Orientation-specific aftereffects and illusions in the perception of brightness

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.     

Stimulus control and response bias in an analogue prey-detection procedure

The present study compared the performance of 6 pigeons trained to detect luminance differences in two different signal-detection procedures. Exposed to a three-key array, the pigeons were trained to peck the left key when the brighter of two light intensities had been presented on the center key and to peck the right key when the dimmer of two light intensities had been presented on the center key. Procedure A was a standard signal-detection procedure in which left/bright and right/dim responses produced food reinforcement and left/dim and right/bright responses produced periods of timeout. Procedure B was designed to simulate some of the contingencies operating in a prey-detection situation. Left-key responses produced reinforcement following the brighter center-key stimulus and a period of timeout following the dimmer center-key stimulus. Right-key responses always produced a short period of timeout irrespective of the stimulus. Within each procedure, the duration of timeout arranged for false alarms (left/dim responses) was varied between 3 s and 120 s. Measures of accuracy and response bias were compared between the two procedures. The timeout manipulation produced systematic, but relatively small, changes in these measures when right/dim responses (i.e., correct rejections) produced reinforcement (Procedure A). Arranging timeout for right/dim responses in Procedure B produced greater variability in accuracy and response bias than did arranging reinforcement, but this variability was not related to timeout duration. Overall, discrimination accuracy was considerably higher when right/dim responses produced timeout than when they resulted in reinforcement, and accuracy was accompanied by a large bias toward the response associated with reinforcement. These results are consistent with a recently proposed model of signal detection.     

The glare effect does not give rise to a longer-lasting afterimage

The glare effect is an illusion in which a region appears self-luminous when flanked by gradients that decrease in luminance with distance from the region (Zavagno, 1999 Perception 28 835-838). This region also appears brighter than a surface of the same luminance. We investigated, using the paradigm of afterimages, whether a low-level mechanism at the level of the retina or LGN could account for this apparent brighter sensation. We first replicated the result from the literature that brighter and longer-lasting physical stimuli generate longer-lasting afterimages. We then compared the glare-effect stimuli with their counterpart controls, and found that the glare-effect stimuli did not give rise to longer-lasting afterimages. This suggests that the apparent brighter sensation of the glare effect is not due to a retinal or LGN mechanism, but must have a cortical origin.     

Specularity,brightness, achromatic color—and orthogonality

For objects under the same illumination, the more specular object appeared brighter. This occurred irrespective of an object’s apparent achromatic color, the distance at which it was viewed, the level of illumination, the method used for collecting observations, and other conditions. Also, when identical specular objects were differently illuminated, the one under the higher illumination, with the higher maximal luminances, appeared brighter. In the five experiments, which involved 340 subjects subdivided into 17 groups, large fields of view and real spaces were used. The results supported the conclusion that apparent brightness and achromatic color are orthogonal phenomena, and that in four of these experiments increased apparent brightness was correlated to, if not determined by, maximal specular luminances without regard to achromatic colors and diffuse luminances. This conclusion was necessarily modified by a fifth experiment, which showed that if placed under sufficiently high levels of illumination, less specular surfaces appeared brighter than more specular surfaces. This was taken to mean that a total account of the apparent brightness of surfaces would depend on an undiscovered algorithm involving maximal specular and diffuseluminances and their areal extents. With regard to the validity of the studies, the subjects were shown to have phenomenally discriminated brightness from glossiness and glare. Finally, phenomenal gloss and glare were found to be correlated to a surface’s level of specularity.     

Apparent movement and metacontrast suppression: A decisional analysis

Three test and three mask energies were varied orthogonally and randomly over trials. The stimulus onset asynchrony (ISOA) separating test and mask was varied between trial blocks within each of two display conditions, apparent movement (two-object) and metacontrast (threeobject). Subjects were required to makebrightness judgments of both test and mask energies by responding “bright,” “medium,” or “dim” with respect to the apparent intensity of each stimulus. The accuracy and the coherence lconsistencyt of test judgments were U-shaped functions of SOA for both apparent movement and metacontrast situations. However, the accuracy and the coherence of mask judgments did not vary with SOA for either apparent movement or metacontrast. It was noted that substantially the same results have been reported previously when subjects were required to makecontour judgments. Hence, it is argued that apparent movement and metacontrast suppression are intimately related.     

Illusions of size change in dynamic displays

A line of constant width viewed against an expanding or contracting grating appears to become narrower or wider, respectively. This effect was studied using a computer-controlled video system with a motorized zoom lens. The magnitude of the illusory size (width) change with a vertical line did not differ when viewed against a horizontal or an oblique background, and the size change was not due to static size contrast between the line and the background. The illusory narrowing observed with an expanding background was equivalent to the widening observed with a contracting background. The apparent change in the width of lines that actually expanded was consistently underestimated, either on a similarly changing background or when viewed against a gray field. Although the transformation of the background and line produced changes in their perceived distance, this did not occur for a constant line against a transforming background, even though this condition induced the perceived size change.     

Some temporal properties of behavioral contrast

Pigeons were rewarded on a variable time interval for pecking a translucent key illuminated with either a 45 degrees or a vertical line. The key illumination changed every 2 min during daily 1-hr sessions. When the rates of pecking were stable, reinforcement was omitted in the presence of the 45 degrees line. Responding in the presence of the vertical line increased. This increase did not disappear when responses to the 45 degrees line were once more reinforced, but when reinforcements for responses in the presence of the 45 degrees line were again omitted, responding to the vertical line increased again. After the second alternation of these two procedures, the increased responding to the vertical line appeared when responses were not reinforced in the presence of the 45 degrees line, and disappeared when reinforcement was available during both stimuli. In a second experiment, the key illumination changed between sessions only, so that 1-hr sessions of reinforcement and non-reinforcement occurred on alternate days. Responding to the vertical line still increased when responding to the 45 degrees line was not reinforced, but the increase tended to disappear during the session.     

The constancy of object orientation: Compensation for ocular rotation

When a thin horizontal line is displaced, either left or right of straight ahead, or when a vertical line is displaced up or down, systematic changes occur in the binocular disparity associated with the target. In threeexperiments, Ss matched the orientation of displaced targets with a variable comparison line. Estimates of apparent displacement with a pointing technique were also made. Since head position was fixed, apparent displacement was mediated by the angle of ocular rotation. Near perfect matches were made with vertical targets, but horizontal targets produced errors suggestive of underestimation of apparent displacement. However, the pointing data did not yield clear evidence for this view. Control data denien the possible role of the induced effect (IE) in matching the horizontal targets, and the result were discussed in the context of orientation constancy based upon compensation for displacement.     

Matching person identity from facial line drawings

The processing of facial line drawings was investigated in either simultaneous or sequential matching trials with either the same or different viewpoint, showing pictures of faces either in the same modes (both photographs or line drawings) or different modes (one in each mode). Line drawings were particularly difficult to match in memory rather than under perceptual conditions, and line drawings did not allow the creation of efficient structural codes. These deficits of line representations underline the assumption that the face-processing system is inflexible when it is confronted with edge-based material.     

Perceptual organization and apparent brightness in subjective contour figures

According to a number of theories subjective contours arise from brightness contrast and/or assimilation. The apparent brightness gradients generated by these effects are assumed to give rise to the perception of contours delineating the gradients. A study is reported in which naive observers were shown a subjective contour display and asked to report what they saw. They were then asked to judge whether the center or the surround of the display appeared brighter. Subjects whose reports indicated that they had seen the subjective contour figure showed an overwhelming preference for the center of the display being brighter than the surround. However, subjects who did not see the subjective contour figure did not differ significantly in their selection of the center over the surround. This finding presents difficulties for any theory which derives subjective contours from the apparent brightness difference.     

Key pecking in pigeons produced by pairing keylight with inaccessible grain

In Experiment I, keylight was paired with inaccessible grain delivery (under two conditions of keylight intensity) to determine if autoshaping would occur in the absence of primary reinforcement. In Experiment II, the procedure was repeated with accessible grain, for comparison. In Experiment III, the procedures were repeated with explicitly unpaired presentations of keylight and either inaccessible or accessible grain. The results indicated that key pecking occurred as quickly in the presence of keylight pairings with inaccessible grain as with accessible grain, though (except for one bird) key pecking was not maintained with inaccessible grain. Furthermore, compared to the dim keylight, the bright keylight greatly suppressed key pecking when paired with inaccessible grain, and reduced the rate of key pecking when paired with accessible grain. Little key pecking occurred in groups exposed to explicitly unpaired presentations of keylight (whether bright or dim) and grain (whether accessible or inaccessible). When the birds in Experiment III were retested with explicitly paired presentations of keylight and grain, little key pecking was observed, suggesting suppressive effects of prior explicitly unpaired presentations. It is suggested that the effects of key-brightness manipulation were produced by the association of grain with cues other than the response key, or by distraction produced by partial illumination of the grain hopper.     

Increment thresholds against pulsed and steady backgrounds

Cornsweet and Pinsker reported that ΔL/L was constant at all values of L for briefly pulsed fields (ΔL was the luminance increment required for discriminating which of two nearby flashed fields was brighter). We obtain the same result when the subject must detect which of two briefly (4-msec) flashed backgrounds contains an increment at its center. However, when the background and increment are flashed at a longer duration (200 msec) ΔL/L decreases with increasing L as is obtained with a steady background. Two previous models which handle earlier work do not readily accommodate this result; we suggest a theoretical approach that does.     

Response priming driven by local contrast, not subjective brightness

We demonstrate qualitative dissociations of brightness processing in visuomotor priming and conscious vision. Speeded keypress responses to the brighter of two luminance targets were performed in the presence of preceding dark and bright primes (clearly visible and flanking the targets) whose apparent brightness values were enhanced or attenuated by a visual illusion. Response times to the targets were greatly affected by consistent versus inconsistent arrangements of the primes, relative to the targets (response priming). Priming effects could systematically contradict subjective brightness matches, such that one prime could appear brighter than the other but could prime as if it were darker. Systematic variation of the illusion showed that response-priming effects depended only on local flanker-background contrast, not on the subjective appearance of the flankers. Our findings suggest that speeded motor responses, as opposed to conscious perceptual judgments, access an early phase of lightness and brightness processing prior to full lightness constancy.     

Test of the effect of attention on judged length of a line

Studies to assess experimentally whether attention affects the judged length of a line have produced discordant results. This paper reports the results of a test designed to avoid factors that were not controlled in previous studies. Stimuli were either two vertical lines or two horizontal lines of equal physical length presented briefly on opposite sides of a fixation cross. Subjects were asked to direct their attention to one line when the arm of the cross pointing to the line changed in luminance. This arm was used either as a precue or as a postcue. Subjects judged lines to be longer when a precue preceded the line than when a postcue followed the line.     

Knowledge of spatial relations: Varying the precision with which locations must be specified

In previous studies, subjects have been asked whether one named town is north (say) of another. The results have shown that as the distance between the towns increases, reaction times decrease. Explanations have focused on a subject's disposition to derive an answer by using less precise information about location when the towns are far apart. In the present experiments, subjects who had memorized a map of Europe were asked whether an imaginary line connecting two named towns passes through a named country. The country was either large or small in area, and the distance between the towns was either long or short. It was expected that in the large country condition less precise information would be used when the distance was long than when it was short, as inferred in previous experiments. Accordingly, reaction times decreased as the distance increased. In the small country condition it was supposed that some degree of precision would be required to determine that the line ran through the country, regardless of the distance between the towns. Therefore, it was expected that the size of the country would limit the degree to which a decrease in precision could occur with an increase in distance. Consistent with this expectation, distance and size of country interacted, such that when the country was small an increase in distance did not cause a decrease in reaction time. Similar results were found in a second experiment.     

The poggendorff illusion with subjective contours

Gregory (1972) has claimed that the Poggendorff misalignment effect occurs when the collinear obliques are separated by subjective rather than real contours. He used two figures to demonstrate this variant of the illusion. Two experiments to test the claim are reported. The first showed that apparent misalignment in one of the two original figures is no greater than that with two obliques alone (the oblique line effect), but misalignment in the other is greater than with two oblique lines and than with a control without subjective contours. The second experiment showed that apparent misalignment in the second figure was less than in two control figures without subjective contours. Since this reduced effect was probably due to the nature of the intersection between the oblique and a semi-circular element, the role of subjective contours remains unsettled.     

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.     

Apparent combined length of two-line and four-line sets

Recent studies, using single-line stimuli, show apparent length to be a power function, with exponent 1.0, of objective or physical length. If apparent lengths are additive, then, given the 1.0 exponent, two lines should appear to S to have the same total length as the single line they would form if physically joined. When S adjusted the length of one line to match the combined length of two other lines, however, he generally made the variable line much longer than the actual combined length of the two lines. Dividing the total length equally between the two lines, so that each had 50%, represents the greatest departure from a single line, yet the largest overestimation occurred when one of the two lines had 65% to 75% of the total length. The overestimation was greater when the lines were spaced farther apart, suggesting that the amount of area occupied by the lines affected apparent combined length.     

Stimulus generalization from feeder to response key in the acquisition of autoshaped pecking

During autoshaping, a 6-second presentation of one stimulus and a variable time 30-second presentation of a second stimulus alternated in appearance on a pigeon key. Grain always was delivered for 3 seconds at the end of the first stimulus interval. In the first experiment, autoshaped pecking of the stimulus preceding grain delivery began much sooner when that stimulus was a black vertical line on a white background and the other stimulus was green than when the opposite stimulus arrangement was used. Because these two stimuli differed in form, hue, brightness, and similarity in hue and brightness to the illumination of the raised feeder, three subsequent experiments examined whether the differential speed of autoshaping in the two groups was due to a feature-positive, feature-negative effect, a preference for brighter over darker stimuli, a simple preference for white over green, or stimulus generalization from the brightness or hue of the illuminated, raised feeder to the stimulus on the key preceding grain delivery. The data from these experiments showed that the first autoshaped key peck was most likely to be made to the stimulus of the same hue as that illuminating the feeder, regardless of whether that stimulus was positively or negatively associated with grain delivery. At least under some conditions, therefore, stimulus-generalization mediated response transfer of pecking grain in the presence of the hue illuminating the feeder to pecking the key illuminated by a similar hue appears to account for the occurrence of autoshaped key pecking.     

A transient processing deficit following selection of an auditory target

Two experiments were conducted to examine the possibility that a transient processing deficit might result following selection of an auditory target. In both experiments, listeners were presented with a sequence of tones and required to make separate detection judgments, one regarding a target and the second regarding a subsequent probe. The target was presented following a variable number of tones, and the probe was presented in one of the seven or eight possible temporal positions following the target. The experiments differed only in that in Experiment 1 the first post-target item could be either a distractor tone or a probe, whereas in Experiment 2 the first post-target item was replaced by a silent period. In both experiments, a deficit in processing of the probe was apparent when it was presented in one of the first few positions following the target.