation. Tw o functional im aging experim ents using PET are presented that suggest that two physiologically distinct types o f change in sensory activity are seen in humans. The relationship between these changes and those seen at a single cell level are discussed. M oreover, in visual and auditory cortex the results show that even ignored stimuli evoke rate correlated brain activity, suggesting processing o f these unattended stimuli.

3.1.1.

Single neuron mechanisms of selection

M oran and D esim one (M oran and D esim one, 1985) investigated w hether responses of single cells in visual areas differed depending on w hether or not a m onkey was attending to a stim ulus. In prim ary visual cortex, there w as no difference, b ut in area V 4 the resp on se to an irrelevan t stim ulus was reduced by about tw o-thirds. M oreov er, this atten uation only occurred w hen both attended and u n atten ded stim uli w ere located sim ultaneously w ithin the receptive field (RF) o f the neuron. W hen the u n atten ded stim ulus was outside the receptive field and the attended stim ulus inside it, no m odulation o f neural responses was observed. In inferotem poral cortex (IT), attenuation w as always observed, as the receptive fields in this area are large and cover m ost of the visual field. The effects o f attention are invariably seen when both relevant and irrelevant stim uli are p re se n te d sim u ltan e o u sly , but effects are also seen w hen stim uli are p resen te d sequentially, suggesting that com petition takes place in both space and tim e (Luck et a i ,

Neural correlates of selective attention

D esim one and D uncan (D esim one and D uncan, 1995) argued that these findings were com patible with a ‘biased com petition’ m odel o f attention, w here objects in the visual field com pete for processing at the receptive field level. The role o f top-dow n selective influences is to bias the processing at a cellular level tow ards one stim ulus or the other. T he results o f M oran & D esim one are consistent with this as they show that attention only has an effect w hen relevant and irrelevant stim uli are com peting for the c e ll’s response. A ccording to the ‘biased com p etition ’ m odel, perceptual selection is a tw o- stage process (D esim one and D uncan, 1995). Initially, top-dow n signals bias activity in neurons representing the relevant object or location. C onsistent w ith this, elevations of baseline firing rates are seen in V4 neurons w henever attention is directed tow ards their receptive field (Luck et al., 1997); similarly in IT, neurons show elevated activity in the delay period of a delayed m atch-to-sam ple task (Chelazzi et al., 1993). These findings are con sistent with an extrinsic ‘b ia s’ signal. In the second stage o f perceptual selection, neuro ns th at have receiv ed a bias signal gain an ad v an tag e in th e ir co m p etitiv e interactions (m ediated through local intrinsic connections) with other neurons. C onsistent w ith this, attention to non-spatial features o f objects in flu en ces not only b aseline (spontaneous) firing rates but also m odulates stim ulus-evoked responses (Ferrera et al., 1994; H aenny et a i , 1988; Luck et al., 1997). Thus it seem s that there are at least two different m echanism s at a single cell level that m ight m ediate selective attention. In visual cortex, elevations in baseline firing rate and m odulation of stim ulus-evoked activity both appear to occur during selective visual discriminations.

3.1.2.

Evoked potentials in humans

E lectrophysiological studies in humans have been discussed briefly in section 1.3.2. and a m ore com prehensive review is available (H illyard et al., 1998). The m ost im portant finding from ER P studies is that com ponents arising very soon after the presentation of a stim ulus clearly differ as a function o f w hether the stim ulus is attended or ignored (H illyard et al., 1995). The visually evoked com ponent P I is sm aller when a stim ulus is ignored because o f its location. W hen selection is based on stim ulus attributes other than location, such as colour, an additional com ponent is superim posed on the N1 and P I. This colour selection com ponent is substantially reduced when the stim ulus is in an ignored sp atial lo catio n, su g g estin g th at the lo catio n and co lo u r selectio n are o rg an ised hierarchically with location-selection dom inant (M angun et al., 1993). The location o f the

N eural correlates o f selective attention

generators o f these com ponents in early sensory processin g areas suggests th at the m echanism o f selection not only operates very soon after stim ulus presentation but also very early in the anatom ical hierarchy o f processing areas (W oldorff et a l , 1993). It is d ifficu lt to relate these ER P findings to the single cell data, because stim uli in ER P studies are frequently presented alone to aid interpretation o f the resultant w aveform s. F urtherm ore w here ER P attention effects have been seen as a function o f com petition betw een relevant and irrelevant stim uli, the stim ulus conditions have never led to the sim ultaneous presence of attended and ignored stim uli inside a single receptive field in extrastriate cortex.

3.1.3.

Functional imaging studies

In hum ans, functional im aging studies have shown that attention to different attributes of visually presented stim uli changes evoked activity in areas o f cortex con cerned with processing those attributes. For exam ple, attention to the colour o f visual stim uli changes evoked activity in cortical area V4, com pared to attention to the motion o f identical visual stim uli (Corbetta et a l , 1990; Corbetta et al., 1991b) Sim ilarly, attention to visual motion m odulates the stim ulus-evoked activity in cortical area V5 (Buchel and F riston, 1997; O 'C raven et a l , 1997). These functional im aging experim ents have dem onstrated the general principle that attention m odulates functionally segregated and stim ulus-specific regions o f visual cortex, but have not provided any insight into the m echanism o f such processes. The reason for this lies in the experim ental design; typically investigators have com pared a state w here the subject attends to a certain aspect o f a display with a state w here the subject receives identical visual stim ulation but does not attend. T he difference in activity betw een these tw o states has been attrib uted to the effects o f attention. H ow ever in light o f the preceding discussion it can be seen that such a difference between ‘atten d ’ and ‘no attend ’ conditions necessarily conflates both additive and interactive (stim ulus-evoked) com ponents o f attention. Intuitively this can be seen by im agining perform ing the experim ent again but w ithout using a stim ulus. W ould com paring ‘attend’ and ‘no attend’ reveal a difference in evoked activity (that could be produced by a change in b aselin e firing rates or bias signal), or is such a difference contingent on stim ulus- evoked activity (a true m odulatory effect of attention)? Such a thought experim ent serves to illu strate the theoretical p o in t that to ch aracterise the effects o f atten tio n using functional im aging requires an experim ental m anipulation o f stim ulus-evoked activity ^ that is independ ent o f the experim ental m anipulation o f attention. T he independent *

N eural correlates o f selective attention

m a n ip u la tio n o f a tte n tio n an d stim u lu s -e v o k e d a c tiv ity allo w s th e se p a ra te characterisation o f attentional effects due to m odulation of the stim ulus evoked activity p e r se, or due sim ply to changes in baseline activity independently of that evoked by the stim u lu s. N ote th a t th e re q u ire m en t fo r a m e asu re o f stim u lu s-ev o k ed activ ity independent o f the attentional m anipulation is largely a consequence o f the tim e over w hich functional im aging m easurem ents are acquired (of the order of seconds) relative to the tim e scale over w hich the neuronal effects are m anifest (of the order of m illiseconds). This m eans that in any given trial, both stim ulus evoked and baseline activity are lum ped to g eth er. To sep arate the effects o f attention on each requ ires som e in d ep en d en t m anipulation o f stim ulus-evoked activity.

F rith & Friston (Frith and Friston, 1996) proposed that such an independent m easure of stim ulus-evoked activity m ight be elicited by varying the rate o f presentation o f visual and auditory stim uli. The basis for this proposal is the observation that cerebral blood flow in prim ary sensory, prim ary m otor and higher order association cortices increases w ith increasing stim ulus presentation (Frith and Friston, 1996; Price et a l , 1996a; Rees et al., 1997c; Sadato e t a l , 1997). The m ost likely interpretation o f these results is that each stim ulus produces a transient increase in blood flow such that the total increase in blood flow is directly related to the num ber of stim uli presented during the scan. This m eans that the slope o f the line relating activity to presentation rate is an index o f the am ount of transient activity associated with the presentation of a single stim ulus. A ttention can now be investigated by studying how the slope or intercept o f this line varies with the direction o f attention (Figure 3-1). A change in slope im plies a true m odulatory effect o f attention; the am ount o f activity associated with each stim ulus presentation is directly changed. On the other hand, a change in intercept im plies that the activity associated w ith each stim ulus presentation is unchanged, but instead a ‘b ias’ signal, constant across different presentation rates, is added. This pattern im plies that there may be activity in an area even in the absence o f stim ulus presentation.

T he approach outlined here relies on m easuring the relationship betw een rC B F and a stim ulus variable (presentation rate), and then exam ining how a m anipulation o f attention changes this relationship. Certain assum ptions are im plicit in this approach that should be m ade clear. M ost im portantly, the assum ption is m ade that w ithin an area, the gain co n tro l or bias signal factor is co nstant across d ifferen t presen tation rates. If this assum ption is incorrect then the m easured changes in rCBF, although rem aining a correct

N eural correlates of selective attention

description o f the effects o f attention on rCBF, will be falsely attributed to a single underlying m echanism . In other words this approach m akes the sim plifying assum ption o f a unitary neurobiological m echanism o f attention with a constant gain control or bias signal across the range o f presentation rates studied. In these initial exp erim en ts a restricted range o f presentation rates is used for this reason.

Neural correlates o f selective attention

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