Linking dynamical perceptual decisions at different levels of description in motion pattern formation: psychophysics

The relationship between local-level motion detection and higher level pattern-forming mechanisms was investigated with the motion quartet, a bistable stimulus for which either horizontal or vertical motion patterns are perceived. Local-level perturbations in luminance contrast affected the stability of the perceived patterns and, thereby, the size of the pattern-level hysteresis obtained by gradually changing the motion quartet's aspect ratio. Briefly eliminating luminance contrast (so nonmotion was perceived during the perturbation) eliminated pattern-level hysteresis, and briefly increasing luminance contrast (so motion was perceived during the perturbation) increased pattern-level hysteresis. Partially reducing luminance contrast resulted in bistability during the perturbation; pattern-level hysteresis was maintained when motion was perceived, and eliminated when nonmotion was perceived. The results were attributed to local motion/nonmotion perceptual decisions in area V1 affecting the magnitude of the activation feeding forward to motion detectors in area MT, where the stability of pattern-level perceptual decisions is determined by activation-dependent, future-shaping interactions that inhibit soon-to-be-stimulated detectors responsive to competing motion directions.     

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Sch?ner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.     

The effect of perceptual load on attention-induced motion blindness: the efficiency of selective inhibition

Recent visual marking studies have shown that the carry-over of distractor inhibition can impair the ability of singletons to capture attention if the singleton and distractors share features. The current study extends this finding to first-order motion targets and distractors, clearly separated in time by a visual cue (the letter X). Target motion discrimination was significantly impaired, a result attributed to the carry-over of distractor inhibition. Increasing the difficulty of cue detection increased the motion target impairment, as distractor inhibition is thought to increase under demanding (high load) conditions in order to maximize selection efficiency. The apparent conflict with studies reporting reduced distractor inhibition under high load conditions was resolved by distinguishing between the effects of "cognitive" and "perceptual" load.     

The first pictures: perceptual foundations of Paleolithic art.

Paleolithic representational art has a number of consistent characteristics: the subjects are almost always animals, depicted without scenic background, usually in profile, and mostly in outline; the means of representation are extremely economical, often consisting of only a few strokes that indicate the salient features of the animal which are sufficient to suggest the whole form; and it is naturalistic to a degree, but lacks anything like photographic realism. Two elementary questions are raised in this essay: (i) why did the earliest known attempts at depiction have just these characteristics and not others? and (ii) how are objects so minimally represented recognizable? The answers seem to lie with certain fundamental features of visual perception, especially figure-ground distinction, Gestalt principles of closure and good continuation, line surrogacy, component feature analysis, and canonical imaging. In the earliest pictures the graphic means used are such that they evoke the same visual responses as those involved in the perception of real-world forms, but eschew redundancies of color, texture, linear perspective, and completeness of representation.     

Representational momentum,internalized dynamics,and perceptual adaptation

In this paper I look at dynamic mental representations, motion detection under conditions of certainty or uncertainty, perceptual adaptation, and priming of motion direction. The goal is to bridge the boundaries created in part by the use of different terminology within different literatures. The most fruitful parallel may be between the phenomenon of dynamic mental representation and representational momentum on the one hand, and perceptual adaptation as revealed by motion priming on the other. I suggest an overlap between the two phenomena.     

Evidence for a common motion mechanism of luminance-modulated and contrast-modulated patterns: selective adaptation.

Selective adaptation effects were measured with contrast-modulated patterns and sine-wave gratings in order to determine the extent to which the two patterns are processed by common mechanisms. Direction-specific adaptation effects were measured for a contrast-modulated adapting pattern and a test pattern. The contrast-modulated adapting pattern was composed of a sine-wave grating of 8 cycles deg-1 whose contrast was spatially modulated by a sinusoid of 1 cycle deg-1 at one of four levels: 100%, 60%, 30%, or 0%. The results showed that contrast-modulation thresholds for contrast-modulated gratings were raised by 0.3 to 0.5 log units following adaptation to a contrast-modulated grating moving in the same direction as the test pattern, relative to thresholds obtained following adaptation to a contrast-modulated grafting moving in the opposite direction. Cross-adaptation effects were also measured with a sine-wave adapting pattern and a contrast-modulated test pattern. The sine-wave adapting pattern was a sine-wave grating of 1 cycle deg-1 whose contrast was set to one of three levels: 16.4%, 1.25%, or 0%. The contrast-modulated test pattern was a sine-wave grating of 8 cycles deg-1 whose contrast was modulated by a sinusoid of 1 cycle deg-1. The results revealed that contrast-modulation thresholds for contrast-modulated gratings were raised by approximately 0.25 log units following adaptation to moving sine-wave gratings, relative to thresholds obtained following adaptation to a uniform field. Cross-adaptation effects were also obtained with a contrast-modulated adapting pattern and a sine-wave test pattern. The results support the view that signals generated from luminance-domain stimuli and from contrast-domain stimuli are processed by a common motion mechanism.     

The cognitive foundations of cultural stability and diversity

The existence and diversity of human cultures are made possible by our species-specific cognitive capacities. But how? Do cultures emerge and diverge as a result of the deployment, over generations and in different populations, of general abilities to learn, imitate and communicate? What role if any do domain-specific evolved cognitive abilities play in the emergence and evolution of cultures? These questions have been approached from different vantage points in different disciplines. Here we present a view that is currently developing out of the converging work of developmental psychologists, evolutionary psychologists and cognitive anthropologists.     

The formation of linked perceptual classes

Multiple-exemplar training with stimuli in four domains induced two new fill-based (A1' and A2') and satellite-image-based (B1' and B2') perceptual classes. Conditional discriminations were established between the endpoints of the A1' and B1' classes as well as the A2' and B2' classes. The emergence of linked perceptual classes was evaluated by the performances occasioned by nine cross-class probes that contained fill variants as samples and satellite variants as comparisons, along with nine other cross-class probes that consisted of satellite variants as samples and fill variants as comparisons. The 18 probes were first presented serially and then concurrently. Class-consistent responding indicated the emergence of linked perceptual classes. Of the linked perceptual classes, 70% emerged during the initial serial test. An additional 20% of the linked perceptual classes emerged during the subsequently presented concurrent test block. Thus, linked perceptual classes emerged on an immediate or delayed basis. Linked perceptual classes, then, share structural and fuctional similarities with equivalence classes, generalized equivalence classes, cross-modal classes, and complex maturally occurring categories, and may clarify processes such as intersensory perception.     

The perceptual buildup of three-dimensional structure from motion

We present a set of psychophysical experiments that measure the accuracy of perceived three-dimensional (3-D) structure derived from relative motion in the changing two-dimensional image. The experiments are motivated in part by a computational model proposed by Ullman (1984), called the incremental rigidity scheme, in which an accurate 3-D structure is built up incrementally, by considering images of moving objects over an extended time period. Our main conclusions are: First, the human visual system can derive an accurate model of the relative depths of moving points, even in the presence of noise in their image positions; second, the accuracy of the 3-D model improves with time, eventually reaching a plateau; and third, the 3-D structure currently perceived appears to depend on previous 3-D models. Through computer simulations, we relate the results of our psychophysical experiments with the predictions of Ullman's model.     

Wholistic and analytic stimulus processing: The development of selective perceptual strategies

This experiment reports an investigation of the development of selective processing strategies as subjects become increasingly more practised at a serial self-paced RT task. It demonstrates the effect a preceding stimulus can have on the analysis of a current signal and the development of active analytic strategies in favour of passive wholistic processing, with practice. The stimuli used were letters with irrelevant visual noise dot patterns superimposed on them. The letter, or the dot pattern, or both, could be repeated on successive trials. Early in practice repetition of both stimulus components simultaneously produced short reponse latencies relative to repetition of the letter alone. The number of noise dots markedly affected RT. Late in practice, however, letter repetition RTs were small, irrespective of whether or not the noise dot pattern was repeated. Furthermore, the number of noise dots no longer had an effect on the RT to these stimulus transitions. The results suggest that subjects appear to be able to select relevant information to process as they become progressively more practised, even though there is evidence that they compare the current stimulus with an iconic representation of the immediately preceding one.     

The role of selective attention in perceptual and affective priming

Two kinds of perceptual priming (word identification and word fragment completion), as well as preference priming (that may rely on special affective mechanisms) were examined after participants either read or named the colors of words and nonwords at study. Participants named the colors of words more slowly than the colors of nonwords, indicating that lexical processing of the words occurred at study. Nonetheless, priming on all three tests was lower after color naming than after reading, despite evidence of lexical processing during color naming shown by slower responses to words than to nonwords. These results indicate that selective attention to (rather than the mere processing of) letter string identity at study is important for subsequent repetition priming.     

The selective adaptation effects of burst-cued stops

One of the most compelling arguments that selective adaptation affects a phonetic level of processing is the demonstration that adaptation with burst-cued stimuli (which have no formant transitions) affects the perception of transition-cued stimuli (which have formant transitions but no bursts). Experiment 1 showed that adaptation with burst-cued [pi] and [ti] affects the perception of a (transition-cued) [mi-ni] test series, as well as a [bi-di] test series. Inasmuch as nasals never contain bursts, this demonstrates that the adaptation effect of burst-cued stops is not limited to those stimuli which normally contain bursts. If adaptation with burst-cued stops affects the perception of transition-cued stops at a more central, phonetic level of processing, their adaptation effect should transfer interaurally completely. Experiment 2 showed that the adaptation effect of both burst and transition-cued stops transfers interaurally only about one-third. These results suggest that the adaptation effect of burstcued stops is mediated by the fatigue of peripheral auditory detectors which are sensitive to both bursts and formant transitions.     

A computational and perceptual account of motion lines

To indicate motion in a static drawing, artists often include lines trailing a moving object. The use of these motion lines is notable because they do not seem to be related to anything in the optic array. The dynamic behavior of a neural-network model for contour detection is analyzed and it is shown that it generates trails of oriented responses behind moving stimuli. The properties of the oriented response trails are shown to correspond to motion lines. The model generates trails of different orientations depending on the speed and length of the movement, and thereby predicts different uses of two types of motion lines. The model further predicts that motion lines should bias real motion in some situations. An experiment relating motion lines to ambiguous motion percepts demonstrates that motion lines contribute to motion percepts.     

The role of transparency in perceptual grouping and pattern recognition.

The shortest stimulus exposure time for which transparency can be seen was examined. In the first experiment, overlapping digits were presented for 120 ms and the luminance in the overlapping regions was varied. Subjects reported, in separate blocks of trials, either the apparent transparency of the digits or the identity of the digits. When the luminance was set so that one set of digits appeared to be seen through the other, recognition of the digits was high. When the luminance in the overlapping regions did not produce impressions of transparency, digit recognition was low. In the second experiment, digit identification at several stimulus durations was compared between stimuli that had luminance that was valid for transparency and stimuli that had invalid luminance. Performance was found to be higher in the valid luminance condition than in the invalid condition after as little as 60 ms exposure duration. This result suggests that the impression of transparency requires only relatively short exposure durations.     

Different forms of causation in dynamical systems: Determinism,pattern generation,and information

The standpoint that three fundamentally inequivalent classes of system behavior exist, which deserve separate tools for their characterization, is developed. These classes are: determinism, pattern generation, and information. A simple explanation for the origin of these classes is given. The three classes are shown to correspond to the different possible ways in which the properties of the components of a system unfold in the course of the system's processes. Determinism will be defined as a form of causation in which the properties causing the behavior are factually the same as the ones which for the observer specify the current conditions of the system. As a consequence, deterministic systems can be characterized by state equations that describe how the observable properties change. Pattern generation is characterized here as a form of causation in which the observable properties and the behavior‐generating properties correspond to each other as one to the many. Consequently, we experience in terms of the observable properties a branching behavior which is characterized by a pattern of selections at the branching points, a pattern that stands in close correspondence with the visually perceivable patterns and forms, generated by the system's dynamics. Information will be understood as a name for a process and an instance of causation where properties that cause behavior differ from those which we directly observe. The behavior of the system appears to be a consequence of the content of information hidden in the components. This information is, in turn, epistemologically equivalent to the very causal process in which it is unfolded.     

The impact of Parkinson's disease on sequence learning: perceptual pattern learning and executive function

The current study examined the contribution of brain areas affected by Parkinson's disease (PD) to sequence learning, with a specific focus on response-related processes, spatial attentional control, and executive functioning. Patients with mild PD, patients with moderate PD, and healthy age-matched participants performed three tasks-a sequence learning task with a spatial pattern that was incidental to response selection, a spatial cuing task, and neuropsychological tests of executive function. Whereas moderate PD patients failed to show significant sequence learning, mild PD patients performed comparably with controls. Neither group of PD patients was impaired in the control of spatial attention. Sequence learning was correlated with neuropsychological measures of executive function but not with the ability to control spatial attention. These results suggest that the contribution of the brain areas affected by PD to sequence learning extends beyond motor learning to include the learning of perceptual patterns and involves executive function, including cognitive flexibility and set shifting.     

The time course of perceptual grouping: the role of segregation and shape formation

The time course of perceptual grouping was examined in two experiments, using a primed matching task. In different conditions, elements were grouped into columns/rows by common lightness, into a shape (triangle/ arrow or square/cross) by common lightness, and into a shape without segregation of elements. The results showed an early and rapid grouping into columns/rows by common lightness and into a shape when no segregation from other elements was involved. Goodness of shape (i.e., triangle/arrow vs. square/cross) had no influence on how early grouping was evident, but the relatively poorer shapes appeared to consolidate with time. In contrast, grouping into a shape that involved segregation and required resolving figure-ground relations between segregated units, as grouping into a shape by common lightness, consumed time, regardless of shape goodness. These results suggest that the time course of grouping varies as a function of the processes involved in it (e.g., segregation and shape formation) and the conditions prevailing for each process.