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A Pantle

Publications and source records attributed to A Pantle.

At least 19 recordsLinked to original sources
PubMed1997

Probing visual motion signals with a priming paradigm.

The perceived motion of a vertical sine-wave luminance grating which undergoes an abrupt 180 deg phase shift (motion step) is ambiguous. The grating sometimes appears to move rightward; sometimes leftward. However, when the 180 deg step follows closely upon an unambiguous grating step, the 180 deg step appears to be in the same direction as the unambiguous step. This phenomenon is termed visual motion priming (VMP), and some of the characteristics of the phenomenon were investigated in a series of experiments. The main findings were that priming (1) lasted for hundreds of msec; (2) was at a maximum when the magnitude of the priming step was 90 deg; (3) was scarcely affected by spatial frequency in the range 0.7-2.8 c/deg; and (4) at suprathreshold contrasts depended upon the relative contrast, not the absolute contrasts, of the frames comprising the priming step. The experiments were conducted within the framework of a motion energy model (Adelson & Bergen, 1985) which possessed an extra stage which summed motion signals over time. Some of the results could be explained by the second-stage integrator. Other nonlinear relationships between VMP and contrast require some form of motion signal compression, and perhaps even a mechanism of dynamic contrast processing.

Analysis of Variance↗
PubMed1994

An energy model of interframe interval effects in single-step apparent motion.

A computational model was developed to explain the effects of an interframe interval (IFI) in single-step apparent motion experiments. In these experiments a stimulus appears in one position, disappears, and then reappears in a shifted position after a short or long IFI. If the luminance during the IFI matches the mean luminance of the stimulus frames, long IFIs result in perceived motion opposite the short-IFI conditions. Brighter or darker IFIs, however, do not support the reversed motion effect. The model possess the following defining characteristics: (1) a biphasic ("transient") channel whose signalled direction of motion reverses with changes of IFI duration; (2) a combined direction-opponent output which is the sum of directional responses developed in two channels--biphasic ("transient") and monophasic ("sustained"); (3) a signal/noise weighting of the contributions of the two channels to the final directional output of the system. Predictions of the model about the effects of IFI intensity and viewing eccentricity were tested and confirmed in two new psychophysical experiments. The interpretations of past studies which included a role for second-order motion mechanisms in explaining IFI duration effects were reexamined. Further empirical tests of the model were outlined.

Humans↗
PubMed1994

Detection and direction discrimination performance with flicker gratings in peripheral vision.

Detection and direction discrimination experiments were conducted with luminance and flicker gratings. The flicker gratings had bars made up of static random pixels interspersed between other bars with flickering random pixels. All experiments were carried out in peripheral vision with grating images centered at 8 deg eccentricity in the superior retina. Detection of flicker gratings (i) was independent of pixel size, (ii) declined with spatial frequency in the range 1-4 c/deg, and (iii) improved with stimulus area (number of grating cycles). Detection performance with a flicker grating was comparable to that obtained with a low-contrast (0.01) luminance grating, and the results suggest that the spatial structure of a flicker-domain stimulus is based upon signals which are weak compared to the maximum signals attainable with a luminance-domain stimulus. With the detectability of flicker and luminance gratings equated, d' for discriminating the direction of motion of a luminance grating increased with step size (1/12 to 1/4 cycle) whereas direction discrimination performance with a flicker grating remained unchanged and at chance levels. Under the conditions tested, there was no evidence that the motion of a flicker-domain stimulus could be processed peripherally. Constraints on alternative models of motion processing are discussed.

Contrast Sensitivity↗
PubMed1992

Visual resolution of motion ambiguity with periodic luminance- and contrast-domain stimuli.

Visual motion processes were studied with luminance- and contrast-modulated gratings. A sine-wave luminance grating was displaced abruptly back and forth by 3/16 cycle. The display sequence is ambiguous in that each 3/16-cycle phase shift (short-path motion) could just as readily be seen as a 13/16-cycle shift (long-path motion) in the opposite direction. By varying the duration of the interval (IFI) between the two phase positions, the luminance of the IFI, or the spatial frequency of the grating, it was possible to bias the ambiguous percept in favor of short-path motions or long-path motions. A contrast-modulated grating displaced through 3/16 cycle always appeared t undergo short-path motion. Current motion models which incorporate Reichardt-type/energy mechanisms and certain types of auxiliary signal transformations which precede those mechanisms do not adequately explain the effects of IFI intensity on the perceived motion of a sinusoidal grating or the effect of IFI duration on the perceived motion of a contrast-modulated grating.

Contrast Sensitivity↗
PubMed1992

Immobility of some second-order stimuli in human peripheral vision.

Real-world objects and events are often demarcated and defined by changes of luminance. Such stimuli are easily noticed by human beings whether the stimuli occur in central or in peripheral vision. It is also possible to use differences of color, texture, contrast, or temporal characteristics to create visual stimuli (so-called non-luminance-domain or second-order stimuli). Here I report the discovery that a variety of spatially periodic, second-order patterns, when set in motion in peripheral vision, appear stationary even though their structure is clearly visible. The immobility of the second-order stimuli is evidenced by the striking inability of observers to report their direction of motion. Special signal transformations are required to reveal the motion of second-order stimuli to higher-stage motion mechanisms, and the immobility of second-order stimuli in peripheral but not in central vision suggests that the processing of the transformed signals by higher-stage motion mechanisms changes radically from central to peripheral vision.

Color Perception↗
PubMed1989

On the mechanism that encodes the movement of contrast variations: velocity discrimination.

Motion can be perceived when a moving pattern is defined by variations in luminance. Motion can also be perceived when a moving pattern is defined by variations in contrast. The central focus of the present study was the isolation and description of the mechanism that is responsible for the processing of the movement of contrast variations. We measured velocity discrimination thresholds to determine whether the coding of the movement of contrast-modulated (CM) patterns is mediated by the same motion mechanism as that which underlies the processing of the movement of luminance variations (sine-wave gratings). The results of the velocity discrimination studies showed that an observer's ability to detect small differences in velocity of both CM patterns and sine-wave (SW) gratings decreased (1) when variations of contrast or luminance approach detection threshold, (2) when the frame duration of an apparent motion display was lengthened, and (3) when patterns were slowed to a standard velocity of 0.75 deg/sec. Three hypotheses were discussed in light of the results. Henning's hypothesis, a two-stage model in which a higher-level system receives input from the output of a lower-level system, was best supported by the results of the present experiments.

Contrast Sensitivity↗
PubMed1985

Using low-level filters to encode spatial displacements of visual stimuli.

Directional responses to visual stimuli were analysed with the aid of a minimal computational model. The model is based upon arrays of motion sensors whose receptive fields are modified versions of those (difference-of-Gaussians) used to describe mechanisms in popular spatial vision models. In the model antagonistic influences on each motion sensor were assumed to: (1) arise from spatially non-aligned areas of the retina; and (2) to follow different time courses. Implications of the model were explored with simulations, and parallel psychophysical data were collected. Visual behaviours chosen for relatively detailed analysis were judgments of the temporal order of onset of two spatially displaced stimuli and motion aftereffects generated with discontinuously moving, sine-wave gratings.

Humans↗
PubMed1985

Discontinuity limits for the generation of visual motion aftereffects with sine- and square-wave gratings.

Visual motion aftereffects (MAE's) were produced with adapting gratings that underwent repeated, abrupt displacements in a uniform direction. MAE's could be generated with sine-wave gratings even when the magnitude of each displacement approached a phase angle as large as 1/2 cycle. The maximum spatial step for generating a MAE with a square-wave grating was less than 1/4 cycle. If a dark pause was introduced between the successive positions (phases) of the adapting grating, MAE's disappeared when the pause was longer than 60-70 msec. The results can be used to define the spatiotemporal-response limits of a short-range motion process (or system of directionally selective motion sensors), and the results also suggest that the individual Fourier components of complex spatial patterns are capable of producing independent signals for direction of motion.

Adult↗
PubMed1978

On the capacity of directionally selective mechanisms to encode different dimensions of moving stimuli.

Direction-specific losses of contrast sensitivity for sinusoidal test gratings as a function of the contrast of a sinusoidal adapting grating were found to be similar to those measured previously with square-wave gratings. Furthermore, both relationships were similar to that between motion aftereffect duration and the contrast of sinusoidal adapting gratings, and all three sets of data can be fit by a single function. The function shows that the magnitude of direction-specific adaptation effects increases linearly with the logarithm of adapting contrast in the low contrast region, but is essentially independent of contrast once the contrast exceeds threshold by more than a factor of five-six. In addition, it was found that direction-specific losses of contrast sensitivity are restricted to limited ranges of spatial frequency.

Adaptation, Ocular↗
PubMed1976

A multistable movement display: evidence for two separate motion systems in human vision.

Two competing sensations of apparent movement were produced by the rapid alternation of two multielement stimulus frames. Either sensation could be made dominant by, appropriate manipulations of the stimulus display. The results suggest that there are two systems capable of generating movement signals in man. One system depends on preliminary processing of form, and the second system does not.

Humans↗