Biomedical subjects

K K Niall

Publications and source records attributed to K K Niall.

9 recordsLinked to original sources

PubMed2000

Some plane truths about pictures: notes on Wagemans, Lamote, and van Gool (1997).

Wagemans, Lamote, and van Gool (1997) have attempted to show that observers can determine the geometric equivalence of shapes seen in perspective, or in projective transformation. Artifacts of measurement in their procedures forestall such a conclusion. Their experiment fails to control projective properties adequately, and also confounds the transformations of shear and compression. Their evidence that observers can discern the equivalence of shapes under perspective can be reconstrued as evidence for sensitivity to different, unrelated properties: to the plane compression that follows from the depiction of flat shapes in perspective, and to gross differences in shape not specific to projective geometry. An improved set of procedures is proposed for the measurement of stimuli in the study of visual shape constancy.

Artifacts↗

PubMed1999

Estimates of shape by eye, or, the little invariant that could.

Five experiments were conducted to test the hypothesis that observers apprehend specific constancies under change in perspective. The constancies were projective properties of ellipses pictured to slant and tilt in depth. Observers were asked to reproduce the static upright view of a moving pair of ellipses, using a computer graphics display and interface. Projective invariants for pairs of conics were computed on the observers' productions. A few experimental conditions revealed near-perfect performance. When pairs of coplanar ellipses were viewed under dynamic transformation in perspective, then invariants calculated on the observers' productions were a match--in value on average--to the invariants of the transforming ellipse pairs. It is proposed that measures of projective properties afford a family of techniques that can be applied to gauge acuity for complex shapes in the study of visual form perception.

Analysis of Variance↗

PubMed1999

Distance estimation with night vision goggles: a little feedback goes a long way.

Immediate feedback was given to correct observers' estimates of distance in an experiment in which those estimates were made outdoors at night while observers wore night vision goggles (NVGs). Initially observers made unguided estimates of distances between marked positions in an open field. Those distances ranged from 7.6 m (25 ft) to 64 m (210 ft). Later the same observers made more estimates. After each of these they were told the measured distance between the positions. During this training, the observers' height from the ground plane was either at a standing position or at an elevated position raised 2.3 m (7 ft 7 in) from standing position. After the training--either immediately after, a week later, or at both times--observers made unguided estimates of distance for a second time. These latter estimates of ground distance made with the NVGs were improved. Average improvement of the observers' estimates persisted for at least one week after training. This training can be applied to improve clearance estimates and estimates of hover height for pilots of rotary-wing aircraft.

Aerospace Medicine↗

PubMed1999

The art of descrying distance.

How naive can one experiment be? Imagine asking observers to judge distance -- yes, literally asking them. And in asking them, no visual evoked potentials were measured, and changes in blood flow to the occipital cortex were ignored. But serendipity occurs even to the well-prepared: One's best thought for a new experiment can prove to be one's first unbiased, uncomplicated thought.Here's a cursory review of the events: A number of observers were asked to judge distances between marked posts in a field at night. They made their judgments with the help of night vision goggles (NVG). At first the observers underestimated the distances on average. Then an experimenter began to correct the observers after each judgment. The observers' judgments of distance became accurate. On average, the observers began to estimate those distances accurately. Is this and incredible story or a difficult one?

Aerospace Medicine↗

PubMed1995

Conventions of measurement in psychophysics: von Kries on the so-called psychophysical law.

A translation of von Krie's (1882) paper "On the measurement of intensive magnitudes and on the so-called psychophysical law" is accompanied by a commentary. Von Kries claims that intensive magnitudes are not measurable in themselves, because the establishment of an equivalence between different steps in a scale of intensity does not make any sense without further clarification. Where intensive magnitudes are determined in a domain of the natural sciences, he claims this is only a matter of counting, and of the measurement of temporal and spatial magnitudes. Every measurement of intensity should then be reduced to these operations by explicit conventions. Likewise, we can only speak of the measurement of sensations once we have established an arbitrary convention that determines what we will consider equal. The debate whether sensation varies with the logarithm of stimulus intensity or in direct proportion to stimulus intensity, is then not a difference over matters of fact. Instead it is an empty dispute over words that is rooted in misunderstanding.

Humans↗

PubMed1992

Projective invariance and the kinetic depth effect.

Seven experiments test the assumption that, in the kinetic depth effect, observers have reliable and direct access to the equivalence of shapes in projective geometry. The assumption is implicit in 'inverse optics' approaches to visual form perception. Observers adjusted a comparison shape to match a standard shape; both standard and comparison were portrayed as in continuous rotation in space, using a graphics computer. The shapes were either plane quadrilaterals or solid prisms. The angular difference of the planes of the shapes was varied, as was the dot density of a texture in those planes. Departure from projective equivalence was measured in six studies by measuring the planar analogue of cross ratio, and in a seventh by measuring the cross ratio for points in space. Projective equivalence was not found to be perceived uniformly, except in one experiment that did not involve rotation in depth. Otherwise changes in orientation of up to 180 degrees about a single coordinate axis had no significant effect on matches in shape, while changes in orientation about more than one coordinate axis produced significant effects. The addition of texture and a change in rotation speed did not correct departures from projective equivalence.

Adult↗

PubMed1990

Projective invariance and picture perception.

Four experiments test the assumption that, in the visual perception of pictures, observers have reliable and direct access to the equivalence of shapes in projective geometry. The assumption is that perception of projective equivalence is the basis of shape constancy ('the projective thesis'). Observers matched or reproduced abstract planar shapes under conditions of rotation in the picture plane, and pictured rotation in depth. Departure from projective equivalence was assessed in each study by measuring the planar analogue of cross ratio. Projective equivalence was not found to be perceived uniformly where Euclidean equivalence was not judged uniformly, either in recognition tasks or in production tasks. When the projective thesis is put to a suitably general test, confidence in the thesis is undermined.

Adult↗

PubMed1989

Projective invariance and visual shape constancy.

As one moves about a table, the projection of its shape on the retina varies enormously, yet the table's shape appears constant. The various retinal images of a single object are nearly congruent in projective geometry. To explain apparent constancy, standard theories of vision assume that the visual system has access to this projective congruence. We present four experiments that undermine this assumption (i.e., the projective thesis). The basic result is that observers' estimates of shape in a simple production task represent gross departures from correct projection, even when observers are given aids to fixation. We manipulate both observer sample and experimental procedure in an attempt to find a source of these persistent errors. Our present hypothesis is that observers lack the sensitivity or implicit knowledge of projective geometry that has been attributed to them.

Attention↗

PubMed1988

On the trichromatic and opponent-process theories: an article by E. Schrödinger.

A translation of Schrödinger's paper, 'On the relation of the tetrachromatic theory to the trichromatic theory' (1925), is accompanied by a commentary. Schrödinger applies a projective transformation to a standard chromaticity diagram, to demonstrate the common geometry of the chromaticity diagrams derived from the trichromatic and opponent-process theories of color vision.

Color Perception↗