Terry Caelli

Discrimination thresholds in the two-dimensional spatial frequency domain

In two experiments we have determined the discriminability between two sinusoidal gratings as a function of orientation and spatial frequency differences. Twelve orientation (15 degrees steps) and four spatial frequencies (2, 4, 8, 12 c/deg) were considered and corresponding discrimination thresholds were determined. Results indicated that: (a) spatial frequency discrimination thresholds did not significantly vary over all frequency and orientation positions, and averaged at +/- 1/8 octave limits. (b) Orientation thresholds only slightly increased with spatial frequency (from +/- 5 degrees at 2 c/deg to +/- 6 degrees at 12 c/deg). (c) A large and consistent anisotropy occurred with orientation thresholds showing most sensitivity in the horizontal and vertical directions, with largest thresholds at the oblique angles. No oblique effect was observed with spatial frequency thresholds. These results confirm past observations, providing an extensive set of threshold measurements in the two-dimensional spatial domain.

On discriminating visual textures and images

Recent developments in modeling image discrimination by feature analytic and frequency selective methods are discussed. Some issues relating to the design of two-dimensional spatial frequency filters are developed within the context of two experiments on texture discrimination using artificial and naturally occurring textures. Results of these experiments indicate that, given an adequately formulated relationship between spatial frequency and orientation tuning parameters of the filter, one can predict a variety of texture discriminations using only amplitude-specific models. Finally, guidelines are established for ascertaining when phase transmission characteristics do become critical in two-dimensional image processing by the human observer.

Visual sensitivity to two-dimensional spatial phase

We investigate some limits to phase processing in the human visual system with two-dimensional textured images. Results indicate that, although phase sensitivity increases with contrast and energy components of the image, observers cannot discriminate between images and their 45 degrees phase-quantized versions under brief exposure and lower- (less than or equal to 30%) contrast conditions. These results seem to be frequency independent although modulated by different energy levels at each two-dimensional frequency band. Finally, we have analyzed some past texture-discrimination results that occur under identical amplitude-spectra conditions. Here also phase-quantization differences seem to constitute an adequate explanation of discrimination performance.

The discrimination of structure in vectorgraphs: Local and global effects

Many recent electrophysiological studies have demonstrated specificity for orientation and position tuning in single units of the visual system. Psychophysical investigations have produced complementary evidence for orientation and position coding in vision which is usually interpreted in terms of the underlying neuronal properties. To date, no studies have endeavored to relate such “local” receptive field response profiles to more “global” or field-specific aspects of the input signal. In a series of experiments, we have investigated orientation/position sensitivity as a function of general orientation and position rules governing line element, or “vectorgraph,” displays. We have discovered with such vectorgraph images that individual element acuities are determined by the field structures present. These results show that there are specific “top-down” components even to basic orientation/position coding in the visual system

ON DIFFICULTIES IN LOCALIZING AMBULANCE SIRENS

This project investigated how difficult it is to localize sirens under normal driving conditions. It considered various positions and movement directions of ambulances with respect to the driver, and compared traditional types of sirens with a new electronic range of sirens. Results indicate that judgments of distance, location, and direction of the siren source are unreliable, since they are particularly dependent on the driving conditions and the type of siren employed. In all, one type of siren seems to produce more accurate responses than other types and, therefore, may be regarded as a safer device.

Frequency, Phase, and Colour Coding in Apparent Motion: 2

We present some results which indicate that the known spatiotemporal limits for apparent motion are consistent with the motion being sinusoidal or a result of filtering. Given this we investigated how two such motions interact as a function of their relative temporal phase differences. This was accomplished by inducing two independent motions from complementary coloured event pairs. Results indicated critical phase limits for perceiving the two motions (red and green) which were consistent with the frequency specificity of the effect. The results are discussed within the framework of a filtering process for the perception of apparent motion.

The Waggon-Wheel Effect

The waggon-wheel effect was studied by use of three wheels with different numbers of spokes (4, 8, 16) and a wide range of strobe temporal frequencies. The results obtained are discussed in terms of a model in which: (i) nearest-neighbour relationships predict the direction and speed of movement, (ii) persistence and masking occur over ranges consistent with values reported in the literature, (iii) apparent motion is generated between currently illuminated spokes and persisting images of spokes, (iv) duration of spoke illumination (sweep) is a determining factor and, (v) a top-down process finds the best fit (a sort of ‘simple structure’) for the complex spatiotemporal display.

Intensity, Spatial Frequency, and Temporal Frequency Determinants of Apparent Motion: Korte Revisited

The authors have earlier found support for the notion that apparent motion (AM) is mediated by a low-pass temporal-frequency filter operating over the spatial domain of the AM sources. Experiments have been carried out to test how this response is changed, or unchanged, through varying the source intensities or contrasts. The results indicate no intensity or contrast effects on the spatiotemporal limits for AM. These results are related to the early formulations of Korte and indicate that there is no amplitude-modulation or contrast-sensitivity function for AM—as there is for threshold movement detection. The filter mechanism seems to be largely restricted to the spatiotemporal domain.

Facilitative and inhibitory factors in visual texture discrimination

In this paper we consider some spatial and temporal properties of visual textures which either inhibit or enhance their discrimination. From exposure time and texture mixing studies two findings emerge. First, it is clear that the spatial summation process involved in texture discrimination has a time course consistent with counting models for decision making. Secondly, the receptive field structures of salient texture features, in particular-orientation, seem to involve simple excitatory center and inhibitory surround mechanisms.

Probing the spatial frequency spectrum for orientation sensitivity in stochastic textures

Abstract In this paper the question as to the amount and type of orientation information identified and discriminated at brief exposures is investigated as a function of (radial) frequency with two-dimensional grey-scaled stochastic textures. Results indicate that no more than 18 orientation classes are detected with the low frequency components of such textures. The results on high frequency ( f ⩾ 8 c deg) components are less clear, due to lower energy and less uniform orientation distributions within this range.