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Christopher P Benton

Publications and source records attributed to Christopher P Benton.

7 recordsLinked to original sources
PubMed2006

Viewpoint dependence in adaptation to facial identity.

We produced morph sequences between identities at a variety of viewpoints, ranging from the three quarter leftward facing view, to the three quarter rightward facing view. We measured the strength of identity adaptation as a function of changing test viewpoint whilst keep the adaptation viewpoint constant, and as a function of adaptation viewpoint whilst keeping test viewpoint constant. Our results show a substantial decrease in adaptation as the angle between adaptation and test viewpoint increases. These findings persisted when we introduced controls for low-level retinotopic adaptation, leading us to conclude that our results show strong evidence for viewpoint dependence in the high-level encoding of facial identity. Our findings support models in which identity is encoded, to a large degree, by viewpoint dependent non-retinotopic neural mechanisms. Functional imaging studies suggest the fusiform gyrus as the most likely location for this mechanism.

Adaptation, Physiological↗
PubMed2006

Test stimulus characteristics determine the perceived speed of the dynamic motion aftereffect.

Using a speed-matching task, we measured the speed tuning of the dynamic motion aftereffect (MAE). The results of our first experiment, in which we co-varied dot speed in the adaptation and test stimuli, revealed a speed tuning function. We sought to tease apart what contribution, if any, the test stimulus makes towards the observed speed tuning. This was examined by independently manipulating dot speed in the adaptation and test stimuli, and measuring the effect this had on the perceived speed of the dynamic MAE. The results revealed that the speed tuning of the dynamic MAE is determined, not by the speed of the adaptation stimulus, but by the local motion characteristics of the dynamic test stimulus. The role of the test stimulus in determining the perceived speed of the dynamic MAE was confirmed by showing that, if one uses a test stimulus containing two sources of local speed information, observers report seeing a transparent MAE; this is despite the fact that adaptation is induced using a single-speed stimulus. Thus while the adaptation stimulus necessarily determines perceived direction of the dynamic MAE, its perceived speed is determined by the test stimulus. This dissociation of speed and direction supports the notion that the processing of these two visual attributes may be partially independent.

Figural Aftereffect↗
PubMed2004

A role for contrast-normalisation in second-order motion perception.

Two low-level motion models are applied to a second-order stimulus, a translating contrast modulation of static binary noise. Both models have been used to demonstrate equivalence between energy and gradient algorithms and can be split into a motion-opponent stage followed by a contrast-normalised stage. Analysis of results shows no directional bias at the motion-opponent stage but a strong bias, indicating the correct direction of second-order motion, at the contrast-normalised stage. This demonstrates that the intrinsically non-linear process of contrast-normalisation may play a part in the detection of second-order motion.

Algorithms↗
PubMed2003

Direction repulsion goes global.

When viewing two superimposed, translating sets of dots moving in different directions, one overestimates direction difference. This phenomenon of direction repulsion is thought to be driven by inhibitory interactions between directionally tuned motion detectors. However, there is disagreement on where this occurs-at early stages of motion processing, when local motions are extracted; or at the later, global motion-processing stage following "pooling" of these local measures. These two stages of motion processing have been identified as occurring in area V1 and the human homolog of macaque MT/V5, respectively. We designed experiments in which local and global predictions of repulsion are pitted against one another. Our stimuli contained a target set of dots, moving at a uniform speed, superimposed on a "mixed-speed" distractor set. Because the perceived speed of a mixed-speed stimulus is equal to the dots' average speed, a global-processing account of direction repulsion predicts that repulsion magnitude induced by a mixed-speed distractor will be indistinguishable from that induced by a single-speed distractor moving at the same mean speed. This is exactly what we found. These results provide compelling evidence that global-motion interactions play a major role in driving direction repulsion.

Humans↗
PubMed2003

Speed tuning of direction repulsion describes an inverted U-function.

Direction repulsion describes the phenomenon in which observers typically overestimate the direction difference between two superimposed motions moving in different directions (Marshak & Sekuler, Science 205 (1979) 1399). Previous research has found that, when a relatively narrow range of distractor speeds is considered, direction repulsion of a target motion increases monotonically with increasing speed of the distractor motion. We sought to obtain a more complete measurement of this speed-tuning function by considering a wider range of distractor speeds than has previously been used. Our results show that, contrary to previous reports, direction repulsion as a function of distractor speed describes an inverted U-function. For a target of 2.5 deg/s, we demonstrate that the attenuation of repulsion magnitude with high-speed disractors can be largely explained in terms of the reduced apparent contrast of the distractor. However, when we reduce target motion speed, this no longer holds. When considered from the perspective of Edwards et al.'s (Edwards, Badcock, & Smith, Vision Research 38 (1998) 1573) two global-motion channels, our results suggest that direction repulsion is speed dependent when the distractor and target motions are processed by different global-motion channels, but is not speed dependent when both motions are processed by the same, high-speed channel. The implications of these results for models of direction repulsion are discussed.

Adaptation, Ocular↗
PubMed2002

Gradient-based analysis of non-Fourier motion.

A gradient-based image analysis technique is applied to a class of non-Fourier stimuli. To create the stimuli, n translating sine waves with identical spatial and temporal frequencies, but separated by 2pi/n radians, are spatially randomly sampled to produce a P(n) stimulus. For n>or=2, the stimuli are non-Fourier. Local image gradients are represented in the form of a gradient plot, a histogram which shows the frequency of ranges of temporal gradient/spatial gradient pairs occurring. It is shown that the gradient plots contain features, oriented in gradient space, which indicate correct non-Fourier velocity. As n increases, so too does the complexity of the gradient plots, a finding which may account for the concomitant decrease in perceived coherent motion [Vision Res 37 (1997) 1459]. This paper demonstrates that the gradient plot and associated velocity plots are a useful way of assessing gradient-based motion information. Compared to the traditional Fourier based approach, gradient-based analysis can lead to different judgement of the motion information available to standard models of low-level motion processing.

Computational Biology↗