Illusions of visual orientation: comparisons between perceptual and visuo-motor tasks

The Milner and Goodale (1995) model of dual cortical visual systems suggests that, in the primate cortex, separate neural substrates dominate the tasks of visual perception and visuo-motor control. This model derives from a number of independent sources of evidence: anatomical, physiological and behavioural. Neuropsychological evidence in humans suggests that visual perception and visuo-motor control can be selectively impaired through damage to the ventral and dorsal visual streams respectively. Evidence has emerged that in the healthy human visual cortex, differentiable effects of visual illusions can be found between the two measures of perception and visuo- motor control. This evidence has been cited to support the Milner and Goodale (1995) model. The series of studies reported in this dissertation used a similar, but methodologically revised application of the illusion paradigm in the novel domain of orientation. Using two types of visual illusions, the simultaneous tilt illusion (STI) and the rod-and-frame illusion (RFI), a series of studies found patterns of association, dissociation and interaction that strongly support the Mihier and Goodale model. The critical issue, in terms of predicting the pattern of effects across perception and visuo-motor control tasks, was found to be the siting of the causal mechanisms underlying the illusion employed

Asymmetrical representation of body orientation

The perceived orientation of objects, gravity, and the body are biased to the left. Whether this leftward bias is attributable to biases in sensing or processing vestibular, visual, and body sense cues has never been assessed directly. The orientation in which characters are most easily recognized-the perceived upright (PU)-can be well predicted from a weighted vector sum of these sensory cues. A simple form of this model assumes that the directions of the contributing inputs are coded accurately and as a consequence participants tilted leftor right-side-down relative to gravity should exhibit mirror symmetric patterns of responses. If a left/right asymmetry were present then varying these sensory cues could be used to assess in which sensory modality or modalities a PU bias may have arisen. Participants completed the Oriented Character Recognition Test (OCHART) while manipulating body posture and visual orientation cues relative to gravity. The response patterns showed systematic differences depending on which side they were tilted. An asymmetry of the PU was found to be best modeled by adding a leftward bias of 5.68 to the perceived orientation of the body relative to its actual orientation relative to the head. The asymmetry in the effect of body orientation is reminiscent of the body-defined left-leaning asymmetry in the perceived direction of light coming from above and reports that people tend to adopt a right-leaning posture

Tilted frames of reference have similar effects on perception of the gravitational vertical and the planning of vertical saccadic eye movements

We investigated the effects of a tilted reference frame (i.e., allocentric visual context) on perception of the gravitational vertical and saccadic eye movements along a planned egocentric vertical path. Participants (n=5) in a darkened room fixated a point in the center of a circle on an LCD display, and decided which of two sequentially presented dots was closer to the unmarked ‘6 o’clock’ position on that circle (i.e., straight down towards their feet). The slope of their perceptual psychometric functions showed that participants were able to locate which dot was nearer the vertical with a precision of 1-2°. For three of the participants, a square frame centered at fixation and tilted (in the roll direction) 5.6° from the vertical caused a strong perceptual bias, manifest as a shift in the psychometric function, in the direction of the traditional ‘rod and frame’ effect, without affecting precision. The other two participants showed negligible or no equivalent biases. The same subjects participated in the saccade version of the task, in which they were instructed to shift their gaze to the 6 o’clock position as soon as the central fixation point disappeared. The participants who showed perceptual biases showed biases of similar magnitude in their saccadic end points, with a strong correlation between perceptual and saccadic biases across all subjects. Tilting of the head 5.6° reduced both perceptual and saccadic biases in all but one observer, who developed a strong saccadic bias. Otherwise, the overall pattern and significant correlations between results remained the same. We conclude that our observers' saccades-to-the-vertical were dominated by perceptual input, which outweighed any gravitational or head-centered input

Observer Variation of 2-Deoxy-2-[F-18]fluoro-d-Glucose-Positron Emission Tomography in Mediastinal Staging of Non-Small Cell Lung Cancer as a Function of Experience, and its Potential Clinical Impact

Purpose: To test the extent of variation among nuclear medicine physicians with respect to staging non-small cell lung cancer with positron emission tomography (PET). Procedures: Two groups of nuclear medicine physicians with different levels of PET experience reviewed 30 PET scans. They were requested to identify and localize suspicious mediastinal lymph nodes (MLN) using standardized algorithms. Results were compared between the two groups, between individuals, and with expert reading. Results: Overall we found good interobserver agreement (kappa 0.65). Experience with PET translated into a better ability to localize MLN stations (68 % vs. 51%, respectively), and experienced readers appeared to be more familiar with translating PET readings into clinically useful statements. Conclusions: Although our results suggest that clinical experience with PET increases observers _ ability to read and interpret results from PET adequately, there is room for improvement. Experience with PET does not necessarily improve the accuracy of image interpretation

The Poggendorff illusion affects manual pointing as well as perceptual judgements

Pointing movements made to a target defined by the imaginary intersection of a pointer with a distant landing line were examined in healthy human observers in order to determine whether such motor responses are susceptible to the Poggendorff effect. In this well-known geometric illusion observers make systematic extrapolation errors when the pointer abuts a second line (the inducer). The kinematics of extrapolation movements, in which no explicit target was present, where similar to those made in response to a rapid-onset (explicit) dot target. The results unambiguously demonstrate that motor (pointing) responses are susceptible to the illusion. In fact, raw motor biases were greater than for perceptual responses: in the absence of an inducer (and hence also the acute angle of the Poggendorff stimulus) perceptual responses were near-veridical, whilst motor responses retained a bias. Therefore, the full Poggendorff stimulus contained two biases: one mediated by the acute angle formed between the oblique pointer and the inducing line (the classic Poggendorff effect), which affected both motor and perceptual responses equally, and another bias, which was independent of the inducer and primarily affected motor responses. We conjecture that this additional motor bias is associated with an undershoot in the unknown direction of movement and provide evidence to justify this claim. In conclusion, both manual pointing and perceptual judgements are susceptible to the well-known Poggendorff effect, supporting the notion of a unitary representation of space for action and perception or else an early locus for the effect, prior to the divergence of processing streams

The effect of the “rod-and-frame” illusion on grip planning in a sequential object manipulation task

We investigated the effect of visual context (i.e., a visual illusion) on the planning of a sequential object manipulation task. Participants (n = 13) had to grasp a rod embedded in a “rod-and-frame” illusion and insert the rod-end into a tight hole in a pre-defined way. The grip type (defined by start posture, either pronated or supinated; and end posture, either comfortable or uncomfortable) used to grasp the rod was registered as a macroscopic variable of motor planning. Different rod orientations forced the participants to switch between grip types. As expected, most participants switched between pronated and supinated start postures, such that they ended the movement with a comfortable end posture. As it has been argued that planning is dependent on visual context information, we hypothesized that the visual illusion would affect the specific rod orientation at which participants would switch into a different grip type. This hypothesis was confirmed. More specifically, the illusion affected the critical spatial information that is used for action planning. Collectively, these findings are the first to show an effect of an illusion on motor planning in a sequential object manipulation task

Vestibular Facilitation of Optic Flow Parsing

Simultaneous object motion and self-motion give rise to complex patterns of retinal image motion. In order to estimate object motion accurately, the brain must parse this complex retinal motion into self-motion and object motion components. Although this computational problem can be solved, in principle, through purely visual mechanisms, extra-retinal information that arises from the vestibular system during self-motion may also play an important role. Here we investigate whether combining vestibular and visual self-motion information improves the precision of object motion estimates. Subjects were asked to discriminate the direction of object motion in the presence of simultaneous self-motion, depicted either by visual cues alone (i.e. optic flow) or by combined visual/vestibular stimuli. We report a small but significant improvement in object motion discrimination thresholds with the addition of vestibular cues. This improvement was greatest for eccentric heading directions and negligible for forward movement, a finding that could reflect increased relative reliability of vestibular versus visual cues for eccentric heading directions. Overall, these results are consistent with the hypothesis that vestibular inputs can help parse retinal image motion into self-motion and object motion components

Rapid eye movements to a virtual target are biased by illusory context in the Poggendorff figure.

In order to determine the influence of perceptual input upon oculomotor responses, we examined rapid saccadic eye movements made by healthy human observers to a virtual target defined by the extrapolated intersection of a pointer with a distant landing line. While corresponding perceptual judgments showed no evidence of systematic bias, eye movements showed a strong bias, in the direction of assimilation of the saccade trajectory to the shortest path between the end of the pointer and the landing line. Adding an abutting vertical inducing line to make an angle of 45 deg with the pointer led to a larger bias in the same direction as the classical Poggendorff illusion. This additional Poggendorff effect was similar in direction and magnitude for the eye movements and the perceptual responses. Latency and dynamics of the eye movements were closely similar to those recorded for a control task in which observers made a saccade from the start fixation to an explicit target on the landing line. Further experiments with inducing lines presented briefly at various times during the saccade latency period showed that the magnitude of the saccade bias was affected by inducer presentation during the saccade planning process, but not during the saccade itself. We conclude that the neural mechanisms for extrapolation can feed into the control of eye movements without obvious penalties in timing and accuracy and that this information can instantaneously modify motor response throughout the planning phase, suggesting close association between perceptual and motor mechanisms in the process of visuo-spatial extrapolation