Perception

Following adaptation to a field of light which was modulated by a rising ramp so that it repetitively grows gradually brighter, a steady test field of light appears to be gradually growing dimmer. In this study, if a small grey spot of constant luminance was centred in the brightening field, it appeared to be growing gradually dimmer by simultaneous contrast. This apparent dimming led to a brightening aftereffect in the spot. It was shown that this spot aftereffect had two independent components: the apparent dimming of the adapting spot produced its own aftereffect (contrast produced an aftereffect) and also the dimming aftereffect in the surround field spatially induced an aftereffect into the spot during the test period (aftereffect produced simultaneous contrast). Thus simultaneous contrast can both precede and follow successive contrast in the visual system.

The R-index approach to the signal-detection P(A) index of sensitivity was used to measure taste sensitivity to NaCl solutions. The R-index is the predicted probability of the correct choice of a signal, for a given signal – noise pair. For flow-wise and sipwise presentation, R-indices did not fluctuate significantly, provided subjects could not see the stimuli yet to be presented, indicating a lack of systematic sensitivity drift. The simultaneous measurement of more than one signal strength with reference to a common noise stimulus was seen to be a viable and sensitive procedure. Comparison of flow-wise and sipwise presentation of stimuli using R-indices, indicated that the former elicited greater subject sensitivity to NaCl taste.

Visual acuity for the detection of gratings at four orientations was measured for groups of ten boys and ten girls aged five to seven years, from the following four populations: Scots in Glasgow, Pakistanis in Glasgow, Gaels in Stornoway (Outer Hebrides), and East Anglians in Littleport (Cambridgeshire fenlands). The Glaswegians, both Scottish and Pakistani, showed the normal pattern of anisotropy, with poorest acuity for oblique orientations; the East Anglians showed no significant anisotropy; while the Gaels were unusual in showing poorest horizontal acuity. A group of fourteen Pakistani children in Stornoway differed slightly from a matched group of Gaels. The group differences bore little relation to the visual environments, and were probably due to genetic or cultural factors. The relatively poor horizontal acuity of the Gaels was not correlated with astigmatism. Sex differences were also found, with the boys showing higher mean acuity and a higher ratio between vertical and oblique acuity.

The tilt aftereffect (TAE) was studied with adapting and test stimuli consisting of black or white bars (experiment 1), and of luminance edges (experiment 2). Both experiments failed to demonstrate selectivity of the TAE to the polarity of luminance contrast.

The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg⁻¹ gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg⁻¹ gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.

Selective adaptation was used to explore the characteristics of a metacontrast masking stimulus which contribute to its effectiveness in masking the test stimulus. Subjects adapted for 10 s to a configuration like the masking stimulus that was either continuously on or flickering. Following this they viewed a metacontrast presentation and estimated the brightness of the test stimulus. Prior adaptation to a continuously present stimulus did not appreciably affect metacontrast masking; however, masking was greatly reduced following adaptation to flickering stimuli. These results are consistent with recent models of metacontrast masking based on transient and sustained visual channels.

Contrast thresholds for the detection of flicker and the detection of pattern were measured for nasal and temporal retinae of each eye separately. While confirming that these two types of thresholds can differ, depending on spatial frequency, the results provide no support for hemiretinal or hemispheric asymmetries in the distribution of the putative pattern and movement channels.

The exposure durations of a vertical test line and a tilted inducing grating were varied and the tilt illusion thus generated was found to change as a function of this variation. Significant direct effects (acute-angle expansion) and indirect effects (acute-angle contraction) were found to occur at times consistent with Andrews's estimate of the time course of inhibition in the visual system when the inducing grating had a spatial frequency of 10 cycles deg⁻¹. However, a 2 · 71 cycles deg⁻¹ grating gave significant effects at exposure durations of 10 as well as 1000 ms, while in a further experiment a 10 · 91 cycles deg⁻¹ grating gave significant effects at 1000 ms only. These results seem to suggest that orientation interactions thought to be due to inhibition (direct effect) and disinhibition (indirect effect) may occur within both sustained and transient channels with concomitant differences in time constants.

Spatial illusions which occur in angle-matching tasks were examined in six experiments, using two different kinds of display. In experiments 1 and 2, illusory errors generally were in the direction predicted by Lennie's hypothesis which states that angle arms are attracted perceptually towards the oblique axes of space, although the display used in these experiments differed from Lennie's. However, experiment 3 showed that these errors might equally be explained by the addition of interactive effects between angle arms (tilt illusions). Parametric investigation of Lennie's figures (experiments 4 and 5) showed that the largest angular illusion occurred with the largest angle used (45°) and an intermediate line length (2 deg 7 min). These angular illusions were not explicable by the addition of tilt illusions (experiment 6), suggesting that different judgmental processes may underlie orientation and angle estimations.

An investigation of ninety-five university admission candidates failed to replicate the finding by Yin of a negative correlation between the ability to recognise upright and inverted faces. A zero correlation was obtained when unknown faces were both learned and recognised upside down, but when well-known faces were presented normally and upside down for identification, a significant positive correlation appeared. Rock has suggested that inverted faces are difficult to recognise because they overtax a mechanism for correcting disoriented stimuli. This explanation satisfactorily accounts for the data with the proviso that, when inverted faces are to be remembered, the best strategy is not to attempt to correct their orientation, but to learn isolated features of the face. This describes the data more parsimoniously than Yin's face-specific mechanism.

A simple technique for producing random-dot stereograms of real objects is described. It is then shown that stereograms of inside-out faces (moulds) cease to be perceptually reversible only when the presentation is truly cyclopean. Further possible applications of the technique are outlined.

A distinction is drawn between diffuse regions of apparent brightness and illusory, but abrupt, brightness gradients. Differences between the two phenomena are sufficient to disqualify conclusions about one based on observations about the other. It is suggested that diffuse contours may be attributable to peripheral mechanisms while sharp contours may be central or cognitive in origin.

In an earlier paper, Caelli and Finlay demonstrated that the perception of apparent motion was contingent on critical phase relationships between inducing sources, and their spatio-temporal frequency ranges. In their original stimulus red-green centre-surround sources were used and phase-specific inhibition was found at 60°–90° phase angle differences between the complementarily coloured elements. In this experiment we show that, when centre-surround sources are replaced by vertically-horizontally displaced complementary (or identically coloured) sources, symmetrical phase inhibition effects occur with both motions.