Online research in philosophy

Theories of human vision have generally assumed that the features underlying visual search and texture segmentation correspond to simple measurements made at the first stages of visual processing. In this paper, we describe a series of visual search experiments that refute this assumption. Using several variants of the Mueller-Lyer figure, we show that an illusion of length exists in preattentive vision -- search is easy when items contain line segments of equal length, but becomes difficult when these segments are adjusted to have the same apparent length. This illusion cannot be reduced by selective inhibition of features, such as that used to facilitate the rapid detection of feature conjunctions. For example, subjects are unable to ignore the wings when making judgements of the test line, even when it is advantageous to do so. This rules out explanations based on interactions among the features themselves. We also show that spatial filtering cannot account for this illusion, since these effects are indifferent to the sign of contrast of the line segments and can occur for textured lines having the same first-order statistics. The illusion, however, can be explained by a model in which line length is determined via grouping operations acting at a level prior to the formation of preattentive features.

Previous theories of early vision have assumed that visual search is based on simple two-dimensional aspects of an image, such as the orientation of edges and lines. It is shown here that search can also be based on three-dimensional orientation of objects in the corresponding scene, provided that these objects are simple convex blocks. Direct comparison shows that image-based and scene-based orientation are similar in their ability to facilitate search. These findings support the hypothesis that scene-based properties are represented at preattentive levels in early vision.

Berkeley thinks that we only see the size, shape, location, and orientation of objects in virtue of the correlation between sight and touch. Shadows have all of these spatial properties and yet are intangible. In Seeing Dark Things (2008), Roy Sorensen argues that shadows provide a counterexample to Berkeley's theory of vision and, consequently, to his idealism. This paper shows that Berkeley can accept both that shadows are intangible and that they have spatial properties.

We describe an update to our visual search software for the Macintosh line of computers. The new software, VSearch Color, gives users access to the full-color capabilities of the Macintosh II line. One of the key features of the new software is its ability to treat graphics information separately from color information. This makes it easy to study color independently of form, to design experiments based on isoluminant stimuli, and to incorporate texture segregation, visual identification, number discrimination, adaptation, masking, and spatial cuing into the basic visual search paradigm.

I discuss a solution to the Yale shadow puzzle, due to Roy Sorensen, based on the actual process theory of causation, and argue that it does not work in the case of a new version of the puzzle, which I call "the Bilkent shadow puzzle". I offer a picture of the ontology of shadows that constitute the basis for a new solution that uniformly applies to both puzzles.

The eclipse riddle -- Seeing surfaces -- The disappearing act -- Spinning shadows -- Berkeley's shadow -- Para-reflections -- Para-refractions : shadowgrams and the black drop -- Goethe's colored shadows -- Filtows -- Holes in the light -- Black and blue -- Seeing in black and white -- We see in the dark -- Hearing silence.

2. Invariant Sensorimotor Features ("Affordances"). To say this is not to declare oneself a Gibsonian, whatever that means. It is merely to point out that what a sensorimotor system can do is determined by what can be extracted from its motor interactions with its sensory input. If you lack sonar sensors, then your sensorimotor system cannot do what a bat's can do, at least not without the help of instruments. Light stimulation affords color vision for those of us with the right sensory apparatus, but not for those of us who are color-blind. The geometric fact that, when we move, the "shadows" cast on our retina by nearby objects move faster than the shadows of further objects means that, for those of us with normal vision, our visual input affords depth perception. From more complicated facts of projective and solid geometry it follows that a 3-dimensional shape, such as, say, a boomerang, can be recognized as being the same shape Ð and the same size Ð even though the size and shape of its shadow on our retinas changes as we move in relation to it or it moves in relation to us. Its shape is said to be invariant under these sensorimotor transformations, and our visual systems can detect and extract that invariance, and translate it into a visual constancy. So we keep seeing a boomerang of the same shape and size even though the shape and size of its retinal shadows keep changing.

It has generally been assumed that parallel visual search can only be based on the presence of simple features -- the spatial relations between features do not influence this process. We describe a series of visual search experiments that contradict this assumption. Search for line drawings of opaque polyhedra is greatly influenced by some line relations. In particular, search is rapid for line drawings (i) that have arrow- and Y-junctions corresponding to corners formed from orthogonal surfaces, and (ii) that do..

The Yale shadow puzzle1 purports to involve the ultimate incompatibility of the following plausible looking principles: (1) If X casts any shadow, then some light is falling directly on X. (2) Shadows cannot be cast through an opaque object. (3) Every shadow is the shadow of something. Imagine a high wall casting a shadow on the ground on a sunny day, and a little bird standing on the ground within the shadow cast by the wall. Is the shadow that is present on the ground under the bird –let’s mark it with S- the shadow of the bird, or (part of) the shadow of the wall? It cannot be the bird’s, by proposition (1), as the bird does not block any light. Neither can it be the wall’s, by proposition (2), as the wall cannot cast its shadow through the opaque bird. Yet, by proposition (3), it has to be the shadow of something. Roy Sorensen has offered neat looking solution to the puzzle; however, part of my goal in this paper is to show that this solutions doesn’t work for a new version of the Yale shadow puzzle, which I call “The Bilkent shadow puzzle”, in acknowledgment of my own academic affiliation. The remaining issue will be to offer a new solution that would cover both puzzles. As it will turn out, the solution is based on a peculiar understanding of the ontology of shadows.

The task of visual search is to determine as rapidly as possible whether a target item is present or absent in a display. Rapidly detected items are thought to contain features that correspond to primitive elements in the human visual system. In previous theories, it has been assumed that visual search is based on simple two-dimensional features in the image. However, visual search also has access to another level of representation, one that describes properties in the corresponding three-dimensional scene. Among these properties are three dimensionality and the direction of lighting, but not viewing direction. These findings imply that the parallel processes of early vision are much more sophisticated than previously assumed.