Biomedical subjects

D R Pope

Publications and source records attributed to D R Pope.

9 recordsLinked to original sources

PubMed2000

First- and second-order processing in transient stereopsis.

Large-field stimuli were used to investigate the interaction of first- and second-order pathways in transient-stereo processing. Stimuli consisted of sinewave modulations in either the mean luminance (first-order stimulus) or the contrast (second-order stimulus) of a dynamic-random-dot field. The main results of the present study are that: (1) Depth could be extracted with both the first-order and second-order stimuli; (2) Depth could be extracted from dichoptically mixed first- and second-order stimuli, however, the same stimuli, when presented as a motion sequence, did not result in a motion percept. Based upon these findings we conclude that the transient-stereo system processes both first- and second-order signals, and that these two signals are pooled prior to the extraction of transient depth. This finding of interaction between first- and second-order stereoscopic processing is different from the independence that has been found with the motion system.

Depth Perception↗

PubMed1999

Orientation and luminance polarity tuning of the transient-vergence system.

Previously, Edwards, Pope & Schoor, Vision Research, 38, 705-717, demonstrated that transient disparity vergence appears to be mediated by a system that employs a single low-pass sensitive spatial channel whose performance is not reduced by dichoptic mixed contrasts (no contrast paradox) or dichoptic mixed spatial frequencies. This broadband tuning to both contrast and spatial frequency may be indicative of a second-order or non-linear envelope extraction system. The current study tests for lack of tuning to orientation and luminance polarity which are typically taken as evidence of a second-order system. We found that when the transient vergence system was simultaneously presented with both convergent and divergent disparities, there was a small but distinct bias in favor of responding in the direction defined by matched orientations or luminance polarities over unmatched pairs. Although less frequent, responses to orthogonal carriers or opposite luminance polarities were possible. The vergence system could match a horizontal with a vertical carrier, or a light gaussian with a dark gaussian. The degree of orientation or luminance polarity tuning varied inversely with the disparity magnitude over the range of 2.5-5 degrees, and the orientation tuning peaked at a spatial frequency about 2 cpd. At all disparities tested, however, the tuning was very broad, and other candidate features for mediating transient-vergence need to be investigated.

Contrast Sensitivity↗

PubMed1999

Orientation tuning of the transient-stereopsis system.

Stereo-perception appears to be mediated by at least two systems: a transient system that processes stimuli presented briefly and a sustained one that processes stimuli presented for longer durations. In this paper we investigated the tuning of the transient-stereopsis system to stimulus orientation. Narrowband-gabor targets with a constant envelope size (Gaussian standard deviation of 1 degree) were presented for brief (140 ms) durations at large (from 4 to 8 degrees) disparities. The results were as follows: (1) while observers could extract depth from orthogonally-oriented gabors at above chance levels, their performance was worse than that with gabors of matched orientation; (2) varying the relative contrasts of the two orthogonally oriented gabors of the same spatial frequency resulted in a reduction in performance; (3) varying the relative spatial frequencies of the orthogonally-oriented gabors impaired performance, relative to that for matched frequencies; and (4) varying the relative contrasts of orthogonal gabors that were at different spatial frequencies could improve performance. These results indicate that transient stereo-performance in the orthogonal condition was not mediated by the channels that extracted depth in either the horizontal- or vertically-matched gabor conditions. This apparent lack of orientation tuning is indicative of a second-order pathway. That this performance was mediated by a binocular, as opposed to a monocular channel, is supported by the finding that performance decreased as the contrast of one of the gabors was reduced. The finding that performance with orthogonal gabors of unmatched spatial frequency (0.5 and 4 cpd) could be improved by varying their relative contrasts suggests that the binocular spatial-frequency tuning exhibited by this channel is broadband in nature. Finally, the observation that lowering the contrast of either the high or low spatial-frequency gabor improved performance suggests the presence of at least two broadband channels: one with its peak sensitivity at a low and the other at a high spatial-frequency.

Contrast Sensitivity↗

PubMed1999

Extraction of depth from opposite-contrast stimuli: transient system can, sustained system can't.

The ability of observers to extract depth from opposite luminance-contrast-polarity stimuli was investigated. The stimuli consisted of two dichoptic-pairs of Gaussians, with one of the Gaussians in each pair having a positive contrast-polarity and the other a negative contrast-polarity. Stimulus durations ranging from 0.2 to 4 s were used. This range of durations was employed to reveal stereo mechanisms that were preferentially sensitive to transient or sustained stimuli. Stimuli were presented in a raised-cosine temporal envelope. Performance with stimuli of the same contrast-polarity was also tested. Observers could easily perceive depth with the same-polarity stimuli, at both long and short durations. Depth could be perceived with low-contrast opposite-polarity stimuli only at short durations. However, depth could be perceived with long-duration stimuli presented within a raised cosine temporal-envelope if a high contrast was used. Depth could also be perceived with low-contrast long-duration stimuli if they were presented within a rectangular temporal-envelope. These findings suggest there are separate sustained and transient mechanisms for stereopsis and that the transient-stereoscopic system can extract depth from opposite-contrast stereograms while the sustained system cannot. Further, it is likely that depth perception with opposite-contrast stereograms found in many previous studies was mediated by the transient-stereopsis system.

Contrast Sensitivity↗

PubMed1999

Noise and its effects on photoreceptor temporal contrast sensitivity at low light levels.

We studied photoreceptors in the locust (Schistocerca americanus) visual system to determine the extent to which quantal noise and intrinsic neural noise limit temporal sensitivity. Typical computational models of the temporal contrast sensitivity function are deterministic, reflect only filter characteristics, and lack explicit noise sources [J. Opt. Soc. Am. 58, 1133 (1968); Vision Res. 32, 1373 (1992)]. We report here that the temporal contrast sensitivity function, at low light levels, is not simply the reflection of a filter function. Our evidence suggests that, at low backgrounds, noise, in conjunction with temporal filtering, plays a role in shaping the temporal contrast sensitivity function. At a given low adaptation level, quantal noise limits sensitivity at low temporal frequencies, while intrinsic noise limits sensitivity at relatively higher temporal frequencies.

Animals↗

PubMed1999

Noise location and the slope of the psychometric function for simple motion stimuli.

We measured subject performance as a function of luminance for both detection and discrimination of increment stimuli; some were static, and some were arranged to give two-step apparent motion. Our aim was to examine a prediction for the shape of the psychometric function for motion: an accelerating function due to the presence of a multiplicative nonlinearity contained in many low-level motion models. For the tasks with static stimuli we found psychometric function slopes (of log d' versus log luminance plots) between 1.9 and 2.4 in two subjects, as previously reported. For the tasks with apparent motion stimuli in the same range of detectability, however, the slopes are between 1.2 and 1.7. The lower slopes indicate that many low-level motion models are either incorrect or incomplete as currently specified, and changes in nonlinearities and noise placement are discussed.

Artifacts↗

PubMed1998

Luminance contrast and spatial-frequency tuning of the transient-vergence system.

Vergence has transient components that are stimulated by brief presentations of stimuli at large disparities (up to several degrees). The question that we have addressed is what stimulus features are encoded by this system. A competition paradigm [Jones & Kerr, (1972)]. Vision Research, 12, 1425-1430) was used in which three gabors were presented. A single Gabor was presented to the fovea of one eye and two gabors, 2.5 deg to either side of the fovea, to the other; one of which, when paired with the single Gabor defined a convergent direction, the other a divergent direction. First we determined if increasing the luminance contrast of the Gabor pair whose disparity was opposite to the observer's response-bias direction (variable-contrast pair) relative to the remaining Gabor (reference) could alter the observer's response direction. Secondly, we determined if the contrast required for such a change in response was affected by the relative spatial frequency of the convergent and divergent Gabors. The reference Gabor was held at 2 cpd and the variable Gabor pair was varied between 5.6 and 0 (a gaussian) cpd. Results demonstrated that increasing the luminance contrast of the variable pair relative to the reference Gabor could alter the observer's response direction, even when the contrast of only one of the variable-pair Gabors was increased. The luminance contrast required for this change to occur was directly related to the spatial frequency of the variable pair over the entire frequency range tested. Vergence responses were preferentially made to lower spatial frequencies, even when a low spatial frequency was pared with a high one. We conclude that transient-vergence responses are not reduced by mixed contrasts (i.e. no contrast-paradox effect) and appear to be mediated by a system that employs a single lowpass sensitive channel.

Contrast Sensitivity↗

PubMed1998

Spatial-frequency and contrast tuning of the transient-stereopsis system.

The tuning of the transient-stereopsis system to luminance contrast and spatial-frequency (SF) was investigated with narrow-band gabor targets with a constant sigma of 1 degree. They were presented for brief (140 ms) durations and subtended a large (6 degrees) disparity. When dichoptic gabor stimuli were matched in SF (0-5 cpd), transient stereo performance was either uniform across SF or greater at frequencies below 1 cpd. When dichoptic stimuli had unmatched SF (0.5 + 0-5 cpd) and matched contrast (100%), stereo performance was impaired below that of the matched SF condition. Stereo performance with matched SF at 0.5 cpd was impaired when contrast of one eye's image was reduced, demonstrating a contrast-paradox effect (i.e. contrast tuning) for transient stereopsis. Performance with three dichoptic unmatched SF conditions (0.5 and 1.0 cpd; 0.5 and 5.0 cpd; 1.5 and 3.5 cpd) was improved when the contrasts of the low SF gabor was reduced while holding the contrast of the high SF gabor constant at 100%. However stereo performance was not improved by reducing the contrast of a high SF gabor (3.5 cpd) while holding the contrast of the lower SF gabor (1.5 cpd) constant at 100%. We interpret these findings as indicating that transient-stereopsis performance is mediated by a single spatial-channel that has low-pass spatial-frequency sensitivity and which compares the ocular based signals prior to binocular combination so that signals that are not balanced in terms of their strength lead to a weaker binocular signal, as per the model proposed by Kontsevich and Tyler (Vis Res 1994; 3417:2317-2329) for sustained stereopsis.

Contrast Sensitivity↗

PubMed1996

Relative efficiency for the detection of apparent motion.

We measured the relative efficiency for motion and position discriminations of brief, localized spot stimuli with a technique that makes no assumptions about sites of noise or information loss in the visual system. In one task, the observer had to discriminate whether an increment was located at one (left) or another (right) closely spaced spots. In the other task, the observer had to discriminate two successive brief increments of the left spot from a left spot increment followed by a right spot increment. Ideal observer theory predicts identical performance on the two tasks. Observers' thresholds, however, were significantly lower in the motion task at all intervals between flashes (ISIs) less than 60 msec in one observer and all ISIs less than 150 msec in two other observers (P < 0.01, t-test). We conclude that this apparent motion stimulus is seen more efficiently than a non-moving stimulus, and that the higher efficiency may be due to use of a motion sensitive channel in addition to independent position sensitive channels.

Discrimination, Psychological↗