Is the perception of brightness different in poor readers?

The transient system deficit hypothesis (TSDH) of specific reading disability [Percept. Psychophys. 40 (1986) 440] remains contentious. As part of a study examining multiple measures of transient and sustained system function, heterochromatic flicker matching (HFM) and brightness matching (HBM) were assessed in 30 poor readers (9.11+/-0.68 years) and 30 age, grade and sex matched controls (9.24+/-0.73 years). HBM and HFM are known to reflect the processing of brightness and luminance information and have been related to the function of magnocellular and parvocellular visual sub-systems. Flicker and brightness matches were determined for blue, green, yellow and red stimuli on Macintosh colour displays using 2AFC and double interleaved random staircases. A ratio of the luminances for brightness and flicker matches represented performance. A significant difference between controls and poor readers in performance for red and blue stimuli was found indicating different visual function in poor readers. While not providing direct support for the transient system deficit hypothesis, this effect implies a mismatch between those achromatic systems that subserve HFM and those more complex mechanisms involved in HBM. The most important aspect of this finding is that poor readers and normal controls could be differentiated on the basis of a paradigm known to be contingent upon magnocellular and parvocellular functioning.

Human exposure to 4.0-Tesla magnetic fields in a whole-body scanner.

Details are given for the design, construction, properties, and performance of a large, highly homogeneous magnet designed to permit whole-body magnetic resonance imaging and spectroscopy at 4 T. The magnet has an inductance of 1289 H and a stored energy of 33.4 MJ at rated field. The health of a group of 11 volunteers who had varying degrees of exposure to this field was followed over a 12-month period and no change that could be associated with this exposure was detected. A mild level of sensory experiences, apparently associated with motion within the field of the magnet, was reported by some of the volunteers during some of their exposures. A questionnaire regarding sensory effects associated with magnetic resonance scanners and possibly caused by the static magnetic field of these instruments, was given to nine respondents who had experience within both 1.5-T scanners and this 4-T scanner and to another group of 24 respondents who had experience only within 1.5-T scanners. For the sensations of vertigo, nausea, and metallic taste there was statistically significant (p less than 0.05) evidence for a field-dependent effect that was greater at 4 T. In addition, there was evidence for motion-induced magnetophosphenes caused by motion of the eyes within the static field. These results indicate the practicality of experimental whole-body body scanners operating at 4 T and the possibility of mild sensory effects in humans associated with motion within a static magnetic field. The results also indicate the likelihood of a wide margin of safety for the exposure of noncompromised patients to the static fields of conventional magnetic resonance scanners operated at 1.5 to 2 T and below.