The effect of feedback on luminance thresholds for peripherally presented stimuli

The effect of a central task on peripheral vision was determined by obtaining absolute luminance thresholds for peripherally presented stimuli. A competing central task was provided by interrupting the fixation light and requiring the S to reilluminate it. Data were obtained over five sessions, with feedback information provided on each trial. Thresholds were highest for the initial session and declined as a function of practice. The magnitude of the change depended upon the interruption rate. When central and peripheral stimuli compete for attention, priority is given initially to the central task, but, with practice, the differences are reduced.     

The effect of warning interval on the electric phosphene and auditory thresholds

Using different warning signals and threshold stimuli, the thresholds, as determined by a method of limits, were found to rise monotonically as the interval between warning signal and threshold stimulus increased from I to 9 sec. It was found that the variability of the threshold did not increase as the threshold increased. Similar results were obtained for phosphene and auditory thresholds and with visual and auditory warnings; therefore the effect was considered to be central. Motokawa's finding of a minimum in the phosphene threshold 2 sec. after a flash of white light was not repeated. The rise in threshold was not obtained when the warning intervals were randomized and so seemed to depend on the use of fixed warning intervals. A model was developed relating threshold level to accuracy of anticipation of the end of the warning interval.     

Perceptual learning effect on decision and confidence thresholds

Practice can enhance of perceptual sensitivity, a well-known phenomenon called perceptual learning. However, the effect of practice on subjective perception has received little attention. We approach this problem from a visual psychophysics and computational modeling perspective. In a sequence of visual search experiments, subjects significantly increased the ability to detect a “trained target”. Before and after training, subjects performed two psychophysical protocols that parametrically vary the visibility of the “trained target”: an attentional blink and a visual masking task. We found that confidence increased after learning only in the attentional blink task. Despite large differences in some observables and task settings, we identify common mechanisms for decision-making and confidence. Specifically, our behavioral results and computational model suggest that perceptual ability is independent of processing time, indicating that changes in early cortical representations are effective, and learning changes decision criteria to convey choice and confidence.     

The effect of prior experience on recognition thresholds for plane-disoriented pictures of familiar objects

We investigated plane rotation effects on the minimum presentation duration that is required in order to recognize pictures of familiar objects, using the method of ascending limits. Subjects made unspeeded verification responses, selecting from 126 written alternatives. Replicating similar identification studies in which brief, masked pictures (Lawson & Jolicoeur, 1998) were presented, disorientation reduced the efficiency of recognition. Mirroring the findings in speeded picture naming studies (e.g., Jolicoeur, 1985; Jolicoeur & Milliken, 1989), but in contrast to those of Lawson and Jolicoeur (1998), orientation effects were found over a wide range of views and were attenuated but not eliminated with experience with a given object. The results bridge the findings from unspeeded verification and speeded naming tasks. They suggest that the same orientation-sensitive processes are tapped in both cases, and that practice effects on these processes are object specific.     

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.     

A comparison of sniff bottle staircase and olfactometer-based threshold tests

Olfactometers have been gaining popularity as research tools, but they have yet to replace established testing procedures in a variety of laboratory and clinical settings, including absolute threshold tests. In this research, we designed and operated a simple olfactometer with which to assess threshold. To do this, we used a method-of-adjustment test that was compared to the three-alternative forced choice ascending sniff bottle staircase method, which is currently a standard threshold test procedure. We found that the olfactometer threshold test correlated highly with the staircase method, and that it possessed suitable test–retest reliability. The advantages of the olfactometer threshold test include faster test time and reduced cleaning and reassembly demands. Future use of the olfactometer in olfactory identification and/or detection thresholds amongst odors is also outlined.     

An oblique effect of chromatic gratings measured by color-mixture thresholds

Contrast sensitivity is lower for obliquely oriented achromatic gratings than for vertical or horizontal gratings at high spatial and low temporal frequencies. Although this response is suggestive of mediation by P-pathway cortical correlates, no clear sensory (i.e. class 1) oblique effect has been demonstrated with isoluminant chromatic stimuli. In the present experiment, a two-alternative forced-choice detection task was used to measure observers' sensitivity to spatiotemporal sinusoids varying in orientation and color contrast. A maximum-likelihood method fit ellipses to the thresholds, with the length of each ellipse taken as a measure of chromatic contrast sensitivity at isoluminance, and the width as luminance contrast threshold. A chromatic oblique effect was observed at about 3 cycles deg-1 suggesting an orientation bias within the cortical stream conveying P-cell activity.     

Vibrotactile difference thresholds for intensity and the effect of a masking stimulus

Vibrotactile difference thresholds for intensity were measured at several intensity levels of a test stimulus in the absence of a masking vibration and in the presence of three different amplitudes of a masking vibration. The test stimulus was a 160-Hz vibration delivered to the right index finger. The masking stimulus was a 160-Hz vibration delivered to the right little finger. For the same amplitudes of the test stimulus, △I varied as a direct function of the amplitude of the masking vibration. The smallest △Is resulted from measurements made in the absence of the masking stimulus. The Weber fraction, △I/I, was constant only for the more intense test stimuli in the absence of any masking stimuli. Independent of the presence or level of the masker, the Weber fraction for all stimuli approached approximately the same value, .25, when the test stimuli were raised to 20-dB sensation level. A model is proposed to account for the increase in the Weber fraction as a function of masker intensity and to predict masked thresholds.