Perception

Reported differences in neuronal contrast processing between the parallel magnocellular (M) and parvocellular (P) visual pathways invite the hypothesis that contrast discrimination in the human visual system is more sensitive at low contrasts and less sensitive at high contrasts, for stimuli modulated at high compared with low temporal frequencies. In the present study, an edgeless temporally modulated uniform field was selected as the stimulus for psychophysical contrast discrimination, and contrast-increment thresholds for pedestal contrasts ranging from 5.5% to 78.2% were determined with a temporal two-alternative forced-choice staircase procedure. The increment thresholds for five normal subjects were adequately fit by power functions with exponents that shifted continuously from about 0.5 (square-root-law behavior) to about 1.0 (Weber's-law behavior) as stimulus temporal frequency increased from 1 to 30 Hz. A neural simulation, with the use of published contrast-response functions of magnocellular and parvocellular neurons, adjusted with an estimate of response variance, produced two distinct ‘neural increment-threshold functions’ that were similar to the psychophysical results obtained at the highest and the lowest temporal frequencies, respectively. A shift from a relatively more noise-limited neural mechanism to one whose response is predominantly determined by gain is suggested to account for the change of the contrast-increment-threshold function with increasing temporal frequency.

Analysis of recently published human contrast-sensitivity data obtained along the cardinal and major oblique visual-field meridians of a single subject has demonstrated a consistently greater sensitivity at a given eccentricity to horizontally oriented as compared with obliquely oriented gratings. This difference was evident not only at foveal but also at several eccentric loci over a range of low to medium spatial frequencies. This observation is to be distinguished in extrafoveal fixation from the well-documented oblique effect, which describes the variation in sensitivity with orientation at a single visual-field locus. With periodic stimuli which were well localised in space and frequency, and had comparable spatial-summation properties, a spatial-frequency dependency of what could be termed the global oblique effect could be demonstrated along isoeccentric contours centred on the fovea (eccentricity 0 deg) out to an eccentricity of at least 40 deg.

The ability to set the angle of a ‘V’ to a designated value in the following three conditions was compared: (1) verbal designation of V angle; (2) initial 30 s visual demonstration of the designated V angle; (3) verbal designation of V angle plus feedback after every setting. The designated angles were 90° and 45° plus three arbitrary angles (65°, 125°, and 145°). Each run comprised thirty consecutive settings. To ensure that our observers based their settings entirely on V angle it was arranged that line orientation did not provide a reliable cue to V angle. In condition (1), accuracy of setting V angle was significantly worse when the designated angle was other than 90° or 45°. This was not the case in condition (2), indicating that observers maintained a memory of the initial demonstration throughout a run of thirty settings. Setting error was not significant in condition (3) for any of the five angles. However, even in condition (3), setting-to-setting variability was significantly lower for the 90° angle than for the other angles.

In previous work with a neural-network model of boundary segmentation and reset, the percept of persistence was linked to the duration of a boundary segmentation after stimulus offset. In particular, the model simulated the decrease of persistence duration with an increase in stimulus duration and luminance. Further evidence is revealed for the neural mechanisms involved in the theory. Simulations show that the model reset signals generate orientational afterimages, such as the MacKay effect, when the reset signals can be grouped by a subsequent boundary segmentation that generates illusory contours through them. Simulations also show that the same mechanisms explain properties of residual traces, which increase in duration with stimulus duration and luminance. The model hereby discloses previously unsuspected mechanistic links between data about persistence and afterimages, and helps to clarify the sometimes controversial issues surrounding distinctions between persistence, residual traces, and afterimages.

Open-loop reaching for locations within figural illusions was measured in three experiments. The experiments differed with respect to whether subjects were provided a visible target toward which to direct their reaching or were required to form a mental representation of the intended target. In the first experiment, subjects' reaching errors for vertices of a Müller-Lyer figure were similar to those for a nonillusory control stimulus. In experiment 2, subjects' errors while reaching to the imaginary bisector of the Judd illusion were consistent with the presence of an illusion of bisector location. However, when a bisector line was added to the Judd figure, reaching errors were similar to those obtained with a control figure. In experiment 3, subjects' open-loop reaching at the perceived midpoint of a triangle was biased toward its illusory perceptual midpoint. When a mark was placed at the midpoint between a vertex and the opposite side, reaching errors were similar to those obtained with a control figure. The results of the experiments indicate that hand–eye coordination is biased in the direction of illusions of bisector location only when no target is present at the intended goal of the reaching response and subjects are required instead to form a mental image of the target. Under these conditions, reaching responses appear to utilize the spatial map of the visual system, and are influenced by figural illusions of bisector location. The present data can be understood without invoking the notion of visual–motor dissociation.

An investigation was undertaken into whether judgments of time-to-contact between a laterally moving object and a bar are based on the direct perception of an optical variable (tau), or on the ratio between the perceived distance and perceived velocity of the object. A moving background was used to induce changes in the perceived velocities without changing the optical variables that specify time-to-contact. Background motion induced large systematic errors in the estimated time-to-contact. It is concluded that the judgment of time-to-contact is primarily based on the ratio between the perceived distance and the perceived velocity, and not on tau.

The use of nonspatial attentional mechanisms in search tasks was investigated by presenting observers with stimuli that contained 4–12 elements located on a circle around the fixation point. The elements differed in one of six nonspatial ‘dimensions’, namely orientation, contrast, scale, number of cycles, ‘shape’, and place in the alphabet. The target element of the search task differed from trial to trial but was always presented to the observer as a nonspatial, visual cue. This cue was displayed either before the stimulus (precue) or after the stimulus (postcue). Whereas a precue creates optimal conditions for the use of nonspatial attentional mechanisms, a postcue precludes benefits from their use. The fact that performance was better in the case of precued stimuli than in the case of postcued stimuli indicates that observers employed nonspatial attentional mechanisms. In the final analysis, however, the effect of nonspatial attention reduces to spatial attention in combination with limited storage capacity.

A signal-processing model is proposed in which the phenomenon of ‘aliasing’ is invoked to explain certain phenomena in the perception of musical tones, for which a really satisfactory explanation has not hitherto been available. It is shown that this model offers a reason why the harmonic series appears to play such a central role in tone and pitch perception, and can throw light on ‘virtual pitch’, ‘harmonic beats’, etc. Some preliminary results from a computer simulation of the model are described which are consistent with empirical data on tone perception.