Global Shape Processing Research Articles

Apparent size of an object remains uncompressed during presaccadic compression of visual space

It is well known that compression of visual space occurs near the saccade goal when visual stimuli are briefly flashed at various locations on a visual reference just before a saccade. We investigated how presaccadic compression of visual space affected the apparent size of an object. In the first experiment, subjects were instructed to report the apparent number of multiple bars briefly presented around the time of saccade onset. The reported number of four bars began to decline at the 50 ms mark before a saccade and reached a minimum near the saccade onset. This confirms that the compression of visual space occurs just before saccades. In the second experiment, subjects judged the apparent width of a rectangle (a single element) or four bars (four elements) presented just before saccades. The apparent width of the four-bar stimulus was compressed just before saccades, but that of the rectangle stimulus was not compressed. Experiment 3 shows that the width compression of the four-bar stimulus is consistent with the width change predicted by compression of position. These findings indicate that the shape of a single object is not distorted at the saccade goal during presaccadic compression of visual space. In addition, experiment 4 indicates that the apparent width of a flashed stimulus just before saccades depends on the processing of global shape. This extends the definition of a visual object during presaccadic compression of visual space to not only a solid element but also a constellation of multiple elements. Furthermore, the results from these experiments suggest that presaccadic compression of visual space does not prevent object recognition underlying an attentional mechanism in generating saccadic eye movements.

Differential effects of word length and visual contrast in the fusiform and lingual gyri during reading.

Previous studies have shown differential responses in the fusiform and lingual gyri during reading and suggested that the former is engaged in processing local features of visual stimuli and the latter is engaged in global shape processing. We used positron emission tomography in order to investigate how these regions are modulated by two common variables in reading: word length (three, six and nine letters) and perceptive similarity to the background (high and low contrast). Increasing both word length and visual contrast had a positive monotonic effect on activation in the bilateral fusiform. However, in the lingual gyrus, activation increased with increasing word length but decreased with increasing contrast. On the basis of previous studies, we suggest that (i) increasing word length increases the demands on both local feature and global shape processing, but (ii) increasing visual contrast increases the demands on local feature processing while decreasing the demands on global shape processing.

Global word shape processing in off-line recognition of handwriting

Off-line recognition of handwriting may be achieved using simplified profiles of word shapes. These profiles consist of approximations of the word's upper and lower contour. Training and recognition are based on n-gram extraction and identification. The lexicons used extend to 16,000 words.

Perceptual sensitivity maps within globally defined visual shapes.

An unsolved problem of biology is the processing of global shape in natural vision. The known processes of early vision are spatially restricted (or local) operations, and little is known about their interactions in organizing the visual image into functionally coherent (or global) objects. Here we introduce a human psychophysical method which allows us to measure the effect of perceptual organization on the activity pattern of local visual detectors. We map differential contrast sensitivity for a target across regions enclosed by a boundary. We show that local contrast sensitivity is enhanced within the boundary even for large distances between the boundary and the target. Furthermore, the locations of maximal sensitivity enhancement in the sensitivity maps are determined by global shape properties. Our data support a class of models which describe shapes by the means of a medial axis transformation, implying that the visual system extracts 'skeletons' as an intermediate-level representation of objects. The skeletal representation offers a structurally simplified shape description which can be used for higher-level operations and for coding into memory.

The effects of noise on the processing of global shape and local detail.

Two experiments investigated the effects of noise on the processing of global shape and local detail. Both experiments showed that noise improved ordered recall of local features but impaired recall of global shape, although the extent of the effect of noise was influenced by the size of the compound stimuli. Noise did not produce the same effects as those obtained when attention is manipulated by instructions, and it is possible that noise produced a change in the more passive aspect of attention rather than altering the active allocation of attentional resources.