Perception

Evidence for the proposition that human time perception is determined by an internal clock is largely indirect. It would strengthen the case for this hypothesis if a model for the internal clock were available from which predictions could be derived and tested, and if the basic parameter of such a model, the frequency at which the clock runs, could be estimated. A model for an internal temporal pacemaker is briefly described and its properties are explored by computer simulation. Results are obtained that provide a basis for predicting that, under appropriate conditions, interference between an imposed rhythm and the frequency of a temporal oscillator may cause perturbations in temporal judgment which are related to the characteristic frequency of that oscillator. Experimental data are reported which appear to demonstrate such an interference pattern. These results allow some estimates of the characteristic frequency of the temporal oscillator to be obtained.

Incremental threshold versus field-intensity curves (tvi) and spectral sensitivity functions were measured for 500 nm and 667 nm test flashes, either under the standard two-colour threshold conditions of Stiles (target: 1.0 deg, 200 ms)—which are known to favour detection by the chromatically-opponent pathways—or with a smaller (0.1 deg), shorter-duration (10 ms) target—chosen to favour detection by the non-opponent (achromatic) pathway. The data reveal differences between the two conditions: for the small, brief target, the tvi curves were shallower and less dependent upon wavelength, and the spectal sensitivity functions were narrower than for the standard target.

Measurements of the perceived spatial frequency of stationary sinewave gratings were made with the gratings presented at the same eccentricity in the left, right, upper, and lower visual hemifields. Ten subjects performed the task binocularly with spatial frequencies of 1, 2, and 4 cycles deg⁻¹. Two of these subjects also performed the task monocularly at 2 cycles deg⁻¹. In the majority of cases, the spatial frequency of stimuli presented in the left and lower visual hemifields was overestimated relative to stimuli presented in the right and upper visual hemifields. The results were similar for all spatial frequencies tested, and the direction of the asymmetry was the same whether viewing was with the left eye, right eye or binocular, suggesting that the differences in perceived spatial frequency are not retinal in origin.

When a rotating 3-D wireframe object passes behind a narrow slit, it is often perceived as a 3-D object, even though only a small portion of it is visible at any one instant. This result constitutes a new finding in connection with both anorthoscopic perception and the perception of structure-from-motion.

Recent studies using Brennan's computerized caricature generator have demonstrated distinctiveness effects consistent with the idea that faces are coded in terms of their individual distinctive properties. Based on these findings it is suggested that, for homogeneous classes whose members share a common configuration, distinctive configural information may be coded as metric deviations from a spatial norm. Experiments are described which demonstrate similar distinctiveness effects in bird identification. Transformations that increase distinctiveness (caricatures) produced faster identification and a higher recognition proportion, for both experts and nonexperts, than transformations that reduce distinctiveness (anticaricatures). This distinctiveness advantage is consistent with the norm-based coding idea. Furthermore, within certain limits, increasing distinctiveness did not impair performance relative to that for veridical drawings. For experts there was also a caricature advantage, such that 50% caricatures of birds in a highly homogeneous and familiar class (passerines) were identified more quickly, provided that they were recognized at all, than uncaricatured veridical drawings. The significance of a caricature advantage for the visual coding of configural information is discussed.

Infants' tactual discriminative abilities and intermodal transfer were investigated in two research studies which endeavoured to answer two questions: can infants aged 4–5 months process and discriminate tactually information about different spatial arrangements of elements of an object, in the absence of visual control, and secondly, can they transfer this information from touch to vision? Two experiments were conducted involving a habituation procedure and a procedure which tests reaction to novelty. Infants were found to be able to discriminate tactually between different spatial arrangements of the three elements of an object (experiment 1), and to transfer this information to the visual mode (experiment 2). These findings constitute new data on intermodal functioning in babies.

Saccadic reaction times (SRT) were measured in a simple task: subjects had to make saccades from a central fixation point to peripheral targets, which appeared randomly 4 deg to the left or to the right. In the first test the fixation point went off before the target appeared (gap trials); in the second test it remained on the screen (overlap trials). The distribution of SRTs for trained normal adults (N = 4), untrained normal adults (N = 11), untrained normal children (aged 9–11 years, N = 9), untrained normal teenagers (aged 15–17 years, N = 8), dyslexic children (aged 9–11 years, N = 15), and dyslexic teenagers (aged 15–17 years, N = 5) were compared, with special emphasis on the number of express saccades, ie saccades with extremely short reaction times (100–120 ms, under the present conditions). In normal adults with the gap paradigm, the distribution of reaction times typically exhibits two or three modes (express saccades, fast regular saccades, and very few slow regular saccades), whereas in the overlap paradigm only one or two modes (few fast regular saccades and many slow regular saccades) are obtained. On average, normal children produce more express saccades than naive normal adults. Dyslexic children produce more express saccades than the normal age-matched controls. Among the dyslexic children, four different types of abnormalities in their reaction times were encountered. The group of dyslexic teenagers was characterized by a larger number of express saccades at the expense of fast regular saccades in gap trials and by fewer express saccades and fewer fast regular saccades in overlap trials when compared to the age-matched control group. It is concluded that the abnormal patterns of saccadic reaction times reflect defects in the system of visual attention and/or in its control over the oculomotor system, rather than indicating a defect in the oculomotor system itself. In this context, symptoms of dyslexia appear as a combination of attentional deficits and irregular timing of saccadic eye movements.

Based on anatomical and evolutionary conceptions of the human ear, an experiment was conducted in which forty-eight human subjects were asked to localize sounds (a human voice) emitted by one of twenty-seven stationary loudspeakers in an anechoic chamber. The position of the active loudspeaker varied with respect to azimuth, distance, and elevation in three steps each. The position of a single sound-reflecting surface (about 6 m²) was varied: on the floor, on the ceiling, to the left, and to the right. The accuracy of identifying the active loudspeaker for each position of the sound-reflecting surface was compared intraindividually with the absence of reflection. The results show an overall increase in correct localizations with a sound-reflecting surface on the floor. Especially the elevation of the sound source can be detected with greater precision. Additionally, the percentage of correct localizations decreased systematically with the presence of a sound-reflecting ceiling, while the presence of sound-reflecting walls did not systematically affect the localization performance. Judgments in the horizontal plane and those of distance were not systematically influenced by the presence of a sound-reflecting surface.