Robert Fendrich

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 thegap 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.

Visual-auditory interactions in sensorimotor processing: saccades versus manual responses

Reaction times (RTs) to bimodal (visual and auditory) stimuli were examined using 3 different response systems: saccades, directed manual responses, and simple manual responses. The observed levels of intersensory facilitation exceeded race model predictions and therefore support summation (coactivation) models of bimodal processing. However, response-dependent differences suggest that the processing of bimodal targets also depends on the relevant sensorimotor pathways and requirements of the task. Coactivation of response mechanisms might account for the effects found using simple RTs. The results for saccades are consistent with known patterns of auditory-visual convergence in the oculomotor system.

The temporal cross-capture of audition and vision.

We report that when a flash and audible click occur in temporal proximity to each other, the perceived time of occurrence of both events is shifted in such a way as to draw them toward temporal convergence. In one experiment, observers judged when a flash occurred by reporting the clock position of a rotating marker. The flash was seen significantly earlier when it was preceded by an audible click and significantly later when it was followed by an audible click, relative to a condition in which the flash and click occurred simultaneously. In a second experiment, observers judged where the marker was when the click was heard. When a flash preceded or followed the click, similar but smaller capture effects were observed. These capture effects may reveal how temporal discrepancies in the input from different sensory modalities are reconciled and could provide a probe for examining the neural stages at which evoked responses correspond to the contents of conscious perception.

Global versus local processing in the absence of low spatial frequencies

When observers are presented with hierarchical visual stimuli that contain incongruous coarse (global) and fine (local) pattern attributes, the global structure interferes with local pattern processing more than local structure interferes with global pattern processing. This effect is referred to as global precedence. The present experiments tested the hypothesis that global precedence depends on the presence of low spatial frequencies using stimuli constructed from contrast balanced dots. Stimuli composed of contrast balanced dots are largely devoid of low-frequency content. Choice reaction time to identify either the local or global pattern information was the dependent measure. Global precedence was found only for control stimuli that contained low spatial frequencies. In the absence of low-frequency information, local precedence was obtained. These findings suggest that global precedence is heavily dependent on the low spatial frequency content of the patterns.

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.

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 subjects 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.

Nasotemporal overlap at the retinal vertical meridian: investigations with a callosotomy patient.

To evaluate nasotemporal overlap at the retinal vertical midline, we asked a callosotomy patient to compare the orientation of square wave gratings that were presented in his opposing visual fields. The gratings were horizontally or vertically oriented and had spatial frequencies ranging from 1 to 8 cycles per degree (cpd). Retinal stabilization ensured the gratings remained properly lateralized during sustained presentations. In accord with previous investigations, when the gratings were presented for only 200 msec, or their medial edges were 2 from the vertical meridian, performance was generally at chance. However, when presentations lasted 2 sec and the medial edges of the gratings were 1 from the vertical meridian, above chance performance was obtained. Accuracy rates were highest with 2 and 4 cpd gratings, and dropped at 1 and 8 cpd. Unexpectedly, the subject performed significantly better when the gratings were offset vertically from each other than when both were displayed on the horizontal meridian. Since this improvement did not occur when gratings were presented horizontally aligned above or below the horizontal meridian, it must be distributed to the relative offset between the gratings. The data suggest a narrow zone of nasotemporal overlap at the retinal vertical midline where very limited visual information is encoded by crossed temporal and uncrossed nasal retinal ganglion cells. An experiment in which only one grating in a pair was close to the vertical meridian indicates that this zone may be more pronounced in the nasal hemiretina.

Subfield-specific loss of hippocampal N-acetyl aspartate in temporal lobe epilepsy.

Purpose: In patients with mesial temporal lobe epilepsy (MTLE) it remains an unresolved issue whether the interictal decrease in N‐acetyl aspartate (NAA) detected by proton magnetic resonance spectroscopy (1H‐MRS) reflects the epilepsy‐associated loss of hippocampal pyramidal neurons or metabolic dysfunction.

Islands of residual vision in hemianopic patients

Following damage to primary visual cortex, some patients our initial findings. The data reveal a patchy distribution of redemonstrate a limited ability to respond to stimuli they do not sidual visual abilities in the absence of acknowledged awareness. acknowledge seeing. This residual vision, which has been referred to as blindsight, has been attributed to secondary visual pathways. We previously reported an isolated island of blindsight in one patient and argued it was a likely consequence of cortical sparing in V1. We now report an extension of our initial findings. The data reveal a patchy distribution of residual visual abilities in the absence of acknowledged awareness. Variable patterns of cortical sparing appear to be the most parsimonious way to account for this outcome, suggesting that blindsight is generally mediated by the primary visual pathway.

Illusory Contour Perception and Amodal Boundary Completion: Evidence of a Dissociation Following Callosotomy

A fundamental problem in form perception is how the visual system can link together spatially separated contour fragments to form the percept of a unitary shape. Illusory contours and amodal completion are two phenomena that demonstrate this linking process. In the present study we investigate these phenomena in the divided hemispheres of two callosotomy (split-brain) patients. The data suggest that dissociable neural mechanisms are responsible for the generation of illusory contours and amodal completion. Although both cerebral hemispheres appear to be equally capable of perceiving illusory contours, amodal completion is more readily utilized by the right hemisphere. These results suggest that illusory contours may be attributable to low-level visual processes common to both hemispheres, whereas amodal completion reflects a higher-level, lateralized process.