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W F Bischof

Publications and source records attributed to W F Bischof.

17 recordsLinked to original sources
PubMed1998

Common reference frame for neural coding of translational and rotational optic flow.

Self-movement of an organism through the environment is guided jointly by information provided by the vestibular system and by visual pathways that are specialized for detecting 'optic flow'. Motion of any object through space, including the self-motion of organisms, can be described with reference to six degrees of freedom: rotation about three orthogonal axes, and translation along these axes. Here we describe neurons in the pigeon brain that respond best to optic flow resulting from translation along one of the three orthogonal axes. We show that these translational optic flow neurons, like rotational optic flow neurons, share a common spatial frame of reference with the semicircular canals of the vestibular system. The three axes to which these neurons respond best are the vertical axis and two horizontal axes orientated at 45 degrees to either side of the body midline.

Animals↗
PubMed1996

Psychophysical evidence of a sustained input to directionally selective motion mechanisms.

Human psychophysical evidence congruent with neurophysiological findings of a sustained input to directionally selective motion sensors in cat visual cortex is reported. Apparent motion was produced by displaying a group of dots in two frames (F1 and F2), where F2 was a translated version of F1. All stimulus sequences included a period during which F1 and F2 were displayed concurrently (combined images) and a period during which only F1 or F2 was on display (single images). There were three stimulus sequences: a display beginning with combined and ending with single image, a display beginning with single and ending with combined image, and a display beginning with F1, continuing with combined image, and ending with F2. Six durations of single and of combined images (10, 20, 40, 80, 160, and 320 ms) were crossed factorially in each stimulus sequence. Directional motion was seen easily at long durations of the single image in all stimulus sequences, as would be expected on the basis of a sustained input to the directional-motion-sensing mechanisms. Perception of directional motion improved with the duration of single images, but declined as the duration of combined images was increased. Baker and Cynader's model could account for the effect of duration of single images, but not for the effect of duration of combined images. An elaborated version of the model provides a good qualitative match to all empirical findings.

Humans↗
PubMed1996

Machine learning paradigms for pattern recognition and image understanding.

In this paper some issues are considered related to the encoding of spatial information and associated perceptual learning algorithms which, it is claimed, are necessary for robust pattern and object recognition in multi-object (natural) scenes. The types of learning requirements within a 'recognition-by-parts' paradigm are contrasted with findings from alternative models.

Form Perception↗
PubMed1995

Inverse-intensity effect in duration of visible persistence.

Duration of visible persistence can vary inversely with stimulus intensity. This inverse-intensity effect is obtained by varying the intensity of the stimuli or of the background, provided that the variations extend into the mesopic range. A similar relationship--known as the Ferry-Porter law--holds for the critical frequency at fusion (CFF). The authors propose that studies of CFF, 2-pulse threshold, and visible persistence can be encompassed within 1 conceptual framework in which the effect is modeled by the progressive reduction in the temporal extent of the positive phase of the system's response as the level of light adaptation changes from scotopic to photopic. In this context, the authors present an integrative scheme in which G. Sperling and M. M. Sondhi's (1968) formal model and M. Coltheart's (1980) neurophysiological conjecture are shown to be compatible and complementary accounts of the effect.

Attention↗
PubMed1995

Motion and metacontrast with simultaneous onset of stimuli.

Coherent directional motion can be seen if an image is displayed in two sequential frames (F1 and F2), where F2 is a translated version of F1. A similar two-frame sequence can produce metacontrast masking: the visibility of a leading target (F1) is reduced by a trailing, spatially nonoverlapping mask (F2). Strict temporal succession of the stimuli has been considered essential for both motion and masking. This requirement for a minimum stimulus-onset asynchrony (SOA) is known as the SOA law. Contrary to the SOA law, we found that motion and masking can be obtained with simultaneous onsets of the stimuli, provided that F2 outlasts F1. We compared motion and metacontrast with simultaneous onsets of the stimuli (SIM paradigm) with the traditional paradigm in which an interstimulus interval (ISI) is inserted between the leading and the trailing stimuli (ISI paradigm). We studied the effects in light-adapted and in dark-adapted viewing, each over a wide range of stimulus intensities. Homologous results were obtained with the two paradigms, thus disconfirming the SOA law. Models of motion sensors, such as that proposed by Reichardt [in Sensory Communication, W. A. Rosenblith, ed. (MIT Press, Cambridge, Mass., 1961), p. 303], are inherently capable of explaining the motion results obtained with both paradigms. The masking results with the SIM paradigm disconfirm theories based on onset-locked slow excitatory and fast inhibitory responses but can be explained in terms of Bridgeman's network model [Bull. Math. Biol. 40, 605 (1978)]. In light of the results obtained with the two paradigms, we discuss, and tentatively support, the suggestion that motion and metacontrast may be complementary parts of a unitary perceptual system.

Contrast Sensitivity↗
PubMed1994

Computational approaches to human pattern recognition.

This paper consolidates recent findings on how humans detect and recognize patterns and considers computational procedures which reflect observed performance. A multi-level correlation model for spatial information processing is proposed and used to interpret past results on human psychophysical performance.

Computer Graphics↗
PubMed1993

On the confounding effects of phosphor persistence in oscilloscopic displays.

Phosphor persistence has been a source of confounding in studies of temporal integration in vision. We examined the confounding by assessing the effects of the persistence of two commonly-used phosphors (P15 and P31) on performance of a temporal-integration task. In one experiment we eliminated the visibility of phosphor persistence by closing two mechanical shutters upon display termination. In a second experiment we estimated the duration of phosphor persistence by displaying the image behind closed shutters which opened upon display termination. No detectable persistence was every produced by P15 phosphor. By contrast, P31 phosphor produced persistence that lasted several hundred milliseconds even when a veiling light was projected on the screen. We ascribe the earlier instances of confounding to inadequate interpretation of the technical data on phosphor decay.

Humans↗
PubMed1992

Automated detection and classification of breast tumors.

This paper presents a new method for the mammographic detection and classification of two types of breast tumors, stellate lesions and circumscribed lesions. The method assumes that both types of tumors appear as approximately circular, bright masses with a fuzzy boundary and that stellate lesions are in addition surrounded by a radiating structure of sharp, fine lines. Experimental results for a set of 27 mammograms are presented and the method is shown to have a high detection rate and an extremely low false positive rate.

Automation↗
PubMed1991

Automated detection of breast tumors using the asymmetry approach.

A method for automated detection of breast tumors in mammograms is presented. The method uses the asymmetry principle: Strong structural asymmetries between corresponding regions in the left and right breast are taken as evidence for the possible presence of a tumor in that region. Asymmetry detection is achieved in two steps. First, mammograms are aligned, compensating for possible differences in size and shape between the two breasts. Second, asymmetry between corresponding positions is determined using a combination of several asymmetry measures, each responding to different types of asymmetries. Results obtained with a set of mammograms indicate that this method can improve the sensitivity and reliability of systems for automated detection of breast tumors.

Breast Neoplasms↗
PubMed1991

On the half-cycle displacement limit of sampled directional motion.

We employed filtered random-dot kinematograms to determine the maximum displacement (dmax) at which sampled directional motion was reliably detected. The images were produced using ideal band-pass filters that varied in lower cut-off frequency (f1), in bandwidth, and in range (alpha) of component orientations that were passed. Results showed that dmax, expressed in cycles of f1, increased with f1 and alpha, and decreased with bandwidth. In many conditions, dmax exceeded half a cycle of f1, a result that appears to contradict predictions from quadrature models of motion detection. However, an account that does not violate the half-cycle limit can be given on two assumptions. First, motion perception is mediated by a population of orientation and frequency-selective sensors that respond correctly to displacements up to half a cycle in the preferred direction. Second, the outputs from all sensors (notably including off-axis sensors) are linearly summated to yield perception of motion. A computer simulation based on these assumptions provided a remarkably close fit to the psychophysical data.

Computer Simulation↗
PubMed1991

Displacement limit (dmax) of sampled directional motion: direct and indirect estimates.

The maximum displacement at which directional motion can be seen, known as dmax, has been said to define the spatial limits of the short-range motion system. Turano and Pantle (1985) used duration of motion aftereffect (MAE) to estimate the spatial limit of the short-range system, the assumption that dmax (a direct measure of motion perception) and MAE (an indirect measure) are equivalent indices of the same underlying perceptual process. In a series of four experiments, we examined this assumption by measuring dmax and duration of MAE across a range of displacements, stimulus waveforms (sine- or square-wave gratings), and spatial frequencies. We found that dmax and duration of MAE were affected differently by changes in the same variables. Therefore, we concluded that the two indices cannot be regarded as equivalent measures of the spatial limits of the short-range process. Two novel effects that separated MAE from motion detection are described, and suggestions for exploring them are outlined.

Adult↗
PubMed1990

Perception of directional sampled motion in relation to displacement and spatial frequency: evidence for a unitary motion system.

Perception of directional motion was studied by displaying two images (F1 and F2) in rapid succession. The two images were identical except for a horizontal displacement of F2 with respect to F1. Observers reported the direction of horizontal motion over a wide range of displacements. The stimuli in Experiment 1 were one-dimensional gratings with spatial frequency between 0.125 and 6 c/deg. Motion was seen at all displacements to almost 0.5 cycles (counterphase) and remained invariant across spatial frequencies. In Experiment 2 the stimuli were band-pass filtered random-dot patterns. The bandwidth of the filters was 1 octave, and centre frequencies ranged from 0.75 to 12 c/deg. In every case, the response functions exhibited quasi-periodic oscillations related to structural properties of the images. One-dimensional analyses based on autocorrelation did not provide a satisfactory account of the data. By contrast, the data were fitted successfully by a two-dimensional analysis that integrated the responses of neighbouring motion detectors so as to yield a smooth motion flow field from which left-right directional motion could be derived. Practically and conceptually, the outcome supports a unitary motion system as distinct from separate systems subserving short-range and long-range motion.

Contrast Sensitivity↗
PubMed1988

A model of visible persistence and temporal integration.

A quantitative model of temporal integration and visible persistence is described and tested. The model treats visible persistence as resulting from processing activity within sustained visual channels whose temporal response is modelled using a second-order control system. Temporal integration of two successive stimuli is assumed to be enabled by the overlap between the two periods of activity and to be impaired by the non-overlapping activity. The model successfully predicts the effects of inter-stimulus interval and of stimulus duration on goodness of temporal integration.

Afterimage↗
PubMed1987

Bootstrap variance estimators for the parameters of small-sample sensory-performance functions.

The bootstrap method, due to Bradley Efron, is a powerful, general method for estimating a variance or standard deviation by repeatedly resampling the given set of experimental data. The method is applied here to the problem of estimating the standard deviation of the estimated midpoint and spread of a sensory-performance function based on data sets comprising 15-25 trials. The performance of the bootstrap estimator was assessed in Monte Carlo studies against another general estimator obtained by the classical "combination-of-observations" or incremental method. The bootstrap method proved clearly superior to the incremental method, yielding much smaller percentage biases and much greater efficiencies. Its use in the analysis of sensory-performance data may be particularly appropriate when traditional asymptotic procedures, including the probit-transformation approach, become unreliable.

Animals↗
PubMed1985

Beyond the displacement limit: an analysis of short-range processes in apparent motion.

If a circle of random dots is presented in two successive displays in which the second is rotated in relation to the first, then observers are able to accurately discriminate the direction of apparent rotation as long as the rotation is small. Rotations beyond this short-range apparent motion can produce the impression of motion in the reverse direction. The performance in identifying the direction of rotation further depends on the eccentricity of stimulation and the density of the random dots. Simulations of the experiments using the Marr and Ullman model of motion detection are in good quantitative agreement with the data except for low dot density patterns and large displacements. In these situations perception seems to be dominated by the operation of long-range processes.

Discrimination, Psychological↗
PubMed1985

Selective internal operations in the recognition of locally and globally point-inverted patterns.

Performance in discriminating rotated 'same' patterns from 'different' patterns may decrease with rotation angle up to about 90 degrees and then increase with angle up to 180 degrees. This anomalously improved performance under 180 degrees pattern rotation or point-inversion can be explained by assuming that patterns are internally represented in terms of local features and their spatial-order relations ('left of', 'above', etc.), and that, in pattern comparison, an efficient internal sense-reversal operation occurs (transforming 'left of' to 'right of', etc.). Previous experiments suggested that local features and spatial relations could not be efficiently separated in some pattern-comparison tasks. This hypothesis was tested by measuring 'same-different' discrimination performance under four transformation: point-inversion 1 of the whole pattern, point-inversion 1F of local features alone, point-inversion 1P of local-feature positions alone, and identity transformation Id. The results suggested that internal sense-reversal operations could be applied selectively and efficiently, provided that local features were well separated. Under this condition performances for 1F and 1 were about the same whereas performance for 1P was significantly worse, the latter performance resulting possibly from an attempt to apply internal global and local sense-reversal operations serially.

Adult↗