Biomedical subjects

C P Benton

Publications and source records attributed to C P Benton.

10 recordsLinked to original sources

PubMed2001

A new approach to analysing texture-defined motion.

It has been widely accepted that standard low-level computational approaches to motion processing cannot extract texture-defined motion without applying some pre-processing nonlinearity. This has motivated accounts of motion perception in which luminance- and texture-defined motion are processed by separate mechanisms. Here, we introduce a novel method of image description where motion sequences may be described in terms of their local spatial and temporal gradients. This allows us to assess the local velocity information available to standard low-level motion mechanisms. Our analysis of several texture-motion stimuli shows that the information indicating correct texture-motion velocity and/or direction is present in the raw luminance measures. This raises the possibility that luminance-motion and texture-motion may be processed by the same cortical mechanisms. Our analysis offers a way of looking at texture-motion processing that is, to our knowledge, new and original.

Electrophysiology↗

PubMed2001

Computational modeling of non-Fourier motion: further evidence for a single luminance-based mechanism.

It is generally assumed that the perception of non-Fourier motion requires the operation of some nonlinearity before motion analysis. We apply a computational model of biological motion processing to a class of non-Fourier motion stimuli designed to investigate nonlinearity in human visual processing. The model correctly detects direction of motion in these non-Fourier stimuli without recourse to any preprocessing nonlinearity. This demonstrates that the non-Fourier motion in some non-Fourier stimuli is directly available to luminance-based motion mechanisms operating on measurements of local spatial and temporal gradients.

Fourier Analysis↗

PubMed2000

Computational modelling of interleaved first- and second-order motion sequences and translating 3f+4f beat patterns.

Despite detailed psychophysical, neurophysiological and electrophysiological investigation, the number and nature of independent and parallel motion processing mechanisms in the visual cortex remains controversial. Here we use computational modelling to evaluate evidence from two psychophysical studies collectively thought to demonstrate the existence of three separate and independent motion processing channels. We show that the pattern of psychophysical results can largely be accounted for by a single mechanism. The results demonstrate that a low-level luminance based approach can potentially provide a wider account of human motion processing than generally thought possible.

Humans↗

PubMed1999

Induced motion at texture-defined motion boundaries.

When a static textured background is covered and uncovered by a moving bar of the same mean luminance we can clearly see the motion of the bar. Texture-defined motion provides an example of a naturally occurring second-order motion. Second-order motion sequences defeat standard spatio-temporal energy models of motion perception. It has been proposed that second-order stimuli are analysed by separate systems, operating in parallel with luminance-defined motion processing, which incorporate identifiable pre-processing stages that make second-order patterns visible to standard techniques. However, the proposal of multiple paths to motion analysis remains controversial. Here we describe the behaviour of a model that recovers both luminance-defined and an important class of texture-defined motion. The model also accounts for the induced motion that is seen in some texture-defined motion sequences. We measured the perceived direction and speed of both the contrast envelope and induced motion in the case of a contrast modulation of static noise textures. Significantly, the model predicts the perceived speed of the induced motion seen at second-order texture boundaries. The induced motion investigated here appears distinct from classical induced effects resulting from motion contrast or the movement of a reference frame.

Humans↗

PubMed1999

Concurrent measurement of perceived speed and speed discrimination threshold using the method of single stimuli.

Velocity matching using the method of Constant Stimuli shows that perceived velocity varies with contrast [Thompson, P. (1982). Perceived rate of movement depends upon contrast. Vision Research, 22, 377-380]. Random contrast jitter would therefore be expected to increase the slopes of psychometric functions, and thus the velocity discrimination threshold. However, McKee, S., Silverman, G., and Nakayama, K. [(1986) Precise velocity discrimination despite random variation in temporal frequency. Vision Research, 26, 609-620] found no effect of contrast jitter on thresholds, using the method of single stimuli. To determine whether this apparent discrepancy is due to the difference in methodology, or to the different ranges of temporal frequencies used in the two studies, we used the method of single stimuli to measure psychometric functions at three different velocities (0.5, 2.0 and 4.0 degrees/s). We found that contrast jitter increased thresholds at low but not at high velocities. Separate analysis of the psychometric functions at each contrast level showed that increases in contrast increased perceived velocity at low standard speeds (0.5 degree/s) but not at high. We conclude that the effect of contrast on perceived speed is real, and not a methodological artefact, but that it is found only at low temporal frequencies.

Contrast Sensitivity↗

PubMed1999

Contrast inconstancy across changes in polarity.

In this study, we show that negative polarity noise patterns appear to have a higher contrast than positive polarity noise patterns with identical expected Fourier amplitude spectra. This demonstrates a failure of contrast constancy over changes in pattern polarity. An examination of local contrast measures shows that negative polarity noise has a wider distribution of local contrast values than positive polarity noise. We propose that the difference in apparent contrast between the two patterns may be based upon spatial non-linearities in the combination of local contrast measures.

Contrast Sensitivity↗

PubMed1997

Speed discrimination thresholds for first--and second-order bars and edges.

Speed discrimination thresholds were measured for first- and second-order Gaussian bars and edges as a function of speed and the spatial scale of the modulation signal. Discrimination thresholds were generally higher for the second-order patterns when compared with modulations of luminance. There were no systematic effects of variations in the width of the bars and edges. The results are discussed in relation to mechanisms for the explicit recovery of contrast modulations and the influence of the form of the carrier signal on visual performance in second-order motion tasks.

Contrast Sensitivity↗

PubMed1997

Perception of motion direction in luminance- and contrast-defined reversed-phi motion sequences.

Nonlinear processing can be used to recover the motion of contrast modulations of binary noise patterns. A nonlinear stage has also been proposed to explain the perception of forward motion in motion sequences which typically elicit reversed-phi. We examined perceived direction of motion for stimuli in which these reversed motion sequences were used to modulate the contrast of binary noise patterns. A percept of forward motion could be elicted by both luminance-defined and contrast-defined stimuli. The perceived direction of motion seen in the contrast-defined stimuli showed a profound carrier dependency. The replacement of a static carrier by a dynamic carrier can reverse the perceived direction of motion. Forward motion was never seen with dynamic carriers. For luminance- and contrast-defined patterns the reversed motion percept increasingly dominated, with increases in the spatial frequency and temporal frequency of the modulation. Differences in the patterns of responses to the two stimuli over spatial and temporal frequency were abolished by the addition of noise to the luminance-defined stimulus. These data suggest the possibility that a single mechanism may mediate the perception of luminance- and contrast-defined motion.

Contrast Sensitivity↗

PubMed1997

First-order motion from contrast modulated noise?

The class of microbalanced motion stimuli is thought to contain no systematic directional biases in motion energy. The fact that we can see motion in such stimuli implies that models of human motion perception based on Fourier decomposition need to be revised. The validity of one widely studied class of microbalanced stimuli, contrast modulated noise, has recently been questioned. It has been proposed that stochastic local biases in the noise carrier give rise to luminance artifacts detectable by a Fourier energy mechanism. However, in this study we show that the response of a motion energy system to contrast modulated noise shows no directional bias over a number of carrier configurations. We conclude that this class of stimuli remains an important tool for researchers wishing to study non-Fourier motion.

Fourier Analysis↗

PubMedDate not supplied

Biological computation of image motion from flows over boundaries.

A theory of early motion processing in the human and primate visual system is presented which is based on the idea that spatio-temporal retinal image data is represented in primary visual cortex by a truncated 3D Taylor expansion that we refer to as a jet vector. This representation allows all the concepts of differential geometry to be applied to the analysis of visual information processing. We show in particular how the generalised Stokes theorem can be used to move from the calculation of derivatives of image brightness at a point to the calculation of image brightness differences on the boundary of a volume in space-time and how this can be generalised to apply to integrals of products of derivatives. We also provide novel interpretations of the roles of direction selective, bi-directional and pan-directional cells and of type I and type II cells in V5/MT.

Animals↗