Robert D. Rafal

Effects of parietal injury on covert orienting of attention

The cognitive act of shifting attention from one place in the visual field to another can be accomplished covertly without muscular changes. The act can be viewed in terms of three internal mental operations: disengagement of attention from its current focus, moving attention to the target, and engagement of the target. Our results show that damage to the parietal lobe produces a deficit in the disengage operation when the target is contralateral to the lesion. Effects may also be found on engagement with the target. The effects of brain injury on disengagement of attention seem to be unique to the parietal lobe and do not appear to occur with our frontal, midbrain, and temporal control series. These results confirm the close connection between parietal lobes and selective attention suggested by single cell recording. They indicate more specifically the role that parietal function has on attention and suggest one mechanism of the effects of parietal lesions reported in clinical neurology.

Inhibition of return: Neural basis and function

Abstract A goal of neuropsychology is to connect cognitive functions with underlying neural systems. Posner (1984; in press) has proposed a framework for doing so in which elementary mental operations in cognitive models are expressed in terms of component facilitations and inhibitions in the performance of normal persons. Studies of brain-injured patients are used to link these components to underlying neural systems. In the area of spatial attention one such component is the tendency to inhibit orienting towards visual locations which have been previously attended (inhibition of return). Here we report studies in patients and normals which demonstrate the relationship of this component to neural systems which generate saccades. The first experiment showed that midbrain lesions impairing saccade generation produced a concurrent loss of the inhibition of return, whereas cortical components shown to impair facilitatory components did not. The second and third experiments show that the inhibition of return ...

How do the parietal lobes direct covert attention

In cases of unilateral parietal damage patients have trouble with stimuli in the visual hemifield on the side opposite the lesion (contralesional). This deficit is clear when they are first cued to attend to a location on the side of the lesion (ipsilesional) and then given a target on the opposite side of fixation. Our first experiments indicated that these patients do worse when cued to a location in either field and then given a target in the contralesional as compared to the ipsilesional direction. The results of this experiment can be accounted for by a representational or a directional view. A second experiment seeks to discriminate between the two. The results tend to favor the directional view.

Spatial attention deficits in humans: a comparison of superior parietal and temporal-parietal junction lesions.

Although clinical evidence of spatial attention deficits, such as neglect and extinction, is typically associated with lesions of the right temporal-parietal junction, recent evidence has suggested an important role for the superior parietal lobe. Two groups of patients, selected for lesions at the temporal-parietal junction including the superior temporal gyrus (TPJ group), or for lesions involving the parietal but not the superior temporal region (PAR group), performed cued-target detection tasks in 2 experiments. An extinction-like response time pattern was found for the TPJ but not the PAR group. In addition, both groups were able to use expectancy information, in the form of cue predictiveness, suggesting that separate mechanisms mediate exogenous and endogenous processes during attention shifts.

Competition between endogenous and exogenous orienting of visual attention

The relation between reflexive and voluntary orienting of visual attention was investigated with 4 experiments: a simple detection task, a localization task, a saccade toward the target task, and a target identification task in which discrimination difficulty was manipulated. Endogenous and exogenous orienting cues were presented in each trial and their validity was manipulated orthogonally to examine whether attention mechanisms are mediated by separate systems and whether they have additive and independent effects on visual detection and discrimination. The results showed that each orienting mechanism developed its typical and independent effect in every case except for the difficult identification task. A theoretical framework for understanding the relationship between endogenous and exogenous orienting of attention is proposed, tested, and confirmed.

Neural fate of seen and unseen faces in visuospatial neglect: A combined event-related functional MRI and event-related potential study

To compare neural activity produced by visual events that escape or reach conscious awareness, we used event-related MRI and evoked potentials in a patient who had neglect and extinction after focal right parietal damage, but intact visual fields. This neurological disorder entails a loss of awareness for stimuli in the field contralateral to a brain lesion when stimuli are simultaneously presented on the ipsilateral side, even though early visual areas may be intact, and single contralateral stimuli may still be perceived. Functional MRI and event-related potential study were performed during a task where faces or shapes appeared in the right, left, or both fields. Unilateral stimuli produced normal responses in V1 and extrastriate areas. In bilateral events, left faces that were not perceived still activated right V1 and inferior temporal cortex and evoked nonsignificantly reduced N1 potentials, with preserved face-specific negative potentials at 170 ms. When left faces were perceived, the same stimuli produced greater activity in a distributed network of areas including right V1 and cuneus, bilateral fusiform gyri, and left parietal cortex. Also, effective connectivity between visual, parietal, and frontal areas increased during perception of faces. These results suggest that activity can occur in V1 and ventral temporal cortex without awareness, whereas coupling with dorsal parietal and frontal areas may be critical for such activity to afford conscious perception.

Visual extinction and stimulus repetition

Five patients with visual extinction following unilateral brain injury were briefly presented with colored letters in either or both visual fields, and required to report and locate the colors or the shapes. On double simultaneous stimulation, they tended to miss the event contralateral to their lesion. This extinction was increased when the two stimuli were the same on the reported dimension, Similarity on the irrelevant dimension had no effect. These data suggest that extinguished colors and shapes may be correctly extracted by the visual system (when task-relaant) even though they are unavailable for verbal report. An analogy is made with the phenomena of repetition blindness in normal observers, and it is proposed that extinction may reflect failure in a token-individuation process for correctly extracted visual types.

Endogenously generated and visually guided saccades after lesions of the human frontal eye fields

Nine patients with chronic, unilateral lesions of the dorso-lateral prefrontal cortex including the frontal eye fields (FEF) made saccades toward contralesional and ipsilesional fields. The saccades were either voluntarily directed in response to arrows in the center of a visual display, or were reflexively summoned by a peripheral visual signal. Saccade latencies were compared to those made by seven neurologic control patients with chronic, unilateral lesions of dorsolateral prefrontal cortex sparing the FEF, and by 13 normal control subjects. In both the normal and neurologic control subjects, reflexive saccades had shorter Latencies than voluntary saccades. In the FEF lesion patients, voluntary saccades had longer latencies toward the contralesional field than toward the ipsilesional field. The opposite pattern was found for reflexive saccades: latencies of saccades to targets in the contralesional field were shorter than saccades summoned to ipsilesional targets. Reflexive saccades toward the ipsilesional field had abnormally prolonged latencies; they were comparable to the latencies observed for voluntary Saccades. The effect of FEF lesions on saccacles contrasted with those observed in a second experiment requiring a key press response: FEF lesion patients were slower in making key press responses to signals detected in the contralesional field. To assess covert attention and preparatory set the effects of precues providing advance information were measured in both saccade and key press experiments. Neither patient group showed any deficiency in using precues to shift attention or to prepare saccades. The FEF facilitates the generation of voluntary saccatles and also inhibits reflexive saccades to exogenous signals. FEF lesions may disinhibit the ipsilesional midbrain which in turn may inhibit the opposite colliculus to slow reflexive saccades toward the ipsilesional field.

Extrageniculate contributions to reflex visual orienting in normal humans: A temporal hemifield advantage

Evidence is presented that the phylogenetically older retin-otectal pathway contributes to reflex orienting of visual attention in normal human subjects. The study exploited a lateralized neuroanatomic arrangement of retinotectal pathways that distinguishes them from those of the geniculostriate system; namely, more direct projections to the colliculus from the temporal hemifield. Subjects were tested under monocular viewing conditions and responded to the detection of a peripheral signal by making either a saccade to it or a choice reaction time manual keypress. Attention was summoned by noninformative peripheral precues, and the benefits and costs of attention were calculated relative to a central precue condition. Both the benefits and costs of orienting attention were greater when attention was summoned by signals in the temporal hemifield. This temporal hemifield advantage was present for both saccade and manual responses. These findings converge with observations in patients with occipital and midbrain lesions to show that the phylogenetically older retinotectal pathway retains an important role in controlling visually guided behavior; and they demonstrate the usefulness of temporal-nasal hemifield asymmetries as a marker for investigating extrageniculate vision in humans.

Deficits in spatial coding and feature binding following damage to spatiotopic maps in the human pulvinar

We report a patient with unilateral damage to the rostral part of the pulvinar who was impaired in localizing stimuli in the inferior visual field contralateral to the lesion and who made errors in the binding of shape and color in that quadrant. The findings demonstrate the importance of the pulvinar in spatial coding and provide support for the function of the thalamus in binding of features. They also provide evidence for a homology between the visual field maps of the inferior and lateral subdivisions of the pulvinar in monkeys and in humans, such that the inferior visual field is represented in the rostral part of the nucleus.