Learning to see random-dot stereograms.

In the present study some specific properties of the learning effects reported for random-dot stereograms are examined. In experiment 1 the retinal position-specific learning effect was reproduced and in a follow-up experiment it was shown that the position specificity of learning can be accounted for by selective visual attention. In experiments 2 and 3 evidence was obtained that suggests that observers can learn, to a certain degree, monocular random-dot patterns and that this learning facilitates the depth percept. This result indicates that the traditional belief that random-dot stereograms are devoid of monocularly recognizable or useful forms should be reconsidered. In the second set of experiments the learning of two binocular surface properties of random-dot stereograms, depth edges and internal depth regions, was investigated. It was shown in experiment 4 that the depth edges of random-dot stereograms are not learned, whereas the results of experiment 5 indicate that the internal depth regions are learned. Finally, in experiment 6 it was shown that depth edges are learned when the internal depth regions of the stereogram are ambiguous. The results are discussed in terms of the importance of the particular type of stimulus used in the learning process and in terms of perceptual learning and attention.     

Patent stereopsis with diplopia in random-dot stereograms

Fusional limits for an RD (random-dot) stereogram with overall horizontal retinal disparity due to the temporalward pulling of its two constituent RD patterns beyond the divergence limits of the eyes have been determined by using an afterimage method. Two criteria for fusion were used, viz, the perception ofsingle local RD elements and the perception of stereoscopic depth in the “hidden” square of side 1.38 deg with 8-min relative horizontal disparity. The diplopia thresholds were found to be in the range of 0.15–0.3 deg, and hence do not exceed the “classical” upper limit of about 0.3 deg, which has been reported for elementary line stereograms. The stereoscopic limits were found to be in the range of 0.5–1.3 deg, which is compatible with the precision of patent stereopsis from double images which has been reported for elementary line stereograms. The results of the present third look at the experiments performed by Fender and Julesz (1967) suggest that there are no special neuronal processes raising the fusional limits for RD stereograms above those for elementary line stereograms. Previous claims that such special neuronal processes may occur seem to be based on an evaluation of fusional limits obtained with different criteria for fusion. It is further argued that the major hysteresis effect that has been observed for fusional limits for RD stereograms should not be ascribed to a raising of the classical size of the diplopia threshold due to special neuronal processes initiated by RD stereograms, but to a lowering of the classical limits of patent stereopsis from double images due to the increased difficulty of solving the correspondence problem in RD stereograms.     

Hysteresis,cooperativity, and depth averaging in dynamic random-dot stereograms

Experiments were performed to assess the response of the human visual system to dynamic random-dot patterns composed of disparity mixtures. In Experiment 1, the perceived depth and relative stability of two patterns were compared; one pattern depicted two transparent layers of dots, and the other depicted a volume of dots. Two effects were found: (1) the volume pattern exhibited a large degree of disparity averaging; and (2) asymmetries were observed in the relative stability of these two patterns. Experiment 2 was designed to determine whether these findings could be attributed to spatially localized processes occurring-at the location of disparity discontinuities. This was accomplished by introducing unpaired noise points localized either along the disparity discontinuities or in the center of the layered and volume patterns. The amount of depth averaging and the direction of the asymmetry did not appear to depend on processes localized along the disparity discontinuities. Results of these experiments, taken in conjunction with those of previous studies, suggest that hysteresis is independent of cooperative persistence mechanisms.     

Hysteresis, cooperativity, and depth averaging in dynamic random-dot stereograms.

Experiments were performed to assess the response of the human visual system to dynamic random-dot patterns composed of disparity mixtures. In Experiment 1, the perceived depth and relative stability of two patterns were compared; one pattern depicted two transparent layers of dots, and the other depicted a volume of dots. Two effects were found: (1) the volume pattern exhibited a large degree of disparity averaging; and (2) asymmetries were observed in the relative stability of these two patterns. Experiment 2 was designed to determine whether these findings could be attributed to spatially localized processes occurring at the location of disparity discontinuities. This was accomplished by introducing unpaired noise points localized either along the disparity discontinuities or in the center of the layered and volume patterns. The amount of depth averaging and the direction of the asymmetry did not appear to depend on processes localized along the disparity discontinuities. Results of these experiments, taken in conjunction with those of previous studies, suggest that hysteresis is independent of cooperative persistence mechanisms.     

How longer saccade latencies lead to a competition for salience

It has been suggested that independent bottom-up and top-down processes govern saccadic selection. However, recent findings are hard to explain in such terms. We hypothesized that differences in visual-processing time can explain these findings, and we tested this using search displays containing two deviating elements, one requiring a short processing time and one requiring a long processing time. Following short saccade latencies, the deviation requiring less processing time was selected most frequently. This bias disappeared following long saccade latencies. Our results suggest that an element that attracts eye movements following short saccade latencies does so because it is the only element processed at that time. The temporal constraints of processing visual information therefore seem to be a determining factor in saccadic selection. Thus, relative saliency is a time-dependent phenomenon.     

Illusory concomitant motion in ambiguous stereograms: evidence for nonstimulus contributions to perceptual organization

Three experiments were performed to test whether perceptual organization is cognitively or motivationally penetrable. In Experiments 1 and 2, subjects viewed a reversible stereogram while instructed to hold one depth organization. Responses about depth were recorded indirectly by recording responses about direction of the illusory concomitant motion that is perceptually coupled to depth in a stereogram. Inasmuch as perceptually coupled variables covary without necessary stimulus covariation, a post-perceptual locus for any intention effects they exhibit is unlikely. Experiments 2 and 3 examined the possibility that instructed intention might influence perception indirectly by influencing eye movements: Viewers' vergence position was measured directly through responses about alignment of a vernier nonius fixation. In all three experiments, a residual effect of instructed intention was found. Therefore, instructed intention may influence perceptual organization by influencing internal nonstimulus components integral to the perceptual process.     

Depth inversion despite stereopsis: the appearance of random-dot stereograms on surfaces seen in reverse perspective.

Inside-out relief masks of faces can be depth-inverted (i.e. seen in reverse perspective) during close-up binocular viewing. If a random-dot stereogram is projected onto such a mask, stereopsis can be achieved for the stereogram, and its depth planes are correctly seen while the mask itself, including the region covered by the stereogram, is simultaneously perceived as depth-inverted. This demonstration shows that binocular depth inversion cannot be explained by a complete loss of stereoscopic information (e.g. through monocular suppression), or by a process analogous to pseudoscopic viewing whereby retinal disparities are incorporated into perception, but with their signs uniformly reversed.     

The use of numerical and graphical statistical methods in the analysis of data on learning to see complex random-dot stereograms

Several numerical and graphical statistical methods are illustrated in an analysis of data from an experiment that investigated a hypothesis of Julesz that giving a person a priori information about the structure of a complex random-dot stereogram reduces the time needed to perceive it when it is viewed. The data are divided into two groups, one consisting of those observers who received no cue or verbal cues (NV) and the other consisting of those who received verbal-visual cues (VV). A quantile-quantile plot shows that the NV times (mean = 7.6) are longer than the VV times (mean =5.6). By using probability plots, it is shown that the perception times have an exponential probability distribution. A hypothesis test based upon this distribution is used to show that the difference between the NV and VV times has significance slightly below 0.05.     

Perceptual latencies to discriminate surface orientation in stereopsis

The difference in sensitivity to stereoscopic surfaces oriented horizontally or vertically (the stereoscopic orientation anisotropy) can be redescribed as a difference in sensitivity to shear or compression transformations that relate the binocular images. The present experiment was designed to test this by dissociating the image transformation from the orientation of the surface. Surfaces were presented in isolation or in the presence of a surrounding frame that formed step and gradient discontinuities in the disparity field. Without discontinuities, observers required considerably more time to discriminate between surfaces differing in compression than between those differing in shear, irrespective of surface orientation. Disparity discontinuities facilitated the perception of the disparity gradients; minimum stimulus durations were reduced by over an order of magnitude when the reference frame was present. These results support the hypothesis that the disparity field is decomposed into different primitives during the recovery of depth and surface structure.     

Selective attention in perceptual adjustments to voice

The effects of perceptual adjustments to voice information on the perception of isolated spoken words were examined. In two experiments, spoken target words were preceded or followed within a trial by a neutral word spoken in the same voice or in a different voice as the target. Over-all, words were reproduced more accurately on trials on which the voice of the neutral word matched the voice of the spoken target word, suggesting that perceptual adjustments to voice interfere with word processing. This result, however, was mediated by selective attention to voice. The results provide further evidence of a close processing relationship between perceptual adjustments to voice and spoken word recognition.     

Having to identify a target reduces latencies in prosaccades but not in antisaccades

In a princeps study, Trottier and Pratt (2005) showed that saccadic latencies were dramatically reduced when subjects were instructed to not simply look at a peripheral target (reflexive saccade) but to identify some of its properties. According to the authors, the shortening of saccadic reactions times may arise from a top-down disinhibition of the superior colliculus (SC), potentially mediated by the direct pathway connecting frontal/prefrontal cortex structures to the SC. Using a “cue paradigm” (a cue preceded the appearance of the target), the present study tests if the task instruction (Identify vs. Glance) also reduces the latencies of antisaccades (AS), which involve prefrontal structures. We show that instruction reduces latencies for prosaccade but not for AS. An AS requires two processes: the inhibition of a reflexive saccade and the generation of a voluntary saccade. To separate these processes and to better understand the task effect we also test the effect of the task instruction only on voluntary saccades. The effect still exists but it is much weaker than for reflexive saccades. The instruction effect closely depends on task demands in executive resources.     

Development of differences in response latencies to right and left visual fields

Robust lateralization developed in right-handed adults who were asked to judge letter pairs as "same" or "different" during 4608 trials. By the end of the first two blocks (768 trials) "same" responses were favored when presented in the RVF (transmitted directly to the left hemisphere) and "different" responses were favored when presented in the LVF (transmitted direction to the right hemisphere). This gradually reversed over sessions with "same" responses becoming faster for letters presented in the LVF, and "different" responses becoming faster for stimuli presented in the RVF. The laterality acquired under these conditions was cumulative and reproducible, appeared in all 16 subjects, and was preserved between sessions a week apart. The data suggest that laterality is a flexible and reversible characteristic of the human brain even when stimulus and task remain constant.     

Inhibitory associations contribute to perceptual learning in humans

Perceptual learning in humans was examined in 2 experiments. In Experiment 1, participants received intermixed exposure to 2 similar compounds (AX, BX, AX, BX, . . .) and blocked exposure to a 2nd pair of similar compounds (CY, CY, . . . , DY, DY, . . .). Aversions established to AX and CY generalized less to BX than to DY. In Experiment 2, 1 pair of compounds was presented in a forward order (i.e., AX3BX), whereas the 2nd pair of compounds was presented in a backward order (i.e., DY-->CY). Aversions established to AX and CY generalized less to BX than to DY. These results indicate that inhibitory associations contribute to perceptual learning in humans and thereby establish a fundamental similarity between the mechanisms that underlie perceptual learning in humans and rats.     

Age differences in contingency judgement linked to perceptual segregation

We demonstrate large differences in judging positive and null contingencies between younger and older adults with a task commonly used to explore cue competition in both contingency and causality judgements. The one-phase blocking task uses two cues, with separate contingencies with the same outcome. The age differences persisted even when participants knew in advance which of the two contingencies to judge. The age differences disappeared, however, when the stimulus display contained markers aiding perceptual segregation. We suggest that the age differences elicited in the one-phase blocking task are linked to decrements in perceptual segregation.     

Age differences in contingency judgement linked to perceptual segregation

We demonstrate large differences in judging positive and null contingencies between younger and older adults with a task commonly used to explore cue competition in both contingency and causality judgements. The one-phase blocking task uses two cues, with separate contingencies with the same outcome. The age differences persisted even when participants knew in advance which of the two contingencies to judge. The age differences disappeared, however, when the stimulus display contained markers aiding perceptual segregation. We suggest that the age differences elicited in the one-phase blocking task are linked to decrements in perceptual segregation.