Biomedical subjects

H P Snippe

Publications and source records attributed to H P Snippe.

12 recordsLinked to original sources

PubMed1998

Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain: comment.

The recent paper by Pokorny and Smith [J. Opt. Soc. Am. A 14, 2477 (1977)] describes psychophysical data that correlate well with the physiology of the magnocellular and the parvocellular pathways. One of the critical differences between cells in these two pathways is the contrast of the visual input for which the responses of these cells start to saturate. In the analysis of their data, Pokorny and Smith have to assume values for the saturation contrast of the putative pathways that mediate detection. Here I reanalyze their model and show that this assumption is unnecessary, since values of the saturation contrast follow directly from their data.

Animals↗

PubMed1997

Dynamics of adaptation at high luminances: adaptation is faster after luminance decrements than after luminance increments.

As is well known, dark adaptation in the human visual system is much slower than is recovery from darkness. We show that at high photopic luminances the situation is exactly opposite. First, we study detection thresholds for a small light flash, at various delays from decrement and increment steps in background luminance. Light adaptation is nearly complete within 100 ms after luminance decrements but takes much longer after luminance increments. Second, we compare sensitivity after equally visible pulses or steps in the adaptation luminance and find that detectability is initially the same but recovers much faster for pulses than for increment steps. This suggests that, whereas any residual threshold elevation after a step shows the incomplete luminance adaptation, the initial threshold elevation is caused by the temporal contrast of the background steps and pulses. This hypothesis is further substantiated in a third experiment, whereby we show that manipulating the contrast of a transition between luminances affects only the initial part of the threshold curve, and not later stages.

Adaptation, Ocular↗

PubMed1996

Parameter extraction from population codes: a critical assessment.

In perceptual systems, a stimulus parameter can be extracted by determining the center-of-gravity of the response profile of a population of neural sensors. Likewise at the motor end of a neural system, center-of-gravity decoding, also known as vector decoding, generates a movement direction from the neural activation profile. We evaluate these schemes from a statistical perspective, by comparing their statistical variance with the minimum variance possible for an unbiased parameter extraction from the noisy neuronal ensemble activation profile. Center-of-gravity decoding can be statistically optimal. This is the case for regular arrays of sensors with gaussian tuning profiles that have an output described by Poisson statistics, and for arrays of sensors with a sinusoidal tuning profile for the (angular) parameter estimated. However, there are also many cases in which center-of-gravity decoding is highly inefficient. This includes the important case where sensor positions are very irregular. Finally, we study the robustness of center-of-gravity decoding against response nonlinearities at different stages of an information processing hierarchy. We conclude that, in neural systems, instead of representing a parameter explicitly, it is safer to leave the parameter coded implicitly in a neuronal ensemble activation profile.

Algorithms↗

PubMed1994

Discrimination of geometric angle in the fronto-parallel plane.

This study determines the sensitivity of human observers to 2D fronto-parallel angles. Angle discrimination thresholds vary as a function of base angle and stimulus configuration orientation. However, these variations can all be understood from the well-known meridional anisotropy for orientation discrimination of the orientations that define the angle. Specifically, observers do not show any special sensitivity to angles of 90 degrees and 180 degrees (straightness). Instead it is claimed that observers measure geometric angle by comparing the visual orientations that define the angle, although it is shown that they are not fully efficient in this comparison operation. An explicit visual reference angle does not improve discrimination thresholds (that is, observers can perfectly well supply one from memory), nor do observers need an explicit visual reference orientation in an orientation discrimination task.

Form Perception↗

PubMed1994

Extraction of optical velocity by use of multi-input Reichardt detectors.

We study the possibility of a metrical readout of velocity from an ensemble of Reichardt correlators. We show that with a suitable choice of spatial and temporal prefiltering of the correlator input it is possible to devise reliable (nonaliasing) velocity-tuned Reichardt detectors. However, because of the well-known covariance of spatial and velocity tuning of velocity detectors in biological motion vision, an ensemble consisting of these detectors has problems in extracting a pattern-invariant velocity. We find that pattern invariance of the motion estimate can be closely approximated with Reichardt correlators that sample the luminance pattern at more than the minimum number of two locations.

Humans↗

PubMed1994

Detection of temporal order of noise-like luminance functions.

We study the capacities of human observers to time order light sources that emit dynamic noise, identical for the different light sources, except for an adjustable delay. There is a range of temporal delays for which human observers are perfectly able to perform this task, using the direction of the motion percept that is evoked by the stimulus as a cue. An optimal delay between light sources at which the observers are most robust against any deterioration of the stimulus is defined. We claim that optimal delays (15-25 msec) correspond to the time delay of a putative Reichardt correlation mechanism in human motion vision. Contrary to the ability of human observers to sense temporal correlations in noise sequences, observers are totally unable to detect anticorrelation between noise sequences. This inability rules out motion opponency as a viable model for human front-end ("early") motion vision.

Adult↗

PubMed1992

Information in channel-coded systems: correlated receivers.

Noise correlation can easily occur in the densely connected systems observed in biological information processing. We study the consequences of noise correlation for a statistically optimal processing of noise-perturbed receptor array outputs. We find a critical importance of the noise correlation length as compared to the receptors' tuning width for both the structure and the performance of the ideal observer. We show the general consistency of our scheme with psychophysical discrimination thresholds obtained in human spatial vision.

Animals↗

PubMed1992

Visual processing of optic acceleration.

We present data on the human sensitivity to optic acceleration, i.e. temporal modulations of the speed and direction of moving objects. Modulation thresholds are measured as a function of modulation frequency and speed for different periodical velocity vector modulation functions using a localized target. Evidence is presented that human detection of velocity vector modulations is not directly based on the acceleration signal (the temporal derivative of the velocity vector modulation). Instead, modulation detection is accurately described by a two-stage model: a low-pass temporal filter transformation of the true velocity vector modulation followed by a variance detection stage. A functional description of the first stage is a second order low-pass temporal filter having a characteristic time constant of 40 msec. In effect, the temporal low-pass filter is an integration of the velocity vector modulation within a temporal window of 100-140 msec. A non-trivial link of this low-pass filter stage to the temporal characteristics of standard motion detection mechanisms will be discussed. Velocity vector modulations are detected in the second-stage, whenever the variance of the filtered velocity vector exceeds a certain threshold variance in either the speed or direction dimension. The threshold standard deviations for this variance detection stage are estimated to be 17% for speed modulations and 9% for motion direction modulations.

Fixation, Ocular↗

PubMed1990

Effects of element orientation on apparent motion perception.

We present an ambiguous motion paradigm that allows us to quantify the influence of aspects of form relevant to the perception of apparent motion. We report on the role of bar element orientation in motion paths. The effect of orientation differences between bar elements in a motion path is small with respect to the crucial role of the orientation of bar elements relative to motion direction. Motion perception between elements oriented along the motion direction dominates motion perception between elements oriented perpendicularly to motion direction. The perception of apparent motion is affected by bar length and width and is anisotropic.

Attention↗

PubMed1988

Local spatial scale for three-dot alignment acuity.

Three-dot alignment discrimination thresholds were determined for blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. Thresholds were determined as a function of the blur parameter of the stimuli. This was done for a range of eccentricities in the visual field (from 45 degrees nasal to 65 degrees temporal). The thresholds were corrected for variations of the stimulus extent with the blur parameter. The results were used to estimate the local spatial scale for three-dot alignment acuity. This was done by a method recently introduced by Watson (1987). It was found that the local spatial scale for three-dot alignment acuity is approximately linearly proportional to eccentricity.

Adult↗

PubMed1988

Effects of blur and eccentricity on differential spatial displacement discrimination.

Differential spatial displacement discrimination thresholds were determined for stimuli consisting of blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. The tasks were similar to the two-point discrimination acuity task and the three-dot alignment hyperacuity task. Thresholds were determined as a function of eccentricity along the horizontal meridian of the visual field (from 45 degrees nasal to 65 degrees temporal). The spatial spread or blur parameter of the blobs was adopted as a scale parameter. The results show that the performance of the visual system in differential spatial displacement discrimination tasks becomes progressively more homogeneous for a progressive increase in the blur parameter of the stimuli. Scaling (i) the three-blob alignment results with estimates of the cortical magnification factor and (ii) the two-blob separation discrimination results with their corresponding neural blur parameter shows an impressive isotropy and blur scale-invariance for the mechanisms mediating differential spatial displacement discrimination across the visual field. These results are interpreted in terms of a scaled sampling lattice model of the visual system, in combination with an automatic scale-selection mechanism.

Adult↗

PubMedDate not supplied

Pulse modulation detection in human motion vision.

We present data on the human sensitivity to temporal pulse modulations of target velocity. We measured threshold detection modulation amplitudes for pulse-shaped speed modulations, as a function of pulse duration and temporal frequency. At short pulse durations (up to 50 msec) and low modulation frequency (1 Hz), detection amplitudes are ruled by Bloch's law: the product of pulse duration and threshold modulation amplitude is a constant. This constant corresponds to a position modulation with an amplitude of 3 arc min in a coordinate frame that moves at the average speed (3 deg/sec) of the target. At longer pulse durations we find deviations from Bloch's law. Speed modulation thresholds are not critically dependent on target luminance contrast. These results are modeled by a modulation detection process in two stages. A functional description of the first stage is filtering of the true speed modulation signal by a second order low-pass filter with a characteristic time constant of 20-25 msec. The second (decision) stage is variance detection: modulations are detected when the variance of the filtered modulation function exceeds a certain threshold variance. The square-root threshold variance is estimated 8-10%. This two-parameter model accurately predicts the measured dependence of pulse modulation detection thresholds on pulse duration and pulse density.

Filtration↗