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P C Dodwell

Publications and source records attributed to P C Dodwell.

At least 19 recordsLinked to original sources
PubMed1994

Experiments on the afterimages of stimulus change (Dvorák 1870): a translation with commentary.

In 1870 Dvorák rejected Helmholtz's eye-movement account of motion aftereffects (MAEs) on the grounds that it was inconsistent with previous reports of nonuniform rotation in MAEs induced with Plateau spirals. Subsequent observations with spirals that were modified to induce both expanding and contracting MAEs simultaneously, together with the use of stationary negative afterimages during induction and test, were offered as further counter-examples to the eye-movement hypothesis. Dvorák's conjectures that perception (and misperception) of movement involves a unitary perceptual dimension of stimulus change also led him to investigate whether aftereffects comparable to MAEs could be induced along other stimulus dimensions in vision (luminance gradients), and in audition (gradients of pitch and intensity). It is suggested that Dvorák's observations, taken as a whole, may be interpreted as an attempt to provide evidence challenging the Helmholtzian traditions underpinning eye-movement accounts of MAEs. The nature and outcomes of these observations are provided in a translation of the original work, and are subsequently discussed in relation to some contemporary empirical counterparts.

Auditory Perception↗
PubMed1994

On raising our sights.

There are some very strong traditions in contemporary research on spatial vision that have arisen, in large measure, through advances in technology. This is especially true of research on the neurophysiology of vision, e.g. in recording from single cells, and in developments in computing that have promoted the study of machine vision. No one would dispute that these are major advances in the field, but one may ask whether they have unduly constrained the sorts of question that we ask about the visual processes. It is argued that they have, and that it is timely to stand back to see what other problems and questions have suffered neglect. Two items seem to be quite ripe for reconsideration: the question of how to understand the enormous adaptability of visual systems to sensory rearrangement (by means of distorting lenses, for example); and the very broad matter of keeping our perceptual processing in close and coherent touch both with stimulation, and with the properties of the physical/biological environment--what Gibson called the problem of veridical perception. It is argued that the two items are closely allied, and can be approached from a single theoretical stance. The one I favour is outlined, and I claim that it is one that has little to do, in the first instance, with either neurophysiology or computation. In that sense tackling the two questions involves raising our sights above the current traditions of vision research, but that of course does not in any way mean attempting to belittle the immense advances that those traditions have brought about.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗
PubMed1991

Pattern-specific contrast threshold elevation.

Visual channels are defined psychophysically; stimuli that interact share information in the same channel, and those that do not interact are processed in different channels. Channels are often investigated by means of adaptation to one stimulus, testing contrast threshold elevation with one (or more) others. Much recent work has tested the tuning of channels for orientation and spatial frequency, using simple line gratings. This study examined the pattern-specificity of such adaptation, testing the hypothesis that the fundamental operators of the Lie Transformation Group Theory of Neuropsychology (LTG/NP) define psychophysical channels. In Experiment I the three basic pattern pairs of LTG/NP were used as adaptation and test stimuli in a conventional contrast threshold-elevation experiment. Threshold elevation was pattern-specific, thus supporting the hypothesis. In subsequent experiments various 'fractured' patterns, and patterns generated by combinations of Lie operators were used both for adaptation and test. The results were mixed; some supported the original hypothesis, but many did not. Relations between local contour orientations in adaptation and test patterns could explain some results, but not all. The hypothesis that adaptation occurs to the oriented spatial-frequency components of the test patterns, on the other hand, gave a good fit to the data. It is concluded that there is pattern-specificity in contrast threshold elevation, but it is a form of specificity that can be explained without recourse to a model of geometrical pattern processing, at least for the simple patterns used here.

Adaptation, Ocular↗
PubMed1990

A functional theory of the McCollough effect.

Pattern-contingent color aftereffects, or McCollough effects (MEs), are used to probe the visual brain's operations psychophysically. Their neural substrate is unknown, however, and theories about them are weak. Our theory proposes a strong functional role for MEs and a neuropsychological basis that accounts for "top-down" (global) constraints ignored by other theories. The functional aspect of the theory is based on the concept of contingent adaptation level, following Helson (1964), and on the "error-correcting device" of Andrews (1964), which tracks and adjusts internal representation to external-world contingencies. The neuropsychological part of the theory postulates that global properties are the result of MEs being generated not at the individual detector level but in vectorfields of which the detectors are elements. It is an implementation of Lie transformation group theory (Hoffman, 1966). Evidence for this model is assessed.

Brain↗
PubMed1990

Angular frequency filtering: a basis for pattern decomposition.

Much insight has been gained into the nature of early vision by postulating that the system encodes patterns by means of channels, or filters, tuned to specific orientations and spatial frequencies. We argue that a different decomposition, in terms of radially and rotationally symmetrical filters, is equally plausible, both in terms of the geometry of the eye and the ecology of visual pattern recognition. We demonstrate experimentally some of the properties of angular frequency filtering.

Contrast Sensitivity↗
PubMed1989

Pattern-contingent color aftereffects on noninduced patterns.

In a series of experiments, we found that in addition to expected reports of color aftereffects on patterns viewed during induction, reliable and predictable reports of color were given by subjects to patterns they did not view during induction. These reports to noninduced patterns were generally to patterns that were orthogonal to the patterns seen during induction. Induction with, for example, a red vertical grating led to appropriate aftereffects (i.e., green) on that vertical pattern and to the complementary aftereffect (i.e., pink) on a horizontal grating. We suggest that such color aftereffects on noninduced patterns are based on a shift in the activity of orientation coding mechanisms as a result of viewing the inducing patterns. We further propose that the results are consistent with the Lie transformation group theory of neuropsychology and that they add to a growing body of research demonstrating the applicability of this theory to the understanding of pattern-contingent color aftereffects.

Attention↗
PubMed1987

Global factors generate the McCollough effect.

It has recently been demonstrated that certain globally orthogonal pattern pairs derived from Hoffman's Lie Transformation Group Model of Neuropsychology (LTG/NP) are sufficient to generate McCollough effects (pattern-contingent colour aftereffects: PCCAES). We now provide evidence that the global factors postulated to generate these particular PCCAEs are also necessary. In Experiment 1 observers made controlled eye movements over the induction patterns and subsequently showed PCCAEs which were as strong as those obtained under central fixation. The induction process therefore did not depend on selectively adapting local oriented and colour-specific edge detectors. The global structure of the inducing patterns appears to be a necessary component of the generation of such aftereffects. In Experiment 2, a number of pattern pairs having various degrees of global organization were used as induction patterns, but in every case there was an equal degree of local orthogonality. Local orthogonality was shown not to be sufficient to induce PCCAEs, but global orthogonality was. These results lend strong support to the notion that McCollough-type PCCAEs are generated in a structure of the visual system somewhat above the level of strictly local operations, whether defined as edge and bar detectors or, equivalently, as units selectively tuned to different spatial frequencies and orientations. The global nature of the effects, and the particular pattern structures involved, lead us to suggest that Hoffman's LTG/NP provides a sound basis for explaining our findings.

Adolescent↗
PubMed1986

Pattern perception in infants: effects of structure and transformation.

Four experiments on pattern perception in 4-month-old infants are reported. The first two experiments examined the perception of pattern structure as defined by Garner (1974). Although the infants did not show an immediate preference for well-structured patterns (Experiment 1), during habituation they showed a reliable decrement in looking only to well-structured patterns (Experiment 2). On recovery trials in Experiment 2 infants were presented with two types of pattern change: a new configuration of the habituation pattern and a change in the orientation of the habituation pattern. The infants recovered looking to the orientation change, but generally not to the changes in configuration. It is argued that the results of Experiment 2 highlight the role of symmetry, especially vertical symmetry, in infant pattern perception. Recovery of looking to changes in the orientation, size, and position of the habituation pattern was examined in Experiments 3 and 4 where it was demonstrated that such changes are salient to the infant, arguably because of the presence of a surrounding, external frame.

Attention↗
PubMed1983

Pattern recognition in kittens: performance on Lie patterns.

Despite a vast accumulation of knowledge about the anatomy and physiology of the cat's visual system in recent years, and about its early development, there has been very little experimental study of the development of visual behaviour in this species. This is especially true of the kitten's ability to recognize patterns. Two experiments are reported that aim to remedy some part of this deficiency, and that also serves to examine a particular hypothesis about the basis of pattern analysis in the young organism. This is Hoffman's hypothesis that the orbits of elementary Lie transformation groups (a species of continuous transformation group) represent the basis for coding pattern information.

Age Factors↗