The importance of being upright: Use of environmental and viewer-centered reference frames in shape discriminations of novel three-dimensional objects

We investigated the frame of reference that people use to make shape discriminations when their heads are either upright or tilted. Observers madesame-different judgments of pairs of novel threedimensional objects that were aligned along their length within the frontal-parallel plane and rotated in depth around an axis parallel to their own axes of elongation. The aligned objects were displayed vertically, tilted 45°, or horizontally with respect to the environmental upright, but the distance of each pair from the upright was irrelevant to resolving the angular disparity between the stimuli for thesame-different judgment. Nevertheless, when observers’ heads were upright, the time to encode the stimuli was a linear function of the distance of the stimuli from the environmental upright, whereas when observers’ heads were tilted 45°, encoding times for tilted and vertical stimuli did not differ and were faster than the times to encode horizontal stimuli. We interpreted these data to mean that observers either rotate or reference the top of an object to the environmental upright, and they can use either a gravitational or retinal reference frame to do so when either they or the objects are not upright.     

The effects of concentrated and distributed attention on peripheral acuity

Two experiments studied the peripheral discriminability of a target differing in its line slope (a tilted T) and in its line arrangement (an L) when presented in briefly flashed displays of upright Ts. The results showed that: (a) an L and a tilted T were equal in discriminability when attention was focused or concentrated on one display position, (b) the discriminability of an L decreased while the discriminability of a tilted T was not statistically significantly affected as the number of display positions that attention needed to be paid to increased, and (c) the reaction time to find a disparate tilted T was less than that to find a disparate L. The results are interpreted as supporting the hypothesis that, under distributed attention in peripheral vision, the visual system is more sensitive to differences in line slope than to differences in line arrangement The results are discussed in connection with hypotheses of how selective attention affects the discriminability of a target.     

The Beck effect is back,now in color: A demonstration

It has been repeatedly found (e.g., Beck, 1982; Beck & Ambler, 1973) that a tilted T is detected better than an L when presented on the background of a number of upright Ts, although in isolation both are discriminated from an upright T about equally well. We demonstrate that an analogous interaction between context presence and stimulus type can be obtained with color stimuli: A light green was discriminated from a dark green better than an olive brown was, yet that occurred only in the context of a background of three dark green disks. This indicates that the effects, both original and novel, probably are not due to anything peculiar to form perception, such as line arrangement or supraelement configurations. It is suggested that the presence of collateral background stimuli probably enables the perceptual system to capitalize on relative judgment involving contrasts or deviations from a presented anchor value.     

Tactile memory in sighted and blind observers: the influence of orientation and rate of presentation

Sighted, early blind, and late blind subjects attempted to identify numerals or number sequences printed on their palms. The numerals were either upright, or inverted, or rotated perpendicular to the arm axis. Stimulus rotation degraded recognition in the early blind subjects, suggesting the influence of experience with visual frames of reference. Slower rates of presentation with upright number sequences improved recall in both sighted and blind observers. An experiment on tactual-visual braille recognition in the sighted observers showed that tilt degraded pattern identification, but visual guidance of the fingertip and ballpoint minimized this loss. A further experiment was performed to distinguish between visual imagery and visual frame of reference explanations of the visual guidance effect on recognition of rotated braille. Subjects explored upright or tilted braille characters while viewing only a light emitting diode on the exploratory fingertip. Sight of scanning movements did not aid pattern recognition with tilt. The results indicate that the benefits of visual guidance on recognition of tilted patterns were probably due to frame of reference information. It is concluded that spatial reference information may aid tactile memory in the sighted and late blind, since the early blind performed at a lower level in the retention task. It is proposed that visual imagery may only explain the superiority of the sighted and late blind when familiar stimuli are studied.     

Effects of response disparity on stimulus and reinforcer control in human detection tasks

In two detection experiments, university students reported whether the second of two sequentially presented tones was longer or shorter than the first by responding to stimuli presented on a touch screen. Stimulus disparity and response disparity were manipulated to compare their effects on measures of discrimination and response bias when the reinforcement ratio for correct responses was asymmetric. Choice stimuli consisted of squares filled with different pixel densities. Response disparity was manipulated by varying the difference in density between the two choice stimuli. In both experiments, decreasing stimulus disparity reduced discrimination but had no consistent effect on bias. Decreasing response disparity also reduced discrimination in both experiments, and often reduced estimates of bias. The effects of response disparity on bias were most clear in Experiment 2, in which a greater overall level of response disparity was arranged. The data show that, like corresponding research with pigeons, detection performance of human subjects can be conceptualized as discriminated operants.     

Establishing global and local correspondence between successive stimuli: the holistic nature of backward alignment

Reflection decisions on alphanumeric characters display systemic effects of disorientation, suggesting that subjects mentally rotate the stimulus to the upright (the uprighting process). However, response time also increases with increasing angular disparity between the current and preceding orientations. This occurs only when the current stimulus is brought into congruence with the preceding one (the backward alignment process). In the present study, we examined the hypothesis that the transformation that occurs in backward alignment in holistic even in tasks in which the uprighting process is likely to be piecemeal. Evidence supporting this hypothesis is presented on the basis of tasks requiring either classification of numbers (Experiments 1 and 3) and words (Experiment 2), or mirror image discrimination on letter pairs (Experiment 4). The results indicated that backward alignment establishes global correspondence between successive stimuli and is indifferent to local correspondence at the level of the constituent elements. The establishment of this global correspondence decreases with the number of elements in the stimulus (Experiment 5), but its effects are still observed for four-letter strings (Experiment 6).     

Mental rotation under head tilt: Factors influencing the location of the subjective reference frame

We report five experiments on the effect of head tilt on the mental rotation of patterns to the “upright.” In Experiment 1, subjects rotated alphanumeric characters, displayed within a circular surround. Experiment 2 was similar except that the character was an unfamiliar letter-like symbol. In Experiment 3, subjects again rotated alphanumeric characters, but they were displayed within a rectangular frame tilted 60° to the right. Experiment 4 was similar, except that the subjects were instructed to rotate the characters to the “upright” defined by the tilted frame. In all four experiments, the subjects performed the task with their heads either upright or tilted 60°. In Experiment 5, subjects had their heads and bodies tilted 90°, and rotated alphanumeric characters displayed within a circular surround. In all except Experiment 4, analysis of response latencies revealed that the subjective vertical lay closer to the gravitational than to the retinal vertical, although it was somewhat displaced in the direction of the head tilt—more so in Experiments 2 and 3 than in Experiment 1, and more so still in Experiment 5. In Experiment 4, instructions to adopt the axes of the frame land thus of the retina) succeeded in bringing the subjective vertical closer to the retinal than to the gravitational vertical, although the subjective vertical was still some 20° on average from the gravitational vertical. The results show that the subjective reference frame is distinct from both gravitational and the retinal frames, and that the gravitational frame exerts the stronger influence. They also argue against the primacy of a “retinal factor” in the perception of orientation.     

Must egocentric and environmental frames of reference be aligned to produce spatial S-R compatibility effects?

Four experiments were conducted to determine the effects of misaligning egocentric and environmental frames of reference on spatial S-R compatibility effects. In Experiments 1 and 3, subjects looked at two lights that were aligned horizontally, one each on either side of the body midline. They held their head upright or tilted 90 degrees to the left or right. In the upright condition the hands were uncrossed and rested opposite the lights (frames of reference aligned), whereas in the head tilt condition the hands were either crossed or uncrossed but positioned perpendicular to the lights (frames of reference not aligned). Manual choice reaction times to the lights produced spatial S-R compatibility effects that were as large when the frames of reference were aligned as when they were not. In Experiments 2 and 4, which also used upright and tilted conditions, we found generally similar results when the lights were displayed vertically and the hands disposed horizontally. The results indicate that under conditions of head rotation and with stimulus and response arrays perpendicular to each other, spatial S-R compatibility effects still occur. By taking into account both frames of reference, the subject classifies the stimuli as left or right whether they are horizontally or vertically disposed and maps them onto the responding hand, thereby producing the observed compatibility effects.     

Maintaining readiness for mental rotation interferes with perceptual processes in children but with response selection in adults

Previous work revealed that mental rotation is not purely inserted into a same-different discrimination task. Instead, response time (RT) is slowed to upright stimuli in blocks containing rotated stimuli compared to RT to the same upright stimuli in pure upright blocks. This interference effect is a result of maintaining readiness for mental rotation. In two experiments we investigated previous evidence that these costs depend upon distinct sub-processes for children and for adults. In Experiment 1, the maintaining costs turned out to be independent of the visual quality of the stimulus for adults but not so for children. Experiment 2 revealed that the maintaining costs were greatly reduced for adults when they performed mental rotation as a go-no-go task, but not so for children. Taken together, both experiments provide evidence that whereas perceptual processes seem to be important for school-age children to maintain readiness for mental rotation, response selection is relevant for adults.     

The effect of orientation on tactual braille recognition: Optimal touching positions

Subjects in five experiments matched tangible braille against a visible matching code. In Experiment 1, braille recognition suffered when entire lines of braille characters were tilted in varying amounts from the upright. Experiment 2 showed that tilt lowered performance for tangible, large embossed letters, as well as for braille. However, recognition was better for print letters than it was for braille. In Experiment 3, subjects attempted to match the upright array against embossed braille that was left/right reversed, inverted up/down, or rotated +180°. Performance was close to that for normal braille in the left/right reversal condition, and very low for the +180° rotation group. These results on braille tilt in the “picture plane” may reflect difficulty in manipulating the tangible “image.” Braille recognition performance was not lowered whenthe visible matching array was tilted ?45° or ?90° from the upright but the tangible stimuli were upright. In Experiment 4, recognition of left/right reversed braille that was physically horizontal (on the bottom of a shelf) was compared with that of braille left/right reversed due to its location on the back of a panel, in the vertical plane. Braille recognition accuracy was higher with braille located vertically. An additional experiment showed the beneficial effect of locating braille in the vertical, frontoparallel plane, obtained with +90° degree rotated braille. It is proposed that optimal tactual performance with tangible arrays might depend on touching position, and on the physical position of stimuli in space. Just as there are good and poor viewing positions, there may be optimal touching positions. The effects of tilt on braille identification were diminished for blind subjects, suggesting the importance of tactile experience and skill.     

The effect of orientation on tactual braille recognition: optimal touching positions.

Subjects in five experiments matched tangible braille against a visible matching code. In Experiment 1, braille recognition suffered when entire lines of braille characters were tilted in varying amounts from the upright. Experiment 2 showed that tilt lowered performance for tangible, large embossed letters, as well as for braille. However, recognition was better for print letters than it was for braille. In Experiment 3, subjects attempted to match the upright array against embossed braille that was left/right reversed, inverted up/down, or rotated +180 degrees. Performance was close to that for normal braille in the left/right reversal condition, and very low for the +180 degrees rotation group. These results on braille tilt in the "picture plane" may reflect difficulty in manipulating the tangible "image." Braille recognition performance was not lowered when the visible matching array was tilted -45 degrees or -90 degrees from the upright but the tangible stimuli were upright. In Experiment 4, recognition of left/right reversed braille that was physically horizontal (on the bottom of a shelf) was compared with that of braille left/right reversed due to its location on the back of a panel, in the vertical plane. Braille recognition accuracy was higher with braille located vertically. An additional experiment showed the beneficial effect of locating braille in the vertical, frontoparallel plane, obtained with +90 degree rotated braille. It is proposed that optimal tactual performance with tangible arrays might depend on touching position, and on the physical position of stimuli in space. Just as there are good and poor viewing positions, there may be optimal touching positions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Categorical perception of stereoscopic stimuli

Two studies of the stereopsis model of Richards were conducted using a between-subjects balanced design and a repeated trials procedure rather than the within-subjects procedure of Richards. Measures of the accuracy of discrimination between classes of retinal disparity were defined by the method of successive intervals in place of the d’ index of SDT. The results support the predictions (1) of symmetric discrimination functions for convergent and divergent stimuli, (2) of greater discrimination for stimuli of 45 min of retinal disparity than for the stimuli of 15 min disparity, (3) of greater variability for the stimuli of greater disparity, and (4) of the use by nonanomalous subjects of at least two different processing mechanisms for stimuli of different disparity classes. The results do not support the prediction (1) of different processing mechanisms for foveal stimuli differing in figure-ground contrast or (2) of greater usefulness for monocular stimuli as a null disparity reference condition, as compared with binocular null disparity stimuli. Similar results were found for two stimulus shapes, a bar and a disk of equal area. Unreliability of subjects’ response systems was suggested as a factor in the prediction of figure-ground contrast changes and as a factor in the classification of subjects as stereo anomalous. The pattern of accuracy of discrimination between classes of disparity stimuli also raises questions of the adequacy of the simple stereopsis model.     

The face-inversion effect as a deficit in the encoding of configural information: direct evidence

We report four experiments leading to conclusions that: (i) the face-inversion effect is mainly due to the deficits in processing of configural information from inverted faces; and (ii) this effect occurs primarily at the encoding stage of face processing, rather than at the storage stage. In experiment 1, participants discriminated upright faces differing primarily in configuration with 81% accuracy. Participants viewing the same faces presented upside down scored only 55%. In experiment 2, the corresponding discrimination rates for faces differing mainly in featural information were 91% (upright) and 90% (inverted). In experiments 3 and 4, the same faces were used in a memory paradigm. In experiment 3, a delayed matching-to-sample task was used, in which upright-face pairs differed either in configuration or features. Recognition rates were comparable to those for the corresponding upright faces in the discrimination tasks in experiments 1 and 2. However, there was no effect of delay (1 s, 5 s, or 10 s). In experiment 4, we repeated experiment 3, this time with inverted faces. Results were comparable to those of inverted conditions in experiments 1 and 2, and again there was no effect of delay. Together these results suggest that an 'encoding bottleneck' for configural information may be responsible for the face-inversion effect in particular, and memory for faces in general.     

Orientation Constancy in Neurons of Monkey Visual Cortex

We studied the effect of body tilt on the orientation selectivity of single neurons in the visual cortex of an alert monkey. The monkey performed a visual fixation task either in the upright position or with its whole body tilted about the naso-occipital (roll) axis by ±25° or ±30°. We determined the preferred stimulus orientation for 51 of 117 neurons in two or, if possible, three body positions (i.e. with the whole body upright, and tilted either left ear or right ear down). In striate cortex, most of the neurons were of a non-compensatory type, showing a change in the preferred orientation according to the body tilt and the estimated counterrolling of the eye. By contrast, about 40% of the neurons in prestriate cortex were of a compensatory type, preferring similar orientations in all body positions. This suggests that mechanisms which produce orientation constancy with respect to the direction of gravity are implemented at an early stage of cortical processing.     

Conditional discrimination in mentally retarded adults: the development of generalized skills.

The development of generalized conditional discrimination skills was examined in adults with retardation. Two subjects with histories of failure to acquire arbitrary matching under trial-and-error procedures were successful under procedures that trained one or more prerequisite skills. The successive discrimination between the sample stimuli was established by training the subjects to name the stimuli. The simultaneous discrimination between the comparison stimuli was established using either (a) standard simple discrimination training with reversals or (b) a procedure in which each of the two sample-comparison relations in the conditional discrimination was presented in blocks of trials, with the size of the blocks decreasing gradually until sample presentation was randomized. The amount of prerequisite training required varied across subjects and across successive conditional discriminations. After acquiring either two or three conditional discriminations with component training, both subjects learned new conditional discriminations under trial-and-error procedures. In general, each successive conditional discrimination was acquired more rapidly. Tests showed that conditional responding had become a generalized skill. Symmetry was shown for almost all trained relations. Symmetry trial samples were ultimately named the same as the stimuli to which they were related in training.     

Anticipation of visual form independent of knowing where the form will occur

We investigated how selective preparation for specific forms is affected by concurrent preknowledge of location when upcoming visual stimuli are anticipated. In three experiments, participants performed a two-choice response time (RT) task in which they discriminated between standard upright and rotated alphanumeric characters while fixating a central fixation cross. In different conditions, we gave the participants preknowledge of only form, only location, both location and form, or neither location nor form. We found main effects of both preknowledge of form and preknowledge of location, with significantly lower RTs when preknowledge was present than when it was absent. Our main finding was that the two factors had additive effects on RTs. A strong interaction between the two factors, such that preknowledge of form had little or no effect without preknowledge of location, would have supported the hypothesis that form anticipation relies on depictive, perception-like activations in topographically organized parts of the visual cortex. The results provided no support for this hypothesis. On the other hand, by an additive-factors logic Sternberg (Sternberg, Acta Psychologica 30:276?C315, 1969), the additivity of our effects suggested that preknowledge of form and location, respectively, affected two functionally independent, serial stages of processing. We suggest that the two stages were, first, direction of attention to the stimulus location and, subsequently, discrimination between upright and rotated stimuli. Presumably, preknowledge of location advanced the point in time at which attention was directed at the stimulus location, whereas preknowledge of form reduced the time subsequently taken for stimulus discrimination.     

Effects of multimodal presentation and stimulus familiarity on auditory and visual processing

Two experiments examined the effects of multimodal presentation and stimulus familiarity on auditory and visual processing. In Experiment 1, 10-month-olds were habituated to either an auditory stimulus, a visual stimulus, or an auditory-visual multimodal stimulus. Processing time was assessed during the habituation phase, and discrimination of auditory and visual stimuli was assessed during a subsequent testing phase. In Experiment 2, the familiarity of the auditory or visual stimulus was systematically manipulated by prefamiliarizing infants to either the auditory or visual stimulus prior to the experiment proper. With the exception of the prefamiliarized auditory condition in Experiment 2, infants in the multimodal conditions failed to increase looking when the visual component changed at test. This finding is noteworthy given that infants discriminated the same visual stimuli when presented unimodally, and there was no evidence that multimodal presentation attenuated auditory processing. Possible factors underlying these effects are discussed.     

Processing emotional stimuli: Comparison of saccadic and manual choice-reaction times

We compared the speed of discrimination for emotional and neutral faces in four experiments, using a forced-choice saccadic and manual reaction time task. Unmasked, brief (20 ms) bilateral presentation of schematic (Exp. 1) or naturalistic (Exp. 2) emotional/neutral face pairs, led to shorter discrimination of emotional stimuli in saccadic localisation task. When the effect of interference from emotional stimuli is ruled out by showing a pairing of the emotional or neutral face with an outline face, faster saccadic discrimination was obtained for fearful facial expression only (Exp. 3). The manual discrimination reaction time was not significantly different for emotional versus neutral stimuli. To explore the absence of a manual RT effect we manipulated the stimulus duration (20 ms vs. 500 ms: Exp. 4). Faster saccadic discrimination of emotional stimuli was observed at both durations. For manual responses, emotional bias was observed only at the longest duration (500 ms). Overall, comparison of saccadic and manual responses shows that faster discrimination of emotional/neutral stimuli can be carried out within the oculomotor system. In addition, emotional stimuli are processed preferentially to neutral face stimuli.     

Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect.

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).     

S-R compatibility effects due to context-dependent spatial stimulus coding

Responses are faster with spatial S-R correspondence than with noncorrespondence (spatial compatibility effect), even if stimulus location is irrelevant (Simon effect). In two experiments, we sought to determine whether stimuli located above and below a fixation point are coded as left and right (and thus affect the selection of left and right responses) if the visual context suggests such a coding. So, stimuli appeared on the left or right eye of a face’s image that was tilted by 90° to one side or the other (Experiment 1) or varied between upright and 45° or 90° tilting (Experiment 2). Whether stimulus location was relevant (Experiment 1) or not (Experiment 2), responses were faster with correspondence of (face-based) stimulus location and (egocentrically defined) response location, even if stimulus and response locations varied on physically orthogonal dimensions. This suggests that object-based spatial stimulus codes are formed automatically and thus influence the speed of response selection.