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Biomedical subjects

Douglas B Tweed

Publications and source records attributed to Douglas B Tweed.

6 recordsLinked to original sources
PubMed2003

Motion parallax is computed in the updating of human spatial memory.

As we move through space, stationary objects around us show motion parallax: their directions relative to us change at different rates, depending on their distance. Does the brain incorporate parallax when it updates its stored representations of space? We had subjects fixate a distant target and then we flashed lights, at different distances, onto the retinal periphery. Subjects translated sideways while keeping their gaze on the distant target, and then they looked to the remembered location of the flash. Their responses corrected almost perfectly for parallax: they turned their eyes farther for nearer targets, in the predicted nonlinear patterns. Computer simulations suggest a neural mechanism in which feedback about self-motion updates remembered locations of objects within an internal map of three-dimensional visual space.

Adult↗
PubMed2003

Interaction of retinal image and eye velocity in motion perception.

When we move our eyes, why does the world look stable even as its image flows across our retinas, and why do afterimages, which are stationary on the retinas, appear to move? Current theories say this is because we perceive motion by summation: if an object slips across the retina at r degrees/s while the eye turns at e degrees/s, the object's perceived velocity in space should be r + e. We show that activity in MT+, the visual-motion complex in human cortex, does reflect a mix of r and e rather than r alone. But we show also that, for optimal perception, r and e should not summate; rather, the signals coding e interact multiplicatively with the spatial gradient of illumination.

Confidence Intervals↗
PubMed2003

Optimal transsaccadic integration explains distorted spatial perception.

We scan our surroundings with quick eye movements called saccades, and from the resulting sequence of images we build a unified percept by a process known as transsaccadic integration. This integration is often said to be flawed, because around the time of saccades, our perception is distorted and we show saccadic suppression of displacement (SSD): we fail to notice if objects change location during the eye movement. Here we show that transsaccadic integration works by optimal inference. We simulated a visuomotor system with realistic saccades, retinal acuity, motion detectors and eye-position sense, and programmed it to make optimal use of these imperfect data when interpreting scenes. This optimized model showed human-like SSD and distortions of spatial perception. It made new predictions, including tight correlations between perception and motor action (for example, more SSD in people with less-precise eye control) and a graded contraction of perceived jumps; we verified these predictions experimentally. Our results suggest that the brain constructs its evolving picture of the world by optimally integrating each new piece of sensory or motor information.

Humans↗
PubMed2003

Influence of eye position on stereo matching.

In animals with binocular depth vision, or stereopsis, the visual fields of the two eyes overlap, shrinking the overall field of view. Eye movements increase the field of view, but they also complicate the first stage of stereopsis: the search for corresponding images on the two retinas. If the eyes were stationary in the head, corresponding images would always lie on retina-fixed bands called epipolar lines. Because the eyes rotate, the epipolar lines move on the retinas. Therefore, the stereoptic system has a choice: it may monitor eye position to keep track of the epipolar lines, or it may give up on tracking epipolar lines and instead search for matches over retina-fixed regions, but in that case the search regions must be 2-D patches, large enough to encompass all possible locations of the epipolar lines in all usual eye positions. We use a new type of random-dot stereogram to show that human stereopsis uses large, retina-fixed search zones. We show that the brain somewhat reduces the size of these search zones by rotating the eyes about their lines of sight in a way that reduces the motion of the epipolar lines. These findings show the link between sensory and motor processes: by considering eye motion we can understand why the brain searches for matching images over 2-D retinal regions rather than along epipolar lines; and by considering retinal correspondence we appreciate why the eyes rotate as they do about their lines of sight.

Depth Perception↗
PubMed2002

Rotational remapping in human spatial memory during eye and head motion.

The brain uses vision and other senses to compute the locations of objects relative to the body, and then must update these locations when the body moves. How geometrically sophisticated is this internal updating? It has been suggested that updating simply shifts the stored locations of all objects uniformly, by a common vector, when the eye or head turns. For horizontal and vertical turns, a uniform shift would often approximate the real changes in location of objects in front of the subject. But for torsional rotations, a shift would be inadequate: accurate updating would call for a more geometrically exact remapping, not shifting but rotating the stored locations through the inverse of the rotation of the eye in space. Here we asked human subjects to make eye saccades to remembered targets after torsional head rotations. Their accuracy showed that spatial updating works in the torsional dimension and operates by rotation rather than shifting.

Adult↗