Further evidence for two components in visual persistence.

The phenomenal durations of tachistoscopically presented letter arrays at five levels of redundancy were measured using a subtractive reaction-time technique. Phenomenal duration was inversely related to redundancy only when report of the letters was required. The duration of functional availability of stimulus information for stimuli at the five levels of redundancy was determined in a backward masking paradigm. The measurements of the duration of functional availability and phenomenal presence were convergent, indicating that one process is responsible for both the phenomenal and functional duration of a persisting representation of a visual stimulus. The results are interpreted as providing further evidence for two components in visual persistence: a physical component whose duration is unrelated to stimulus parameters and an informational component whose duration is inversely related to the efficiency of encoding stimulus information.     

Apparent movement in tridimensional space

The space-time relationships satisfying Korte’s third law of apparent movement were studied in three experiments. The results generally confirm and extend Corbin’s conclusion that apparent movement is not a projection-level phenomenon, but one involving spatial constancy, i.e., some representation of tridimensional space. Between situations in which the stimuli were retinally matched, the time interval necessary, for apparent movement varied w~th their phenomenal separation, whether the latter was determined by the distance of stimuli from the O, deviation of sttmuli from the frontal plane, or illusory slant of the stimulus plane produced by a perspective gradient.     

Strong phenomenal wholes are associated with fast “same” and slow “different” responses and superior overall performance

“Same” and “different” responses to stimuli that result in strong and weak single phenomenal wholes were investigated. Present and also previous results indicate that strong phenomenal wholes tend to be associated with a fast “same” and slow “different” response pattern. This result suggests that the parts of strong phenomenal wholes are perceived as more similar (e.g., because it should be easier to respond “same” to more similar stimuli). This suggestion is consistent with results based on other measures of perceived similarity. Present and also previous results additionally indicate that strong phenomenal wholes are frequently associated with superior overall (“same” plus “different”) performance. This result is consistent with evidence that the identification of simultaneously occurring multiple targets improves when they belong to the same phenomenal whole. The present results also hint that attention can affect whether two or three objects form the effective (functional) whole and support the assimilation explanation of the Müller Lyer illusion.     

PERCEPTION AND THE REACH OF PHENOMENAL CONTENT

The phenomenal character of perceptual experience involves the representation of colour, shape and motion. Does it also involve the representation of high-level categories? Is the recognition of a tomato as a tomato contained within perceptual phenomenality? Proponents of a conservative view of the reach of phenomenal content say 'No', whereas those who take a liberal view of perceptual phenomenality say 'Yes'. I clarify the debate between conservatives and liberals, and argue in favour of the liberal view that high-level content can directly inform the phenomenal character of perception.     

Relationship between visual binding, reentry and awareness

Visual feature binding has been suggested to depend on reentrant processing. We addressed the relationship between binding, reentry, and visual awareness by asking the participants to discriminate the color and orientation of a colored bar (presented either alone or simultaneously with a white distractor bar) and to report their phenomenal awareness of the target features. The success of reentry was manipulated with object substitution masking and backward masking. The results showed that late reentrant processes are necessary for successful binding but not for phenomenal awareness of the bound features. Binding errors were accompanied by phenomenal awareness of the misbound feature conjunctions, demonstrating that they were experienced as real properties of the stimuli (i.e., illusory conjunctions). Our results suggest that early preattentive binding and local recurrent processing enable features to reach phenomenal awareness, while later attention-related reentrant iterations modulate the way in which the features are bound and experienced in awareness.     

The feeling of fluent perception: a single experience from multiple asynchronous sources

Zeki and co-workers recently proposed that perception can best be described as locally distributed, asynchronous processes that each create a kind of microconsciousness, which condense into an experienced percept. The present article is aimed at extending this theory to metacognitive feelings. We present evidence that perceptual fluency-the subjective feeling of ease during perceptual processing-is based on speed of processing at different stages of the perceptual process. Specifically, detection of briefly presented stimuli was influenced by figure-ground contrast, but not by symmetry (Experiment 1) or the font (Experiment 2) of the stimuli. Conversely, discrimination of these stimuli was influenced by whether they were symmetric (Experiment 1) and by the font they were presented in (Experiment 2), but not by figure-ground contrast. Both tasks however were related with the subjective experience of fluency (Experiments 1 and 2). We conclude that subjective fluency is the conscious phenomenal correlate of different processing stages in visual perception.     

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.     

Perceived shifts of flashed stimuli by visible and invisible object motion

Perceived positions of flashed stimuli can be altered by motion signals in the visual field-position capture (Whitney and Cavanagh, 2000 Nature Neuroscience 3 954-959). We examined whether position capture of flashed stimuli depends on the spatial relationship between moving and flashed stimuli, and whether the phenomenal permanence of a moving object behind an occluding surface (tunnel effect; Michotte 1950 Acta Psychologica 7 293-322) can produce position capture. Observers saw two objects (circles) moving vertically in opposite directions, one in each visual hemifield. Two horizontal bars were simultaneously flashed at horizontally collinear positions with the fixation point at various timings. When the movement of the object was fully visible, the flashed bar appeared shifted in the motion direction of the circle. But this position-capture effect occurred only when the bar was presented ahead of or on the moving circle. Even when the motion trajectory was covered by an opaque surface and the bar was flashed after complete occlusion of the circle, the position-capture effect was still observed, though the positional asymmetry was less clear. These results show that movements of both visible and 'hidden' objects can modulate the perception of positions of flashed stimuli and suggest that a high-level representation of 'objects in motion' plays an important role in the position-capture effect.     

Novel Colours and the Content of Experience

Abstract: I propose a counterexample to naturalistic representational theories of phenomenal character. The counterexample is generated by experiences of novel colours reported by Crane and Piantanida. I consider various replies that a representationalist might make, including whether novel colours could be possible colours of objects and whether one can account for novel colours as one would account for binary colours or colour mixtures. I argue that none of these strategies is successful and therefore that one cannot fully explain the nature of the phenomenal character of perceptual experiences using a naturalistic conception of representation.     

Two types of phenomenal instructions for size and distance judgments of objects presented on a two-dimensional plane

Demand characteristics were studied in a simple situation in which observers made size and distance estimates of stimuli presented on a two-dimensional linear perspective drawing. Half the Ss were asked questions stressing phenomenal report: The other half were asked the same questions preceded by instructions stressing the objective nature of the stimulus configuration but requesting, nonetheless, phenomenal report. Instructions resulted in a significantly greater perspective size illusion but did not affect the distance estimates. No meaningful correlations between size and distance estimates were found.     

The effects of familiarity in a perceptual matching task

Latencies of same-different judgments to pairs of two-digit numerals were recorded for stimuli presented in familiar or unfamiliar (inverted) orientation. Familiar stimuli were responded to more quickly than unfamiliar. For both stimulus types, latencies were correlated with the syllable length of the verbal representation of the numerals, allowing the interpretation that the effect of stimulus orientation is on encoding processes. In two other experiments, it was found that familiarity had no effect on different judgments when the stimuli were relatively simple (e.g., a single digit), but did affect different judgments with more complex stimuli. These results were related to the hypothesis that the complexity of verbal material determines whether different judgments are instigated by visual or by verbal representations of the stimuli.     

Local and global processing: The role of sparsity

It has recently been proposed that global processing precedes local processing of a visual scene even when the local and the global aspects are similar in nature (e.g., both alphabetic). The two types of processing were compared here in four different ways, for stimuli with many and with few local elements (i.e., differing sparsities). These methods consisted of assessing naming latency, intrastimulus Stroop-like interference, intermodality Stroop-like interference, and phenomenal judgment. The results of four experiments were consistent in demonstrating global processing priority for many-lement stimuli but local processing priority for few-element stimuli.     

Influence of the instructions on the performance and establishment of memorization strategies in space judgments

Studies of visual space perception have been assuming that people have an internal representation of the physical space that surrounds them. A variety of psychophysical procedures has been used in an attempt to measure the properties of visual space. The goal of the present study was to evaluate the accuracy of the mental representation and the strategies adopted to acquire and retain visuo-spatial information of a configuration as a function of two types of instructions. Thirty-eight undergraduate and graduate students participated in the study and were distributed in perceptive and mnemonic experimental conditions. The effect of the instructions (intentional and incidental) on the representation of the distances among the objects of the scene was estimated using exponents of power function, based on the reproduction of the distances among the stimuli of the scene. The results revealed that judgments made under intentional instructions were more frequently based on strategies related to the location of the stimuli, whereas judgments originating from incidental instructions were based on strategies related to the name of the stimuli. It was observed that the intentional instruction facilitated a more accurate mental representation of the observed experimental configuration, enhancing participants' performance.     

Subliminal Gestalt grouping: Evidence of perceptual grouping by proximity and similarity in absence of conscious perception

Previous studies making use of indirect processing measures have shown that perceptual grouping can occur outside the focus of attention. However, no previous study has examined the possibility of subliminal processing of perceptual grouping. The present work steps forward in the study of perceptual organization, reporting direct evidence of subliminal processing of Gestalt patterns. In two masked priming experiments, Gestalt patterns grouped by proximity or similarity that induced either a horizontal or vertical global orientation of the stimuli were presented as masked primes and followed by visible targets that could be congruent or incongruent with the orientation of the primes. The results showed a reliable priming effect in the complete absence of prime awareness for both proximity and similarity grouping principles. These findings suggest that a phenomenal report of the Gestalt pattern is not mandatory to observe an effect on the response based on the global properties of Gestalt stimuli.     

Interference and Facilitation in Spoken Word Production: Effects of Morphologically and Semantically Related Context Stimuli on Picture Naming

We report two picture–word interference experiments investigating conceptual and lexical activation, and response selection, in speaking. We varied stimulus onset asynchrony to investigate potential fine-grained activation and competition effects. Morphologically related existing and pseudoword adjectives, as well as associatively related adjectives, served as context stimuli in Experiment 1. In Experiment 2, we focused on semantic interference by using morphologically related and unrelated subordinates of the target concept as context stimuli. Morphologically complex pseudowords were also included as context stimuli. Pseudowords should not interfere, given that they have no lexical or conceptual representation. We consistently obtained facilitation with all morphologically related context stimuli, irrespective of their lexical status. We argue that effects originate at the word-form level, and discuss how our results may help decide among the many explanations of semantic interference in picture naming.     

How could phenomenal consciousness be involved in mental function?

In a phylogenetic perspective, the phenomenal and the functional aspects of consciousness cannot be separated because consciousness, as a phenomenal experience, must be causally effective. The hypothesis I propose is that the fundamental property of consciousness consists of a self-organizing process: the differentiation of a content. The differentiation of a content occurs on the basis of the relations internal to a representational whole, which behaves like a field and tends towards a condition of equilibrium. This hypothesis can be somehow considered an extension of Gestalt visual perceptual theory. Unlike neurocomputational processes, which are non-conscious and extrinsic to the representation, conscious processes are intrinsic to the representational whole. Consciousness, as an intrinsically self-organizing process interwoven with its phenomenal aspects, can be more than epiphenomenal and it can be involved in mental function. The paper then discusses the implications of this hypothesis for subjectivity and the explanatory gap.     

Neural representation of consciously imperceptible speech sound differences

The concept of subliminal perception has been a subject of interest and controversy for decades. Of interest in the present investigation was whether a neurophysiologic index of stimulus change could be elicited to speech sound contrasts that were consciously indiscriminable. The stimuli were chosen on the basis of each individual subject's discrimination threshold. The speech stimuli (which varied along an F3 onset frequency continuum from /da/ to /ga/) were synthesized so that the acoustical properties of the stimuli could be tightly controlled. Subthreshold and suprathreshold stimuli were chosen on the basis of behavioral ability demonstrated during psychophysical testing. A significant neural representation of stimulus change, reflected by the mismatch negativity response, was obtained in all but 1 subject in response to subthreshold stimuli. Grand average responses differed significantly from responses obtained in a control condition consisting of physiologic responses elicited by physically identical stimuli. Furthermore, responses to suprathreshold stimuli (close to threshold) did not differ significantly from subthreshold responses with respect to latency, amplitude, or area. These results suggest that neural representation of consciously imperceptible stimulus differences occurs and that this representation occurs at a preattentive level.     

Property Dualism,Epistemic Normativity and the Limits of Naturalism*

This paper examines some consequences of the (quasi‐)epiphenomenalism implied by a property dualistic view of phenomenal consciousness. The focus is upon the variation of phenomenal content over time. A thought‐experiment is constructed to support two claims. The weaker claim exhibits an incompatibility which arises in certain logically possible situations between a conscious subject’s epistemic norms and the requirement that one be aware of one’s conscious experience. This could be interpreted as providing some epistemic grounds for the postulation of bridging laws between the physical/functional and phenomenal domains. The stronger claim has it that the ontology of property dualism is not properly able to account for the certainty I have of being phenomenally conscious. The problem is viewed as resulting from the neglect of the intensional context involved in a proper representation of the argument for property dualism. It is argued that only a transcendental move can do justice to this certainty I have.     

The Continuum of Consciousness

Research in a wide variety of domains provides converging evidence for the psychological unconscious—percepts, memories, and other mental contents that influence experience, thought, and action outside of phenomenal awareness. Studies of preconscious processing indicate that two continua underlie conscious experience—one having to do with the quality of the stimulus event or its mental representation, and the other having to do with the cognitive resources brought to hear on the processing of that representation. However, evidence of subconscious processing violates these conclusions and suggests that something more is involved—perhaps a link between mental representations of events and of the self as the agent or experiencer of them.