Biomedical subjects
R E Fredericksen
Publications and source records attributed to R E Fredericksen.
An analysis of the temporal integration mechanism in human motion perception.
We present a model for the temporal integration of apparent motion information. The model is constructed by considering psychophysical and neurophysiological data, and consists of the leaky integration of pulsatile motion detector responses to apparent motion stimuli. Each pulse represents a motion detector populational response to a discrete spatial displacement of the spatial pattern. Temporal contrast sensitivity determines the shape of constant-stimulus-duration threshold curves for image frame exposure durations less than about 133 msec. The shape of the threshold curve for image frame exposure durations greater than about 133 msec is determined by the leaky integrator time constant and the shape of the pulses emitted by the motion detectors. The leaky integrator model exhibits threshold saturation behaviour (the reaching of a maximum sensitivity or minimum threshold) seen in psychophysical data as well as dependence of saturation time on the frame rate of the apparent motion stimulus. A low frame rate results in a longer time-to-saturation because the leaky integrator discharges more between detector output pulses. When the motion detector output pulses are far enough apart there is effectively no temporal integration and therefore no threshold improvement over time. Finally, the behaviour of the psychophysical threshold curves across spatial displacement sizes is consistent with a populational-response threshold mechanism combined with spatial summation over a non-uniform distribution of detector types across the visual field.
Spatial summation and its interaction with the temporal integration mechanism in human motion perception.
The combination of visual motion information over visual space (spatial summation) and stimulus duration (temporal integration) was investigated using a random-pixel array (spatiotemporally broad-band) apparent motion stimulus designed to isolate specific populations of visual motion detectors. The results indicate that, in agreement with results from spatiotemporally narrow-band stimuli, spatial summation follows the form of linear probabilistic summation rather than non-linear probabilistic summation. Linear probabilistic summation holds for a wide range of stimulus parameters and when changing either motion stimulus height or width. Linear probabilistic summation breaks down when the motion display region approaches a height and/or width that is related to the spatial displacement size, not the speed, of the random-pixel array. This height and width (termed the critical height and width, or critical dimension), increases with spatial displacement size and can be interpreted as a measure of the basic dimensions of the selected motion detector population's receptive field. The critical height is smaller than the critical width, a result that is consistent with a motion detector receptive field that is elongated in the direction of motion. Perhaps most importantly, the mechanisms of temporal integration and spatial summation can work independently under a wide range of conditions. Finally, the results provide evidence for a short-term inhibitory phenomenon from the edges of the useful display area that affects the visibility of the motion.
Movement aftereffect of bi-vectorial transparent motion.
Two moving random-pixel arrays (RPAs) were presented simultaneously in the same target field. These RPAs are perceived as two superimposed transparent moving sheets. Although two directions are perceived simultaneously during stimulus presentation, the movement aftereffect (MAE) is unidirectional. The visual system averages both motion signals in the MAE. For motion vectors of equal magnitude and perpendicular direction the MAE direction is the inverse of the sum of both vectors. In the first experiment we measured perceived direction of the MAE of transparent motion for a range of speed combinations. Results indicate that vector summation only predicts the correct MAE direction for combinations of equal speeds. It is suggested that the direction of the MAE of transparent motion is a resultant of the weighted summation of the component inducing vectors. The question then arises what determines the weighting factors. Directional sensitivity and MAE duration of the individual vectors under transparent conditions were measured and used to weigh the vectors and predict the MAE direction of transparent motion. Statistical analyses showed that MAE duration is a better basis to determine the weighting factors predicting the direction of the MAE of transparent motion than component sensitivity. The direction of the MAE of transparent motion thus seems to be determined by the amount of adaptation to the component vectors as reflected by MAE duration. The results suggest that this gain control cannot be located in the individual motion detectors and must be situated at or after some subsequent cooperation stage of the human motion analysis system.
Temporal integration of random dot apparent motion information in human central vision.
Human motion perception is assumed to be functionally described by an array of bi-local detectors feeding later, higher order computational stages. Using this model as a guide, improvement of spatio-temporal displacement sensitivity by temporal integration (summation) was measured in human central vision using random dot pattern apparent-motion stimuli. Our results agree with previous experiments with regard to improvement of maximum perceivable spatial displacement but show that contrary to previous results the minimum perceivable spatial displacement can be improved in a similar manner. Furthermore, stimulus duration is a more accurate predictor of sensitivity than the number of frames in the stimulus over a wide range of stimulus parameter values. Finally, our results indicate that temporal tuning of motion detectors is inversely related to the size of the spatial pattern displacement.
Recovery from motion adaptation is delayed by successively presented orthogonal motion.
Following a period of adaptation to a pattern moving in a particular direction, a subsequently viewed stationary pattern appears to move in the opposite direction for some time: the movement after effect (MAE). The MAE lasts longer when the test pattern is not immediately or not continuously presented after adaptation. This phenomenon is called storage. So far research indicates that storage only occurs when textured visual stimulation is absent during part of the test phase or if the processing of a stationary test stimulus is prevented (e.g. by binocular rivalry). We present evidence that storage-like phenomena can occur even while a textured and moving visual stimulus is phenomenally present. We adapted binocularly to uni-directional motion of a random-pixel array M1 for 60 sec. This stimulus was immediately followed by another moving pattern M2. Its motion direction was orthogonal to that of M1. The presentation time of M2 was the independent variable. A stationary pattern was presented immediately after presentation of M2. The direction of the resulting integrated uni-directional MAE was measured. For short presentation times of M2 there is an integrated uni-directional MAE, which shows an interaction of the output of units stimulated by both moving patterns. However, it appeared that the effect of M1 on the direction of this combined uni-directional MAE is much longer present than would be expected from the MAE duration of M1, when tested in isolation.
A transparent motion aftereffect contingent on binocular disparity.
Under transparent motion conditions overlapping surfaces are perceived simultaneously, each with its own direction. The motion aftereffect (MAE) of transparent motion, however, is undirectional and its direction is opposite to that of a sensitivity-weighted vector sum of both inducing vectors. Here we report a bidirectional and transparent MAE contingent on binocular disparity. Depth (from retinal disparity) was introduced between two patterns. A fixation dot was presented at zero disparity, that is, located between the two adaptation patterns. After adaptation to such a stimulus configuration testing was carried out with two stationary test patterns at the same depths as the preceding moving patterns. For opposite directions a clear transparent MAE was perceived. However, if the adaptation directions were orthogonal the chance of a transparent MAE being perceived decreased substantially. This was subject dependent. Some subjects perceived an orthogonal transparent MAE whereas others saw the negative vector sum-an integrated MAE. In addition the behavior of the MAE when the distance in depth between adapting and test patterns was increased was investigated: it was found that the visibility of the MAE then decreased. Visibility is defined in this paper as: (i) the percentage of the trials in which MAEs are perceived and (ii) the average MAE duration. Both measures decreased with increasing distance. The results suggest that segregation and integration may be mediated by direction-tuned channels that interact with disparity-tuned channels.
Spatial integration in coherent motion detection and in the movement aftereffect.
Sensitivity characteristics and spatial integration properties of the motion-detection system are compared with those of the system responsible for the movement aftereffect (MAE), elicited by the same stimulus. This provides new information about the mechanisms involved in MAE generation. A screen was divided into a chequerboard where the squares were filled with random-pixel arrays moving in opposite directions. Changing the size of the squares produced drastic changes in the percept during the adaptation phase and in the MAE during the test phase. One striking new phenomenon that is described is 'structure from MAE.' The results indicate that the receptive fields of units involved in eliciting the MAE are larger than the receptive-field sizes of units involved in detection and segregation of motion components in the stimulus. Furthermore, the results suggest that the receptive fields contributing to the MAE are involved in complex interactions in which different local motion directions are integrated in pattern-specific ways.
Spatio-temporal characteristics of human motion perception.
A bi-local detector array model was assumed to describe the functional performance of monocular motion perception. Distributions of model parameters were measured in human vision at several positions in the visual field. The stimulus paradigm was designed to measure directional motion perception thresholds for individual combinations of spatial displacement and temporal delay in random dot apparent motion stimuli. The resulting data support previous results on perceivable spatial displacement limits in human vision but also indicate that both minimum and maximum perceivable spatial displacement thresholds in human observers have a similar dependence on temporal delay. This dependence changes with eccentricity in the visual field in a qualitatively similar manner but by quantitatively different factors. A description of possible biological properties of the bi-local detector population is presented that may explain how detection of spatio-temporal pattern displacements can be performed by a single system. Such a system also predicts that minimum and maximum perceivable spatial displacement thresholds should scale with visual field eccentricity in a manner consistent with our results.