Faculty Bibliography

Many individuals with normal visual acuity are unable to discriminate the direction of 3-D motion in a portion of their visual field, a deficit previously referred to as a stereomotion scotoma. The origin of this visual deficit has remained unclear. We hypothesized that the impairment is due to a failure in the processing of one of the two binocular cues to motion in depth: changes in binocular disparity over time or interocular velocity differences. We isolated the contributions of these two cues and found that sensitivity to interocular velocity differences, but not changes in binocular disparity, varied systematically with observers' ability to judge motion direction. We therefore conclude that the inability to interpret motion in depth is due to a failure in the neural mechanisms that combine velocity signals from the two eyes. Given these results, we argue that the deficit should be considered a prevalent but previously unrecognized agnosia specific to the perception of visual motion.

Sensory systems are faced with an essentially infinite number of possible environmental events but have limited processing resources. Posed with this challenge, it makes sense to allocate these resources to prioritize the discrimination of events with the most behavioral relevance. Here, we asked if such relevance is reflected in the processing and perception of motion. We compared human performance on a rapid motion direction discrimination task, including monocular and binocular viewing. In particular, we determined sensitivity and bias for a binocular motion-in-depth (three-dimensional; 3D) stimulus and for its constituent monocular (two-dimensional; 2D) signals over a broad range of speeds. Consistent with prior work, we found that binocular 3D sensitivity was lower than monocular sensitivity for all speeds. Although overall sensitivity was worse for 3D discrimination, we found that the transformation from 2D to 3D motion processing also incorporated a pattern of potentially advantageous biases. One such bias is reflected by a criterion shift that occurs at the level of 3D motion processing and results in an increased hit rate for motion toward the head. We also observed an increase in sensitivity for 3D motion trajectories presented on crossed rather than uncrossed disparity pedestals, privileging motion trajectories closer to the observer. We used these measurements to determine the range of real-world trajectories for which rapid 3D motion discrimination is most useful. These results suggest that the neural mechanisms that underlie motion perception privilege behaviorally relevant motion and provide insights into the nature of human motion sensitivity in the real world.

Human performance on various visual tasks can be improved substantially via training. However, the enhancements are frequently specific to relatively low-level stimulus dimensions. While such specificity has often been thought to be indicative of a low-level neural locus of learning, recent research suggests that these same effects can be accounted for by changes in higher-level areas--in particular in the way higher-level areas read out information from lower-level areas in the service of highly practiced decisions. Here we contrast the degree of orientation transfer seen after training on two different tasks--vernier acuity and stereoacuity. Importantly, while the decision rule that could improve vernier acuity (i.e. a discriminant in the image plane) would not be transferable across orientations, the simplest rule that could be learned to solve the stereoacuity task (i.e. a discriminant in the depth plane) would be insensitive to changes in orientation. Thus, given a read-out hypothesis, more substantial transfer would be expected as a result of stereoacuity than vernier acuity training. To test this prediction, participants were trained (7500 total trials) on either a stereoacuity (N = 9) or vernier acuity (N = 7) task with the stimuli in either a vertical or horizontal configuration (balanced across participants). Following training, transfer to the untrained orientation was assessed. As predicted, evidence for relatively orientation specific learning was observed in vernier trained participants, while no evidence of specificity was observed in stereo trained participants. These results build upon the emerging view that perceptual learning (even very specific learning effects) may reflect changes in inferences made by high-level areas, rather than necessarily fully reflecting changes in the receptive field properties of low-level areas.

PURPOSE/OBJECTIVE:Perception necessarily entails combining separate sensory estimates into a single coherent whole. The perception of three-dimensional (3D) motion, for instance, can rely on two binocular cues: one related to the change in binocular disparity over time (CD) and the other related to interocular velocity differences (IOVD). Although previous work has shown that neither cue is strictly necessary for the perception of 3D motion, observers are able to judge 3D motion in displays in which one or the other cue has been eliminated, it is unclear whether or how the two cues are combined in situations in which both are present. METHODS:We tested the visual performance of a sample of 81 individuals (Mage = 20.34, 49 females) in four main conditions that measured, respectively, static stereoacuity, CD, IOVD, and combined CD+IOVD sensitivity. RESULTS:We show that the sensitivity to the two binocular cues to 3D motion varies substantially across observers (CD: Md' = 1.01, SDd' = 1.1; IOVD: Md' = 1.16, SDd' = 1.03). Furthermore, sensitivity to the two cues was independent across observers (r[48] = 0.12, P = 0.42). Importantly, however, observed CD+IOVD performance was well-predicted based on the assumption that each observer combines the two cues in a statistically optimal fashion (r[79] = 0.75, P < 0.001). CONCLUSIONS:Our findings provide an explanation for the previously puzzling variability found in 3D perception across observers and laboratories, with some results suggesting that motion-in-depth percepts are largely determined by changes in binocular disparity, whereas others indicate that interocular velocity differences are key. Our results underline the existence of two complementary binocular mechanisms underlying 3D motion perception, with observers relying on these two mechanisms to different extents depending on their individual sensitivity.

We experience our visual world as seen from a single viewpoint, even though our two eyes receive slightly different images. One role of the visual system is to combine the two retinal images into a single representation of the visual field, sometimes called the cyclopean image [1]. Conventional terminology, i.e. retinotopy, implies that the topographic organization of visual areas is maintained throughout visual cortex [2]. However, following the hypothesis that a transformation occurs from a representation of the two retinal images (retinotopy) to a representation of a single cyclopean image (cyclopotopy), we set out to identify the stage in visual processing at which this transformation occurs in the human brain. Using binocular stimuli, population receptive field mapping (pRF), and ultra-high-field (7 T) fMRI, we find that responses in striate cortex (V1) best reflect stimulus position in the two retinal images. In extrastriate cortex (from V2 to LO), on the other hand, responses better reflect stimulus position in the cyclopean image. These results pinpoint the location of the transformation from a retinal to a cyclopean representation and contribute to an understanding of the transition from sensory to perceptual stimulus space in the human brain.

Although we have made major advances in understanding motion perception based on the processing of lateral (2D) motion signals on computer displays, the majority of motion in the real (3D) world occurs outside of the plane of fixation, and motion directly toward or away from observers has particular behavioral relevance. Previous work has reported a systematic lateral bias in the perception of 3D motion, such that an object on a collision course with an observer's head is frequently judged to miss it, with obvious negative consequences. To better understand this bias, we systematically investigated the accuracy of 3D motion perception while manipulating sensory noise by varying the contrast of a moving target and its position in depth relative to fixation. Inconsistent with previous work, we found little bias under low sensory noise conditions. With increased sensory noise, however, we revealed a novel perceptual phenomenon: observers demonstrated a surprising tendency to confuse the direction of motion-in-depth, such that approaching objects were reported to be receding and vice versa. Subsequent analysis revealed that the lateral and motion-in-depth components of observers' reports are similarly affected, but that the effects on the motion-in-depth component (i.e., the motion-in-depth confusions) are much more apparent than those on the lateral component. In addition to revealing this novel visual phenomenon, these results shed new light on errors that can occur in motion perception and provide a basis for continued development of motion perception models. Finally, our findings suggest methods to evaluate the effectiveness of 3D visualization environments, such as 3D movies and virtual reality devices.

Amblyopia is a visual disorder caused by poorly coordinated binocular input during development. Little is known about the impact of amblyopia on the white matter within the visual system. We studied the properties of six major visual white-matter pathways in a group of adults with amblyopia (n=10) and matched controls (n=10) using diffusion weighted imaging (DWI) and fiber tractography. While we did not find significant differences in diffusion properties in cortico-cortical pathways, patients with amblyopia exhibited increased mean diffusivity in thalamo-cortical visual pathways. These findings suggest that amblyopia may systematically alter the white matter properties of early visual pathways.

Stereomotion scotomas are a surprisingly common visual impairment that result in an observer's inability to accurately report the direction of an object's motion in depth in restricted parts of the visual field. In this study we investigated the role of binocular cues to motion in depth. Using stimuli containing only non-stationary cues to stereomotion, we measured sensitivity across the visual field and identified areas of significant impairment in stereomotion processing in over 50% of otherwise healthy observers. These impairments vary idiosyncratically in extent and location between observers. We established that these impairments occur for a variety of visual stimuli, as long as they share the property that stimulus motion is exclusively defined by interocular and velocity differences. We tested for concordant impairments at relatively early stages along the visual pathway, i.e. changes in sensitivity across the visual field to local eye-dominance, monocular motion or instantaneous binocular disparity. Although we find variability in sensitivity across the visual field of our observers for all visual tasks, this variability across visual field locations did not correlate with the impairments in stereomotion processing. We therefore conclude that these stereomotion scotomas are due to impaired processing of dynamic cues after the stage of binocular combination.

Neurons in the visual cortex process a local region of visual space, but in order to adequately analyze natural images, neurons need to interact. The notion of an ‘‘association field’’ proposes that neurons interact to extract extended contours. Here, we identify the site and properties of contour integration mechanisms. We used functional magnetic resonance imaging (fMRI) and population receptive field (pRF) analyses. We devised pRF mapping stimuli consisting of contours. We isolated the contribution of contour integration mechanisms to the pRF by manipulating the contour content. This stimulus manipulation led to systematic changes in pRF size. Whereas a bank of Gabor filters quantitatively explains pRF size changes in V1, only V2/V3 pRF sizes match the predictions of the association field. pRF size changes in later visual field maps, hV4, LO-1, and LO-2 do not follow either prediction and are probably driven by distinct classical receptive field properties or other extraclassical integration mechanisms. These pRF changes do not follow conventional fMRI signal strength measures. Therefore, analyses of pRF changes provide a novel computational neuroimaging approach to investigating neural interactions. We interpreted these results as evidence for neural interactions along cooriented, cocircular receptive fields in the early extrastriate visual cortex (V2/V3), consistent with the notion of a contour association field.

Emotions can color people's attitudes toward unrelated objects in the environment. Existing evidence suggests that such emotional coloring is particularly strong when emotion-triggering information escapes conscious awareness. But is emotional reactivity stronger after nonconscious emotional provocation than after conscious emotional provocation, or does conscious processing specifically change the association between emotional reactivity and evaluations of unrelated objects? In this study, we independently indexed emotional reactivity and coloring as a function of emotional-stimulus awareness to disentangle these accounts. Specifically, we recorded skin-conductance responses to spiders and fearful faces, along with subsequent preferences for novel neutral faces during visually aware and unaware states. Fearful faces increased skin-conductance responses comparably in both stimulus-aware and stimulus-unaware conditions. Yet only when visual awareness was precluded did skin-conductance responses to fearful faces predict decreased likability of neutral faces. These findings suggest a regulatory role for conscious awareness in breaking otherwise automatic associations between physiological reactivity and evaluative emotional responses.