Visual perception

Visual sensitivity underlying changes in visual consciousness

by Robert O'Shea

Alais, D., Cass, J., O’Shea, R. P., & Blake, R. (2010). Visual sensitivity underlying changes in visual consciousness. Current Biology, 20, 1362-1367. doi: 10.1016/j.cub.2010.06.015

When viewing a different stimulus with each eye, we experience the remarkable phenomenon of binocular rivalry:... more When viewing a different stimulus with each eye, we experience the remarkable phenomenon of binocular rivalry: alternations in consciousness between the stimuli [1, 2]. According to a popular theory first proposed in 1901, neurons encoding the two stimuli engage in reciprocal inhibition [3–8] so that those processing one stimulus inhibit those processing the other, yielding consciousness of one dominant stimulus at any moment and suppressing the other. Also according to the theory, neurons encoding the dominant stimulus adapt, weakening their activity and the inhibition they can exert, whereas neurons encoding the suppressed stimulus recover from adaptation until the balance of activity reverses, triggering an alternation in consciousness. Despite its popularity, this theory has one glaring inconsistency with data: during an episode of suppression, visual sensitivity to brief probe stimuli in the dominant eye should decrease over time and should increase in the suppressed eye, yet sensitivity appears to be constant [9, 10]. Using more appropriate probe stimuli (experiment 1) in conjunction with a new method (experiment 2), we found that sensitivities in dominance and suppression do show the predicted complementary changes.

Neuropsychological evidence of high-level processing in binocular rivalry

by Robert O'Shea

Daini, R., Facchin, A., Bignotti, M., Lentini, C., Peverelli, M., O’Shea, R. P., & Molteni, F. (2011). Neuropsychological evidence of high-level processing in binocular rivalry. Behavioural Neurology, 23, 233-235. doi: 10.3233/BEN-2010-0303

Binocular rivalry stimuli are common but rivalry is not

by Robert O'Shea

O’Shea, R. P. (2011). Binocular rivalry stimuli are common but rivalry is not. Frontiers in Human Neuroscience, 5(148), 1-2. doi: 10.3389/fnhum.2011.00148

Recently Arnold (2011) asked “Why is binocular rivalry uncommon?”. He answered in an entertainingly written,... more Recently Arnold (2011) asked “Why is binocular rivalry uncommon?”. He answered in an entertainingly written, provocative article, for which I thank and congratulate him. However, I will argue that Arnold’s answer falls short in two respects and his assumption that rivalry is uncommon is correct for two reasons other than the one he discusses.

Design and ergonomics of package inserts of drugs in Brazil: a reality in construction

by Charles Leite

Published in the Journal "Work: A Journal of Prevention, Assessment and Rehabilitation"

This research deals with the design of leaflets of medicines, evidencing the problems resulting from the lack of... more This research deals with the design of leaflets of medicines, evidencing the problems resulting from the lack of Brazilian normalization to promote the use of the graphical representation of instructional texts warnings. It approaches studies related to the effectiveness and efficiency of information systems, highlighting the semiotics and the cultural and informational ergonomics. The analysis of the context uses as method, an analytical study on selected warnings of thirty leaflets of medicines, followed by interviews lead with the public managers involved with the regulation of the pharmaceutical companies, and two experiments with users performed in city of Recife, in State of Pernambuco: one aiming at to identify how they interact with the leaflets of medicines, and the second one testing their understanding concerning standardized illustrations in the United States and the South Africa. The results show the need for improvements in presentation and graphic representation of leaflets of medicines, powering them to the role of communication, to ensure the consumption of medicine safely by its users. The conclusion congregates parameters and recommendations for the graphic representation of warnings in leaflets of medicines in Brazil

Attention and Perception: Looking without Paint

by Alessandra Tanesini

This is a Mumford Style Handout not for citation without permission. Any comments welcome

An argument against Block's Mental Paint deploying the pre-motor theory of attention An argument against Block's Mental Paint deploying the pre-motor theory of attention

Is a small apple more like an apple or more like a cherry? A study with real and modified sized objects

by Valentina Bazzarin

Co-authored with Anna Maria Borghi

In a categorization experiment we assessed whether seeingobjects automatically activates information on how... more In a categorization experiment we assessed whether seeingobjects automatically activates information on how tomanipulate them. The experiment also aims at investigatingthe role played in a categorization task by online, visualinformation (i.e., of information mediated by the dorsal system), and by information stored in memory (i.e.,information mediated by the ventral system). Participantscategorized photographs of objects manipulable either with apower or a precision grip into artifacts or natural kinds.Target-objects were preceded by primes consisting of photographs of hands in either grasping postures (precision orpower grip) or in a neutral posture (grip). Target-objects couldbe presented either in their real size or in modified size, sothat they activated a different kind of grip. For example, astrawberry was presented both in its real size and with the sizeof an apple, so that it activated a power grip. Results confirm that visual stimuli activate motor information. More importantly, they suggest a crucial role of online, visualinformation even in a categorization task. Results arediscussed in the framework of theories on the role of onlineand offline memory features.

Byzantine Painting and Visual Communication

by Sotiria Kordi

When we look at an icon do we see Christ or an image of Christ? This question continuously emerges in modern... more When we look at an icon do we see Christ or an image of Christ? This question continuously emerges in modern scholarship. Issues such as presence and absence, art and worship, image and art, seem to concern modern scholars who study Byzantine painting. What is the Byzantine interpretation of the above issues and how do modern scholars perceive them?

The hierarchical order of processes underlying the direction illusion and the direction aftereffect

by Kevin Brooks

Farrell-Whelan, M., Wenderoth, P. & Brooks, K. R. (in press). The hierarchical order of processes underlying the direction illusion and the direction aftereffect. Perception, accepted 19 Mar 2012.

Motion perception involves the processing of velocity signals through several hierarchical stages of the visual... more Motion perception involves the processing of velocity signals through several hierarchical stages of the visual cortex. To better understand this process, a number of studies have sought to localize the neural substrates of two misperceptions of motion direction, the direction illusion (DI) and the direction aftereffect (DAE). These studies have produced contradictory evidence as to the hierarchical order of the processing stages from which the respective phenomena arise. We have used a simple stimulus configuration to further investigate the sequential order of processes giving rise to the DI and DAE. To this end, we measured the two phenomena invoked in combination, and also manually parsed this combined effect into its two constituents by measuring the two phenomena individually in both possible sequential orders. Comparing the outcomes from each order to the outcome from the combined effect allowed us to test the tenability of two models: the DAE-first model and the DI-first model. Our results indicate that DAE-invoking activity does not occur earlier in the motion processing hierarchy than DI-invoking activity. Although the DI-first model is not inconsistent with our data, the possible involvement of non-sequential processing may be better able to reconcile these results with those of previous studies.

Sensitivity to feature displacement in familiar and unfamiliar faces: Beyond the internal/external feature distinction

by Kevin Brooks

Brooks, K. R. & Kemp, R. I. (2007). Sensitivity to feature displacement in familiar and unfamiliar faces: Beyond the internal/external feature distinction. Perception, 36, 1646-1659, http://www.perceptionweb.com/abstract.cgi?id=p5675, doi:10.1068/p5675

Previous studies of face recognition and of face matching have shown a general improvement for the processing of... more Previous studies of face recognition and of face matching have shown a general improvement for the processing of internal features as a face becomes more familiar to the participant. In this study, we used a psychophysical two-alternative forced choice paradigm to investigate thresholds for the detection of a displacement of the eyes, nose, mouth, or ears for familiar and unfamiliar faces. No clear division between internal and external features was observed. Rather, for familiar (compared to unfamiliar) faces participants were more sensitive to displacements of internal features such as the eyes or the nose; yet, for our third internal feature--the mouth--no such difference was observed. Despite large displacements, many subjects were unable to perform above chance when stimuli involved shifts in the position of the ears. These results are consistent with the proposal that familiarity effects may be mediated by the construction of a robust representation of a face, although the involvement of attention in the encoding of face stimuli cannot be ruled out. Furthermore, these effects are mediated by information from a spatial configuration of features, rather than by purely feature-based information.

Spatial scale of stereomotion speed processing

by Kevin Brooks

Brooks, K. R. & Stone L. S. (2006). Spatial scale of stereomotion speed processing. Journal of Vision, 6, 1257-1266, http://journalofvision.org/6/11/9, doi:10.1167/6.11.9

To examine the spatial scale of the mechanisms supporting the perception of motion in depth defined by binocular cues,... more To examine the spatial scale of the mechanisms supporting the perception of motion in depth defined by binocular cues, we measured stereomotion speed discrimination thresholds as a function of stimulus size using a two-interval speed comparison task. Stimuli were either random dot stereogram (RDS) bars featuring both the changing disparity (CD) and the interocular velocity difference (IOVD) cues to motion in depth or dynamic random dot stereogram (DRDS) bars featuring the CD cue alone. Monocular speed discrimination performance was also assessed, using half-images of the RDS stimulus. In addition, subjects’ stereoacuity for stationary versions of the binocular stimuli was measured. Stimuli ranged in vertical extent from 1.25 to 40 min. Sensitivity to speed differences was strongly related to stimulus height for DRDS stimuli. Performance decreased rapidly as stimulus size was reduced, becoming nearly random for heights below 5 min. However, for RDS stimuli, speed discrimination performance declined with reductions in stimulus size at a far slower rate, providing superior performance at every stimulus size used. Monocular performance was superior still for the majority of subjects, yet showed a similar rate of decline to binocular RDS stimuli. We conclude that the spatial resolution of the CD mechanism and its static disparity inputs is, on average, nearly nine times more coarse than the IOVD system and its monocular motion inputs. Static stereoacuity controls show that this finding cannot be explained by differences in the disparity signals available in our RDS and DRDS stimuli.

Stereomotion suppression and the perception of speed: accuracy and precision as a function of 3D trajectory

by Kevin Brooks

Brooks, K. R. & Stone L. S. (2006). Stereomotion suppression and the perception of speed: accuracy and precision as a function of 3D trajectory. Journal of Vision, 6, 1214-1223, http://journalofvision.org/6/11/6, doi:10.1167/6.11.6

The precision and accuracy of speed discrimination performance for stereomotion stimuli were assessed for several... more The precision and accuracy of speed discrimination performance for stereomotion stimuli were assessed for several receding 3D trajectories confined to the horizontal meridian. It has previously been demonstrated in a variety of tasks that detection thresholds are substantially higher when subjects observe a stereomotion stimulus than when simply viewing one of its component monocular half-imagesVa phenomenon known as stereomotion suppression (C. W. Tyler, 1971). Using monocularly visible motion in depth targets, we found mean speed discrimination thresholds to be higher for stereomotion,
compared with monocular lateral speed discrimination thresholds for equivalent stimuli, demonstrating a disadvantage for binocular viewing in the case of speed discrimination as well. Furthermore, speed discrimination thresholds for motion in depth were not systematically affected by trajectory angle; hence, the disadvantage of binocular viewing persists even when there are concurrent changes in binocular visual direction. Lastly, there was a tendency for oblique trajectories of stereomotion to be perceived as faster than equally rapid motion receding directly away from the subject along the midline. Our data, in addition to earlier stereomotion suppression observations, are consistent with a stereomotion system that takes a noisy, weighted difference of the stimulus velocities in the two eyes to compute motion in depth.

The swinging doors of perception: Stereomotion without binocular matching

by Kevin Brooks

Brooks, K. R. & Gillam, B. J. (2006). The swinging doors of perception: stereomotion without binocular matching. Journal of Vision, 6, 685-695, http://journalofvision.org/6/7/2, doi:10.1167/6.7.2

Until recently, it was considered necessary for features in the two eyes to be matched before the evaluation of... more Until recently, it was considered necessary for features in the two eyes to be matched before the evaluation of differences in their locations (binocular disparities) could reveal depth information. Motion in depth can also be perceived binocularly from related changes in the locations of matched binocular features. However, unmatched features can arise when a binocular object occludes more distant features in one eye but not the other. The presence and extent of such features can provide quantitative depth information, although perceived depth relative to geometrical predictions may vary from one such arrangement to another. The ability of humans to perceive motion in depth from unmatched stimuli has not previously been explored. Here, we use B. Gillam, S. Blackburn, and K. Nakayama’s (1999) ‘‘monocular gap’’ stimuli to investigate perception of motion in depth simulated by a change in the extent of a monocularly occluded feature in a binocular display. Settings of a motion in depth probe revealed that the magnitude of perceived motion in depth is generally as large as that for a stimulus containing matchable binocular features. We show that our stimuli provide disambiguating information not present in similar static stimuli. We conclude that in the computation of motion in depth, a binocular match is not required. A new cue--dynamic half-occlusion--can be used to reach an accurate percept.

Quantitative perceived depth from sequential monocular decamouflage

by Kevin Brooks

Brooks, K. R. & Gillam, B. J. (2006). Quantitative perceived depth from sequential monocular decamouflage. Vision Research, 46, 605-613. doi:10.1016/j.visres.2005.06.015

We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by... more We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by sequential monocular stimulation (delay P 80 ms). Vertical white lines were occluded as they passed behind an otherwise camouflaged black rectangular target. The location (and instant) of the occlusion event, decamouflaging the target
s edges, differed in the two eyes. Probe settings to match the depth of the black rectangular target showed a monotonic increase with simulated depth. Control tests discounted the possibility of subjects integrating retinal disparities over an extended temporal window or using temporal disparity. Sequential monocular decamouflage was found to be as precise and accurate as conventional simultaneous stereopsis with equivalent depths and exposure durations.

Monocular Transparency and unpaired stereopsis

by Kevin Brooks

Grove, P. M., Brooks, K. R., Anderson, B. L. & Gillam, B. J. (2006). Monocular transparency and unpaired stereopsis. Vision Research, 46, 3041-3053. doi:10.1016/j.visres.2006.05.003

Howard and Duke [Howard, I.P., & Duke, P.A. (2003). Monocular transparency generates quantitative depth. Vision... more Howard and Duke [Howard, I.P., & Duke, P.A. (2003). Monocular transparency generates quantitative depth. Vision Research, 43, 2615–2621] recently proposed a new source of binocular information they claim is used to recover depth in stereoscopic displays. They argued that these displays lack conventional disparity and that the metrical depth experienced results from transparency rather than occlusion relations. Using a variety of modified versions of their stimuli, we show here that the conditions for transparency are not required to elicit the depth experienced in their stereograms. We demonstrated that quantitative and precise depth depended not on the presence of transparency but on the presence of horizontal contours of the same contrast polarity. Depth was attenuated, particularly at larger target offsets, when horizontal contours had opposite contrast polarity for at least a portion of their length. We also show that a demonstration Howard and Duke used to control for the role of horizontal contours can be understood as an example of Gillam et al.’s Gillam, B.J., Blackburn, S., & Nakayama, K. (1999). Stereopsis based on monocular gaps: metrical coding of depth and slant without matching contours. Vision Research, 39, 493–502 monocular gap stereopsis; a form of binocular occlusion. In summary the findings reported by Howard and Duke can be understood by known processes for the computation of binocular disparity and binocular occlusion.

Human discrimination of visual direction of motion with and without smooth pursuit eye movements

by Kevin Brooks

Krukowski, A. E., Pirog, K. A., Beutter, B. R., Brooks, K. R. & Stone, L. S. (2003). Human discrimination of visual direction of motion with and without smooth pursuit eye movements. Journal of Vision, 3, 831-840, http://journalofvision.org/3/11/16/, doi:10.1167/3.11.16.

It has long been known that ocular pursuit of a moving target has a major influence on its perceived speed (Aubert,... more It has long been known that ocular pursuit of a moving target has a major influence on its perceived speed (Aubert, 1886; Fleischl, 1882). However, little is known about the effect of smooth pursuit on the perception of target direction. Here we compare the precision of human visual-direction judgments under two oculomotor conditions (pursuit vs. fixation). We also examine the impact of stimulus duration (200 ms vs. ~800 ms) and absolute direction (cardinal vs. oblique). Our main finding is that direction discrimination thresholds in the fixation and pursuit conditions are indistinguishable. Furthermore, the two oculomotor conditions showed oblique effects of similar magnitudes. These data suggest that the neural direction signals supporting perception are the same with or without pursuit, despite remarkably different retinal stimulation. During fixation, the stimulus information is restricted to large, purely peripheral retinal motion, while during steady-state pursuit, the stimulus information consists of small, unreliable foveal retinal motion and a large efference-copy signal. A parsimonious explanation of our findings is that the signal limiting the precision of direction judgments is a neural estimate of target motion in head-centered (or world-centered) coordinates (i.e., a combined retinal and eye motion signal) as found in the medial superior temporal area (MST), and not simply an estimate of retinal motion as found in the middle temporal area (MT).

Monocular motion adaptation affects the perceived trajectory of stereomotion

by Kevin Brooks

Brooks, K. R. (2002b). Monocular motion adaptation affects the perceived trajectory of stereomotion. Journal of Experimental Psychology: Human Perception and Performance, 28, 1470-1482.

Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or... more Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or nondominant eye to assess the relative contributions of 2 cues: changing disparity and interocular velocity difference. Perceived speed for monocular lateral motion and perceived binocular visual direction (BVD) was also assessed. Unlike stereomotion trajectory perception, the BVD of static targets showed an ocular dominance bias, even without adaptation. Adaptation caused equivalent biases in perceived trajectory and monocular motion speed, without significantly affecting perceived BVD. Predictions from monocular motion data closely match trajectory perception data, unlike those from BVD sources. The results suggest that the interocular velocity differences make a significant contribution to stereomotion trajectory perception.

Hinge versus twist: the effects of “reference surface” and discontinuities on stereoscopic slant perception

by Kevin Brooks

Gillam, B. J., Blackburn, S. & Brooks, K. R. (2007). Hinge versus twist: the effects of “reference surface” and discontinuities on stereoscopic slant perception. Perception, 36, 596-616, doi: 10.1068/p5535

Stereoscopic slant perception around a vertical axis (horizontal slant) is often found to be strongly attenuated... more Stereoscopic slant perception around a vertical axis (horizontal slant) is often found to be strongly attenuated relative to geometric prediction. Stereo slant is much greater, however,
when an adjacent surface, stereoscopically in the frontal plane, is added. This slant enhancement is often attributed to the presence of a `reference surface' or to a spatial change in the disparity gradient (introducing second and higher derivatives of disparity). Gillam, Chambers, and Russo (1988 Journal of Experimental Psychology: Human Perception and Performance 14 163-175) questioned the role of these factors in that placement of the frontal-plane surface in a direction collinear with the slant axis (twist configuration) sharply reduced latency for perceiving slant whereas placing the same surface in a direction orthogonal to the slant axis (hinge configuration) had little effect.We here confirm these findings for slant magnitude, showing a striking advantage for twist over hinge configurations.We also examined contrast slant measured on the frontal-plane surface in the hinge and twist configurations. Under conditions where test and inducer surfaces have centres at the same depth for twist and hinge, we found that twist configurations produced strong negative slant contrast, while hinge configurations produced significant positive contrast or slant assimilation.We conclude that stereo slant and contrast effects for neighbouring surfaces can only be understood from the patterns and gradients of step disparities present. It is not adequate to consider the second surface merely as a reference slant for the first or as having its effect via a spatial change in the disparity gradient.

Stereomotion perception for a monocularly camouflaged stimulus

by Kevin Brooks

Brooks, K. R. & Gillam, B. J. (2007). Stereomotion perception for a monocularly camouflaged stimulus. Journal of Vision, 7, 1-14, http://journalofvision.org/7/13/1, doi:10.1167/7.13.1

Under usual circumstances, motion in depth is associated with conventional stereomotion cues: a change in disparity... more Under usual circumstances, motion in depth is associated with conventional stereomotion cues: a change in disparity and differences between object velocities in each monocular image. However, occasionally these cues are unavailable due to the fact that in one eye the object may be occluded by, or camouflaged against appropriately positioned binocular objects. We report two experiments concerned with stereomotion perception under conditions of monocular camouflage. In Experiment 1, the visible half-image of a monocularly camouflaged object translated laterally. In this binocular context, percepts of lateral motion and motion in depth were equally consistent with the stimulus. Subjects perceived an oblique trajectory of 3D motion, compared to the more direct 3D trajectory experienced for binocularly matched stimuli. In Experiment 2, the perceived velocity of stereomotion was assessed. Again, for the stimulus used in Experiment 1, perceived stereomotion speed was lower than that for matched stimuli. However, when additional background objects were present, tightening the ecological constraints, perceived stereomotion velocity was often equivalent to that for matched stimuli. These results demonstrate for the first time that the motion of a monocularly camouflaged object can result in the perception of stereomotion, and that the perceived trajectory and speed are influenced by the ecological constraints of binocular geometry.

Perceived speed of motion in depth is reduced in the periphery

by Kevin Brooks

Brooks, K. & Mather, G., (2000). Perceived speed of motion in depth is reduced in the periphery. Vision Research, 40, 3507-3516.

The perceived speed of motion in depth (MID) for a monocularly visible target was measured in central and peripheral... more The perceived speed of motion in depth (MID) for a monocularly visible target was measured in central and peripheral vision using a 2AFC speed discrimination task. Only binocular cues to MID were available: changing disparity and interocular velocity difference (IOVD). Perceived speed for monocular lateral motion and perceived depth for static disparity were also assessed, again in both central and peripheral vision. The purpose of the experiment was to assess the relative contributions of changing disparity and IOVD cues to the perceived speed of stereomotion. Although peripheral stimuli appeared to lie at approximately the same depth as their central counterparts, their apparent speed was reduced. Monocular/lateral and binocular/MID speeds were reduced to a similar extent. It seems that reduced apparent monocular speed leads to reduced perceived MID speed, despite the fact that the disparity system appears to be unaffected. These results suggest that the IOVD cue makes a significant contribution to MID speed perception.

Perceptual memory for highly familiar people’s body shape: manipulation of images of the self and friend

by Kevin Brooks

Daury, N., Brooks, K. R. & Bredart, S. (2009). Perceptual memory for highly familiar people’s body shape: manipulation of images of the self and friend. Perception, 38, 261-270.

Previous studies have shown that people's ability to detect, from memory, alterations in highly familiar faces is... more Previous studies have shown that people's ability to detect, from memory, alterations in highly familiar faces is excellent. Indeed, just noticeable differences for the detection of small alterations in a recognition-memory task were not significantly different from the corresponding measures in a perceptual-discrimination task (Bre¨dart and Devue, 2006 Perception 35 101-106; Ge et al, 2003 Perception 32 601-614). The object of the present study was to evaluate whether people's perceptual memory for body shapes of very familiar persons reaches the high level of precision that was reported for face memory. For one group of participants, the task was to detect body shape alterations (an increase or a decrease of 2% to 10% of the waist-to-hip ratio) on photographs depicting either themselves or a friend. For another group of participants who did not know the target persons, the task was to discriminate whether two photographs presented side by side were the same or not. Results showed that the detection of alterations was significantly
better in the perceptual-discrimination task than in the recognition-memory tasks (for the participant's own body as well as for the friend's body). In conclusion, the high fidelity of perceptual memory for very familiar faces does not extend to familiar body shapes.