Visual persistence of spatially filtered images

A low-cost, accurate method of producing large quantities of digitally filtered images

We demonstrate how to produce complex image transformations of bitmap files for vision experiments using the Cogimatic Vision Starter Kit (VSK) library of mathematical routines along with Visual Basic, C++, or the Delphi Pascal compiler. Implementing this system on an IBM-compatible PC running Windows 95, 98, or NT4 enables researchers to quickly and economically manipulate images for vision research. The VSK includes a simple stand-alone image-processing application. In addition, VSK has the ability to automate image transformations and to fully integrate image processing into new experimental software on the PC platform.

An investigation of some sensory and refractive visual factors in dyslexia

The role of visual factors in dyslexia has been a long-standing source of controversy. Recent research has suggested that there may be a deficit of the transient visual subsystem in dyslexia. The evidence for this hypothesis comes principally from investigations of spatial and temporal contrast sensitivity and visual persistence. This evidence is reviewed and it is noted that previous work has never applied two of these purported "tests of transient function" to the same subject group. The hypothesised transient system deficit in dyslexia was investigated in a study comparing 43 control with 39 dyslexic children who were matched for age, sex, and intelligence. Comprehensive psychometric and optometric data were obtained, including visual acuities and refractive errors. The spatial contrast sensitivity function was determined in such a way as to investigate further the findings of Lovegrove, Martin, Bowling, Blackwood, Badcock and Paxton [(1982) Neuropsychology, 20, 309-315] and Martin and Lovegrove [(1984) Neuropsychologia, 22, 73-77]. It might be expected, from the work of Merigan and Maunsell [(1990) Neuroscience, 5, 347-352], that a better test of magno-cellular function would be to investigate the modulation threshold for a virtually uniform field that was flickering sinusoidally at 10 Hz. This temporal contrast sensitivity was studied in a similar way to Brannan and Williams [(1988) Clinical Vision Sciences, 3, 137-142]. A non-verbal simulated reading visual search task was used to investigate the effect of any visual deficits on a test that was, in its low-level visual requirements, similar to reading. The following factors were found to be significantly associated with dyslexia: reduced visual acuity, impaired flicker detection at 10 Hz, reduced low spatial frequency contrast sensitivity, and slightly slower performance at a simulated reading visual search task. The two alleged "tests of transient function" were only weakly correlated with one another (r = 0.183), suggesting that these variables do not measure the same function. Much of the dyslexic group's slightly slower performance at the simulated reading task could be accounted for by the psychometric variable of visual sequential memory. Like reading, the simulated reading task requires the accurate perception of sequential characters. Hence, it seems unlikely that the low-level visual deficits in the dyslexic group were major causes of their poor reading performance. Alternative explanations for the results are discussed.

Apparent duration and spatial structure

In three experiments, we investigated the relative perceived duration of a full bandwidth image and a set of high- and lowpass filtered images of a scene, briefly presented on a visual display unit. In Experiment 1, the various images were compared with each other, using a paired comparison method. All images were presented for 40 msec, and observers were asked to judge which of each pair of images had the longest duration. The results showed that images containing a wide spatial frequency bandwidth were judged to be of longer duration than were images of a narrower bandwidth, regardless of whether the latter were high- or lowpass filtered. In Experiment 2, a 40-msec presentation of each of the images was compared with a presentation of a probe that was 20, 40, 60, or 80 msec in duration. Observers again judged which of each pair of images had the longest duration. The results were very similar to those of Experiment 1, with wide bandwidth images being judged to be of longer duration than were narrow bandwidth images. In Experiment 3, instead of comparing the various filtered versions of the image with each other, we attempted to obtain a direct measure of perceived duration by comparing a flashing LED to a 40-msec flash of a subset of the images used in the previous experiments. The observers' task was to adjust the duration of the LED flash to match the perceived duration of each image.(ABSTRACT TRUNCATED AT 250 WORDS)