A theory of phenomenal geometry and its applications

The geometry of perceived space (phenomenal geometry) is specified in terms of three basic factors: the perception of direction, the perception of distance or depth, and the perception of the observer's own position or motion. The apparent spatial locations of stimulus points resulting from these three factors thereupon determine the derived perceptions of size, orientation, shape, and motion. Phenomenal geometry is expected to apply to both veridical and illusory perceptions. It is applied here to explain a number of representative illusions, including the illusory rotation of an inverted mask (Gregory, 1970), a trapezoidal window (Ames, 1952), and any single or multiple point stimuli in which errors in one or more of the three basic factors are present. It is concluded from phenomenal geometry that the size-distance and motion-distance invariance hypotheses are special cases of the head motion paradigm, and that proposed explanations in terms of compensation, expectation, or logical processes often are unnecessary for predicting responses to single or multiple stimuli involving head or stimulus motion. Two hypotheses are identified in applying phenomenal geometry. It is assumed that the perceptual localization of stimulus points determines the same derived perceptions, regardless of the source of perceptual information supporting the localizations. This assumption of cue equivalence or cue substitution provides considerable parsimony to the geometry. Also, it is assumed that the perceptions specified by the geometry are internally consistent. Departures from this internal consistency, such as those which occur in the size-distance paradox, are considered to often reflect the intrusion of nonperceptual (cognitive) processes into the responses. Some theoretical implications of this analysis of phenomenal geometry are discussed.     

The Ontology of Perception: Bipolarity and Content

The notion of perceptual content is commonly introduced in the analysis of perception. It stems from an analogy between perception and propositional attitudes. Both kinds of mental states, it is thought, have conditions of satisfaction. I try to show that on the most plausible account of perceptual content, it does not determine the conditions under which perceptual experience is veridical. Moreover, perceptual content must be bipolar (capable of being correct and capable of being incorrect), whereas perception as a mental state is not (if it is veridical, it is essentially so). This has profound consequences for the epistemological view that perception is a source of knowledge. I sktech a two-level epistemology which is consistent with this view. I conclude that the analogy between perception and propositional attitudes, from which the notion of perceptual content is born, may be more misleading than it is usually thought. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of the perception of motion concomitant with a lateral motion of the head

This study is concerned with two questions regarding the illusory motion of objects that occurs concomitantly with motion of the head. One is whether this illusory concomitant motion, unlike the perception of real motion, is paradoxical in the sense that, although the object appears to move, it does not appear to go anywhere. The second question is whether illusory concomitant motion can be explained by errors in convergence produced by a tendency for the convergence of the eyes to displace in the direction of the resting state of convergence. Both questions receive a negative answer. In Experiment 1, it is shown that the illusory motion perceptually can add to or subtract from apparent motion resulting from real motion. In Experiment 2, it is shown that, for a binocularly viewed object at a near distance, the error in convergence (fixation disparity) is far too small to be an explanation for the illusory object motion associated with a moving head. The results of both experiments support an interpretation of illusory concomitant motion in terms of errors in the apparent distance of the stimulus object and the veridical perception of its direction.     

Why don’t synaesthetic colours adapt away?

Synaesthetes persistently perceive certain stimuli as systematically accompanied by illusory colours, even though they know those colours to be illusory. This appears to contrast with cases where a subject??s colour vision adapts to systematic distortions caused by wearing coloured goggles. Given that each case involves longstanding systematic distortion of colour perception that the subjects recognize as such, how can a theory of colour perception explain the fact that perceptual adaptation occurs in one case but not the other? I argue that these cases and the relationship between them can be made sense of in light of an existing view of colour perception. Understanding colours as ways in which objects and surfaces modify light, perceived through grasping patterns and variations in colour appearances, provides a framework from which the cases and their apparent disanalogy can be predicted and explained. This theory??s ability to accommodate these cases constitutes further empirical evidence in its favour.     

On the Reality of Illusory Conjunctions

The reality of illusory conjunctions in perception has been sometimes questioned, arguing that they can be explained by other mechanisms. Most relevant experiments are based on migrations along the space dimension. But the low rate of illusory conjunctions along space can easily hide them among other types of errors. As migrations over time are a more frequent phenomenon, illusory conjunctions can be disentangled from other errors. We report an experiment in which series of colored letters were presented in several spatial locations, allowing for migrations over both space and time. The distribution of frequencies were fit by several multinomial tree models based on alternative hypothesis about illusory conjunctions and the potential sources of free-floating features. The best-fit model acknowledges that most illusory conjunctions are migrations in the time domain. Migrations in space are probably present, but the rate is very low. Other conjunction errors, as those produced by guessing or miscategorizations of the to-be-reported feature, are also present in the experiment. The main conclusion is that illusory conjunctions do exist.     

Are children with autistic spectrum disorders susceptible to contour illusions?

Children with autism have been shown to be less susceptible to Kanisza type contour illusions than children without autism ( Happé, 1996 ). Other authors have suggested that this finding could be explained by the fact that participants with autism were required to make a potentially ambiguous verbal response which may have masked whether or not they actually perceived the illusory contours ( Ropar & Mitchell, 1999 ). The present study tested perception of illusory contours in children with autism using a paradigm that requires participants to make a forced choice about the dimensions of a shape defined by illusory contours. It was reasoned that accuracy of the participant on this task would indicate whether or not children with autism could perceive illusory contours. A total of 18 children with autistic spectrum disorder, 16 children with special educational needs not including autism and 20 typically developing children completed an experimental task which assessed perception of Kanisza‐style rectangles defined by illusory contours. There were no significant differences between the performance of the children with autism and either of the two control groups, suggesting that perception of illusory contours is intact in autism.     

In search of perceptual influences of sentence context on word recognition

A major issue in the study of word perception concerns the nature (perceptual or nonperceptual) of sentence context effects. The authors compared effects of legal, word replacement, nonword replacement, and transposed contexts on target word performance using the Reicher-Wheeler task to suppress nonperceptual influences of contextual and lexical constraint. Experiment 1 showed superior target word performance for legal (e.g., "it began to flap/flop") over all other contexts and for transposed over word replacement and nonword replacement contexts. Experiment 2 replicated these findings with higher constraint contexts (e.g., "the cellar is dark/dank") and Experiment 3 showed that strong constraint contexts improved performance for congruent (e.g., "born to be wild") but not incongruent (e.g., mild) target words. These findings support the view that the very perception of words can be enhanced when words are presented in legal sentence contexts.     

Holes in illusory conjunctions

To address the fundamental question “What are primitives of visual perception?” we tested the hypothesis of early topological perception employing illusory conjunctions. Through a series of experiments, which were well controlled for various feature errors, we consistently observed the illusory conjunction of holes, which are a typical kind of topological property. Subjects in our experiments perceived illusory hollow figures in which the conjoined holes underwent geometric transformations. This indicates that the holes were perceived as abstract topological entities available at an early stage, which provides further support for the hypothesis of topological perception.     

Locating and Representing Pain

Two views on the nature and location of pain are usually contrasted. According to the first, experientialism, pain is essentially an experience, and its bodily location is illusory. According to the second, perceptualism or representationalism, pain is a perceptual or representational state, and its location is to be traced to the part of the body in which pain is felt. Against this second view, the cases of phantom, referred and chronic pain have been marshalled: all these cases apparently show that one can be in pain while not having anything wrong in her body. Pain bodily location, then, would be illusory. I this paper I shall defend the representational thesis by presenting an argument against experientialism while conceding that the appearance/reality distinction collapses. A crucial role in such identification is played by deictics. In reporting that we feel pain here, the deictic directly refers to the bodily part as coinciding with the part as represented. So, pain location is not illusory. The upshot is that the body location is part and parcel of the representational content of pain states, a representation build up from the body map.     

Surface perception and the generic view principle

The hypothesis that perceptual experience can be understood in terms of rule-based processing has strongly influenced recent theories of visual surface perception. However, many of the rules that these theories propose apply only in relatively restricted situations. I suggest that more general and robust principles for reducing perceptual ambiguity are available, such as the generic view principle (GVP) described here. According to the GVP, vision assumes that qualitative (e.g. topological) image structure is stable with respect to small changes of viewpoint. Some consequences of the GVP for visual surfaces, including illusory surfaces, are described. I also demonstrate the decisive role of real and illusory background surfaces in specifying the 3-D shape and layout of visual objects and scenes.     

Appearance and reality

There is tension between the appealing idea that visual experience puts us in touch with objective reality and the doctrine of perceptual relativity, the claim that experience varies with such factors as distance, lighting, and angle of view. In this paper I present arguments for relativity, and then go on to propose that in view of relativity, we should believe that the properties presented to us in perception are relational and viewpoint‐dependent. I conclude by sketching a theory of how we nonetheless manage to achieve contact with objective properties. According to the theory, such contact is best explained in terms of perceptual representations acting in concert with cognitive and motoric representations, not in terms of perceptual representations alone.     

Absence of compensation and reasoning-like processes in the perception of orientation in depth.

When errors are present in the perceived depth between the parts of a physically stationary object, the object appears to rotate as the head is moved laterally (Gogel, 1980). This illusory rotation has been attributed either to compensation (Wallach, 1985, 1987) or to inferential-like processes (Rock, 1983). Alternatively, the perceived distances of and directions to the parts of the object are sufficient to explain the illusory perceived orientations and perceived rotations of the stimulus. This was examined in three experiments. In Experiment 1, a perceived illusory orientation of a stimulus object extended in depth was produced by misleading binocular disparity and was measured at two different lateral positions of the head under two conditions. In the static condition, the head was stationary at different times at each of the two measurement positions of the head. In the dynamic condition, continuous motion of the head occurred between these two positions. In Experiment 2, static and dynamic conditions of illusory stimulus orientation were observed with the head stationary. In Experiment 3, a perspective illusion instead of binocular disparity produced the errors in perceived depth. In no experiment did the perceived orientation of the object differ for the static and dynamic conditions. In the absence of head motion, neither compensatory nor inferential-like processes were available. It is concluded that these processes are not needed to explain either illusory or nonillusory perceptions of the orientation or rotation of stimuli viewed with a laterally moving head.     

Absence of compensation and reasoning-like processes in the perception of orientation in depth

When errors are present in the perceived depth between the parts of a physically stationary object, the object appears to rotate as the head is moved laterally (Gogel, 1980). This illusory rotation has been attributed either to compensation (Wallach, 1985, 1987) or to inferential-like processes (Rock, 1983). Alternatively, the perceived distances of and directions to the parts of the object are sufficient to explain the illusory perceived orientations and perceived rotations of the stimulus. This was examined in three experiments. In Experiment 1, a perceived illusory orientation of a stimulus object extended in depth was produced by misleading binocular disparity and was measured at two different lateral positions of the head under two conditions. In the static condition, the head was stationary at different times at each of the two measurement positions of the head. In the dynamic condition, continuous motion of the head occurred between these two positions. In Experiment 2, static and dynamic conditions of illusory stimulus orientation were observed with the head stationary. In Experiment 3, a perspective illusion instead of binocular disparity produced the errors in perceived depth. In no experiment did the perceived orientation of the object differ for the static and dynamic conditions. In the absence of head motion, neither compensatory nor inferential-like processes were available. It is concluded that these processes are not needed to explain either illusory or nonillusory perceptions of the orientation or rotation of stimuli viewed with a laterally moving head.     

Restrictions on Representationalism

Strong representationalism claims that the qualitative character of our phenomenal mental states consists in the intentional content of such states. Although strong representationalism has greatly increased in popularity over the last decade, I find the view deeply implausible. In this paper, I attempt to argue against strong representationalism by a two-step argument. First, I suggest that strong representationalism must be unrestricted in order to serve as an adequate theory of qualia, i.e., it must apply to all qualitative mental states. Second, I present considerations – deriving largely from nonperceptual states – to show that an unrestricted form of strong representationalism is problematic.     

The weight of facts: A puzzle about perception,reasons and deliberation

How should we understand the epistemic role of perception? According to epistemological disjunctivism (ED), a subject’s perceptual knowledge that p is to be explained in terms of the subject believing that p for a factive and reflectively accessible reason. I argue that ED raises far‐reaching questions for rationality and deliberation; I illustrate those questions by setting up a puzzle about belief‐suspension, and I argue that ED does not have the resources to make sense of the rationality of belief‐suspension in cases in which suspending is clearly rational. The conclusion that I draw from the puzzle is mainly negative: the epistemic contribution of perception cannot be explained in terms of a warrant‐conferring relation between perception and belief. However, toward the end, I sketch a positive picture of the epistemic role of perception in terms of a direct explanatory relation between perception and knowledge.     

Late influences on perceptual grouping: illusory figures

Two experiments demonstrate that grouping can be strongly influenced by the presence of figures defined by illusory contours. Rectangular arrays were constructed in which a central column of figures could group either with those on one side, on the basis of perception of figures defined by illusory contours, or with those on the other side, on the basis of physically present inducing elements. In all displays, subjects grouped according to the illusory figures significantly more often than for control displays that contained the same inducing elements, but rearranged so that illusory contours were degraded or eliminated. A second experiment showed that in objectively defined grouping tasks, subjects grouped faster by illusory figures than by inducing elements. These results indicate that grouping can occur after illusory contours have been perceived.     

Do action systems resist visual illusions?

Arguments about the relative independence of visual modules in the primate brain are not new. Recently, though, these debates have resurfaced in the form of arguments about the extent to which visuomotor reaching and grasping systems are insensitive to visual illusions that dramatically bias visual perception. The first wave of studies of illusory effects on perception and action have supported the idea of independence of motor systems, but recent findings have been more critical. In this article, I review several of these studies, most of which (but not all) can be reconciled with the two-visual-systems model.     

When do letter features migrate? A boundary condition for feature-integration theory

Feature-integration theory postulates that a lapse of attention will allow letter features to change position and to recombine as illusory conjunctions (Treisman & Paterson, 1984). To study such errors, we used a set of uppercase letters known to yield illusory conjunctions in each of three tasks. The first, a bar-probe task, showed whole-character mislocations but not errors based on feature migration and recombination. The second, a two-alternative forced-choice detection task, allowed subjects to focus on the presence or absence of subletter features and showed illusory conjunctions based on feature migration and recombination. The third was also a two-alternative forced-choice detection task, but we manipulated the subjects' knowledge of the shape of the stimuli: In the case-certain condition, the stimuli were always in uppercase, but in the case-uncertain condition, the stimuli could appear in either upper- or lowercase. Subjects in the case-certain condition produced illusory conjunctions based on feature recombination, whereas subjects in the case-uncertain condition did not. The results suggest that when subjects can view the stimuli as feature groups, letter features regroup as illusory conjunctions; when subjects encode the stimuli as letters, whole items may be mislocated, but subletter features are not. Thus, illusory conjunctions reflect the subject's processing strategy, rather than the architecture of the visual system.     

Epistemological motivations for anti-realism

Anti-realism is often claimed to be preferable to realism on epistemological grounds: while realists have difficulty explaining how we can ever know claims if we are realists about it, anti-realism faces no analogous problem. This paper focuses on anti-realism about normativity to investigate this alleged advantage to anti-realism in detail. I set up a framework in which a version of anti-realism explains a type of modal reliability that appears to be epistemologically promising, and plausibly explains the appearance of an epistemological advantage to realism. But, I argue, this appearance is illusory, and on closer investigation the anti-realist view does not succeed in explaining the presence of familiar epistemological properties for normative belief like knowledge or the absence of defeat. My conclusion on the basis of this framework is that there is a tension in the anti-realist view between the urge to idealize the conditions in which normative beliefs ground normative facts, and a robust kind of reliability that normative belief can have if the anti-realist resists these idealizations.