Decrease in metacontrast masking following adaptation to flicker

Selective adaptation was used to explore the characterisitcs of a metacontrast masking stimulus which contribute to its effectiveness in masking the test stimulus. Subjects adapted for 10 s to a configuration like the masking stimulus that was either continuously on or flickering. Following this they viewed a metacontrast presentation and estimated the brightness of the test stimulus. Prior adaptation to a continuously present stimulus did not appreciably affect metacontrast masking; however, masking was greatly reduced following adaptation to flickering stimuli. These results are consistent with recent models of metacontrast masking based on transient and sustained visual channels.     

Dominance of persistence over adaptation in forward masking

Persistence of excitation and neural adaptation are competing theories proposed to explain the mechanisms underlying psychophysical forward masking. Previous research has been directed towards finding models that accurately describe the phenomenon but cannot account for the underlying explanation. The current study was designed to determine which theory best accounts for results obtained from behavioral gap duration adjustment tasks. Thirteen adults adjusted the gap within asymmetrical noise markers to be subjectively equal to the gap within equal-intensity-noise markers. The duration of the perceived gap between the asymmetrical markers is expected to vary depending on which theory dominates perception. The persistence of excitation mechanism would lead to longer duration gaps when the second noise marker is lower in intensity than the preceding. Neural adaptation would result in matched gaps that are shorter in duration when the second noise marker was lower in level. The outcome of our data analysis is consistent with the persistence of excitation as a dominant mechanism in forward masking.     

An attempt to simulate letter-by-letter dyslexia in normal readers

We attempted to simulate the main features of letter-by-letter (LBL) dyslexia in normal readers through stimulus degradation (i.e. contrast reduction and removal of high spatial frequencies). The results showed the word length and the letter confusability effects characteristic of LBL dyslexia. However, the interaction of letter confusability and N size (i.e. a facilitatory effect only for low confusability targets) previously observed in LBL dyslexics [Arguin, M., Fiset, S., & Bub, D. (2002). Sequential and parallel letter processing in letter-by-letter reading. Cognitive Neuropsychology, 19, 535-555; Arguin, M., & Bub, D. (2006). Parallel processing blocked by letter similarity in letter dyslexia: a replication. Cognitive Neuropsychology, 22, 589-602; Fiset, D., Arguin, M. & McCabe, E. (2005a). The breakdown of parallel letter processing in letter-by-letter dyslexia. Cognitive Neuropsychology, 22, 1-22] was not found. Our results suggest that the type of visual degradation employed here may only partially correspond to the visual deficit present in LBL dyslexia and that this degradation may have prevented the normal readers from accessing visual information available to LBL dyslexics when they use the compensatory strategy of serial letter processing.     

An attempt to condition components of urine formation in dogs

Previous research on the renal conditional reflex (CR) has resulted in contradictory data. The work of Bykov with dogs and of Hoferet al. with humans indicated that a renal CR is possible, but the experiments of Corsonet al. and Watt and Gantt with dogs having normal or transplanted cervical kidneys showed no clear-cut renal CR’s. The present work supports the latter finding. Three female mongrel dogs were used. One dog (normal) had an externalized ureter; the other two (experimental) had denervated cervical kidneys (autografts). In 155 days of experiments, the investigators found diuresis to occur only on unconditional stimulus,i.e., only when a dog drank diluted milk (one part milk to four parts water)ad lib.; the dogs drank an average of between 500 and 1,100 ml. No diuresis occurred when 30 ml. of the mixture were given (conditional stimulus). Urinalysis indicated a lack of renal conditioning with respect to the excretion rates and the concentrations of creatinine and electrolytes, except for the sodium concentration in the normal dog’s urine, which decreased on bothad lib and test (30 ml) days; the volume was not affected. Conclusion: Drinking a small quantity of diluted milk as a conditional stimulus did not produce a conditional diuretic response either in the normal dog or in the two dogs with transplanted kidneys.     

Loss of wavelength selectivity in contour masking and aftereffect following dichoptic adaptation

Two experiments measured the apparent orientation (aftereffect) and the threshold for detection (masking) of a colored grating viewed by one eye after exposure to a colored grating to the same or the opposite eye (monoptic inspection) or after stimulation of one eye by color and the other eye by contours (dichoptic inspection). Under the monoptic condition, the color relationship between the inspection and test stimuli exerted control over the extent of aftereffect and masking when the two stimuli were viewed with the same eye, but not when they were seen with different eyes. Aftereffect and masking were nonselective to wavelength following dichoptic inspection, irrespective of whether the test stimulus was presented to the color-adapted or to the contour-adapted eye. The results support other claims that visual detectors with chromatic and spatial tuning have monocular specificity.