RETRACTED: Vergence eye movements direct others' attention in three-dimensional space.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the authors. In our Correspondence, we reported that the spatial focus of visual attention can be cued by another's vergence eye movements. However, we subsequently discovered that there was a mistake in the analysis such that reaction time data were systematically mislabeled. When we reran the analysis on the correctly labeled data, the reported cueing effect no longer reached statistical significance. We are therefore retracting our paper and apologize to the scientific community for any inconvenience caused.

A Bayesian approach to person perception.

Here we propose a Bayesian approach to person perception, outlining the theoretical position and a methodological framework for testing the predictions experimentally. We use the term person perception to refer not only to the perception of others' personal attributes such as age and sex but also to the perception of social signals such as direction of gaze and emotional expression. The Bayesian approach provides a formal description of the way in which our perception combines current sensory evidence with prior expectations about the structure of the environment. Such expectations can lead to unconscious biases in our perception that are particularly evident when sensory evidence is uncertain. We illustrate the ideas with reference to our recent studies on gaze perception which show that people have a bias to perceive the gaze of others as directed towards themselves. We also describe a potential application to the study of the perception of a person's sex, in which a bias towards perceiving males is typically observed.

Spatial structure of contextual modulation.

Contextual effects are ubiquitous in vision and provide a means for detectors with localized receptive fields to encode global properties of a stimulus. Although the nature of the neural connections is complex, the majority of evidence supports the Gestalt idea of collinearity; interactions are greatest when the target and surround orientations are spatially aligned to form a contour. Here we create a novel stimulus that simultaneously probes all areas around a detector to determine which spatial positions influence perception in human observers. We find that the surrounding spatial areas that contribute most to contextual effects for our perception of orientation and motion are not confined to a specific location. Rather our results reveal that human perception displays some interobserver variability in the weighting of detector interactions that is largely independent of collinear structure. We propose that these more extensive surround stimuli reveal how complex visual structure may modulate performance in a manner that is not easily predictable using more conventional stimuli.

Differential changes in perceived contrast following contrast adaptation in humans.

Perceived contrast is reduced after prolonged exposure to a textured pattern (contrast adaptation). The size of this effect is dependent on the relationship between the adapting contrast and the test contrast. It is generally accepted that the greatest reductions occur when the adapting contrast is much higher than the test contrast. Here this relationship was examined for a wide range of spatial frequencies. The results show that the effect of the adapt/test ratio on perceived contrast following contrast adaptation is highly spatial frequency dependent. At high spatial frequencies >1cpd perceived contrast was reduced for all adapting contrasts, which is consistent with other studies. However, at low spatial frequencies (<1cpd) the perceived contrast was actually above veridical perception when the adapting contrast was lower than the test contrast. This finding has not been previously reported and has important implications for models of contrast perception.

Discrimination of the local orientation structure of spiral Glass patterns early in human visual cortex.

The local orientation structure of a visual image is fundamental to the perception of spatial form. Reports of reliable orientation-selective modulations in the pattern of fMRI activity have demonstrated the potential for investigating the representation of orientation in the human visual cortex. Orientation-selective voxel responses could arise from anisotropies in the preferred orientations of pooled neurons due to the random sampling of the cortical surface. However, it is unclear whether orientation-selective voxel responses reflect biases in the underlying distribution of neuronal orientation preference, such as the demonstrated over-representation of radial orientations (those collinear with fixation). Here, we investigated whether stimuli balanced in their radial components could evoke orientation-selective biases in voxel activity. We attempted to discriminate the sense of spiral Glass patterns (opening anti-clockwise or clockwise), in which the local orientation structure was defined by the placement of paired dots at an orientation offset from the radial. We found that information within the spatial pattern of fMRI responses in each of V1, V2, V3, and V3A/B allowed discrimination of the spiral sense with accuracies significantly above chance. This result demonstrates that orientation-selective voxel responses can arise without the influence of a radial bias. Furthermore, the finding indicates the importance of the early visual areas in representing the local orientation structure for the perception of complex spatial form.

Dynamic contrast change produces rapid gain control in visual cortex.

During normal vision, objects moving in the environment, our own body movements and our eye movements ensure that the receptive fields of visual neurons are being presented with continually changing contrasts. Thus, the visual input during normal behaviour differs from the type of stimuli traditionally used to study contrast coding, which are presented in a step-like manner with abrupt changes in contrast followed by prolonged exposure to a constant stimulus. The abrupt changes in contrast typically elicit brief periods of intense firing with low variability called onset transients. Onset transients provide the visual system with a powerful and reliable cue that the visual input has changed. In this paper we investigate visual processing in the primary visual cortex of cats in response to stimuli that change contrast dynamically. We show that 1-4 s presentations of dynamic increases and decreases in contrast can generate stronger contrast gain control than several minutes exposure to a stimulus of constant contrast. Thus, transient mechanisms of contrast coding are not only less variable than sustained responses but are also more rapid and flexible. Finally, we propose a quantitative model of contrast coding which accounts for changes in spike rate over time in response to dynamically changing image contrast.

Relationship between contrast adaptation and orientation tuning in V1 and V2 of cat visual cortex.

Previous studies investigating the response properties of neurons in the primary visual cortex of cats and primates have shown that prolonged exposure to optimally oriented, high-contrast gratings leads to a reduction in responsiveness to subsequently presented test stimuli. We recorded from 119 neurons in cat V1 and V2 and found that in a high proportion of cells contrast adaptation also occurs for gratings oriented orthogonal to a neuron's preferred orientation, even though this stimulus did not elicit significant increases in spiking activity. Approximately 20% of neurons adapted equally to all orientations tested and a further 46% showed at least some adaptation to orthogonally oriented gratings, whereas 20% of neurons did not adapt to orthogonal gratings. The magnitude of contrast adaptation was positively correlated with adapting contrast, but was not related to the spiking activity of the cells. Highly direction selective neurons produced stronger adaptation to orthogonally oriented gratings than other neurons. Orientation-related adaptation was correlated with the rate of change of orientation tuning in consecutive cells along electrode penetrations that traveled parallel to the cortical layers. Nonoriented adaptation was most common in areas where orientation preference changed rapidly, whereas orientation-selective adaptation was most common in areas where orientation preference changed slowly. A minority of neurons did not show contrast adaptation (14%). No major differences were found between units in different cortical layers, V1 and V2, or between complex and simple cells. The relevance of these findings to the current understanding of adaptation within the context of orientation column architecture is discussed.

Perceptual and physiological responses to the visual complexity of fractal patterns.

Fractals have experienced considerable success in quantifying the complex structure exhibited by many natural patterns and have captured the imagination of scientists and artists alike. With ever widening appeal, they have been referred to both as "fingerprints of nature" and "the new aesthetics." Our research has shown that the drip patterns of the American abstract painter Jackson Pollock are fractal. In this paper, we consider the implications of this discovery. We first present an overview of our research from the past five years to establish a context for our current investigations of human response to fractals. We discuss results showing that fractal images generated by mathematical, natural and human processes possess a shared aesthetic quality based on visual complexity. In particular, participants in visual perception tests display a preference for fractals with mid-range fractal dimensions. We also present recent preliminary work based on skin conductance measurements that indicate that these mid-range fractals also affect the observer's physiological condition and discuss future directions based on these results.

Fundamental mechanisms of visual motion detection: models, cells and functions.

Taking a comparative approach, data from a range of visual species are discussed in the context of ideas about mechanisms of motion detection. The cellular basis of motion detection in the vertebrate retina, sub-cortical structures and visual cortex is reviewed alongside that of the insect optic lobes. Special care is taken to relate concepts from theoretical models to the neural circuitry in biological systems. Motion detection involves spatiotemporal pre-filters, temporal delay filters and non-linear interactions. A number of different types of non-linear mechanism such as facilitation, inhibition and division have been proposed to underlie direction selectivity. The resulting direction-selective mechanisms can be combined to produce speed-tuned motion detectors. Motion detection is a dynamic process with adaptation as a fundamental property. The behavior of adaptive mechanisms in motion detection is discussed, focusing on the informational basis of motion adaptation, its phenomenology in human vision, and its cellular basis. The question of whether motion adaptation serves a function or is simply the result of neural fatigue is critically addressed.

Hierarchy of spatial interactions in the processing of contrast-defined contours.

Both psychophysical and neurophysiological evidence suggest that there are two visual cortical processing streams, a linear stream that processes first-order stimuli and a nonlinear stream that also processes second-order stimuli. This evidence also suggests that before the extraction of the second-order signal, the nonlinear pathway broadly but not completely pools signals across initial linear filters that encode the orientation of the carrier of the second-order signal. The evidence suggests that such pooling does not occur across carrier spatial frequencies. We show that similar results are obtained with repulsion tilt illusions but not with attraction effects. Attraction effects exhibit complete orientation crossover (while retaining spatial frequency selectivity), perhaps indicating higher-level processing; an experiment on interocular transfer of the effects supported this conclusion.

Interactions between ON and OFF signals in directional motion detectors feeding the not of the wallaby.

An apparent motion stimulus is used to probe the interactions between signals representing brightness increments (ON stimuli) and decrements (OFF stimuli) in the directional motion detectors forming the input to the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii. Direction-selective NOT neurons increase their firing rates during image motion from temporal-to-nasal over the contralateral eye (preferred direction) and their spontaneous activities are inhibited by motion in the opposite, anti-preferred direction. An apparent motion stimulus, consisting of neighboring vertical bars, where the brightness can be manipulated independently, also produces directional responses. Preferred direction sequences of brightness changes of like polarities (ON-ON or OFF-OFF) produce increased firing rates while sequences of opposite polarities (ON-OFF or OFF-ON) in the same direction produce relatively small excitatory responses or inhibit the spontaneous rate. For apparent motion in the anti-preferred direction, these directional properties are reversed, showing that signals for brightness increments and decrements provide inputs to the same motion detectors. There is no evidence for segregation of motion detectors into those receiving only half-wave rectified inputs. Interactions between ON and OFF signals utilize the sign of the incoming signals. An array of Reichardt-type motion detectors receiving inputs represented as positive and negative values for ON and OFF stimuli, respectively, are used to simulate the NOT responses. The brightness signals enter band-pass temporal filters prior to motion detection. By altering the time constants of these prefilters, it was possible to accurately simulate the time courses of each cell's responses.

Characterising temporal delay filters in biological motion detectors.

Motion detection requires the comparison of spatially and temporally displaced samples of the image. Here, we discuss the problems associated with measuring the delay between spatially displaced signals within biological motion detectors. Data are presented from direction-selective neurons in the nucleus of the optic tract of the wallaby, Macropus eugenii. Their motion responses depend on stimulus contrast and the adapted state of the cells. At low contrasts or in an adapted state, it appears that the input to the motion detectors is a temporally low-passed version of the image. At high contrasts or in the unadapted state, the input signals appear to be temporally band-pass-filtered. Contrary to previous claims, we find that neither the response to stimulation with apparent motion nor measurements of temporal frequency response functions provide a direct estimate of the delay filter time constants. Instead, we find that both measures are also dependent on the temporal characteristics of prefiltering stages. A model is proposed to account for the responses of the neurons and their contrast dependence.

Asynchronous processing in vision: color leads motion.

It has been demonstrated that subjects do not report changes in color and direction of motion as being co-incidental when they occur synchronously. Instead, for the changes to be reported as being synchronous, changes in direction of motion must precede changes in color. To explain this observation, some researchers have suggested that the neural processing of color and motion is asynchronous. This interpretation has been criticized on the basis that processing time may not correlate directly and invariantly with perceived time of occurrence. Here we examine this possibility by making use of the color-contingent motion aftereffect. By correlating color states disproportionately with two directions of motion, we produced and measured color-contingent motion aftereffects as a function of the range of physical correlations. The aftereffects observed are consistent with the perceptual correlation between color and motion being different from the physical correlation. These findings demonstrate asynchronous processing for different stimulus attributes, with color being processed more quickly than motion. This suggests that the time course of perceptual experience correlates directly with that of neural activity.

Interaction between first- and second-order orientation channels revealed by the tilt illusion: psychophysics and computational modelling.

This paper examines the interaction between first- and second-order contours in the orientation domain. Using the simultaneous tilt illusion (TI), we show that the apparent rotation of a vertical test grating away from that of a surrounding inducing grating (repulsion effect) occurs when both the inducing and test grating are either first- or second-order. Furthermore, a significant repulsion effect is obtained when a first-order inducing grating surrounds a second-order test. If lateral inhibitory interactions between populations of orientation selective neurons provides a plausible explanation for orientation repulsion effects [Blakemore, C. B. Carpenter, R. H. S. & Georgeson, M. A. (1970) Nature, 228, 37-39], it is likely that the cue-invariant mechanisms that encodes the orientation of first- and second-order contours also exhibit inhibitory interactions. A two-channel computational model of orientation encoding is presented where one channel encodes only first-order stimuli while the second channel encodes both first- and second-order contours. In addition to predicting the orientation repulsion effects we observed, the model also provides a functional account of orientation attraction effects in terms of the responses of populations of orientation-tuned neurons.

Orthogonal adaptation improves orientation discrimination.

We investigated the effect of adaptation on orientation discrimination using two experienced observers, then replicated the main effects using a total of 50 naïve subjects. Orientation discrimination around vertical improved after adaptation to either horizontal or vertical gratings, but was impaired by adaptation at 7.5 or 15 degrees from vertical. Improvement was greatest when adapter and test were orthogonal. We show that the results can be understood in terms of a functional model of adaptation in cortical vision.

Dissociable factors affect speed perception and discrimination.

Factors affecting our judgement of the speed of visual motion were investigated. Two types of judgement were made: perceived speed relative to a standard comparison stimulus, and discrimination between the speeds of similar stimuli. The factors affect ng these two judgements were found to be doubly dissociable, suggesting that they may be constrained by processing at different levels of the visual hierarchy. The results are discussed in terms of the 3-D interpretation of visual image motion, and related to possible neural substrates.

Recursive implementations of temporal filters for image motion computation.

Efficient algorithms for image motion computation are important for computer vision applications and the modelling of biological vision systems. Intensity-based image motion computation proceeds in two stages: the convolution of linear spatiotemporal filter kernels with the image sequence, followed by the non-linear combination of the filter outputs. If the spatiotemporal extent of the filter kernels is large, then the convolution stage can be very intensive computationally. One effective means of reducing the storage required and computation involved in implementing the temporal convolutions is the introduction of recursive filtering. Non-recursive methods require the number of frames of the image sequence stored at any given time to be equal to the temporal extent of the slowest temporal filter. In contrast, recursive methods encode recent stimulus history implicitly in the values of a small number of variables updated through a series of feedback equations. Recursive filtering reduces the number of values stored in memory during convolution and the number of mathematical operations involved in computing the filters' outputs. This paper extends previous recursive implementations of gradient- and correlation-based motion analysis algorithms [Fleet DJ, Langley K (1995) IEEE PAMI 17: 61-67; Clifford CWG, Ibbotson MR, Langley K (1997) Vis Neurosci 14: 741-749], describing a recursive implementation of causal band-pass temporal filters suitable for use in energy- and phase-based algorithms for image motion computation. It is shown that the filters' temporal frequency tuning curves fit psychophysical estimates of the temporal properties of human visual filters.

Response variability and information transfer in directional neurons of the mammalian horizontal optokinetic system.

This study is concerned with how information about the direction of visual motion is encoded by motion-sensitive neurons. Motion-sensitive neurons are usually studied using stimuli unchanging in speed and direction over several seconds. Recently, it has been suggested that neuronal responses to more naturalistic stimuli cannot be understood on the basis of experiments with constant-motion stimuli (de Ruyter van Steveninck et al., 1997). We measured the variability and information content of spike trains recorded from directional neurons in the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii, in response to constant and time-varying motion. While the NOT forms part of the mammalian optokinetic system, we have shown previously that the responses of its directional neurons resemble those of insect H1 in many respects (Ibbotson et al., 1994). We find that directional neurons in the wallaby NOT respond with lower variability and higher rates of information transmission to time-varying stimuli than to constant motion. The difference in response variability is predicted by an inhomogeneous Poisson model of neuronal spiking incorporating an absolute refractory period of 2 ms during which no subsequent spike can be fired. Refractoriness imposes structure on the spike train, reducing variability (de Ruyter van Steveninck & Bialek, 1988; Berry & Meister, 1998). A given refractory period has a greater impact when firing rates are high, as for the responses of NOT neurons to time-varying stimuli. It is in just these cases that variability in experimentally observed spike trains is lowest. Thus, differences in response variability do not necessarily imply that different models are required to predict neuronal responses to constant- and time-varying motion stimuli.

A functional angle on some after-effects in cortical vision.

The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons.