Spatial frequency, orientation and color: interocular effects of adaptation on the perceived duration of gratings

The Poggendorff illusion driven by real and illusory contour: Behavioral and neural mechanisms

The Poggendorff illusion refers to the phenomenon that the human brain misperceives a diagonal line as being apparently misaligned once the diagonal line is interrupted by two parallel edges, and the size of illusion is negatively correlated with the angle of interception of the oblique, i.e. the sharper the oblique angle, the larger the illusion. This optical illusion can be produced by both real and illusory contour. In this fMRI study, by parametrically varying the oblique angle, we investigated the shared and specific neural mechanisms underlying the Poggendorff illusion induced by real and illusory contour. At the behavioral level, not only the real but also the illusory contours were capable of inducing significant Poggendorff illusion. The size of illusion induced by the real contour, however, was larger than that induced by the illusory contour. At the neural level, real and illusory contours commonly activated more dorsal visual areas, and the real contours specifically activated more ventral visual areas. More importantly, examinations on the parametric modulation effects of the size of illusion revealed the specific neural mechanisms underlying the Poggendorff illusion induced by the real and the illusory contours, respectively. Left precentral gyrus and right middle occipital cortex were specifically involved in the Poggendorff illusion induced by the real contour. On the other hand, bilateral intraparietal sulcus (IPS) and right lateral occipital complex (LOC) were specifically involved in the Poggendorff illusion induced by the illusory contour. Functional implications of the above findings were further discussed.

Motion after-effects and word contingency

Stimulus-selectivity in phenomena such as the McCollough effect and other contingent after effects are controversial. Word specific McCollough effects have been reported (Allan et al., Percept Psychophys 1989;45:104-113) that suggest an associative model rather then a neural one. However, failures to replicate make this finding controversial (Humphrey et al., J Exp Psychol: Gen 123:86-90). We applied the same contingency to the motion after-effect. Moving words, words paired with sine wave gratings and words composed of sine wave gratings failed to generate text contingent after-effects in stimulus situations that normally evoke motion after-effects. Thus, there was little evidence that motion adaptation can be made textually contingent.

Inhibitory circuits accounting for development of visual cortical mappings, stimulus preferences, and psychophysical performance

A developmental rationale is proposed for the circuitry underlying the generation of fine retinotopic mappings, the quantitative range of simple-cell stimulus preferences, and the psychophysical performance of the visual system. It is assumed that the retina consists of a mosaic of partially overlapping elements, or hyperfields, which are laid down in a sunflower-seed pattern. These hyperfields project to a corresponding rectilinear mosaic of hypercolumns in the cortex, according to a pattern of chemoaffinities. Each hyperfield, in turn, consists of a sunflower-seed mosaic of nonoverlapping ganglion-cell receptive-field centres, which project to a matching rectilinear mosaic of minicolumns in the corresponding hypercolumn. Retinotopic order is produced in the hyperfield-hypercolumn mapping by radially symmetric inhibitory links, between cortical cells more than two minicolumns apart, which operate on Hebb-modifiable retinocortical excitatory afferent fibres. Under this mapping, hyperfield radii map onto parallel rows of minicolumns (orientation columns), and concentric semicircles of ganglion-cell receptive fields map onto spatial-frequency columns, crossing orientation columns at right angles. The 'scatter' in this mapping is equivalent to one local average receptive-field diameter. Orientation-related stimulus preferences ar produced by asymmetrical inhibitory links between cells more than two minicolumns apart, in the same spatial-frequency columns. A third network of inhibitory circuits, with Hebb-modifiable synapses, is assumed to operate between cells in the same or immediately adjacent minicolumns. This network enhances stimulus selectivity and sensitivity in simple and hypercomplex cells, and is responsible for adaptation aftereffects and sensory information storage.