The role of posterior parietal cortex in visually guided reaching movements in humans.

Positron emission tomography (PET) was used to identify the brain areas involved in visually guided reaching by measuring regional cerebral blood flow (rCBF) in six normal volunteers while they were fixating centrally and reaching with the left or right arm to targets presented in either the right or the left visual field. The PET images were registered with magnetic resonance images from each subject so that increases in rCBF could be localized with anatomical precision in individual subjects. Increased neural activity was examined in relation to the hand used to reach, irrespective of field of reach (hand effect), and the effects of target field of reach, irrespective of hand used (field effect). A separate analysis on intersubject, averaged PET data was also performed. A comparison of the results of the two analyses showed close correspondence in the areas of activation that were identified. We did not find a strict segregation of regions associated exclusively with either hand or field. Overall, significant rCBF increases in the hand and field conditions occurred bilaterally in the supplementary motor area, premotor cortex, cuneus, lingual gyrus, superior temporal cortex, insular cortex, thalamus, and putamen. Primary motor cortex, postcentral gyrus, and the superior parietal lobule (intraparietal sulcus) showed predominantly a contralateral hand effect, whereas the inferior parietal lobule showed this effect for the left hand only. Greater contralateral responses for the right hand were observed in the secondary motor areas. Only the anterior and posterior cingulate cortices exhibited strong ipsilateral hand effects. Field of reach was more commonly associated with bilateral patterns of activation in the areas with contralateral or ipsilateral hand effects. These results suggest that the visual and motor components of reaching may have a different functional organization and that many brain regions represent both limb of reach and field of reach. However, since posterior parietal cortex is connected with all of these regions, we suggest that it plays a crucial role in the integration of limb and field coordinates.

Covert orienting of attention in macaques. II. Contributions of parietal cortex.

1. To understand some of the contributions of parietal cortex to the dynamics of visual spatial attention, we recorded from cortical cells of monkeys performing attentional tasks. We studied 484 neurons in the intraparietal sulcus and adjacent gyral tissue of two monkeys. We measured phasic responses to peripheral visual stimuli while the monkeys attended toward or away from the stimuli or when attention was not controlled. Neurons were tested while the monkeys gazed at a spot of light (simple fixation task), actively attended to a foveal target (foveal attention task), performed a reaction time task (cued reaction time task), made saccadic eye movements to visual targets (saccade task), or responded to a repetitious peripheral target (probability task). 2. In a previous paper we demonstrated that monkeys, like humans, responded more quickly to visual targets when the targets followed briefly flashed visual cues (validly cued targets) (Bowman et al. 1993). It has been hypothesized that the cue attracts attention to its locus and results in faster reaction times (Posner 1980). In the present physiological studies, visual cues consistently excited these neurons when they were flashed in the receptive field. Such activity might signal a shift of attention. Visual targets that fell within the receptive field and that immediately followed the cue evoked relatively weak responses. This response was due to a relative refractory period. 3. Next we tested attentional processes in these tasks that were independent of the visual response to the cue. We placed the cue outside of the receptive field and the target within the receptive field. We found that 23% of these cells had a significant decrease in their firing rate to validly cued targets in their receptive fields under these conditions. Strong responses were evoked by the same target when the cue was flashed in the opposite hemifield (invalidly cued targets). Thus this group of neurons responded best when attention was directed toward the opposite hemifield. 4. For another group of parietal cells (13%) there was an enhanced response to targets in the visual receptive field when the cue was in the same hemifield. For the remaining 64% of the cells there was no significant modulation in this task. 5. The cued reaction time task involved exogenous control of attention; the sensory cue gave spatial and temporal direction to attention. We used several other tasks to test for endogenous control of attention.(ABSTRACT TRUNCATED AT 400 WORDS)

Covert orienting of attention in macaques. III. Contributions of the superior colliculus.

1. The present experiments were conducted to study physiological mechanisms in the superior colliculus and their relation to visual spatial attention. We used a cued reaction time task studied in detail previously (Bowman et al. 1993; Posner 1980). Monkeys learned to fixate a spot of light and release a bar when a target light appeared. Cues on the same side as the target (valid cue) were associated with faster reaction times than those on the opposite side (invalid cue). The difference in reaction times is hypothesized to be a measure of attention. 2. A total of 79 neurons within the superficial layers of the superior colliculi of two monkeys were studied. When the cues and targets were positioned so that both were within the visual receptive field, the cues excited the cells, and this produced a refractoriness to the targets for the following 400 ms. Both the ON and OFF responses to the cue were constant under all conditions. 3. These neurons were also tested with the cue just outside of the visual receptive field. This was done to avoid refractory effects from the cue; there was no significant modulation of the response to the target under these conditions. The visual responses of neurons in the intermediate layers of the superior colliculus also responded equivalently under these conditions. 4. When the activity of cells within the foveal representation was compared during the performance of three tasks, there was differential activity. The appearance of the fixation point during the performance of the cued reaction time task led to a strong, transient discharge.(ABSTRACT TRUNCATED AT 250 WORDS)

Head movement in normal subjects during simulated PET brain imaging with and without head restraint.

UNLABELLED: Head movement during brain imaging is recognized as a source of image degradation in PET and most other forms of medical brain imaging. However, little quantitative information is available on the kind and amount of head movement that actually occurs during these studies. We sought to obtain this information by measuring head movement in normal volunteers. METHODS: Head position data were acquired for 40 min in each of 13 supine subjects with and without head restraint. These data were then used to drive a mathematically simulated head through exactly the same set of movements. The positions of point sources embedded in this head were computed at each location and these data summarized as movement at FWHM in each of the three coordinate directions. RESULTS: Head movement increased with the length of the sampling interval for studies of either type (with or without head restraint), but the amount and rate of increase with restraint was much smaller. In contrast, head movement during consecutive, short sampling intervals was small and did not increase with time. Spatial gradients in head movement were detected within each study type, and significant spatial differences in head movement were found between study types. CONCLUSIONS: Head movements in normal, supine subjects, though small, can cause the effective resolution of a brain imaging study to appear to vary in space and time. These effects can be reduced significantly with head restraint and may also be reduced by dividing the acquisition of a single image into a sequence of short images (instead of a single long image), aligning these images spatially and summing the result.

Covert orienting of attention in macaques. I. Effects of behavioral context.

1. A task was used by Posner (1980) to measure shifts of attention that occurred covertly, in the absence of an eye movement or other orienting response. This paradigm was used here to assess the nature of covert attentional orienting in monkeys to develop an animal model for neurophysiological studies. Shifts of attention were measurable in monkeys and were consistent across a variety of experimental conditions. 2. The paradigm required that monkeys fixate and release a bar at the appearance of a target, which was preceded by a cue. Reaction times to targets that followed peripheral cues at the same location (validly cued) were significantly faster than those that followed cues in the opposite visual field (invalidly cued). This difference was defined as the validity effect, which as in humans, is used as the measure of a covert attentional shift. 3. When the proportion of validly to invalidly cued targets was decreased, no change was seen in the validity effect of the monkeys. This is in contrast to humans, for whom the ratio of validly to invalidly cued targets affected the magnitude of the validity effect. When 80% of the targets were preceded by cues at the same location, the validity effect was greatest. The effect was reversed when the proportions were reversed. From this result, it is concluded that cognitive processes can affect covert orienting to peripheral cues in humans, whereas in trained monkeys, performance was automatic. 4. To test whether cognitive influences on attention could be demonstrated in the monkey, an animal was taught to use symbolic, foveal signals to covertly direct attention. The magnitude of this validity effect was greater than that obtained with peripheral cues. 5. The effects of motivational and perceptual processes were tested. Although overall reaction times could be modified, the facilitating effects of the cues persisted. This constancy across motivational and perceptual levels supports the notion that the monkeys were performing the task in an automatic way, under the exogenous control of peripheral cues. 6. Most visual cuing has been tested with visual landmarks at the locations of cues and targets. These monkeys were trained with such landmarks, and when tested without them, the attentional effect of the cues was nearly abolished. These data suggest that local visual features can be important for covert orienting. 7. To determine the spatial extent of the effect of the cue, monkeys and humans were tested with four cue-target distances (0-60 degrees).(ABSTRACT TRUNCATED AT 400 WORDS)

Visual responses of pulvinar and collicular neurons during eye movements of awake, trained macaques.

1. We recorded from single neurons in awake, trained rhesus monkeys in a lighted environment and compared responses to stimulus movement during periods of fixation with those to motion caused by saccadic or pursuit eye movements. Neurons in the inferior pulvinar (PI), lateral pulvinar (PL), and superior colliculus were tested. 2. Cells in PI and PL respond to stimulus movement over a wide range of speeds. Some of these cells do not respond to comparable stimulus motion, or discharge only weakly, when it is generated by saccadic or pursuit eye movements. Other neurons respond equivalently to both types of motion. Cells in the superficial layers of the superior colliculus have similar properties to those in PI and PL. 3. When tested in the dark to reduce visual stimulation from the background, cells in PI and PL still do not respond to motion generated by eye movements. Some of these cells have a suppression of activity after saccadic eye movements made in total darkness. These data suggest that an extraretinal signal suppresses responses to visual stimuli during eye movements. 4. The suppression of responses to stimuli during eye movements is not an absolute effect. Images brighter than 2.0 log units above background illumination evoke responses from cells in PI and PL. The suppression appears stronger in the superior colliculus than in PI and PL. 5. These experiments demonstrate that many cells in PI and PL have a suppression of their responses to stimuli that cross their receptive fields during eye movements. These cells are probably suppressed by an extraretinal signal. Comparable effects are present in the superficial layers of the superior colliculus. These properties in PI and PL may reflect the function of the ascending tectopulvinar system.

Effects of physostigmine on spatial attention in patients with progressive supranuclear palsy.

We tested patients with progressive supranuclear palsy and control subjects on a task of visuopatial attention. Targets preceded by cues on the same side were termed validly cued; and those on the opposite side, invalidly cued. For all subjects, validly cued targets were responded to faster than those that were invalidly cued. The difference between reaction times for invalidly and validly cued targets, which is hypothesized to measure attentional movement, was significantly increased for the patients. The performance of the controls on certain neuropsychological tests was correlated with their attentional ability. These correlations were altered by progressive supranuclear palsy. Physostigmine treatment of the patients induced a speeding of responses to invalidly cued targets as a function of the duration of the disease. These studies show defects in cognition and attention in patients with progressive supranuclear palsy and demonstrate that physostigmine reduces some of the abnormal visual attentional performance.

Abnormalities in visual spatial attention in men with mirror movements associated with isolated hypogonadotropic hypogonadism.

We studied spatial attentional performance on a visually cued reaction time task in men with isolated hypogonadotropic hypogonadism. A subset of these patients, who displayed mirror movements, have spatial attentional abnormalities. They were slow to respond to targets in the right visual field and especially slow when those targets followed incorrect or diffuse cues. This slowing was present for at least 500 msec after cue onset. They responded equally to targets in the left visual field independent of the spatial cues. The patient population as a whole was significantly faster than controls across all experimental conditions, although the speed of their attentional movement was normal. These data suggest that patients with isolated hypogonadotropic hypogonadism perform reaction time tasks quickly, that faster reaction times do not reflect superior attentional performance, and that a subset of these patients has a spatial attentional abnormality.

Visuospatial attention: effects of age, gender, and spatial reference.

Visual attention is remarkably stable when spatial cuing is used, but non-spatial cues lead to slowing among females and older subjects. Non-spatial cues are associated with poorer performance during the middle stages of the menstrual cycle. Motivation increased overall response speed but not attentional measures, whereas increasing age was associated with generalized slowing and directional asymmetries. Right-eye dominance was correlated with slow responses to downward targets. These data suggest that attentional performance is modified by age, gender, and endocrine status when spatial reference is not present.

Orbital position and eye movement influences on visual responses in the pulvinar nuclei of the behaving macaque.

We studied the influences of eye movements on the visual responses of neurons in two retinotopically organized areas of the pulvinar of the macaque. Cells were recorded from awake, trained monkeys, and visual responses were characterized immediately before and after the animals made saccadic eye movements. A significant proportion of the cells were more responsive to stimuli around the time of eye movements than they were at other intervals. Other cells had response reduction. For some neurons, the change in excitability was associated with orbital position and not the eye movement. For other cells the change was present with eye movements of similar amplitude and direction but with different starting and ending positions. Here it appears that the eye movement is the important parameter. Other cells had effects related to both eye position and eye movements. In all cells tested, the changes in excitability were present when the experiments were conducted in the dark as well as in the light. This suggests that the mechanism of the effect is related to the eye position or eye movement and not visual-visual interactions. For about half of the neurons with modulations, the response showed facilitation for stimuli presented in the most responsive region of the receptive field but not for those at the edge of the field. For the other cells there was facilitation throughout the field. Thus, a gradient of modulation in the receptive field may vary among cells. These experiments demonstrate modulations of visual responses in the pulvinar by eye movements. Such effects may be part of the visual-behavioral improvements at the end of eye movements and/or contribute to spatial constancy.

Size-difference thresholds after lesions of thalamic visual nuclei in pigeons.

Nucleus rotundus and nucleus dorsolateralis posterior (DLP) are the thalamic components of two parallel pathways within the tectofugal division of the pigeon visual system. An earlier study (Hodos, Weiss & Bessette, 1986) had shown that lesions in direct telencephalic recipients of projections from rotundus and DLP produced postoperative elevations in size-difference thresholds only if the lesion included both structures. What was not revealed by their study was whether the integrity of both thalamic components is necessary for pigeons to discriminate small differences in the size of stimuli or whether the birds could still make the discrimination with only one of the two nuclei intact. This question was particularly important because no prior behavioral evidence existed to indicate that DLP plays a role in visual information processing. Therefore, 14 pigeons were tested preoperatively using a variant of the method of constant stimuli to determine the smallest difference between the size of two annuli that the subjects could discern. The comparison stimuli, which were presented in a successive discrimination procedure, ranged from 3.5-15 mm in diameter. After surgery, in which lesions were placed bilaterally in rotundus, DLP, or both structures, the subjects' size-difference thresholds were again determined. Combined lesions of rotundus and DLP resulted in impaired psychophysical performance. The postoperative behavior was characterized by initial elevations in threshold followed by a gradual improvement in performance. Some birds returned to their preoperative level. By comparison, subjects with lesions in rotundus or DLP alone showed an immediate return to their preoperative sensitivity level. These results indicate that both nuclei can process information about the size of visual stimuli. Moreover, the processing that occurs within either nucleus is sufficient for the pigeon to discriminate size differences. The present experiment provides the first behavioral evidence that DLP participates in visual information processing.