Timothy Ledgeway

Deficits to global motion processing in human amblyopia.

We investigated global motion processing in a group of adult amblyopes using a method that allows us to factor out any influence of the known contrast sensitivity deficit. We show that there are independent global motion processing deficits in human amblyopia that are unrelated to the contrast sensitivity deficit, and that are more extensive for contrast-defined than for luminance-defined stimuli. We speculate that the site of these deficits must include the extra-striate cortex and in particular the dorsal pathway.

Constraints on Analogical Mapping: A Comparison of Three Models

Three theories of analogy have been proposed that are supported by computational models and data from experiments on human analogical abilities. In this article we show how these theories can be unified within a common metatheoretical framework that distinguishes among levels of informational, behavioral, and hardware constraints. This framework clarifies the distinctions among three computational models in the literature: the Analogical Constraint Mapping Engine (ACME), the Structure-Mapping Engine (SME), and the Incremental Analogy Machine (IAM). We then go on to develop a methodology for the comparative testing of these models. In two different manipulations of an analogical mapping task we compare the results of computational experiments with these models against the results of psychological experiments. In the first experiment we show that increasing the number of similar elements in two analogical domains decreases the response time taken to reach the correct mapping for an analogy problem. In the second psychological experiment we find that the order in which the elements of the two domains are presented has significant facilitative effects on the ease of analogical mapping. Of the three models, only IAM embodies behavioral constraints and predicts both of these results. Finally, the immediate implications of these results for analogy research are discussed, along with the wider implications the research has for cognitive science methodology.

Dual multiple-scale processing for motion in the human visual System

A number of psychophysical and physiological studies have suggested that first- and second-order motion signals are processed, at least initially, by independent pathways, and that the two pathways both consist of multiple motion-detecting channels that are each narrowly tuned to a different spatial scale (spatial frequency). However, the precise number and nature of the mechanisms that subserve first- and second-order motion perception in human vision remain both controversial and speculative. We sought to clarify this issue by conducting selective adaptation experiments, in which modulation-depth thresholds for identifying the direction of stimulus motion of first-order (luminance-defined) and second-order (contrast-defined) drifting gratings were measured both prior to and following adaptation to motion. The drift direction, spatial frequency and stimulus type (either first- or second-order) of the adaptation and test stimuli were systematically manipulated. When the adaptation and test stimuli were either both first-order gratings or both second-order gratings, robust elevations of direction-identification thresholds were found and, importantly, these aftereffects exhibited both direction-selectivity and spatial-frequency selectivity. Cross-over-adaptation effects between first- and second-order gratings were also sometimes observed, but were very weak and not spatial-frequency selective. These findings give direct support for the existence of multiple-scale processing for first- and second-order motion in the human visual system and provide additional evidence that the two varieties of motion are initially processed by independent pathways.

Separate detection of moving luminance and contrast modulations: fact or artifact?

We have investigated first-order artifacts in second-order motion perception. Subjects were required to identify the orientation and direction of a drifting sinusoidal contrast modulation. When the carrier consisted of static two-dimensional noise, performance often reflected the use of first-order artifacts that arise from stochastic local biases in the noise, rather than the detection of the contrast modulation per se. This stimulus, which has been used widely for studying second-order motion, therefore appears to be inappropriate for that purpose. In contrast, global distortion products arising from luminance non-linearities do not appear to provide usable artifacts. Two manipulations were employed to eliminate local first-order artifacts: the use of dynamic noise and the use of high-pass filtered static noise. These two manipulations gave similar results, which were quite different from those obtained with broadband static noise. We argue that performance with both of these image types reflects the activity of a true second-order motion mechanism. A characteristic property of this mechanism is that it cannot specify direction at the threshold for detecting orientation. Direction thresholds are around 50% higher than orientation thresholds when first-order artifacts are eliminated.

The influences of visibility and anomalous integration processes on the perception of global spatial form versus motion in human amblyopia.

Do amblyopes demonstrate general irregularities in processes of global image integration? Or are these anomalies stimulus specific? To address these questions we employed directly analogous global-orientation and global-motion stimuli using a method that allows us to factor out any influence of the low-level visibility loss [Simmers, A. J., Ledgeway, T., Hess, R. F., & McGraw, P. V. (2003). Deficits to global motion processing in human amblyopia. Vision Research 43, pp. 729-738]. The combination of orientation and motion coherence thresholds reported here provides comparable psychophysical measures of global processing by spatial-sensitive and motion-sensitive mechanisms in the amblyopic visual system. The results show deficits in both global-orientation and global-motion processing in amblyopia, which appear independent of any low-level visibility loss, but with the most severe deficit affecting the extraction of global motion. This provides evidence for the existence of a dominant temporal processing deficit in amblyopia.

The extent of the dorsal extra-striate deficit in amblyopia

Previously, we have shown that humans with amblyopia exhibit deficits for global motion discrimination that cannot be simply ascribed to a reduction in visibility or contrast sensitivity. Deficits exist in the processing of global motion in the fronto-parallel plane that suggest reduced extra-striate function (i.e., MT) in amblyopia. Here, we ask whether such a deficit also exists for rotation and radial components of optic flow that are first processed at higher sites along the dorsal pathway (i.e., MSTd). We show that similar motion processing deficits occur in our amblyopic group as a whole for translation, rotation, and radial components of optic flow and that none of these can be solely accounted for by the reduced visibility of the stimuli. Furthermore, on a subject-by-subject basis there is no significant correlation between the motion deficits for radial and rotational motion and those for translation, consistent with independent deficits in dorsal pathway function up to and including MSTd.

What causes non-monotonic tuning of fMRI response to noisy images?

We are grateful to Gregor Rainer for providing us with information supplemental to his original paper. S.C.D. was supported by the Wellcome Trust.

Adaptation to second-order motion results in a motion aftereffect for directionally-ambiguous test stimuli

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first- or second-order motion was measured in two experiments using a nulling method. The second-order motion adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional random noise by a drifting, 1 c/deg, vertical sine grating. The first-order motion adaptation stimulus was composed of luminance-modulated noise produced by adding, rather than multiplying, the sine grating and noise field. The test stimuli were directionally-ambiguous first- or second-order motion patterns composed of either two oppositely drifting sine gratings added to static noise or its contrast-modulated equivalent. The amplitudes of the two drifting components were manipulated such that as one increased in amplitude the other decreased in amplitude by the same degree. This technique was employed to estimate the null point at which the test no longer appeared to drift in the direction opposite the adaptation direction. In the first experiment all stimuli were equated for visibility by presenting them at the same multiple of threshold and all possible combinations of first- and second-order motion adaptation and test stimuli were examined. The results were similar for all conditions: following adaptation the amplitude of the test component drifting in the same direction as adaptation needed to be approximately twice that of the oppositely drifting component in order to null the perception of unidirectional motion of the test. In a second experiment, the effects of manipulating the amplitude (visibility) of the first- and second-order motion adaptation stimuli on MAE magnitude were investigated. This revealed an approximately linear relationship between MAE magnitude and the amplitudes of the adaptation stimuli. The results demonstrate that, contrary to the findings of several previous studies, adaptation to second-order motion does produce a substantial movement aftereffect. Cross-adaptation between first- and second-order motion stimuli also occurs under appropriate conditions and produces aftereffects that are comparable in magnitude when the stimuli are equated for visibility.

Putting order into the development of sensitivity to global motion

We studied differences in the development of sensitivity to first-versus second-order global motion by comparing the motion coherence thresholds of 5-year-olds and adults tested at three speeds (1.5, 6, and 9 degrees s(-1)). We used Random Gabor Kinematograms (RGKs) formed with luminance-modulated (first-order) or contrast-modulated (second-order) concentric Gabor patterns with a sinusoidal spatial frequency of 3c deg(-1). To achieve equal visibility, modulation depth was set at 30% for first-order Gabors and at 100%, for second-order Gabors. Subjects were 24 adults and 24 5-year-olds. For both first- and second-order global motion, the motion coherence threshold of 5-year-olds was less mature for the slowest speed (1.5 degrees s(-1)) than for the two faster speeds (6 and 9 degrees s(-1)). In addition, at the slowest speed, the immaturity was greater for second-order than for first-order global motion. The findings suggest that the extrastriate mechanisms underlying the perception of global motion are different, at least in part, for first- versus second-order signals and for slower versus faster speeds. They also suggest that those separate mechanisms mature at different rates during middle childhood.

Sensitivity to second-order motion as a function of temporal frequency and eccentricity

There is considerable evidence that second-order motion, such as motion consisting of a drifting contrast modulation, is detected separately from first-order motion. Some previous studies have shown that the rate at which sensitivity declines as either drift speed or eccentricity increases is the same for both types of motion. However, these studies have used second-order motion stimuli based on static noise carriers, which we have shown (Smith & Ledgeway, 1997) may be inappropriate because they can give rise to local first-order artifacts. By using dynamic noise carriers, we isolate the second-order motion mechanism and show that its temporal response is much worse than that of the first-order system but that its rate of sensitivity loss with increasing stimulus eccentricity is indeed similar to that of the first-order motion system.