Biomedical subjects

William A Simpson

Publications and source records attributed to William A Simpson.

10 recordsLinked to original sources

PubMed2005

The two-pulse experiment and cross-correlation.

Rashbass [Rashbass, C. (1970). The visibility of transient changes of luminance. Journal of Physiology, 210, 165-186] presented pairs of flashes having various contrasts separated by a delay, and found that the thresholds for detecting the pairs fell on an ellipse. He fit the data using a model that computed the filtered energy of the pulses. Although this Rashbass model is phase-insensitive, many other experimental results show that humans can perform phase-sensitive detection consistent with a template-matching mechanism. We show that an observer who uses a form of template-matching produces thresholds that fall on an ellipse, just like the Rashbass model. The results from two-pulse experiments are consistent with the idea that humans cross-correlate the stimulus (signal or noise) with a filtered version of the expected signal rather than the signal itself. In symbols, we propose that observers compute integral r(t)[s(t) *h(t)]dt where r(t) is the received stimulus on a given trial [s(t)+n(t) or n(t)], s(t) is the signal, h(t) is the visual filter, and * is convolution.

Humans↗

PubMed2005

Human cortical responses to contrast modulations of visual noise.

We studied visual evoked potentials (VEPs) elicited by second-order contrast modulations of binary dynamic noise and first-order luminance modulations. Using a 3-point Laplacian operator centred on Oz, we found that contrast modulations of both low and higher spatial frequencies elicited a negative component whose latency was about 200 ms. The latency of this component was significantly longer than that of the early Laplacian components to first-order luminance modulations. These findings could be due to slower first-stage linear filters and additional processing stages of the second-order pathway. The topographical analysis of scalp recorded VEPs to central and half-field stimulation has suggested that the responses to second-order patterns are likely to be generated by neuronal structures within the primary visual cortex which may have inputs from extrastriate neurons via feedback connections.

Brain Mapping↗

PubMed2005

Spatial frequency channels derived from individual differences.

Contrast sensitivity functions differ from observer to observer. We propose that these differences arise because each observer has unique weights for the outputs of the neural channels that underlie the contrast sensitivity function. By applying principal components analysis to individual contrast sensitivity functions of 297 observers, estimates of the channel tuning curves were found. We find evidence for three broadly tuned bandpass channels with peaks at 4, 8, and 16c/deg and bandwidth near 1.3 octaves. These channel tuning curves were reproduced in a cross-validation study of 56 observers.

Adolescent↗

PubMed2005

Why is second-order vision less efficient than first-order vision?

Research has shown that the sensitivity to second-order modulations of carrier contrast is lower than that to first-order luminance modulations stimuli. We sought to compare the efficiency of processing first- and second-order information. Employing a phase-discrimination paradigm we found that when humans were given sufficient a priori information of signal parameters they detected both luminance and contrast modulations of 0.6 and 2c/deg by a phase-sensitive algorithm. The overall detection efficiency for second-order patterns, however, was lower that that for first-order stimuli. To study the factors which limit the efficiency of first- and second-order vision, we measured detection performance for luminance and contrast modulations of 0.6 and 2c/deg embedded in Gaussian noise. The results showed that the detection of second-order patterns had lower sampling efficiency and higher additive internal noise as compared to the detection of first-order stimuli. Classification images for detecting contrast modulations of 2c/deg resembled the side-band component of the contrast modulations which suggests that human observers may detect contrast modulations of a sinusoidal carrier using first-order luminance channels. The lower sensitivity of the mechanism detecting second-order patterns might be due to higher levels of additive internal noise and lower sampling efficiency than those of the mechanism analysing first-order patterns.

Contrast Sensitivity↗

PubMed2004

Illusory percepts of moving patterns due to discrete temporal sampling.

Continuously, moving objects under continuous illumination can be seen to move in a direction opposite to their actual motion. This illusory reversed motion can be explained as due to discrete temporal sampling of the moving stimulus by the visual system. If temporal sampling lies behind the illusory motion, then the probability of illusory motion should depend on the temporal frequency of the motion stimulus. By presenting contracting bull's-eye gratings of various spatial frequencies we were able to tease apart the drift speed and temporal frequency. The prevalence of illusory percepts depended on the temporal frequency, not the speed. The data suggest that the human visual system samples the incoming stimulation at a rate near 16 Hz.

Contrast Sensitivity↗

PubMed2004

Factors limiting suprathreshold vision measured by a flash-sound simultaneity paradigm.

Internal noise and sampling efficiency are the main factors which limit visual performance. In a previous study [Vis. Res. 43 (2003) 1103] we compared the variance of human reaction time to that of an ideal observer and found that the sampling efficiency to suprathreshold stimuli was much lower than that obtained in detection experiments. In order to bypass the effects of the motor system on visual performance, we used a flash-sound simultaneity paradigm. We found that the sampling efficiency for 0.4- and 4-c/deg near-threshold Gabor patches is higher only by a factor of 2.5 than that to above-threshold patterns. The signal-dependent multiplicative internal noise was similar to the additive internal noise at lower signal contrast levels and exceeded it at higher signal contrast levels. The results show that real observers' performance for detecting suprathreshold stimuli can be accounted for by a model taking into account the non-linear visual-signal transduction and multiplicative components of the internal noise induced by the signal and external noise. In addition, this model assumes that performance depends on the response duration, rather than signal duration. The results imply that the multiplicative internal noise induced by high contrast visual signals determines performance for suprathreshold visual detection.

Acoustic Stimulation↗

PubMed2003

Efficiency and internal noise for detection of suprathreshold patterns measured using simple reaction time.

Studies of the detection of simple visual patterns at threshold contrast have found that human performance is limited by the addition of internal noise and by the sub-optimal sampling efficiency of the visual system. Many common visual tasks require the detection of a signal having a contrast well above threshold, and we sought to measure the internal noise and sampling efficiency for such signals using simple reaction time (RT). Observers were presented with suprathreshold Gabors in dynamic Gaussian white noise and were required to hit a button as soon as each was detected. By comparing the RT variances from humans to those of an ideal observer, visuomotor internal noise and sampling efficiency were measured. The internal noise remains constant and the sampling efficiency increases as the signal contrast increases.

Contrast Sensitivity↗

PubMed2003

Temporal properties of the visual responses to luminance and contrast modulated noise.

Vision is sensitive to first-order luminance modulations and second-order modulations of carrier contrast. Our knowledge of the temporal properties of second-order vision is insufficient and contradictory. Using temporal summation and reaction time paradigms, we found that the type of visual noise (static or dynamic) determines the temporal properties of the responses to luminance and contrast modulations. In the presence of static noise, the temporal responses to both types of modulation of low and higher spatial frequencies were transient. When dynamic noise was used, the temporal responses to luminance and contrast modulations of higher spatial frequencies were sustained. At low spatial frequency, however, luminance modulations elicited transient responses, while contrast modulated dynamic noise produced sustained responses. The reaction times to near-threshold contrast modulations of low spatial frequency were slower than those to first-order patterns and they did not significantly differ at modulations of higher spatial frequency. The results suggest that the temporal characteristics of first-stage linear filters which feed the second-order pathway may determine the temporal responses to contrast modulated noise.

Contrast Sensitivity↗

PubMed2003

Sampling efficiency and internal noise for motion detection, discrimination, and summation.

By comparing real observers to an ideal observer, previous studies have found that the detection of static patterns is limited by internal noise and by imperfect sampling efficiency. We developed and applied ideal observer models for the detection, discrimination, and summation of oppositely drifting gratings in Gaussian white noise. The three tasks share a common source of internal noise. The sampling efficiencies were on the order of 1-2% except for much lower efficiency in direction discrimination for faster moving gratings. The efficiency of direction discrimination relative to detection systematically declines as the speed is increased from 1 to 6 Hz. These results suggest that observers use mismatched filters tuned to slow speeds regardless of the signal speed. Human visual motion sensing appears to use distorted representations of the incoming signals, and this distortion is a major limitation to visual performance.

Discrimination, Psychological↗

PubMed2003

Parametric modeling of reaction time experiment data.

A simple parametric model is proposed for data from a point-process version of a reaction time experiment. It is used to statistically check for the presence and nature of nonlinear inhibition in the eye-brain-hand system, as well as to study the nature of the reaction time delay distribution. The model tells us that, in principle, the second-order intensity estimate can be used to determine whether the experimental subject is systematically observing the first or the second of two flashes transmitted in short succession. Nonparametric estimates of second-order intensity functions are used in conjunction with this model. In particular, the model allows for the computation of good bandwidths for intensity curve estimation. A parametric bootstrap can also be implemented. Our methods are illustrated with 12 runs of data from a real reaction time experiment. It is found that nonlinear inhibition is present in the eye-brain-hand system. However, there are insufficient data to distinguish between log-normality and normality in the reaction time distribution, due partly to confounding with the particular kind of nonlinear inhibition present in the system.

Biometry↗