Comparisons between modally and amodally completed regions show that perceptual filling-in is not merely the ignoring of absences. Illusory filled-in colour arises for modal completion, but not for amodal completion in comparable displays. We find that attention spreads automatically to modally but not amodally completed regions from their inducers, revealing a functional effect of filled-in colour

Davis, G.

Driver, J.

English

UCL Discovery

Pessoa et al. must mean that Dennett erroneously concludes thatthe Marilyn case involves a sort of perceptual illusion, for they ev-idently agree that the alleged intellectual errors really are errors.)Pessoa et al. think Dennett draws the erroneous conclusion –that the Marilyn case involves perceptual illusion – by reasoningcorrectly from a crucial assumption, the “uniformity thesis.” Thatis why they side with Gibson (1979), and against Dennett (1978),in rejecting the uniformity thesis.1 So, although these exegeticalmatters are not significant, what is of some importance is Pessoaet al.’s argument that the uniformity thesis is false because it hasfalse consequences.According to the uniformity thesis, “perceptual content at thepersonal level just is the content of brain states at the subpersonallevel” (sect. 9.2, para. 4). In other words: when a person has a per-ceptual experience with such-and-such propositional content, thisfact is constituted by the presence, in some part of the person, ofa certain sort of representation with that content (as it might be,by the tokening of a neural representation in the subject’s “visionbox”).2 The uniformity thesis is therefore entailed by the repre-sentational theory of mind.Pessoa et al. think the uniformity thesis leads to trouble in theMarilyn case for two reasons. First, they claim that the thesis en-tails (or at least motivates) the conclusion that “in the absence ofa brain-level pictorial representation of each of the identical Mar-ilyns the person cannot have a percept with the content that thereare hundreds of identical Marilyns” (sect. 9.2, para. 4).3(Pessoa et al. interpret Dennett as drawing the conclusion justmentioned. But this does not make any sense. For Dennett cer-tainly holds that there are no such pictorial representations, andthat the content of the experience is that there are hundreds ofidentical Marilyns. If he went on to draw the above conclusion, hisposition would be glaringly inconsistent.)This parenthetical difficulty aside, it is obvious that the unifor-mity thesis has no such unwanted consequence. According to it,in order to have an experience with the content that there are hun-dreds of identical Marilyns, one just needs an appropriate innerrepresentation with exactly that content, and plainly that does notrequire a representation, pictorial or otherwise, of each Marilyn.Second, Pessoa et al. claim that the uniformity thesis entails (ormotivates) the conclusion that, when looking at the Marilyn wall-paper, “it seems to one that the Marilyns are all there in one’s mindor brain” (sect. 9.2, para. 5). Since there are no such pictures (atany rate according to Dennett), the person’s experience must,again, be illusory. Pessoa et al. say that “the reasoning depends onthe idea that visual experience is pictorial, in the sense that to havea visual experience that is really of hundreds of identical Marilynsis to have a picture in the mind or brain with precisely that con-tent,” which in turn “depends on” the uniformity thesis (sect. 9.2,para. 5). But the uniformity thesis is clearly not guilty. First, it doesnot lend support to the view that inner representations are picto-rial (as opposed to, say, sentential). Second, even if we grant thatthe representation is pictorial, the uniformity thesis does not im-ply that the person is aware of this inner picture, only that the con-tent of the person’s experience is the content of the appropriateinner representation. In the example, the content is that there arehundreds of Marilyns (more specifically: hundreds of Marilyns onthe wall). To think that the person must be aware of the repre-sentation itself would be to confuse what is represented (what theperson is aware of: Marilyns on the wall) with what does the rep-resenting (a neural picture, which the person need not be awareof). It would be to confuse, that is, represented properties (e.g.,being a Marilyn on the wall) with properties of representations(e.g., being a neural picture of a Marilyn). As noted, Dennett him-self has emphasized the distinction.Pessoa et al. correctly state that “at the personal level, there areno . . . representations in visual perception; there is simply expe-rience of the world” (sect. 9.2, penultimate para.; see also sect. 9.1and Thompson 1995, p. 235).4 What I do not understand is whythis shows that Dennett, and most of us, are wrong, and Gibson isright.ACKNOWLEDGMENTThanks to David Hilbert for discussion.NOTES1. For another Gibsonian attack on Dennett, see McDowell 1994.2. The assumption that the content of a perceptual experience is thecontent of a single inner representation is merely for expository conve-nience. Dennett (1991) appears to hold that the content of a perceptualexperience is the conjunction of the contents of many inner representa-tions, and is never the content of a single representation. That is not in con-flict with a more careful statement of the uniformity thesis.3. Pessoa et al. in fact present this line of reasoning in two steps: fromthe uniformity thesis to “analytic isomorphism,” and from the latter to theconclusion. I agree with them that analytic isomorphism should be re-jected.4. Unlike Dennett, philosophers like Block (1995) would at least wantto add a qualification here, but Block’s reasons are not relevant to the pres-ent discussion.The functional effects of modal versusamodal filling-inGreg Davis and Jon DriverDepartment of Psychology, Institute of Cognitive Neuroscience, UniversityCollege London, London WC1E 6BT, United Kingdom.gjd1000@cus.cam.ac.uk j.driver@ucl.ac.ukAbstract: Comparisons between modally and amodally completed re-gions show that perceptual filling-in is not merely the ignoring of absences.Illusory filled-in colour arises for modal completion, but not for amodalcompletion in comparable displays. We find that attention spreads auto-matically to modally but not amodally completed regions from their in-ducers, revealing a functional effect of filled-in colour.The target article provides a useful summary and discussion of theextensive literature on visual “filling-in.” Like many others in thisfield, we were already convinced of the main empirical conclusion,namely, that neural filling-in goes beyond the mere ignoring of ab-sent information, thus challenging Dennett’s (1991) account. Wewere somewhat disappointed that the article focuses mainly on thechallenge from neural filling-in alone, and less on the equally persuasive challenge from the psychological reality of perceptualfilling-in itself. This seemed a missed opportunity, particularlygiven Pessoa et al.’s extensive discussion of how neural activitymight best be related to perceptual states (and also given that fewresearchers would be so interested in neural filling-in if none aroseperceptually!). The authors seem to assume that the existence offilling-in at the perceptual level is merely an uncontroversial andtheoretically neutral starting point. However, we think that per-ceptual measures, not only neural measures, can help establishwhether (and when) filling-in goes beyond the mere ignoring ofabsences.A direct comparison of modal versus amodal completion maybe particularly revealing on this issue. As Pessoa et al. note, sev-eral recent authors (e.g., Shipley & Kellman 1992) have shownthat these two forms of completion can be induced by very simi-lar displays, and also show several intriguing parallels regardingthe completed shape that will be perceived. However, a criticaldifference remains. No illusory colour or brightness is filled inperceptually for amodally completed regions, which are seen as ly-ing behind an occluder. By contrast, modal completion leads toperceptual filling in of colour and brightness for the completed re-gion, seen in the front plane. The accompanying Figure 1 illus-trates this, for two displays used in our own work (Davis & Driver1997; 1998a). Each comprised two grey segments, abutting awhite bar on a dark screen (as in A). Only the depth of the whitebar differed between the two displays. When the bar appeared tobe closer than the grey segments (due to stereoscopic disparity),the latter were amodally completed as a single grey ellipse that waspartly occluded by a white bar (much as it appears for the two-Commentary/Pessoa et al.: Perceptual completion752 BEHAVIORAL AND BRAIN SCIENCES (1998) 21:6dimensional illustration in A). No illusory colour was seen in theamodally completed region itself. By contrast, when the grey re-gions appeared to be closer than the white bar, they now becamemodally completed, as a continuous transparent grey ellipse infront of the white bar. Critically, illusory colour and brightness wasnow infused into the completed region, which took on the grey ofthe curved segments (as cartooned in B), even though no grey wasphysically present in the centre (just as in A). Thus, modal andamodal completion differ in terms of perceptual filling-in, with il-lusory colour and brightness arising only in the former case. Thisdifference is not readily accounted for by mere “ignoring of ab-sences,” given the similar absences for the two cases. As Pessoa etal. discuss, there have been several previous neuroscience studieson amodal completion, and also on modal completion but nonehas ever compared them directly, even though this would be particularly revealing regarding the neural basis of perceptual filling-in.In several recent articles, we have sought to compare the func-tional effects of modal and amodal completion directly (Davis &Driver 1994; 1997; 1998a, 1998b; Mattingley et al. 1997) as Pes-soa et al. recommend. Two studies used displays like those in Fig-ure 1, to examine the effects of modal versus amodal completionupon the distribution of visual attention. Davis and Driver (1998a)found that when attention was drawn to the inducing grey seg-ments (by a sudden change in their size) this also drew attentionto the completed region between them in the modal situation.However, this did not arise for the comparable amodal case, thusrevealing a difference in the functional effects of the two forms ofcompletion. Davis and Driver (1997) used a measure of distractorinterference to confirm that this spreading of attention betweeninducers and completed regions, for the modal case only, happenseven when counter to observers’ intentions.We concluded that the filling-in of colour and brightness, atmodally completed regions only, was responsible for this spread-ing of attention. More generally, we proposed that the presence offilled-in colour at modally completed regions signals that they be-long to an unoccluded surface (even when physically absent in thestimulation, due to coincidental camouflage or impoverished illu-mination; see Davis & Driver 1998a). By contrast, the absence ofany colour coding at all for amodally completed regions signalsthat they are occluded from view. The distinction between oc-cluded and nonoccluded regions of objects is crucial for visual ob-ject recognition (Nakayama et al. 1989). Moreover, it is also cru-cial for directing attention towards potentially relevant versus en-tirely irrelevant information when judging a particular object. Anyvisible features at occluded (and amodally completed) regions ofa relevant object cannot reflect properties of that object, but onlythose of the occluder. This restriction does not apply to visible fea-tures that form part of a camouflaged by unoccluded object, as inmodally completed regions. Hence it makes excellent functionalsense that attention should spread from inducers to modally com-pleted regions, but not from inducers to comparable amodallycompleted regions, exactly as we find (Davis & Driver 1997;1998a).Our work thus identifies a difference in the functional effectsof modal versus amodal completion, using perceptual rather thanneural measures. Moreover, the observed difference makes goodsense in terms of the particular task faced by the visual systemwhen required to attend to potentially relevant information andignore irrelevant information. In this restricted sense, we are sym-pathetic to Pessoa et al.’s enthusiasm for “task level” conceptionsof vision in terms of the function(s) served. However, we take is-sue with their more general advocacy of a “personal level” or “an-imal centered” approach to vision as a panacea for philosophicalconfusions in this area. Arguing generically that the “animal . . .simply sees aspects of the world” (sect. 9.1, para. 4) does not ad-vance our understanding of how the animal achieves this: it merelyreplaces the homuncular little-animal-inside-the-head as an allpowerful agent with the larger whole animal. Finally, some of thecriticisms of the Marr (1982) approach seemed inappropriate.There are emphatically no homunculi in any of Marr’s computervision systems; and his work was characterised by careful analysesof the tasks to be solved by particular visual processes.No bridge over the stream of consciousnessDaniel C. DennettCenter for Cognitive Studies, Tufts University, Medford, MA 02155.ddennett@tufts.edu www.tufts.edu/as.cogstud/mainpg.htmAbstract: Pessoa et al.’s target article shows that although filling-in of var-ious kinds does appear to occur in the brain, it is not required in order tofurnish a “bridge locus” where neural events are “isomorphic” to the fea-tures of visual consciousness. Some recently uncovered completion phe-nomena may well play a crucial role in the elaboration of normal visual ex-perience, but others occur too slowly to contribute to normal visualcontent.I find this a very useful essay, a model, in fact, for philosopherswho want to make a substantive contribution to cognitive science:it makes sense of controversy, dispels confusions, and sharpensour understanding of the more distant implications of a wide va-riety of current empirical work, an important task that is typicallybeyond the aspirations (if not the talents) of those working in thelabs.First, let me acknowledge that Pessoa et al. have correctedsome errors on my part, errors that betrayed my ignorance of awealth of empirical and theoretical work that had already been un-dertaken on the vexed issue of filling-in. I am glad they alsopointed out, however, that I did point to the very sorts of empiri-cal experiments that “would” disprove my hunch – some of whichshould already have been known to me. I was not entirely the in-novator I took myself to be, then, but I will settle for the role ofcatalyst, since my rash interloping has served to direct attentionand begin the task of clarification that Pessoa et al. are continuing.Second, I want to propose a friendly amendment to Pessoa etal.’s fine discussion of what they take to be the fundamental mis-take of “analytical isomorphism”: supposing there must be a“bridge locus” where features of experience and features of neuralCommentary/Pessoa et al.: Perceptual completionBEHAVIORAL AND BRAIN SCIENCES (1998) 21:6 753Figure 1 (Davis & Driver). (A) Stimulus pattern employed byDavis and Driver (1997; 1998a) to compare spreading of attentionto and from modally – versus amodally – completed regions. Whenstereoscopic disparity caused the white bar in the stimulus to ap-pear nearer than the grey regions, the grey regions were amodallycompleted to form a partly occluded ellipse that continued behindthe white bar. Conversely, when disparity signalled instead that thegrey regions were nearer to the observer than the white bar, thegrey regions were modally completed to form a transparent ellipsethat continued in front of the white bar. The region where themodally completed ellipse overlapped with the white bar now ap-peared filled-in illusory colour. (B) Cartoon of how the stimulus in(A) appeared when the grey regions underwent modal completionto form a transparent ellipse.

The functional effects of modal versus amodal filling-in

Abstract

Similar works

Full text

Available Versions

UCL Discovery