Adaptation of suprathreshold contrast and luminance stimuli

The temporal and spatial properties of the difference in perceived contrast and brightness of two suprathreshold stimuli presented successively in different retinal locations were determined. Stimulus onset asynchrony (SOA) was varied and the perceived contrast or brightness of the first stimulus (S1) was measured as a function of SOA by matching the contrast or luminance of the second stimulus (S2) to that of S1. The two stimuli overlapped in time for 200 ms to allow the comparison to be made. The adjusted values for S2 could well be fitted with an exponential decay function of SOA. For luminance increments and decrements the time constant for this function was 253 ms; for checkerboards with checks of size 16 min square the time constant was 164 ms. The difference in perceived contrast was dependent on initial contrast in a nonlinear fashion. It increased with increasing check size and was independent of the mean luminance and spatial proximity of the two stimuli. The phenomenon was observed with different pattern types and with dichoptic presentation, but could only be seen when direct comparison of the two stimuli was possible.     

Binocular interactions in suprathreshold contrast perception

Suprathreshold binocular contrast interactions were studied psychophysically. A split-screen CRT display was used to present separate sine-wave gratings to the observer’s left and right eyes. The method of constant stimuli and a modified method of adjustment were used to find sets of binoculartest patterns that matched a given binocularstandard. Test patterns consisted of the simultaneous presentation of sine-wave gratings that differed in contrast to the left and right eyes. Standard patterns consisted of identical sine-wave gratings presented to the two eyes, and had the same spatial frequency as the test patterns. Binocular contrast matching functions were obtained for several standard contrasts at 1 and 8 c/deg. Binocular matching functions were obtained for luminance increments as well. The binocular contrast matching functions departed from a simple binocular averaging rule, and behaved as if the eye receiving the higher contrast disproportionately dominated the binocular contrast percept. Departures from the binocular averaging rule were slightly greater for higher standard contrasts. Spatial frequency had little effect, and the luminance increment matching functions also departed from the binocular averaging rule. There was evidence for a contrast version of Fechner’s paradox and for substantial individual differences in a form of ocular dominance. In a further experiment, additivity of suprathreshold binocular contrast summation was examined by testing the double-cancellation condition. We found no systematic violations of additivity at 1 and 8 c/deg. Models of suprathreshold binocular contrast summation were examined.     

Upright versus upside-down faces: How interface attractiveness varies with orientation

A choice experiment is reported in which all pairs and triples of faces from a set of eight moderately attractive faces were presented, both upright and upside down, to 103 subjects. In each orientation, the subjects had to select the face that appeared more (pairs) or most (triples) attractive to them. For each orientation, the preference probabilities that arose from the pair and triple comparisons could be described by the BTL rule (Luce, 1959). Thus, each face was represented by two scores, one reflecting its attractiveness in the upright orientation and the other reflecting its attractiveness in the inverted orientation. Orientation affected the preference probabilities. Qualitatively, score ratios between faces decreased from upright to inverted orientation, suggesting that the faces became less discriminable by inversion. Quantitatively, the effect of inversion could be described by a simple rule that assumes a face’s two attractiveness scores to be affinely related across orientations. This result indicates that inversion affected all faces about equally. The present findings are discussed with respect to faces’ first- and second-order relational properties, a distinction emphasized in current theories of face perception. They suggest that the processing of first- and secondorder relational properties is impaired by inversion to roughly the same degree.     

Functional significance of stimulus orientation: effects on orientation discrimination.

Children (6- and 9-year-olds) and adults were required to discriminate identical pairs of visual stimuli from mirror images. It was hypothesized that a key factor in performance would be the extent to which orientation was a functionally significant attribute of the stimuli. Two variables were manipulated, type of orientation discrimination and stimulus class. The first variable refers to the fact that the mirror images could be produced by either left/right or top/bottom reversals. Three classes of stimuli, varying in the extent to which a particular orientation was emphasized, were used: mobile objects (for which left/right orientation is assumed to be important); stationary objects (which lack comparable relevance for left/right orientation); and novel, abstract forms. The prediction was that if the discrimination task involved left/right reversals, as contrasted with top/bottom reversals, subjects would show an advantage for mobile objects, producing an interaction between stimulus class and orientation discrimination. In the first study, the subjects were children and performance was measured in terms of error rates. In the second study adults were tested, and reaction times were measured. Both studies manifested the predicted interaction. Results are discussed in terms of an information-processing framework, in which the incorporation of orientation-related features in the code representing a stimulus varies with the functional significance of the orientation to the stimulus class.     

The effect of stimulus duration on the persistence of gratings

The persistence of gratings varying in spatial frequency and exposure duration was measured using a stimulus-blank alternation method. Persistence was found to lengthen with increasing spatial frequency and to shorten with increasing exposure duration. For each spatial frequency, persistence decreased linearly with a slope of approximately —.75 as duration increased for short stimulus durations. For longer stimulus durations, the rate of decline in persistence with increasing duration was reduced, the slope being approximately —.13. The stimulus duration at which the change in slope of the persistence-duration relationship occurred was shown to increase with increasing spatial frequency and was approximately equivalent to the critical duration for each spatial frequency. The data were consistent with an interpretation of persistence in terms of a temporal integration component and a second, possibly cortically located, component.     

Effect of stimulus opening size in global dominance with orientation classification task

The aims of this study is to examine whether global dominance depends on the opening size of the stimulus with concentric hierarchical figures and orientation classification task and to determine the role of the salience of global opening and its coincidence with vertical symmetry axis of context. In the first experiment, participants had to indicate the opening direction of stimuli, which were open-left and open-right figures. Three openings were included: 10, 25 and 50% of the total circle perimeter. The results showed a local advantage with stimuli of 10%, absence of global or local advantage with stimuli of 25% and global advantage with stimuli of 50%. In the second experiment, stimuli with an opening of 50% were presented randomly in several positions in the visual field in order to avoid the coincidence of global opening with the vertical symmetry axis of context. The results showed an absence of global or local advantage. These findings indicate that global dominance with orientation classification task depends on stimulus characteristics such as opening size, and strategies used in visual recognition.     

Pictures in mind: initial encoding of object properties varies with the realism of the scene stimulus

A fundamental question in perception is how we visually encode and retain information about a complex scene in order to allow effective operation within it. Interestingly, the stimuli used to investigate scene perception have varied greatly between studies, ranging from line drawings to coloured drawings, computer-generated scenes, photographs, and real scenes. Are findings from these different types of scene stimulus equally ecologically valid? Two experiments are reported that address this issue. In the first we compared photographic and non-photographic scenes and found that observers perform better in questions testing object memory when viewing photographs, suggesting an initial benefit for encoding information from photographs. In the second we found that whether or not non-photographic scenes obeyed realistic scene-organising properties influenced object-memory formation. Effects varied for the different question types, but again were most prominent early in viewing. We conclude that in the search for an understanding of everyday scene perception we must be very careful in our choice of scene stimuli and in our interpretation of findings from the laboratory.     

Recovery from adaptation as a function of stimulus orientation

These experiments examined the oblique effect in an adaptation paradigm. Reaction times (RT) to the presence of a grating test stimulus were obtained following adaptation to either a blank field or a grating of the same orientation as the test stimulus. Horizontal, vertical, and oblique test and adaptation orientations were employed. Test gratings were presented at several interstimulus intervals following offset of the adaptation stimulus. RTs following grating adaptation were elevated to a greater extent (relative to blank adaptation) for oblique then for horizontal or vertical stimuli, for two grating spatial frequencies. Differences in RT can be related to differences in sensitivity among channels responsible for detection of the various orientations.

     

The effect of exposure duration on the apparent contrast of sinusoidal gratings

Apparent contrast determined for sinusoidal grating targets which were flashed for varying durations appears to follow a temporal course similar to a Broca-Sulzer curve, and this temporal course is also influenced by the spatial frequency of the grating target. These results were discussed in terms of differential latency and temporal integrating characteristics of spatial frequency channels.     

Identification of symmetry: Effects of stimulus orientation and head position

It has been argued that the perceptual advantage of symmetry depends upon the essentially symmetrical properties of the visual system. According to this explanation, the ease of identification of symmetries about different axes of orientation should decrease with increasing distance from the vertical: Reaction times to vertical symmetry should be faster than those to diagonal symmetry, which in turn should be faster than those to horizontal symmetry. Previous research demonstrating this pattern of responding employed stimuli with linear axes. In the present study, the subjects viewed tachistoscopically presented symmetrical and asymmetrical dot patterns (which had no explicit axes) in one of three head positions: upright, 45 deg left, and 45 deg right. The subjects’ performance failed to support the structural explanation: Identification of symmetry is equivalently fast for vertical and horizontal; vertical and horizontal show strong advantages over obliques, and this general advantage follows retinal coordinates. Findings are discussed in light of alternative theories of symmetry processing.     

Effects of stimulus orientation on the identification of common polyoriented objects

Experimental evidence has shown that the time taken to recognize objects is often dependent on stimulus orientation in the image plane. This effect has been taken as evidence that recognition is mediated by orientation-specific stored representations of object shapes. However, the factors that determine the orientation specificity of these representations remain unclear. This issue is examined using a word-picture verification paradigm in which subjects identified line drawings of common mono- and polyoriented objects at different orientations. A detailed analysis of the results showed that, in contrast to mono-oriented objects, the recognition of polyoriented objects is not dependent on stimulus orientation. This interaction provides a further constraint on hypotheses about the factors that determine the apparent orientation specificity of stored shape representations. In particular, they support previous proposals that objects are encoded in stored representations at familiar stimulus orientations.     

Gender differences in response to spatial frequency and stimulus orientation

Male and female subjects with normal or corrected-to-normal visual acuity and less than .25 diopter of corrected astigmatism were asked to make contrast threshold judgments in response to both stationary and drifting grating displays. Results indicate a sex difference in contrast sensitivity as a function of spatial frequency for vertical and oblique orientations.     

Interaction of fixation point and stimulus orientation the appearance of rectilinearity

Data from two experiments indicate that the findings of Prytulak (1973) are generalizable to stimulus lines at all orientations: (a) when setting a response dot so that it appears to lie on an imaginary extension of a stimulus line, Ss misplace the dot on the side of the extension opposite the fixation point (a contralateral response, or CR) when the fixation point is in the vicinity of the stimulus line and on the same side of the extension as the fixation point (an ipsflateral response, or IR) when the fixation point is in the vicinity of the response dot; (b) CR is larger than IR; (c) fixation of the stimulus line or any point on its extension produces relatively accurate settings. New findings were: (a) the magnitude of CR decreased as the orientation of the stimulus line approached the vertical or horizontal; (b) the magnitude of CR on or near the horizontal was weaker than on or near the vertical; (c) the magnitude of IR was greater - for stimulus lines lying in the upper half of the visual field than for stimulus lines lying in the lowerhalf. The difficulty - - of accounting for fixation point effects by means of physiological optics was discussed.     

Weak stimulus generalization using sinusoidal gratings: A cautionary note in animal psychophysics

Psychophysical techniques involving operant and classical conditioning are used commonly to determine the visual thresholds of nonverbal subjects. Typically, subjects are conditioned to respond to a particular stimulus, and once trained sufficiently, the subject’s sensitivity to this and similar stimuli can be determined by decreasing the stimulus intensity until the animal no longer responds. However, this assumes that the animal’s tendency to respond correlates with its sensory abilities. Stimulus generalization to sinusoidal gratings of different spatial frequencies was examined in goldfish. Subjects were classically conditioned to suppress respiration upon presentation of a sinusoidal grating. Animals’ learning curves to each spatial frequency grating were compared in order to determine whether the animals generalized across stimuli. Results indicate that fish show weak stimulus generalization across spatial frequencies. Also, the shape of the contrast sensitivity function, a common measure of the animal’s visual capabilities, is distorted by insufficient training. We conclude that when the goal is to obtain accurate data on visual capabilities, either subjects should be trained to each stimulus to be tested or their generalization gradients should be measured.     

The effects of adaptation to gratings on the perceived orientation of single lines

Subjects were adapted for 45 s to a 5 cycle per degree grating, tilted 10° from the vertical, and at one of four levels of luminance contrast. Estimates of the slope of the grating, or of a single line which replaced the grating, were obtained before and after the period of adaptation by the subject's setting of a single reference line which was located 3° from the edge of the grating. In Experiment I the fixation area was at the centre of the grating, so that half of the grating fell on each retinal hemifield. The reference line was set to a tilt further from the vertical following the period of adaptation, indicating either a change in the apparent orientation of the grating or the reference line. In Experiment II, where the fixation area was midway between the grating and the reference line, so that line and grating appeared in different hemifields, settings indicated no change in the tilt of the grating. In Experiment III, where the fixation area was again midway between grating and setting line, but where the grating was replaced by a single line after the adaptation period, settings indicated that the line had decreased in apparent tilt. The results of the three experiments indicate that while gratings either do not normalize or do so to an insignificant extent they produce a substantial amount of normalization on a single line. Comparison of the results of Experiments I and II shows that spread of this effect outside the directly adapted area is less across the naso-temporal division of the retina than within the same hemifield.     

The effects of adaptation to square-wave gratings as a function of grating orientation

An adaptation technique was used to measure the selectivity or tuning for grating orientation in the visual system for different orientations of the inspection stimulus. Duration thresholds for grating patterns of constant luminance were determined for 13 test gratings oriented from ±5 to 90 deg away from each of five adaptation gratings: 0, 22, 45, 67, and 90 deg. Threshold data obtained for test gratings without prior adaptation indicated higher sensitivity for gratings oriented along the horizontal and vertical axis than along the oblique axis. After adaptation, thresholds increased (sensitivity was reduced) for gratings having similar orientations as the test gratings. However, the functions relating sensitivity reduction to degree of angular disparity between test and adaptation grating did not vary across the five inpsection orientations, i.e., selectivity or tuning for grating orientation appeared to be independent of the orientation of the adapting stimulus.     

Effect on of familiarity and category contrast stimulus and response priming

In a letter-symbol classification task, flanking incompatible symbols slowed response latencies to letter targets, whereas incompatible letter flankers did not slow the classification of symbol targets. The conditions surrounding this asymmetry in response competition were investigated in five experiments. The results showed that: (1) the asymmetry was not related to the familiarity of the symbol targets or to the prime-target interval; (2)when the classification involved familiar and unfamiliar symbols, the asymmetry remained (i.e., there was less interference associated with the unfamiliar symbol targets), but there was now significant response competition associated with both symbol categories; and (3) with a mixed-category task (i.e., letters and symbols assigned to both responses), the symbol targets continued to be less interfered with by both letter and symbol incompatible flankers. These findings were interpreted as suggesting that response competition can be influenced by both classification decision rules and cohesiveness of exemplars comprising a category.