EFECTO SOBRE LA VIABILIDAD DE CÉLULAS DE NEUROBLASTOMA

Typically, disparity extraction can account for short range disparity interactions (or apparent long range effects such as stereo transparency, see Section 2.2.5) – normally these effects are attributed to the size of the receptive fields (see Section 2.2.4). Unfortunately, point by point extraction of disparity (which we call a disparity map) does not explain some longer range interactions. One example of this is disparity capture (or interpolation), where areas of ambiguous depth (for example horizontal lines) are displayed flanked by disparity defined surfaces (Georgeson, Yates, & Schofield, 2009; Wilcox, 1999; Wilcox & Duke, 2005). From a disparity extraction point of view, these regions are undefined in depth, and should be perceived as having no depth. However, studies have found that depth is perceived in the ambiguous region and that it is dependent on the disparity of the flankers or surrounding regions (Georgeson et al., 2009; Harris & Gregory, 1973; Ramachandran, 1986; Wilcox & Duke, 2005; Yang & Blake, 1995).

Unexpected biases in the perceived depth of a region do not only occur in poorly defined regions (Yang & Blake, 1995). If two regions of disparity defined depth are placed next to each other, the observers are good at judging the depth difference between the regions. However, if the two regions of disparity are joined with a ‘Cornsweet depth profile’, as in the solid line in Figure 2.11 (bottom), the perceived depth difference between the two regions is magnified (Anstis, Howard, & Rogers, 1977; Didyk, Ritschel, Eisemann,

Myszkowski, & Seidel, 2012; Rogers & Graham, 1983), as shown by the dashed line in Figure 2.11 (bottom). The effect was originally discovered in the luminance domain (Cornsweet, 1970; Craik, 1966; O’Brien, 1958) – the effect can be seen for luminance in Figure 2.11 (top). This depth effect cannot be predicted from simple disparity extraction, as the size of the regions should be sufficient to allow for disparity extraction unaffected by the Cornsweet profile.

38 Shading and luminance gradients do not only provide analogous effects to the perception of disparity defined depth – as discussed in the introduction, there are many other cues to 3D shape apart from binocular disparity. Some of these cues, such as the size of familiar objects, will not be of much use to detect camouflaged animals, as the camouflaged object has to have already been detected in order to be compared. Others, such as self-motion can cause a perception of depth in an object, but would be self-defeating – the predator may be able to detect the prey item, but the self-motion would give away the predator’s presence to its prey (Srinivasan, 1995). The most relevant to our discussion of camouflage of depth defined objects is shape from shading – this is where shadows and

shading can give cues to the shape of a 3D object due to the direction of the light source (Norman et al., 2006; Todd, 2004). Indeed, there is an entire camouflage mechanism called self-shadow concealment or Wcountershading (Penacchio, Lovell, Sanghera, et al., 2015; Rowland, 2009; Ruxton et al., 2004; Stevens & Merilaita, 2009) that is thought to have developed in order to reduce or eliminate shading cues (see Section 2.1.1).

Shape from shading has been found to give a vivid impression of depth and surface orientation, due to the direction of the light source (Kleffner & Ramachandran, 1992; Koenderink, Doorn, & Kappers, 1992; Norman & Wiesemann, 2007). We know that the simultaneous presentation of shape from shading and binocular depth perception causes a more vivid impression of depth than either cue presented alone (Bülthoff & Mallot, 1988; Todd, Norman, Koenderink, & Kappers, 1997), implying that the combination may be very effective at breaking camouflage techniques such as background matching. Other studies have looked at situations where shape from shading and binocular depth perception are placed in conflict, and have found that depth from disparity is typically dominant, although in situations where the disparity signal is noisy, shape from shading can be dominant (Chen & Tyler, 2015; Lovell et al., 2012). This is the behaviour we would expect if binocular

disparity was used to break monocular camouflage techniques – as discussed above, shape from shading cues can be manipulated by camouflage techniques such as shape from shading or countershading.

As an aside, this combination of shape from shading and binocular depth perception can be looked at from the perspective of cue combination – the brain must take two cues, and

Figure 2.11: Top: Cornsweet illusion in the luminance domain.

Both sides have identical luminance. Image reproduced

with permission, (Fibonacci, 2005).

Bottom: solid line: cross-section of the Cornsweet profile. Dashed

line: representation of the viewer’s perception.

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combine them in an optimal way. The exact way in which cues are combined is often complex, and relies on estimates of how reliable each of the two cues are (Hillis, Watt, Landy, & Banks, 2004; Knill & Saunders, 2003). However, a simplified view is that the less reliable a cue, the less weight the visual system places on this cue, resulting in the

perception of the object being closer the characteristics of the reliable cue. The estimates of reliability are typically a mix of a judgement of the noise present in the stimulus, and long term judgements of cue reliability (Backus et al., 1999; Gillam, 1968; Knill, 2007; Young, Landy, & Maloney, 1993). If shape from shading is typically judged to be a less reliable cue than disparity, then the depth from disparity would be dominant – as has been observed. However, in this thesis we primarily concentrate on the effects of binocular depth

perception and camouflage, so we return to examining the literature on interaction of binocular disparity between different elements.

In the 1980s Mitchison and Mckee did a series of experiments (Mitchison & McKee, 1987a, 1987b) on lines of regularly spaced points and found a the perception of depth in the central dots was dictated by the depth of the dots at the ends of the lines. Under short presentation times the entire line of dots was perceived as having the same disparity as the end points; under longer presentations the depth of the dots in the centre of the line were influenced but not dictated by the disparity of dots at the ends. This goes against the predictions of a point by point depth representation, which would predict the exact estimation of the depth of the central points regardless of the depth of the points at the ends of the lines. Instead, we have a grouping effect, where the perception of entire line is affected by the depth of some of the points. Under short presentations times, the grouping of the points into all having the same depth bears a striking resemblance to the theories of Gestalt grouping, which may provide us with some insight into the origins of these

perceptual biases.

Gestalt grouping concerns the collection of elements into objects – an area where disparity maps seem to have considerable trouble. The Gestalt principles of perceptual grouping state that when a set of elements are grouped into single perceived object or collection, they follow certain principles. These ideas were first laid out by Max Wertheimer in 1923 who identified five main aspects listed below (Wertheimer, 1923), each of which we have accompanied with a companion diagram in Figure 2.12.

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1. Proximity. Elements located close to each other are grouped together. 2. Similarity. Elements that are similar to each other are grouped together.

3. Good continuity. Elements that are arranged to follow established lines or curves are preferentially grouped over sharp changes.

4. Closure. Elements are preferentially grouped to form closed objects as opposed to ones with gaps or holes.

5. Common fate. Elements that move together are grouped.

1. Proximity 2. Similarity 3. Continuity 4. Closure 5. Common Fate

Dots in proximity group

into vertical lines.

Similar dots are grouped into vertical lines. Dots are grouped into a continuous line and arc. Dots are perceived as a closed ring even

when occluded.

Dots that move together are

grouped.

Figure 2.12: Using dots to demonstrate the five main principles of Gestalt Grouping

Several other principles have been added since 1923, for example: symmetry,

connectedness (two elements that are connected by a third element) and common regions (elements contained within a common area, e.g. a box) (Wagemans, 2015). Additionally, there appear to be top-down influences in grouping (Beck & Palmer, 2002). As a whole, grouping and subsequent processing on an object level may explain some of the disparity effects discussed here.

Deas and Wilcox (2014) performed a very interesting study on the effect of grouping and depth perception. Inspired by papers that found horizontal lines or

intermediate dots changed the perception of the depth of vertical lines (Fahle & Westheimer, 1988; Mitchison & Westheimer, 1984), they studied the interaction of disparity defined lines and perceptual grouping – participants were asked to judge the depth (defined by disparity) between two vertical lines. The lines were either displayed individually as in Figure 2.13a, and thus not grouped; or displayed with horizontal lines between them as in Figure 2.13b, thus grouping them via closure.

Deas and Wilcox found that less depth was perceived when the lines were grouped together – a completely unexpected result, as the addition of horizontal lines does not alter the disparity information present in the vertical lines. This result strongly suggests that

disparities are further processed after the elements have been grouped together to from a single object. In a follow-up paper, Deas and Wilcox found objects formed of sets of strongly

a b

Figure 2.13: Two vertical lines displayed individually (a) are perceived with more depth than when the lines are grouped with

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grouped elements were faster to detect than objects formed of poorly grouped elements. This was the case despite a decrease in perceived depth in the object (Deas & Wilcox, 2015). Pilzo et al (Pizlo, Li, & Francis, 2005a) also found a similar result: that perceptual grouping has a marked effect on the perception of depth in an object.

Additional processing after early disparity extraction is further indicated by the interaction of depth from binocular disparity and luminance. As discussed earlier, there are strong analogies between several luminance effects and disparity processing effects. This

potentially implies that the same mechanisms are being used for processing both disparity and luminance. A few rare studies have even found interactions between luminance and disparity, with luminance edges having been found to effect the perceived depth and depth thresholds (Burge, Peterson, & Palmer, 2005; Didyk, Ritschel, Eisemann, Myszkowski, Seidel, et al., 2012; Peterson & Gibson, 1993). Interactions are not limited to luminance and

disparity: depth from motion also affects disparity defined depth (Seymour & Clifford, 2012). If early disparity extraction was the be-all and end- all of disparity processing, then effects such as these would be very unexpected.

These studies give a strong case against a point by point representation of disparity in the visual field being the final stage of disparity processing. We have a variety of different cases in which a simplistic point by point disparity extraction should cause veridical perception. When looked at from the angle of Gestalt grouping, it appears that an additional stage of object processing is dominant over the early levels of disparity extraction. We hypothesize that these biases in perceived depth stem from a mechanism dedicated to the identification of objects. In the first strand of this thesis, we investigate in depth the perception of an object defined solely by disparity to understand how disparity defined objects are

perceived, and if poor grouping of the object may cause difficulties in the perception of the object.