Orientation perception anisotropies indicate functional segregation within the color system.

When stimuli are luminance-defined, the visual system is known to prefer those that are radially oriented with respect to the point of fixation over tangentially oriented ones (the radial bias effect). In two contrast detection experiments and an orientation discrimination experiment, we investigated whether the radial bias effect also exists for chromatic stimuli. The contrast detection experiments revealed the radial bias effect to be color-specific; the effect was present for isoluminant red-green stimuli but absent or in the opposite direction for blue-yellow stimuli with, respectively, low (0.4 c/°) and medium (1 c/°) spatial frequencies. In agreement with previous results, we also found distinct sensitivity distributions for red-green and blue-yellow signals as a function of eccentricity. The results, thus, demonstrate a functional segregation between red-green and blue-yellow signals not only in local but also in nonlocal signal processing.

Distributed processing of color and form in the visual cortex.

To what extent does the visual system process color and form separately? Proponents of the segregation view claim that distinct regions of the cortex are dedicated to each of these two dimensions separately. However, evidence is accumulating that color and form processing may, at least to some extent, be intertwined in the brain. In this perspective, we review psychophysical and neurophysiological studies on color and form perception and evaluate their results in light of recent developments in population coding.

Relationship between neural response and adaptation selectivity to form and color: an ERP study.

Adaptation is widely used as a tool for studying selectivity to visual features. In these studies it is usually assumed that the loci of feature selective neural responses and adaptation coincide. We used an adaptation paradigm to investigate the relationship between response and adaptation selectivity in event-related potentials (ERPs). ERPs were evoked by the presentation of colored Glass patterns in a form discrimination task. Response selectivities to form and, to some extent, color of the patterns were reflected in the C1 and N1 ERP components. Adaptation selectivity to color was reflected in N1 and was followed by a late (300-500 ms after stimulus onset) effect of form adaptation. Thus for form, response and adaptation selectivity were manifested in non-overlapping intervals. These results indicate that adaptation and response selectivity can be associated with different processes. Therefore, inferring selectivity from an adaptation paradigm requires analysis of both adaptation and neural response data.

LifeScience Zurich Learning Center - a new symbiosis of research institutions and schools.

The Life Science Learning Center (LSLC) was officially founded in 2005. It is a branch of the pre-existing Life Science Zurich, an organization created by and belonging to the University of Zurich and the Swiss Federal Institute of Technology Zurich to promote and support life sciences in several central parts of society. The LSLC's primary goals are to offer educational opportunities for school children as well as continuing education for teachers of the primary and secondary school levels. In particular, the LSLC facilitates various types of interactions between schools and the higher educational and research institutions (University of Zurich and Federal Institutes of Technology): it offers practicals for pupils in a special laboratory, tours of professional research laboratories, pedagogical training for future biology teachers, and specialized modules of continuing education for teachers. It also contributes to diverse initiatives promoting life sciences in the general public. It is led by a small team of dedicated people based on the Irchel Campus of the University of Zurich.

Evidence for color and luminance invariance of global form mechanisms.

Human visual cortex contains mechanisms that pool local orientation information over large areas of visual space to support percepts of global form. Initial studies concluded that some of these mechanisms are cue invariant, in that they yield form percepts irrespective of whether the visual signals contain luminance or chromatic information. Later studies reported that these mechanisms are chromatically selective, albeit with a broad tuning in color space. We used Glass patterns and the phenomenon of adaptation to determine whether Glass pattern perception is mediated by mechanisms that are color and/or luminance selective, or not. Subjects were adapted to either a radial or concentric Glass pattern of a given color or luminance polarity. We measured the effect of adaptation on subsequent detection of Glass patterns with the same or different visual attributes. Our results show that adapting to a concentric or radial pattern significantly elevates threshold for the subsequent detection of patterns of the same form, irrespective of their color or luminance polarity, but that adaptation to luminance leads to higher threshold elevations than adaptation to color. We conclude that Glass pattern perception is mediated by perceptual mechanisms that are color invariant but not totally insensitive to the difference between color and luminance information.

An extended drawing test for the assessment of arm and hand function with a performance invariant for healthy subjects.

Impaired hand motor function resulting from neurological, psychiatric or orthopaedic disorders affects patients of all ages. Existing hand function assessment methods, e.g. rating scales, accelerometers and electromyographical devices, are often time-consuming to administer, subjective in interpretation and/or expensive. Graphonomic tests are gaining popularity as a way of avoiding these drawbacks while relating directly to writing and drawing. Here we present a computerized Extended Drawing Test (EDT), which improves on an earlier Drawing Test for stroke patients in three ways. First, it assesses isolated proximal arm movement using a graphics pen in a puck-like pen holder, and in addition combined arm and finger dexterity in movements using a normal writing grip. Secondly, we calibrated our test against 186 healthy subjects (3-70 years), finding significant age- and handedness-related differences in both speed and accuracy of drawing. Thirdly, to simplify assessment we devised an overall performance measure using a variant of Fitts' Law combining speed and accuracy, which we found to be age-independent for healthy subjects above 3 years of age. This result enables us to provide age-independent performance norms using both hands, with and without the pen holder. These norms may assist quantification of specific arm dysfunction by comparing patient performance with the healthy norms, and also by comparing within-patient performance in the dominant and non-dominant hands with and without the pen holder. Using our freely available software, this new test will allow clinicians to rapidly assess arm and hand function across a wide range of patient categories and ages.

Observing virtual arms that you imagine are yours increases the galvanic skin response to an unexpected threat.

Multi-modal visuo-tactile stimulation of the type performed in the rubber hand illusion can induce the brain to temporarily incorporate external objects into the body image. In this study we show that audio-visual stimulation combined with mental imagery more rapidly elicits an elevated physiological response (skin conductance) after an unexpected threat to a virtual limb, compared to audio-visual stimulation alone. Two groups of subjects seated in front of a monitor watched a first-person perspective view of slow movements of two virtual arms intercepting virtual balls rolling towards the viewer. One group was instructed to simply observe the movements of the two virtual arms, while the other group was instructed to observe the virtual arms and imagine that the arms were their own. After 84 seconds the right virtual arm was unexpectedly "stabbed" by a knife and began "bleeding". This aversive stimulus caused both groups to show a significant increase in skin conductance. In addition, the observation-with-imagery group showed a significantly higher skin conductance (p<0.05) than the observation-only group over a 2-second period shortly after the aversive stimulus onset. No corresponding change was found in subjects' heart rates. Our results suggest that simple visual input combined with mental imagery may induce the brain to measurably temporarily incorporate external objects into its body image.

The local and global processing of chromatic Glass patterns.

Glass patterns are a valuable tool to study the cortical stages of form perception. We use circular Glass patterns (cGP) to study the relation between form and color vision. The detection of Glass patterns is thought to be carried out in at least two stages. In the first stage, the local orientation information from the pairs of dots is analyzed. A later stage integrates this local orientation information to yield the global percept of form. Previous work (K. S. Cardinal & D. C. Kiper, 2003) has shown that the second stage is chromatically selective, with a broad tuning in color space. Here we completed our characterization of the integration stage by measuring the size of the spatial integration area. We find that the integration area is similar to the size of V4 receptive fields. Furthermore, we measured the chromatic selectivity and spatial resolution of the first stage mechanisms. First stage mechanisms are more selective for color than the integration stage. Their spatial resolution is consistent with the idea that V1/V2 neurons perform the analysis of the dot pairs' orientation. Our results are consistent with the idea that V1/V2 neurons perform the local analysis, and that spatial integration is achieved at the level of V4.

Motion-induced blindness does not affect the formation of negative afterimages.

Aftereffects induced by invisible stimuli constitute a powerful tool to investigate what type of neural information processing can occur in the absence of visual awareness. This approach has been successfully used to demonstrate that awareness of oriented gratings or translating stimuli is not necessary to obtain a robust orientation-specific or motion-specific aftereffect. We exploit motion-induced blindness (MIB, Bonneh, Cooperman, & Sagi, 2001) to investigate the related question of the influence of visual awareness on the formation of negative afterimages. Our results show that MIB does not affect the persistence and intensity of afterimages. Thus, there is no significant contribution to the formation of afterimages beyond the sites mediating MIB.

The detection of colored Glass patterns.

The detection of many chromatic stimuli is mediated by mechanisms that sum their inputs linearly. As a result, these mechanisms have a broad range of selectivity in color space, as do the majority of cells in the early stages of visual processing. In extrastriate cortex, there are cells with a narrow tuning in color space. The function of these cells is not fully understood: they could be involved in color categorization, or could mediate the detection of stimuli such as Glass patterns, whose properties make them undetectable by early stages of processing. We measured the tuning properties of the mechanisms responsible for the detection of colored Glass patterns and found that they have a broad tuning in color space. Our results suggest that Glass patterns are detected by a multitude of mechanisms that sum their inputs linearly.

Color vision.

Color vision starts with the absorption of light in the retinal cone photoreceptors, which transduce electromagnetic energy into electrical voltages. These voltages are transformed into action potentials by a complicated network of cells in the retina. The information is sent to the visual cortex via the lateral geniculate nucleus (LGN) in three separate color-opponent channels that have been characterized psychophysically, physiologically, and computationally. The properties of cells in the retina and LGN account for a surprisingly large body of psychophysical literature. This suggests that several fundamental computations involved in color perception occur at early levels of processing. In the cortex, information from the three retino-geniculate channels is combined to enable perception of a large variety of different hues. Furthermore, recent evidence suggests that color analysis and coding cannot be separated from the analysis and coding of other visual attributes such as form and motion. Though there are some brain areas that are more sensitive to color than others, color vision emerges through the combined activity of neurons in many different areas.

The effect of moving textures on the responses of cells in the cat's dorsal lateral geniculate nucleus.

Neurons in the dorsal lateral geniculate nucleus (dLGN) of the anaesthetized cat were activated with test stimuli (flashing spots, counterphased gratings and moving bars) in the presence of a moving background texture. Moving texture alone produced mild excitation, as a result of stimulation of the receptive field centre. Fast moving coarse textures were more effective than fine slow moving textures. The predominant effect of texture motion, however, was to reduce the response to all test stimuli displayed in the receptive field centre. The effects were similar for X- and Y-like cells. In the case of flashed spots, the sustained response was more strongly suppressed than the transient response. The direction of motion of the texture and differences in the relative motion of bar and texture had no influence on the degree of suppression. These observations are similar to effects seen on cat retinal ganglion cells, and are probably a form of gain control. Such suppressive effects are transmitted to the cortex and are likely to be evoked by large gratings, textures and by natural stimuli, all of which activate extensive regions of the receptive field surround.

Testing the Bayesian model of perceived speed.

In a recent Bayesian model by Weiss, Simoncelli, and Adelson, motion perception is biased by a prior favoring slow speeds. This model predicts qualitatively an impressive variety of phenomena, including the dependence of perceived speed on contrast. We show that the model can also generate quantitative predictions: for a drifting grating with contrast c, perceived speed is proportional to c(q)/(k(q) + c(q)), with k, q constants. We tested this expression on measurements of perceived speed as a function of contrast. Observers indicated the slower of two drifting gratings, a test and a standard. For each test contrast we found the test speed that appeared to match the standard speed. The model fits the data, but only if q is less than 2, the value it would have if the internal representation of contrast were linear. The Bayesian model can make correct quantitative predictions, but needs to be extended to incorporate a more realistic, nonlinear representation of contrast.

Suppression without inhibition in visual cortex.

Neurons in primary visual cortex (V1) are thought to receive inhibition from other V1 neurons selective for a variety of orientations. Evidence for this inhibition is commonly found in cross-orientation suppression: responses of a V1 neuron to optimally oriented bars are suppressed by superimposed mask bars of different orientation. We show, however, that suppression is unlikely to result from intracortical inhibition. First, suppression can be obtained with masks drifting too rapidly to elicit much of a response in cortex. Second, suppression is immune to hyperpolarization (through visual adaptation) of cortical neurons responding to the mask. Signals mediating suppression might originate in thalamus, rather than in cortex. Thalamic neurons exhibit some suppression; additional suppression might arise from depression at thalamocortical synapses. The mechanisms of suppression are subcortical and possibly include the very first synapse into cortex.