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Biomedical subjects

R Fendrich

Publications and source records attributed to R Fendrich.

14 recordsLinked to original sources
PubMed1992

Residual vision in a scotoma: implications for blindsight.

Blindsight, the ability of some blind patients to describe attributes of stimuli they have no conscious awareness of seeing, has been attributed to a secondary (retinotectal) visual pathway. However, it has also been proposed that blindsight could be due to residual function within the primary (geniculostriate) visual pathway. Data have now been obtained that support the second alternative. With an image stabilizer ensuring the accurate retinal placement of stimuli, dense visual field mapping was carried out with a hemianopic patient. This perimetry revealed, embedded in the patient's scotoma, an isolated 1-degree island of residual vision that was not disclosed by conventional perimetric methods. Stimuli presented to this island could be detected and discriminated, although the subject reported he did not see them. The existence of this island of vision implies a corresponding island of functioning cortex within the patient's lesion. Other instances of blindsight may be mediated by similar islands of functioning cortex.

Eye Movements↗
PubMed1992

Bidirectional control of saccadic eye movements by the disconnected cerebral hemispheres.

The present investigation demonstrates that callosotomy patient J.W. can generate either leftward or rightward saccades in response to color cues presented unilaterally. When asked to name the colors, performance was at chance for left visual field presentations, demonstrating a disability in interhemispheric transfer of chromatic information. The successful control of saccadic direction based on discriminative color cues that appear confined to a single hemisphere may suggest a capacity for bidirectional control of saccadic eye movements in the disconnected cerebral hemispheres.

Brain↗
PubMed1992

Oculomotor readiness and covert orienting: differences between central and peripheral precues.

The costs produced by invalid precues can depend on the spatial relationship between the cued location and the target location. If oculomotor programs mediate attention shifts, then the effect of varying the spatial relation between the cue and target should be the same for covert orienting (indexed by manual responses) and saccadic responses. We found this to be true only for central precues. With central precues, both manual and saccadic costs were greater when cue and target occurred on opposite sides of the vertical meridian than when they occurred on the same side. With peripheral precues, there were no meridian effects in either response condition, but there was a significant dissociation in the pattern of saccadic and manual costs. For manual responses costs were greater when the target was eccentric relative to the cue, whereas for saccades costs were greater when the cue was eccentric to the target. These results provide additional support for the idea that different orienting mechanisms are engaged by central and peripheral precues. They further suggest that the relationship between oculomotor and attentional orienting may depend on the nature of the precue, with the potential for interdependence being greater with central precues.

Adult↗
PubMed1991

The reduction of saccadic latency by prior offset of the fixation point: an analysis of the gap effect.

The latency to initiate a saccade (saccadic reaction time) to an eccentric target is reduced by extinguishing the fixation stimulus prior to the target onset. Various accounts have attributed this latency reduction (referred to as the gap effect) to facilitated sensory processing, oculomotor readiness, or attentional processes. Two experiments were performed to explore the relative contributions of these factors to the gap effect. Experiment 1 demonstrates that the reduction in saccadic reaction time (RT) produced by fixation point offset is additive with the effect of target luminance. Experiment 2 indicates that the gap effect is specific for saccades directed toward a peripheral target and does not influence saccades directed away from the target (i.e., antisaccades) or choice-manual RT. The results are consistent with an interpretation of the gap effect in terms of facilitated premotor processing in the superior colliculus.

Adult↗
PubMed1991

Fixation-point offsets reduce the latency of saccades to acoustic targets.

If an observer's fixation point is extinguished just prior to the onset of a peripheral target, the latency to saccade to that target is reduced. We show that this "gap effect" is not specific to visual targets. Observers made saccades to a light flash or to a white-noise burst. A warning tone was presented on every trial to control for the possible warning effect of the fixation-point offset. For both target modalities, saccade latencies were significantly reduced when the fixation point was extinguished 200 msec prior to the target onset. Implications of this outcome for interpretations of the gap effect are considered. It is argued that the presence of a gap effect for tones, in conjunction with previous findings, is consistent with the hypothesis that the gap effect is produced by a facilitation of premotor processes in the superior colliculus.

Attention↗
PubMed1990

Hemispheric processing of spatial frequencies in two commissurotomy patients.

To test the hypothesis that in humans the left brain hemisphere is specialized for processing high spatial frequencies while the right hemisphere is specialized for processing low spatial frequencies, pairs of Gaussian windowed sinusoidal gratings were presented for 167 msec within the left and right visual fields of two commissurotomy patients. The gratings employed had spatial frequencies ranging from 1 to 8 cycles per degree, and horizontal or vertical orientations. The two gratings in each pair were identical in spatial frequency but could differ in orientation. Subjects reported if their orientations were the same or different. Twelve normal controls were also run. Accuracy data provides no indication of a relative advantage for high frequencies in the RVF or low frequencies in the LVF. One commissurotomy subject showed a trend in the reverse direction; the other was better with LVF presentations for all spatial frequencies. Control subjects failed to show a spatial frequency x visual field interaction. These outcomes suggest that at the processing stages required by the task, the hemispheres are not specialized for particular ranges of spatial frequencies.

Adult↗
PubMed1989

Evidence of foveal splitting in a commissurotomy patient.

A bilateral projection of the central fovea, produced by naso-temporal overlap at the retinal vertical meridian, has been proposed as the anatomical basis of macular sparing. To evaluate this claim, a commissurotomy patient was required to compare target figures presented 1 degrees or less from the retinal midline with reference figures presented 2.5 degrees from the midline in the same or opposite visual field. The subject judged whether the target and reference were the same or different. It was found that the subject's fovea was effectively split with respect to the cerebral hemispheres; targets in the same visual field as the reference were readily matched with the reference, but accuracy dropped to chance for targets in the opposite field. Ways of reconciling this data with anatomical evidence of nasotemporal overlap and reports of macular sparing are discussed.

Adult↗
PubMed1989

Human callosal function: MRI-verified neuropsychological functions.

A commissurotomy patient, with MRI-revealed sparing of some rostral and splenial fibers of the corpus callosum, judged whether pairs of words rhymed. We presented one word in each pair to her left visual field and the other to her right visual field. The 2 words in each pair either sounded and looked alike (R + L +), sounded alike but looked different (R + L -), sounded different but looked alike (R - L +), or both sounded and looked different (R - L -). Although in previous studies the patient has demonstrated little or no ability to transfer information between her brain hemispheres, she was able to perform the rhyming judgment significantly better than chance when the words both looked and sounded alike. However, her accuracy did not differ from chance in the other 3 conditions, or when she was asked to indicate if 2 letters presented to her opposing visual fields were the same or different. A second commissurotomy patient, with an MRI-verified full callosal section, performed at chance in all conditions, and normal control subjects were significantly better than chance in all conditions but R + L -. We discuss the results in terms of the specificity of the information carried by groups of callosal fibers.

Adult↗
PubMed1985

Perceived position and saccadic eye movements.

Data are reported which support the conclusion that saccades which occur 600 msec or more after the brief, presentation of a target stimulus are directed to its perceived position when that differs from both its retinal and spatial position.

Animals↗
PubMed1985

Induced motion and oculomotor capture.

Three experiments investigating the basis of induced motion are reported. The proposition that induced motion is based on the visual capture of eye-position information and is therefore a subject-relative, rather than object-relative, motion was explored in the first experiment. Observers made saccades to an invisible auditory stimulus following fixation on a stationary stimulus in which motion was induced. In the remaining two experiments, the question of whether perceived induced motion produces a straight ahead shift was explored. The critical eye movement was directed to apparent straight ahead. Because these saccades partially compensated for the apparent displacement of the induction stimulus, and saccades to the auditory stimulus did not, we conclude that induced motion is not based on oculomotor visual capture. Rather, it is accompanied by a shift in the judged direction of straight ahead, an instance of the straight ahead shift. The results support an object-relative theory of induced motion.

Attention↗
PubMed1979

Smooth pursuit eye movements: is perceived motion necessary?

It has recently been shown that perceived motion, in the absence of any appropriate retinal motion, is a sufficient stimulus to generate smooth pursuit eye motions. This raises the question of whether perceived motion is necessary for pursuit. In three experiments we obtained a negative answer to this question: retinal motion always governed pursuit.

Eye Movements↗