Perceptual Errors Support the Notion of Masking by Object Substitution

Two experiments examined the effect of object substitution masking (OSM) on the perceptual errors in reporting the orientation of a target. In Experiment 1, a four-dot trailing mask was compared with a simultaneous-noise mask. In Experiment 2, the four-dot and noise masks were factorially varied. Responses were modelled using a mixture regression model and Bayesian inference to deduce whether the relative impacts of OSM on guessing and precision were the same as those of a noise mask, and thus whether the mechanism underpinning OSM is based on increasing noise rather than a substitution process. Across both experiments, OSM was associated with an increased guessing rate when the mask trailed target offset and a reduction in the precision of the target representation (although the latter was less reliable across the two experiments). Importantly, the noise mask also influenced both guessing and precision, but in a different manner, suggesting that OSM is not simply caused by increasing noise. In Experiment 2, the effects of OSM and simultaneous-noise interacted, suggesting the two manipulations involve common mechanisms. Overall results suggest that OSM is often a consequence of a substitution process, but there is evidence that the mask increases noise levels on trials where substitution does not occur.

Implicit Memory Formation during Routine Anesthesia in Children

Background Implicit memory cannot be consciously recalled but may be revealed by changes in behavior. There is evidence for implicit memory formation during anesthesia in adults, but several studies in children have found no evidence for implicit memory. This may be due to insensitive testing. Also many of these tests were undertaken under controlled conditions. It remains unknown whether implicit memory is formed during routine pediatric anesthesia. The aim of this study was to determine whether there is evidence of implicit memory formation during routine anesthesia in children, using a degraded auditory stimulus recognition task. Methods Three hundred and twelve children, aged 5-12 yr, were randomly assigned to be played either a sheep sound or white noise continuously through headphones during general anesthesia. No attempt was made to standardize the anesthetic. On recovery, children were played a sheep sound degraded by a white noise mask that progressively decreased over 60 s, with the outcome being the time taken to correctly recognize the sheep sound. Results Three hundred children completed the task. A comparison of the distribution of recognition times between the two groups found little evidence that exposure to a sheep sound during anesthesia was associated with postoperative time to recognition of a degraded sheep sound (hazard ratio 1.14, 95% CI of 0.90-1.43, P = 0.28). Conclusion No implicit memory formation during routine anesthesia was demonstrated in children. It is increasingly likely that the potential clinical implications of implicit memory formation are less of a concern for pediatric anesthetists.

The Effect of White-Noise Mask Level on Sinewave Contrast Detection Thresholds and the Critical-Band-Masking Model

It is known that visual noise added to sinusoidal gratings changes the typical U-shaped threshold curve which becomes flat in log-log scale for frequencies below 10c/deg when gratings are masked with white noise of high power spectral density level. These results have been explained using the critical-band-masking (CBM) model by supposing a visual filter-bank of constant relative bandwidth. However, some psychophysical and biological data support the idea of variable octave bandwidth. The CBM model has been used here to explain the progressive change of threshold curves with the noise mask level and to estimate the bandwidth of visual filters. Bayesian staircases were used in a 2IFC paradigm to measure contrast thresholds of horizontal sinusoidal gratings (0.25-8 c/deg) within a fixed Gaussian window and masked with one-dimensional, static, broadband white noise with each of five power density levels. Raw data showed that the contrast threshold curve progressively shifts upward and flattens out as the mask noise level increases. Theoretical thresholds from the CBM model were fitted simultaneously to the data at all five noise levels using visual filters with log-Gaussian gain functions. If we assume a fixed-channel detection model, the best fit was obtained when the octave bandwidth of visual filters decreases as a function of peak spatial frequency.

Masking of grating detection in the isoluminant plane of DKL color space

A novel noise-masking technique was used to test D'Zmura and Knoblauch's (1998) idea that subjects employ off-channel looking in detecting chromatic test stimuli embedded in spatiotemporal chromatic noise. Detection thresholds were obtained for stationary, isoluminant, Gaussian-windowed (σx = σy = 2.25 deg; σt = 0.25 s), 135 deg (yellow/blue) or 160 deg (orange/blue–green), sinusoidal test gratings (11 deg × 11 deg; 0.75 cycle/deg) superimposed on each of a series of dynamic, random-check chromatic noise masks varying in azimuth in DKL space. Thresholds for detecting the test in the presence of these variable masks were again measured in the presence of an additional (auxiliary) noise mask created from colors falling on azimuths of 0 deg or 90 deg (135-deg test) or 0 deg or 135 deg (160-deg test). The effectiveness, kvar, of the variable noise masks in elevating grating detection thresholds was determined by fitting the detection data to the Pelli-Legge equation relating test detection energy to variable noise-mask energy: Et = K + kvarNvar. Differences in the calculated values of kvar for detection data obtained with and without the auxiliary masks were consistent with off-channel looking and were well accounted for by a simple model based on the idea that subjects possess a multichannel array of linear chromatic detectors spanning the isoluminant plane of DKL space, and they can choose the channel that has the highest signal-to-noise ratio.