Neuromagnetic correlates of visual motion coherence.

In order to characterize cortical responses to coherent motion we use magnetoencephalography (MEG) to measure human brain activity that is modulated by the degree of global coherence in a visual motion stimulus. Five subjects passively viewed two-phase motion sequences of sparse random dot fields. In the first (incoherent) phase the dots moved in random directions; in the second (coherent) phase a variable percentage of dots moved uniformly in one direction while the others moved randomly. We show that: (i) visual-motion-evoked magnetic fields, measured with a whole-scalp neuromagnetometer, reveal two transient events, within which we identify two significant peaks--the 'ON-M220' peak approximately 220 ms after the onset of incoherent motion and the 'TR-M230' peak, approximately 230 ms after the transition from incoherent to coherent motion; (ii) in lateral occipital channels, the TR-M230 peak amplitude varies with the percentage of motion coherence; (iii) two main sources are active in response to the transition from incoherent to coherent motion, the human medial temporal area complex/V3 accessory area (hMT+/V3A) and the superior temporal sulcus (STS), and (iv) these distinct areas show a similar, significant dependence of response strength and latency on motion coherence.

Perception of three-dimensional shape influences colour perception through mutual illumination.

Objects in the natural world possess different visual attributes, including shape, colour, surface texture and motion. Previous perceptual studies have assumed that the brain analyses the colour of a surface independently of its three-dimensional shape and viewing geometry, although there are neural connections between colour and two-dimensional form processing early in the visual pathway. Here we show that colour perception is strongly influenced by three-dimensional shape perception in a novel, chromatic version of the Mach Card--a concave folded card with one side made of magenta paper and the other of white paper. The light reflected from the magenta paper casts a pinkish glow on the white side. The perceived colour of the white side changes from pale pink to deep magenta when the perceived shape of the card flips from concave to convex. The effect demonstrates that the human visual system incorporates knowledge of mutual illumination-the physics of light reflection between surfaces--at an early stage in colour perception.

Discrimination of cone contrast changes as evidence for colour constancy in cerebral achromatopsia.

One proposed mechanism for underpinning colour constancy is computation of the relative activity of cones within one class--cone ratios, or cone contrasts--between surfaces in a fixed scene undergoing a change in illuminant. Although there is evidence that cone ratios do determine colour appearance under many conditions, the site or sites of their computation is unknown. Here, we report that a cerebrally achromatopsic observer, MS, displayed evidence of colour constancy in asymmetric colour matching tasks and was able to discriminate changes in cone ratios for simple, but not complex scenes. We hypothesise that the site of local cone-ratio computation is therefore early in the visual system, probably retinal.

Motion edges and regions guide image segmentation by colour.

Image segmentation is an important early stage in visual processing in which the visual system groups together parts of the image that belong together, prior to or in conjunction with object recognition. Two principal processes may be involved in image segmentation: an edge-based process that uses feature contrasts to mark boundaries of coherent regions, and a region-based process that groups similar features over a larger scale. Earlier, we have shown that motion and colour interact strongly in image segmentation by the human visual system. Here we explore the nature of this interaction in terms of edge- and region-based processes. We measure performance on a region-based colour segmentation task in the presence of distinct types of motion information, in the form of edges and regions which in themselves do not reveal the location of the colour target. The results show that both motion edges and regions may guide the integrative process required for this colour segmentation task. Motion edges appear to act by delimiting areas over which to integrate colour information, whereas motion similarities define primitive surfaces within which colour grouping and segmentation processes are deployed.

Psychophysical evidence for fast region-based segmentation processes in motion and color.

Theories of image segmentation suggest that the human visual system may use two distinct processes to segregate figure from background: a local process that uses local feature contrasts to mark borders of coherent regions and a global process that groups similar features over a larger spatial scale. We performed psychophysical experiments to determine whether and to what extent the global similarity process contributes to image segmentation by motion and color. Our results show that for color, as well as for motion, segmentation occurs first by an integrative process on a coarse spatial scale, demonstrating that for both modalities the global process is faster than one based on local feature contrasts. Segmentation by motion builds up over time, whereas segmentation by color does not, indicating a fundamental difference between the modalities. Our data suggest that segmentation by motion proceeds first via a cooperative linking over space of local motion signals, generating almost immediate perceptual coherence even of physically incoherent signals. This global segmentation process occurs faster than the detection of absolute motion, providing further evidence for the existence of two motion processes with distinct dynamic properties.

Measurements of colour constancy by using a forced-choice matching technique.

Colour constancy is typically measured with techniques involving asymmetric matching by adjustment, in which the observer views two scenes under different illuminants and adjusts the colour of a reference patch in one to match a test patch in the other. This technique involves an unnatural task, requiring the observer to predict and adjust colour appearance under an illumination shift. Natural colour constancy is more a simple matter of determining whether a colour is the same as or different from that seen under different illumination conditions. There are also technical disadvantages to the method of matching by adjustment, particularly when used to measure colour constancy in complex scenes. Therefore, we have developed and tested a two-dimensional method of constant-stimuli, forced-choice matching paradigm for measuring colour constancy. Observers view test and reference scenes haploscopically and simultaneously, each eye maintaining separate adaptation throughout a session. On each trial, a pair of test and reference patches against multicoloured backgrounds are presented, the reference patch colours being selected from a two-dimensional grid of displayable colours around the point of perfect colour constancy. The observer's task is to respond "same" or "different". Fitting a two-dimensional Gaussian to the percentage of "different" responses yields (1) the subjective colour-constancy point, (2) the discrimination ellipse centred on this point, and (3) a map of changes in sensitivity to chromatic differences induced by the illuminant shift. The subjective colour-constancy point measured in this way shows smaller deviations from perfect colour constancy-under conditions of monocular adaptation-than previously reported; discrimination ellipses are several times larger than standard MacAdam ellipses; and chromatic sensitivity is independent of the direction of the illuminant shift, for broad distributions of background colours.

Visual perception. Knowing is seeing.

New visual illusions provide further evidence for the influence of higher-order analyses of the visual scene on the perceived brightnesses of surfaces within that scene.

Perceptual deficits and the activity of the color-opponent and broad-band pathways at isoluminance.

The deficits in texture, motion, and depth perception incurred in monkeys at isoluminance were compared with the responses of neurons of the color-opponent and broad-band systems in the lateral geniculate nucleus. Texture perception, assumed to be carried by the color-opponent system, and motion and depth perception, ascribed to the broad-band pathway, were all found to be compromised but not abolished at isoluminance. Correspondingly, both the color-opponent and the broad-band systems were affected at isoluminance, but the activity of the neurons in neither system was abolished. These results suggest that impairment of visual capacities at isoluminance cannot be uniquely attributed to either of these systems and that isoluminant stimuli are inappropriate for the psychophysical isolation of these pathways.

Synthesizing a color algorithm from examples.

A lightness algorithm that separates surface reflectance from illumination in a Mondrian world is synthesized automatically from a set of examples, which consist of pairs of input (intensity signal) and desired output (surface reflectance) images. The algorithm, which resembles a new lightness algorithm recently proposed by Land, is approximately equivalent to filtering the image through a center-surround receptive field in individual chromatic channels. The synthesizing technique, optimal linear estimation, requires only one assumption, that the operator that transforms input into output is linear. This assumption is true for a certain class of early vision algorithms that may therefore be synthesized in a similar way from examples. Other methods of synthesizing algorithms from examples, or "learning," such as back-propagation, do not yield a significantly better lightness algorithm.