Talk:Figure-ground perception

Referee A

General Comments

This is a useful contribution to Scholarpedia. I liked it very much as far as it goes, and appreciate the fact that Mary Peterson is a leading expert in this area who has contributed many interesting ideas and facts to it. However, as someone who has also worked on figure-ground perception for a long time, it left me feeling that there is much more known about figure-ground processes than the authors indicate, and quite a few references to other authors that could profitably be included. Where to draw the line is a matter of personal judgment. Although I do not feel that any of the following comments MUST be incorporated in a further revision, I would be very disappointed if they were not, since their inclusion would lead readers to explanations of phenomena that are not sufficiently indicated in the current version of the article.

I organize my comments with respect to specific sentences in the article. I indicate current sentences in the text in BLACK, proposed new text and references in BLUE, and proposed deletions in RED.

1. On p. 3, just above the section on Non-classical geometric configural properties:

Thus, it is unclear whether responses to these configural properties per se are innate, or whether a sophisticated learning mechanism has evolved that allows humans to extract the statistical properties of the environment in which they live (and configural cues are among those properties). It has been shown that development of grouping networks in response to simple statistical properties of the environment may be sufficient to generate figure-ground percepts, indeed bistable 3D percepts such as the Necker cube (Grossberg & Swaminathan, 2004).

NEW REFERENCE:

Grossberg, S., and Swaminathan, G. (2004). A laminar cortical model for 3D perception of slanted and curved surfaces and of 2D images: development, attention and bistability. Vision Research, 44, 1147-1187.

• We thank the reviewer for pointing this out. We added the reference as suggested and changed the suggested sentence to read as follows: “See also Grossberg & Swaminathan (2004) for a model that uses statistical learning to account for some perceptual effects.”

2. On. P. 4, paragraph about the section on Depth cues:

The configural cues are shape cues; they determine where the shape lies with respect to an edge. But recall that the region complementary to the figure is often perceived to complete behind it. The perceptual completion of the ground has not received much attention in the study of figure-ground perception per se. It is possible that at least some of the configural properties may convey depth information as well as shape information (Burge et al. 2005; Grossberg, 1994; Kanizsa, 1985; Nakayama, Shimojo, & Silverman, 1989), and that perceptual completion may be most compelling when those cues are present (see, for instance, Peterson & Salvagio 2008).

NEW REFERENCES:

Kanizsa, G. (1985). Organization in vision: Essays in Gestalt perception. New York: Praeger.

Nakayama, K. Shimojo, S., & Silverman, G.H. (1989). Stereoscopic depth: Its relation to image segmentation, grouping, and the recognition of occluded objects. Perception, 18, 55-68.

• We thank the reviewer for pointing this out; we included the suggested references.

3. First paragraph after the header Depth cues:

The region that appears shaped also tends to appear closer (although this relationship does not always hold, e.g., Palmer 1999; Peterson 2003). Depth cues determine which of two contiguous regions is closer to the viewer even in the absence of the classic configural cues. Closer regions tend to be shaped by the edges they share with contiguous regions in the visual input, and the latter typically appear to continue behind as backgrounds. There are ample empirical investigations of the depth cues: for instance, research investigates the ranges over which different depth cues are most effective (e.g., Cutting & Vishton 1995) and the rules by which depth cues combine (e.g., Landy et al. 1995). More research is needed to investigate Very little research investigates how configural cues and depth cues interact (but see Bertamini, Martinovic, & Wuerger, 2008; Burge et al. 2005; Dresp et al. 2000; Egusa, 1983; Peterson & Gibson 1993). Such research is needed for a full understanding of figure-ground perception.

NEW REFERENCES:

Dresp, B., Durand, S., and Grossberg, S. (2002). Depth perception from pairs of overlapping cues in pictorial displays. Spatial Vision , 15, 255-276.

Egusa, H. (1983). Effects of brightness, hue, and saturation on perceived depth between adjacent regions in the visual field, Perception 12, 167-175.

• We made suggested change to the text. We didn’t include the suggested references because we are discussing the interaction between depth cues and configural cues; the suggested articles are focused on depth. There are many articles on depth cues as we stated at the beginning of the paragraph, we chose to include only those articles that investigated the combination of configural cues and depth cues.

4. Section on Subjective factors:

Subjective factors can also influence figure assignment. For instance, the viewer’s intention to perceive one of two contiguous regions as figure affects figure-ground perception (e.g., Peterson et al. 1991). And regions at which the viewer is looking (fixated regions) are more likely to be seen as figures than adjacent un-fixated regions (Peterson & Gibson 1994). Similarly, an attended region is more likely to be seen as figure than the complementary unattended region, even without fixation (Baylis & Driver 1995; Vecera et al. 2002). Neural models have proposed how attention can influence which bistable percept is seen during percepts of the Necker cube (Grossberg & Swaminathan, 2004) and of bistable transparency (Grossberg & Yazdanbakhsh, 2005). These models predict how attention makes a figural surface look closer, and why there may be a change of brightness when there is a change of figure and its perceived depth, as noted by Tse 2005. Subjective factors can alter the likelihood of seeing the figure on one side of an edge, but typically they tend not to overpower configural cues.

NEW REFERENCES:

Grossberg, S. and Yazdanbakhsh, A. (2005). Laminar cortical dynamics of 3D surface perception: Stratification, transparency, and neon color spreading. Vision Research, 45, 1725-1743.

Tse, P. (2005). Voluntary attention modulates the brightness of overlapping transparent surfaces. Vision Research, 45, 1095-1098.

• These references refer to bistability the topic of this piece is figure-ground perception. Although some figure-ground displays are bistable, it is not clear that all bistable stimuli behave the same. Furthermore, Scholarpedia articles are meant to be short; they are not meant to be exhaustive. Many of the articles recommended by this reviewer were on the one hand going beyond the scope of this Scholarpedia article and on the other hand very specific, such that their relationship to what we wrote would have to be spelled out in detail. Scholarpedia’s information for authors notes: “Brevity is the soul of wit", … an effective Scholarpedia article is 2-3 pages long (2,000 words, though some topics warrant longer articles) not counting figures and animations.”

5. Section on Spatial frequency:

A region filled with a high spatial frequency pattern is more likely to be seen as the shaped figure than a contiguous region filled with a low spatial frequency pattern (see Figure 5; Klymenko & Weisstein 1986). Neural models propose how such a percept occurs and how it influences bistability, as when the Rubin’s vase-faces stimulus of Figure 6 is composed of two regions with different spatial frequencies, or when horizontal bands of high and low spatial frequency sinusoids alternate one above the other  (Brown & Weisstein 1988; Grossberg 1994; Klymenko & Weisstein, 1986).

NEW REFERENCES:

Brown, J.M. & Weisstein, N. (1988) A spatial frequency effect on perceived depth. Perception & Psychophysics, 44, 157-166.

Klymenko, V. & Weisstein, N. (1986). Spatial frequency differences can determine figure-ground organization. Journal of Experimental Psychology: Human Perception & Performance, 12, 324-330.

• Klymenko & Weisstein (1986) is already cited in this section. With respect to the other references we discuss general computational approaches to figure-ground perception later in the piece. In our opinion discussing specific models of specific cues goes way beyond the scope of this Scholarpedia piece.

6. Section on Extremal edges:

An extremal edge (EE) is a self-occluding edge. When shading and texture gradients are used to depict an extremal edge along one side of a border but not the other, observers show a strong bias to report seeing the EE side as nearer than the non-EE side (Palmer & Ghose 2008) A sample is shown in Figure 5b, where the extremal edge lies on the left side of the central border. One possible way to think about this percept is by using a A non-shape based likelihood principle may underlie this bias, in that from many viewpoints the input array is consistent with the interpretation that the EE side is closer, whereas it is consistent with the interpretation that the non-EE side is closer from only one viewpoint. On the other hand, Grossberg & Mingolla (1987) have explained such a percept of 3D shape using a property of perceptual grouping that they call a boundary web. A boundary web is a form-sensitive plexus of amodal emergent boundaries that selectively captures the filling-in of surface brightnesses at multiple depths. This concept has been used to quantitatively explain data about such percepts as 3D shape-from-texture (Grossberg et al., 2007) and 3D shape from the waterfall illusion (Pinna & Grossberg, 2005), which we now discuss.

NEW REFERENCES:

Grossberg, S. and Mingolla, E. (1987). Neural dynamics of surface perception: Boundary webs, illuminants, and shape-from-shading. Computer Vision, Graphics, and Image Processing, 37, 116-165.

Grossberg, S., Kuhlmann, L., and Mingolla, E. (2007). A neural model of 3D shape-from-texture: Multiple-scale filtering, boundary grouping, and surface filling-in. Vision Research, 47(5):634-672.

Pinna, B. and Grossberg, S. (2005). The watercolor illusion and neon color spreading: A unified analysis of new cases and neural mechanisms. Journal of the Optical Society of America A, 22, 2207-2221.

• We realize this was confusing as written and have changed the text to read as follows:”An extremal edge (EE) is a self-occluding edge. When shading and texture gradients are used to depict an extremal edge along one side of a border but not the other, observers show a strong bias to report seeing the EE side as nearer than the non-EE side (Palmer & Ghose 2008). A sample is shown in Figure 5b, where the extremal edge lies on the left side of the central border. Research is needed to determine whether extremal edges are depth cues, figural cues, or both.”

7. Section on Watercolor Effect illusion:

Consider a region bounded by two thin colored lines that are parallel to and touching each other. One of the colored lines contrasts less with the background than the other. Pinna, Brelstaff & Spillman (2001) and Pinna, Werner & Spillmann (2003) showed that under these conditions the low contrast color spreads orthogonally from the line and fills the bounded region; they called this phenomenon the “Watercolor Illusion.” They showed that the region through which color spreads is more likely to be seen as the figure than it would be without the color. Key properties of the watercolor illusion have been explained using mechanisms of figure-ground perception that have also been used to explain other figure-ground percepts, such as 3D neon color spreading (Pinna & Grossberg, 2005). Not much is known about the Watercolor Effect as a figural cue; unlike other figural cues, it has not been examined in isolation, it has always interacted with one or more of the other figural cues.

NEW REFERENCE:

Pinna, B., Brelstaff, G. & Spillmann, L. (2001) Surface color from boundaries: a new “watercolor” illusion. Vision Research, 41, 2669-2676.

• We thank the reviewer for pointing out that Watercolor Effect should be Watercolor Illusion; the text has been changed to Illusion. We do not think it is appropriate to go into great detail on the Watercolor Illusion, because of the nature of this Scholarpedia piece. In addition, we believe as we state that unlike other figural cues not much is know about the watercolor illusion.

8. Section on How does figure-ground perception occur?:

Second paragraph:

Experimental work investigating the competition is relatively new (but see Dresp et al 2002). More studies directed to uncovering the nature of the competition are necessary. For instance, experiments investigating how depth cues alter the between-shape competition are needed to elucidate the mechanisms of figure-ground perception.

• We did not include the suggested reference because Scholarpedia pieces are not meant to cite the literature exhaustively.

9. Section on Open questions:

Second paragraph:

2. Edges separate surfaces in touch as well as in vision, and figure-ground perception occurs (Kennedy 1993). There are analogous percepts in hearing, taste, and smell as well. Do the same mechanisms produce figure-ground perception across the senses. As illustrated by auditory streaming and speech categorization, different grouping mechanisms seem to have evolved to deal with audition and vision in general (e.g., Bregman, 1990; Grossberg, 2003), although perhaps the greatest similarities occur between visual apparent motion processing and auditory streaming (Bregman, 1990; Gjerdingen, 1994).

NEW REFERENCES:

Bregman, A.S. (1990). Auditory scene analysis: The perceptual organization of sound. Cambridge, MA: MIT Press.

Gjerdingen, R.O. (1994). Apparent motion in music? Music Perception, 11, 335-370.

Grossberg, S. (2003). Resonant neural dynamics of speech perception. Journal of Phonetics, 31, 423-445.

• We considered including Bregman’s work in this section, however, in correspondence with Bregman, it was decided that his work deals with grouping effects and not figure-ground perception. There is already a piece on perceptual grouping in Scholarpedia.

Third paragraph:

3. There are many other types of ambiguous figures, including binocular rivalry stimuli and reversible stimuli like the duck/rabbit or Necker cube. Interactions between cooperative and competitive mechanisms have been proposed to play a key role in explaining many Competitive mechanisms have been proposed to account for all of these reversals. For example, Grossberg et al. 2008 have explained a large body of binocular rivalry data using the type of perceptual grouping mechanisms that play a key role in figure-ground perception. These mechanisms include long-range cooperation whereby groupings start to form, competitive mechanisms that prune and decide among possible groupings, and habituative mechanisms that enable active groupings to weaken in an activity-dependent way and thereby lead to a perceptual switch. A major goal of ongoing and future research is to clarity how such combinations of mechanisms may explain a wide range of data about percepts of Can comparisons of the different types of ambiguous stimuli. shed light on where and how competitive mechanisms operate in the brain?

NEW REFERENCE:

Grossberg, S., Yazdanbakhsh, A., Cao, Y., and Swaminathan, G. (2008). How does binocular rivalry emerge from cortical mechanisms of 3-D vision? Vision Research, 48, 2232-2250.

• We added the Necker cube example in the text. And the following sentence was modified to read as follows: “Interactions between cooperative and competitive mechanisms have been proposed to play a key role in explaining many of these reversals.” It seems inappropriate to go into such detail in the open questions section of this piece.

Reviewer B

Second review

This is my review of the revision.

1. I still think Figure 1 loooks strange. I am not sure what the authors are trying to do with this figure. Why can't it be more compact and regular? • We reduced the size of the figure to make it appear more compact, as per the reviewer’s suggestion. The reviewer had previously pointed out that our original stimulus was awkward because it appeared to have handles. We revised the original figure. We created Figure 1 for this article; it illustrates the configural cues of small area, symmetry and closure, and it has convex parts on the left and right. We think it is important to demonstrate these cues with a novel figure; anything more regular (e.g., a square or a circle) would either be familiar and/or not demonstrate all the cues we want to demonstrate in one figure.

2. Under Contents the organisation seems wrong. Surely 1 should be a heading "Configural cues" not "Factors that affect figure assignment" which is the superordinate heading of the whole section. Then the subject headings under 1 make sense; 1.1 Classical configural cues and 1.2 Non-classical configural cues. • We thank the reviewer for noticing this problem. The heading “Factors that affect figure assignment” is now superordinate to 3 headings: configural cues; depth cues; non-geometric factors.

3. On page 3 the statement is made that "the Gestalt Psychologists held that these cues were innate". It is essential to give a reference here. I do not believe that Rubin thought this. There may be such a statement in Kohler or Koffka but it should be specifically cited in the light of the specific warnings by Kanizsa in "Organisation in Vision" (1979) and by Mitchell Ash in "Gestalt Psychology in German Culture" (1995) against the assumption that the gestaltists supported an innate position. We thank the reviewer for pointing this out. We searched for clear statements that the configural cues/organizing principles were innate in Koffka, Koehler, and Wertheimer. We didn't find anything explicit; hence, we dropped the statement that "these cues were innate", although all authors argued against experience accounts of initial organization.

Koffka: On p. 100 of the 1063 Harbinger paperback edition of Koffka's (1935) Principles of Gestalt Psychology, Koffka speaks of conditions "inherent in the nervous structure itself."

Koehler: On p. 118 of the first Meridian printing of Koehler's (1929) Gestalt Psychology, Koehler wrote, "There was a time when the remarkable behavior of the Gestalten,...., was generally interpreted as proving that higher mental processes were involved.... From the present point of view, however, sensory organization appears as a primary fact which arises from the elementary dynamics of the nervous system."

Wertheimer: "And yet, despite its plausibility, the doctrine of past experience brushes aside the real problems of apprehension much too easily. Its duty should be to demonstrate in each of the foregoing cases (1) that the dominant apprehension was due to earlier experience (and to nothing else); (2) that non-dominant apprehensions in each instance had not been previously experienced ; and, in general, (3) that in the amassing of experience none but adventitious factors need ever be involved. It should be clear from our earlier discussions and examples that this programme could not succeed. A single example will suffice to show this. Right angles surround us from childhood (table, cupboard, window, comers of rooms, houses). At first this seems quite self-evident. But does the child's environment consist of nothing but man-made objects? Are there not in nature (e.g. the branches of trees) fully as many obtuse and acute angles ? But far more important than these is the following consideration. Is it true that cupboards, tables, etc., actually present right angles to the child's eye? If we consider the literal reception of stimuli upon the retina, how often are right angles as such involved? Certainly less often than the perception of right angles. As a matter of fact the conditions necessary for a literal "right angle" stimulation are realized but rarely in everyday life (viz. only when the table or other object appears in a frontal parallel plane). Hence the argument from experience is referring. not to repetition of literal stimulus conditions, but to repetition of phenomenal experience and the problem therefore simply repeats itself. "

Finally, we note that Boring (1942), p. 30 writes, [Stumpf] is chiefly important in this tradition, however, because he forms a link between the older nativism and the Gestalt psychologist, who are also, in a sense, nativists."

We changed the sentence to read as follows: “The Gestalt psychologists held that these cues did not depend upon an individual's past experience."

4. Figure 3. Given the point on page 2 about the importance of having the same contrast of the two regions relative to the backdrop, the authors should check on the contrasts in this figure. The seahorse does not look as if it has equal contrasts for black and white regions. This may be an illusion of course. • This is probably an illusion. We double-checked the colors and the contrast is equal with the medium gray background for black and white.

5. On page 3 the authors have not made clear the point I suggested in my previous review should be dropped. What does it mean to say that "past experience is not always instantiated as geometric relationships"? Do they mean suggestion? Again what do they refer to when they say that, "it may not be the case that all forms of past experience influence figure assignment but only those that are embodied geometrically"? Do they refer to verbal experience? This whole notion is still quite unclear. • This was confusing as written, its has been changed to as follows: “It is important to note that effects of familiarity were obtained only for upright stimuli and not for inverted or spatially-rearranged stimuli even when a strong demand was placed on the viewer to see these latter stimuli as alterations of the upright familiar object. Thus, having the parts in their proper spatial relationships is necessary for familiarity effects on figure assignment; instruction is insufficient in both the inverted and spatially-rearranged case. These examples make clear that familiar shape, which can be embodied geometrically, affects figure assignment. Whether or not other types of familiarity can exert an influence remains to be demonstrated.”

6. Page 5. Under extremal edges the authors say that from many viewpoints the input array is consistent with the interpretation that the EE is closer whereas it is consistent with the interpretation that the non-EE side is closer from only one viewpoint. Surely this is not true. The non-EE edge could be nearer and overlapping with the region with the EE edge and this will be the case for many viewpoints. • What we should have said is “A non-shape based likelihood principle may underlie this bias, in that the EE side would be closer from a larger number of viewpoints than the non-EE side.” Palmer & Ghose (2008) make this claim compellingly on page 77 and in figure 1 on page 78.

7. Also on page 5 under "Ambiguous figure-ground perception" the authors say that familiarity favors perceiving the black regions as figures. I do not see why the sideways faces are more familiar than the vase. • The reviewer raises an interesting point. Vases are certainly familiar. We added the following sentence after stating that the “factor of familiarity favors perceiving the black regions [profile faces] as figures” “Vases may also be considered familiar but arguably face profiles are encountered more frequently in the real world.”

Old -fashioned review as requested. (Original)

I think this is a very suitable topic for Scholarpedia and on the whole very well done. I have the following suggestions.

P 1.

Paragr 1. The first sentence seems a bit awkward. The word "adjacent" which means "near" does not seem correct. "Abutting" is the correct word but "contiguous" or "adjoining" would also be better. I suggest, "For two abutting regions of the visual field, the usual perceptual outcome is that the common edge appears to be the boundary for only one region- the figure-and this region appears to have a definite shape. • We thank the reviewer for pointing out this ambiguity. We changed the wording as requested to contiguous. “For two contiguous regions in the visual field…”

I think the third sentence should say "nearer" rather than "closer to the viewer". I suggest "Thus in addition to being shaped, the figure appears nearer than the ground part of which appears occluded by the figure.". This adds a little more and slightly different emphasis to the property of ground that it goes behind figure. • We changed the wording as requested. “The contiguous region – the ground-- appears shapeless near the edge it shares with the figure, and is perceived to continue behind it.”

I suggest omitting the sentence " Figures constitute the two-dimensional shapes we perceive and the three-dimensional objects with which we interact." Apart from implying that we do not perceive three-dimensional shapes or interact with two- dimensional ones it raises questions that the review does not address about the way the typical 2-D figure-ground stimuli map onto real world situations. For example surely most three-dimensional objects e.g. a ball could never be ground. Better to omit the sentence. It does not in my opinion add anything useful. • We modified the sentence to read as follows: ” Figures constitute the objects we perceive and with which we interact.”

The figure chosen for Figure 1 does not seem optimal - the handles make it look a bit strange and distract from the main figure. • We selected a new figure to replace the original as suggested, see comments above under point 1 in the new review.

Paragr 2. I suggest leaving out "as does grouping" The article is not about grouping and does not explain what the term refers to. • We removed this as suggested.

P2 Paragr 1. I am not sure that it is correct to identify the Rubin figure-ground determinants with the "gestalt principles". Some of them are but not all. Rubin's work was certainly taken up by the Gestalt psychologists so a mention is fine (line 1) but it would probably be better to omit the reference identifying the "classical cues" with Gestalt Principles (line 8). • We removed this as suggested.

Paragr 2. The first sentence would read better as " regions that are convex .........are more likely to be seen as figures than contiguous regions that are......." • We changed this as suggested. “Regions that are convex, symmetric, smaller in area, enclosed and/or surrounded are more likely to be seen as figure than contiguous regions that are concave, asymmetric, larger in area, and/or surrounding.”

P3. Under the heading "Non-classical geometric configural properties"

parag 2. I did not understand what the last sentence was getting at "These effects necessarily depend on........yet they can be characterised geometrically in the spatial relationship between the parts on one side of an edge; the configuration of parts is familiar in a particular orientation'. The sentence seems to imply that familiarity can be characterised geometrically in a general sense. This cannot be what is meant but if it means that a particular familiar configuration can be specified geometrically that is surely too obvious to spell out. On the other hand the sentence may mean something else altogether. If so it needs to be clearer. I would omit it. • See comments above under point 5.

Under the heading "Depth Cues" line 7. After Burge et al (2005) and Peterson and Gibson (1993) are quoted you may consider adding a recent relevant paper by Gillam et al (2009). (B.J., Anderson, B.L., & Rizwi, F. (2009) Failure of facial configural cues to alter metric stereoscopic depth. Journal of Vision 9 (1):3 1-5). • We added these references as suggested.

p5. In the example of the Rubin figure given it is very difficult to see either alternaive as shapeless. The open questions were good. • We agree it is hard not to see both the face profiles and the vase simultaneously. Oddly, this was Rubin’s original figure (slightly modified). We have replaced that figure with a different version that we think allows alternations between the perceptions.

Third Review

I now think the paper is now fine- a very good summary of the field- except for one thing. That is the persisting assertion by the authors that: "The Gestalt psychologists held that these cues were innate, and did not depend upon an individual’s past experience. In support of this claim, They showed that novel regions possessing the configural properties were seen as figures (see Figure 1 and Figure 2d, for instance). Evidence that figure-ground perception can proceed without input from past experience for novel shapes does not eliminate the possibility that past experience also exerts an influence when shapes are familiar, however. They demonstrated widespread use of the classical configural cues by adults, at least when displays were exposed for long durations. Such findings cannot demonstrate innateness, however, because high agreement between adults can arise because of learning."

I do not see how adding " an individual's past experience" helps. The argument is still strongly that the gestaltists believed that past experience plays no role. The authors claim that they cannot find any relevant references from the Gestalt psychologists. Yet they include Koffka's " Principles of Psychology" in their references at the end. In my edition (Kegan Paul) there are some very relevant statements by Koffka which I have scanned (since the page numbers will be different in their edition -Harcourt Brace). See Figures 1 and 2. It seems to me that on page 209 (Figure 1) Koffka is certainly arguing strongly against the basic characteristics of figure-ground as a matter of experience (e.g., the ground going behind the figure, the contour belonging to one side etc) but on the page 210 (Figure 2) (more directly relevant to the issue of experience with particular stimuli) he clearly allows for the possibility that previous experience with a particular " articulation" may influence the current "articulation" other things being equal. Thus he does not make the first argument attributed to the gestaltists above. I see no evidence of the second either. The general quotes given by the authors in their response to me (not in the paper ) on this point are not nearly as relevant as those directly about figure-ground from Koffka. (I have underlined the most relevant bits)

Figure 1: Page 209 of Koffka).

Figure 2: Page 210 of Koffka).

Excerpt from Wertheimer VIII: Another Factor is that of past experience or habit. Its principle is that if AB and C but not BC have become habitual (or "associated ") there is then a tendency for ABC to appear as AB/C. Unlike the other principles with which we have been dealing, it is characteristic of this one that the contents A, B, C are assumed to be independent of the constellation in which they appear. Their arrangement is on principle determined merely by extrinsic circumstances (e.g. drill).

There can be no doubt that some of our apprehensions are determined in this way.[8] Often arbitrary material can be arranged in arbitrary form and, after a sufficient drill, made habitual. The difficulty is, however, that many people are inclined to attribute to this principle the fundamental structure of all apprehension. The situation in Sec. VII, they would say, simply involves the prominence of habitual complexes. Straight lines, right angles, the arcs of circles, squares-all are familiar from everyday experience. And so it is also with the intervals between parts (e.g. the spaces between written words), and with uniformity of coloured surfaces. Experience supplies a constant drill in such matters.

And yet, despite its plausibility, the doctrine of past experience brushes aside the real problems of apprehension much too easily. Its duty should be to demonstrate in each of the foregoing cases (1) that the dominant apprehension was due to earlier experience (and to nothing else); (2) that non-dominant apprehensions in each instance had not been previously experienced ; and, in general, (3) that in the amassing of experience none but adventitious factors need ever be involved. It should be clear from our earlier discussions and examples that this programme could not succeed. A single example will suffice to show this. Right angles surround us from childhood (table, cupboard, window, comers of rooms, houses). At first this seems quite self-evident. But does the child's environment consist of nothing but man-made objects? Are there not in nature (e.g. the branches of trees) fully as many obtuse and acute angles ? But far more important than these is the following consideration. Is it true that cupboards, tables, etc., actually present right angles to the child's eye? If we consider the literal reception of stimuli upon the retina, how often are right angles as such involved? Certainly less often than the perception of right angles. As a matter of fact the conditions necessary for a literal "right angle" stimulation are realized but rarely in everyday life (viz. only when the table or other object appears in a frontal parallel plane). Hence the argument from experience is referring. not to repetition of literal stimulus conditions, but to repetition of phenomenal experience and the problem therefore simply repeats itself.

Regardless of whether or not one believes that the relationships discussed in Sec. VII depend upon past experience, the question remains in either case: Do these relationships exhibit the operations of intrinsic laws or not, and if so, which laws? Such a question requires experimental inquiry and cannot be answered by the mere expression "past experience".

With regards to innateness, Wertheimer discusses "natural", "spontaneous" (I in Wertheimer online), and "intrinsic" groupings based on "objective" factors (see VI), "what the objective arrangement dictates" (also I) and places them in contrast to "drill" and "past experience."