Biomedical subjects

Robert F Hess

Publications and source records attributed to Robert F Hess.

41 records · Page 3Linked to original sources

PubMed2002

Rules for combining the outputs of local motion detectors to define simple contours.

We know something about the fidelity with which motion can be detected in local regions of the visual field but nothing about how these local motion signals are combined across space to define contours. To investigate such linking rules, we measured the detectability of motion-defined contours using an adaptation of the paradigm of Field, Hayes, and Hess (Vision Research, 33 (1993) 173) in which subjects are asked to detect the presence of simple contours defined solely by local motion direction that are embedded in a field of otherwise random local motions. We show that contours defined by motion whose direction is along the contour are more detectable than contours defined by motions of any common direction. Furthermore, the contour configuration is important in that straight and moderately curved contours, though not highly curved ones, can support this specialized form of motion integration.

Computer Graphics↗

PubMed2002

Luminance spatial scale and local stereo-sensitivity.

Using filtered, broad band, fractal noise images we measured the dependence of D(min) and D(max) for stereo on luminance spatial frequency. D(min) was found to exhibit a simple dependence on the highest spatial frequency contained in the stimulus. D(max) depended on both image size and spatial frequency in a way that suggests an informational limit. Different rules govern D(min) and D(max) even for first order stereopsis, arguing against a common neural explanation based on independent access to the most pertinent spatial filter.

Depth Perception↗

PubMed2002

Failure of direction identification for briefly presented second-order motion stimuli: evidence for weak direction selectivity of the mechanisms encoding motion.

We sought to investigate why the direction of second-order motion, unlike first-order motion, cannot be identified when the stimulus exposure duration is brief (<200 ms). In a series of experiments observers identified both the orientation (vertical or horizontal) and the direction (left, right, down or up) of a drifting sinusoidal modulation (0.93 c/ degrees ) in either the luminance (first order) or the contrast (second order) of a two-dimensional noise carrier. All motion stimuli were equated for visibility, and the duration was varied using the method of constant stimuli. Performance was measured for second-order motion over a range of drift temporal frequencies (0.63-5.04 Hz) and for first-order motion stimuli composed of two, opposite drifting modulations in luminance of unequal modulation depth. Orientation-identification performance was nearly 100% correct for both first-order and second-order motion stimuli, even at the briefest stimulus duration tested (26.49 ms). Direction identification for first-order motion was also typically good with brief presentations, but was poor for second-order motion when the exposure duration was < approximately 200 ms. Importantly increasing either the drift temporal frequency of second-order motion or the bidirectional nature of the first-order motion patterns produced comparable levels of performance for the two varieties of motion (i.e. the minimum duration required for reliable direction identification could be equated). As orientation-identification performance for the first-order and second-order motion stimuli was comparably good and minimally affected by duration, the marked differences on the direction-identification task must be specific to mechanisms that encode drift direction, rather than spatial structure. We propose that second-order motion detectors are much less selective for stimulus direction than first-order motion sensors, and thus are more susceptible to the deleterious effects of limiting stimulus duration (which introduces spurious motion in the opposite direction, particularly at low drift rates). Alternative explanations based on the delayed propagation of second-order motion signals or the temporal characteristics of the underlying motion mechanisms are not supported by our findings.

Humans↗

PubMed2002

Lateral interactions in amblyopia.

We studied lateral neural interactions in strabismic (n=6) and anisometropic amblyopes (n=3) by measuring reductions in the perceived contrast of a foveally viewed Gabor centred in a horizontal array of closely neighboring Gabors. Strabismic amblyopes, but not anisometropic amblyopes, failed to show the reduction in perceived contrast typical of normal vision [J. Opt. Soc. Amer. A 15 (1998) 1733] when lateral contrast information is available at the same orientation and spatial frequency. The strabismic amblyopes also severely misperceive the regularity of the array of Gabors flanking the test stimulus. A normal eye could model the anomalous contrast perception of the amblyopic eye, by adding an equivalent amount of spatial distortion to the stimulus. The relationship between the observed anomalies for local contrast gain control and positional sensitivity is discussed.

Adult↗

PubMed2001

Amblyopia: site unseen.

The site of the amblyopic deficit has been keenly sought for the past half century. Controversy has raged concerning whether it is retinal or cortical. Neurophysiologists and clinicians have provided compelling evidence to support their respective cases. Its resolution in terms of a primary cortical deficit is relatively recent and provides an interesting insight into the working of science. Currently, another controversy is beginning concerning which cortical areas are affected in amblyopia. Only time will tell if this new debate proves as interesting as its retinal counterpart. This review concentrates on where the site of the amblyopic deficit resides. It intentionally bypasses the issue of the nature of the deficit, an issue worthy of a separate review.

Journal Article↗