Spatial displacement, but not temporal asynchrony, destroys figural binding

What are the elementary features that the brain uses to bind spatially distinct parts in a visual scene into an unitary percept of an “object”? The Gestalt psychologists emphasized the extent to which motion, colour, luminance or spatial arrangement contribute towards object formation. Little is known about the role of timeper se, rather than motion, in constituting an object. In particular, does the visibility or saliency of an object change if the various parts making up the object are not presented simultaneously? Using a simple experimental design, we show that very small spatial displacements can significantly influence the saliency of an object while large temporal asynchrony has no significant effect

Performance asymmetries in visual search demonstrate failure of independent-processing models

We report psychophysical data from orientation-popout experiments that are inconsistent with a rather general decision model. Stimuli consisted of 121 line segments arranged on an 11×11 grid. There were two tasks: in the 1-Singleton Task all lines except one had the same orientation, and observers had to report which quadrant contained the singleton. In the 3-Singleton Task three quadrants contained orientation singletons and observers had to identify the quadrant without singleton. These tasks can be viewed as asymmetric search tasks, in which either a singleton-quadrant has to be found among three homogeneous quadrants, or a homogeneous quadrant has to be found among three singleton-quadrants. Using tools from signal-detection theory we show that the large performance asymmetries between 1-Singleton and 3-Singleton Tasks are inconsistent with any model that makes two (very basic and common) assumptions: (1) independent processing of the four quadrants and (2) an ideal-observer decision. We conclude that at least one of the two assumptions is inadequate. As a plausible reason for the model failure we suggest a global competition between salient elements that reduces popout strength when more than one singleton is present

No transfer of perceptual learning between similar stimuli in the same retinal position

AbstractBackground: Recent experiments have demonstrated a remarkable amount of specificity in the learning of simple visual tasks in humans, as well as considerable plasticity of receptive fields in the visual cortex of adult monkeys. Here, we tested the specificity of improvement through learning in the performance of human observers on two tasks using almost identical stimuli.Results Two groups, of six observers each, were trained in two hyperacuity tasks –  three-dot bisection and three-dot vernier discrimination. The groups started with different tasks, and switched tasks after one hour of training. Training improved performance significantly, in spite of considerable variability between observers, but improvement did not generalize from one of these tasks to the other. This result indicates that perceptual learning can be extremely stimulus specific, and that deviations from the same standard but in orthogonal directions require completely new training.Conclusion Learning is not based on the development of a more exact map of positional information, or on training to fixate or accommodate the eye, but on a better discrimination between the stimuli using one specific stimulus dimension. We also demonstrate that observers differ considerably, not only in their speed of learning, but also in their relative level of performance on the two similar tasks

Computation of Texture and Stereoscopic Depth in Humans

The computation of texture and of stereoscopic depth is limited by a number of factors in the design of the optical front-end and subsequent processing stages in humans and machines. A number of limiting factors in the human visual system, such as resolution of the optics and opto-electronic interface, contrast, luminance, temporal resolution and eccentricity are reviewed and evaluated concerning their relevance for the recognition of texture and stereoscopic depth. The algorithms used by the human brain to discriminate between textures and to compute stereoscopic depth are very fast and efficient. Their study might be beneficial for the development of better algorithms in machine vision

Display symmetry affects positional specificity in same–different judgment of pairs of novel visual patterns

AbstractDeciding whether a novel visual pattern is the same as or different from a previously seen reference is easier if both stimuli are presented to the same rather than to different locations in the field of view (Foster & Kahn (1985). Biological Cybernetics, 51, 305–312; Dill & Fahle (1998). Perception and Psychophysics, 60, 65–81). We investigated whether pattern symmetry interacts with the effect of translation. Patterns were small dot-clouds which could be mirror-symmetric or asymmetric. Translations were displacements of the visual pattern symmetrically across the fovea, either left–right or above–below. We found that same–different discriminations were worse (less accurate and slower) for translated patterns, to an extent which in general was not influenced by pattern symmetry, or pattern orientation, or direction of displacement. However, if the displaced pattern was a mirror image of the original one (along the trajectory of the displacement), then performance was largely invariant to translation. Both positional specificity and its reduction in symmetric displays may be explained by location-specific pre-processing of the visual input

Spatial attention increases performance but not subjective confidence in a discrimination task

Selective attention to a target yields faster and more accurate responses. Faster response times, in turn, are usually associated with increased subjective confidence. Could the decrease in reaction time in the presence of attention therefore simply reflect a shift toward more confident responses? We here addressed the extent to which attention modulates accuracy, processing speed, and confidence independently. To probe the effect of spatial attention on performance, we used two attentional manipulations of a visual orientation discrimination task. We demonstrate that spatial attention significantly increases accuracy, whereas subjective confidence measures reveal overconfidence in non-attended stimuli. At constant confidence levels, reaction times showed a significant decrease (by 15–49%, corresponding to 100–250 ms). This dissociation of objective performance and subjective confidence suggests that attention and awareness, as measured by confidence, are distinct, albeit related, phenomena

Effects of biased feedback on learning and deciding in a vernier discrimination task

AbstractWe investigate the influence of biased feedback on decision and learning processes in a vernier discrimination task. Subjects adjust their decision criteria and hence their responses according to biased external feedback. However, they do not use learning processes to encode incorrectly classified stimuli. As soon as correct feedback is restored observers regain their original performance indicating an involvement of internal criteria. If the external feedback is switched off instead of being corrected, the rebound is less vigorous. The findings contradict predictions of supervised neural network models

Spatial aspects of object formation revealed by a new illusion, shine-through

When a vernier stimulus is presented for a short time and followed by a grating comprising five straight lines, the vernier remains invisible but may bequeath its offset to the grating (feature inheritance). For more than seven grating elements, the vernier is rendered visible as a shine-through element. However, shine-through depends strongly on the spatio-temporal layout of the grating. Here, we show that spatially inhomogeneous gratings diminish shine-through and vernier discrimination. Even subtle deviations, in the range of a few minutes of arc, matter. However, longer presentation times of the vernier regenerate shine-through. Feature inheritance and shine-through may become a useful tool in investigating such different topics as time course of information processing, feature binding, attention, and masking

Shine-through: temporal aspects

If a vernier stimulus precedes a grating for a very short time, the vernier either remains invisible, but may bequeath some of its properties to the grating (feature inheritance), or might shine through keeping its features — depending on the number of grating elements [Herzog, M. H. & Koch, C., 2001. Seeing properities of an invisible element: feature inheritance and shine-through. Proceedings of the Natlional Academy of Science USA 98, 4271–4275]. Feature inheritance and shine-through represent two different states of feature binding [Herzog, M. H., Koch, C., & Fahle, M., Switching binding states. Visual Cognition (in press)], whereas shine-through depends in subtle ways on the spatial layout of the grating [Herzog, M. H., Fahle, M., & Koch, C., (2001). Spatial aspects of object formation revealed by a new illusion, shine-through. Vision Research]. Here, we show that also temporal parameters of the grating influence shine-through. For example, a delayed presentation of certain grating elements can deteriorate performance dramatically

How Much of the “Unconscious” is Just Pre – Threshold?

Visual awareness is a specific form of consciousness. Binocular rivalry, the alternation of visual consciousness resulting when the two eyes view differing stimuli, allows one to experimentally investigate visual awareness. Observers usually indicate the gradual changes of conscious perception in binocular rivalry by a binary measure: pressing a button. However, in our experiments we used gradual measures such as pupil and joystick movements and found reactions to start around 590 ms before observers press a button, apparently accessing even pre-conscious processes. Our gradual measures permit monitoring the somewhat gradual built-up of decision processes. Therefore these decision processes should not be considered as abrupt events. This is best illustrated by the fact that the process to take a decision may start but then stop before an action has been taken – which we will call an abandoned decision process here. Changes in analog measures occurring before button presses by which observers have to communicate that a decision process has taken place do not prove that these decisions are taken by a force other than the observer – hence eliminating “free will” – but just that they are prepared “pre-thresholdly,” before the observer considers the decision as taken