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J Nachmias

Publications and source records attributed to J Nachmias.

At least 19 recordsLinked to original sources
PubMed1999

How is a grating detected on a narrowband noise masker?

Thresholds were measured for detecting 4 cpd gratings added to maskers consisting of nine sinusoidal components spanning 1 octave around the signal frequency. Phases of all mask components were randomized on every presentation. To assess their importance, contrast differences were either rendered unreliable by introducing contrast jitter between-intervals, or eliminated by equating contrast energy within the octave band across intervals and trials. The deleterious effects of contrast jitter and the similarity of grating detection and contrast discrimination thresholds argues that contrast cues are being used. Those cues are not the only ones available, because contrast jitter has less than the expected effect, and equating contrast energy only raises threshold a few dB. Computer simulations reveal that there is sufficient information in several spatial pattern cues to support detection performance.

Computer Simulation↗
PubMed1993

Masked detection of gratings: the standard model revisited.

According to the standard model of masked detection of gratings, the observers's task is to decide whether the response of some critical mechanism is reliably increased by the addition of the test stimulus to the masker. To test this conception, experiments were performed in which the contrast levels in the masker alone and test-plus-masker intervals of a forced-choice trial were randomly perturbed on each presentation. The perturbations in the two intervals of a trial were either independent, positively correlated, or negatively correlated. Such perturbations had the expected effect on the detectability of 10 c/deg test on a 10 c/deg masker. However, they had no effect at all on the detectability of 2 or 8 c/deg test stimuli on the same masker. These results contradict the standard model of masking, but are compatible with the assumption that the test stimulus is detected by some changes it produces in the local spatial features of the phenomenal appearance of the masker.

Adult↗
PubMed1989

Contrast modulated maskers: test of a late nonlinearity hypothesis.

The contrast threshold for detecting a low frequency test grating in the presence of a contrast-modulated high frequency masker is heavily dependent upon the phase relations between the gratings: test stimuli in + or - cosine phase are much more detectable than those in + or -sine phase. The present study tests a new hypothesis of these and related phenomena. It is based on the report by Derrington (1987) that recordings from cat geniculate on-center and off-center X-cells exhibit significant point nonlinearities. The major empirical finding of this study is that for highly practiced observers, sine-phase test stimuli and cosine-phase test stimuli are similar in two important respects: (1) there exist phase uncertainty effects; that is, detection thresholds for test stimuli differing in phase by 180 degrees are elevated when they are intermixed within the same block of trials, rather than being presented in separate blocks; (2) phase identification thresholds for test stimuli differing in phase by 180 degrees are the same as their detection thresholds. The new nonlinearity hypothesis cannot account for the results obtained with sine-phase test stimuli, though it gives a better account of the results with cosine-phase stimuli than does the early nonlinearity hypothesis which was tested and rejected by Nachmias and Rogowitz (1983).

Contrast Sensitivity↗
PubMed1988

Primary giant calculus in urethral diverticulum.

Urethral stone is a rare clinical entity and usually encountered in men with urethral stricture or diverticulum. They are exceedingly rare in females secondary to the low incidence of vesical calculi and short urethra. We report a very rare case of a primary giant calculus in a urethral diverticulum of a female.

Aged↗
PubMed1984

Phase reversal discrimination.

In the Fourier representation of space, the parameter of phase plays a crucial role. In this study, several experiments were performed involving discrimination of various phase relations of fundamental (2 c/deg) to second harmonic (4 c/deg) at low contrast levels. The results were consistent with a model involving four "channels", each optimally sensitive to one of the following phase relations: + cosine (bright bar), -cosine (dark bar), +sine (left edge), and -sine (right edge).

Fourier Analysis↗
PubMed1983

Masking by spatially-modulated gratings.

Contrast- or quasi-frequency-modulated masker gratings consisting of three high frequency components (8.8, 11 and 13.2 c/deg) affect the detectability of a 2.2 c/deg signal grating, to an extent that is strongly dependent upon the relative phase between signal and masker. Unmodulated high frequency maskers have no such phase-dependent effects. This paper explores the possibility that the visual system's nonlinear response to luminance is responsible for these phenomena. A specific hypothesis is proposed according to which the effects of the spatially modulated maskers are due entirely to a distortion product at 2.2 c/deg caused by the visual nonlinearity. Although some of the predictions of this hypothesis are borne out by the experimental findings, others are contradicted.

Form Perception↗
PubMed1979

Adaptation effects on the apparent "squareness" of square-wave gratings.

Adaptation to a 9 cycles deg-1 sine wave makes the apparent brightness profile of a 3 cycles deg-1 square wave look less "square" even when it does not look like that of a sine wave. Adaptation to a 3 cycles deg-1 sine wave has no noticeable effect. Hypotheses based on "excitation patterns" across spatial-frequency-selective channels can account for these results; it is not clear whether variations in local light adaptation can also account for them.

Adaptation, Ocular↗
PubMed1979

Intensity discrimination with gated and continuous sinusoids.

Detectability of sinusoidal signals at several durations was measured in the presence of sinusoidal maskers of the same frequency. Maskers were either continuous or gated on and off with the signal. For all signal durations and both types of maskers, delta I/I varied with I raised to the -0.1 power, approximately. Psychometric functions were two or three times steeper with continuous than with gated maskers. Signal duration had only a small effect that varied with the masking condition.

Acoustic Stimulation↗