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PubMed1998

Perceptual and response bias in unilateral neglect: two modified versions of the milner landmark task.

Perceptual and response bias in estimating the proportion of the two segments of prebisected lines were disambiguated in a group of 121 patients suffering from left neglect by means of two variants of the Milner Landmark task (Milner et al., 1993). The first variant, LANDMARK-V, required a verbal response; the second variant, LANDMARK-M, required manual pointing. The paper reports and discusses the results obtained on each task and their correlations, as well as the relationships between either kind of bias and the intrahemispheric location of the lesion. It is argued that besides their usefulness as a diagnostic tool the proposed variants of the Milner Landmark task provide results that are worth further investigation in their own right.

Adult↗
PubMed1998

Position of the egocentric reference and directional arm movements in right-brain-damaged patients.

We asked 12 right-brain-damaged patients (6 with left neglect signs and 6 without left neglect signs) to perform a straight ahead pointing task and a visual detection task with lateralized motor response, in order to investigate the relationship between the position of the egocentric reference and response time and accuracy in producing lateralized arm movements. Results showed that there was no correlation between the position of the egocentric reference and neglect signs, nor between the position of the egocentric reference and the latencies to direct a motor response toward either side of space. These findings were interpreted within the context of egocentric hypotheses of neglect. In particular, it was suggested that attentional or intentional neglect signs cannot be considered as a direct consequence of an ipsilesional deviation of the egocentric reference.

Adult↗
PubMed1998

Visuospatial neglect: the ultimate deconstruction?

Unilateral visuospatial neglect is now widely acknowledged to be a highly heterogeneous condition: The overt manifestations of visual neglect can vary as a function of task, spatial domain, and mode of response (at least). Double dissociations (sometimes of the strong form) have already been reported between most of the components of what was originally thought to be a relatively stable construct within the visual modality. Nonetheless, throughout successive fractionations of neglect, reported cases of bidirectional task-specific neglect after unilateral brain damage are rare. We now report two such cases. After right hemisphere stroke, the first patient reliably showed severe left neglect on cancellation but right neglect on line bisection. After left hemisphere stroke, the second case showed right neglect on cancellation but left neglect on line bisection. Extensive investigation of case 1 confirmed our previous conjecture that the crucial distinction between these tasks lies in the presence or the absence of an overt target. In contrast to cancellation, line bisection demands the internal computation of the location of the "target" (the midpoint), followed by executing a motor response toward the precise location of that "imaginary" midpoint. The relative attentional and premotor contributions of the intact and damaged hemispheres to these forms of bidirectional neglect are also assessed.

Aged↗
PubMed1998

A search for the optimal stimulus.

How do stimulus size and item number relate to the magnitude and direction of error on center estimation and line cancellation tests? How might this relationship inform theories concerning spatial neglect? These questions were addressed by testing twenty patients with right hemisphere lesions, eleven with left hemisphere lesions and eleven normal control subjects on multiple versions of center estimation and line cancellation tests. Patients who made large errors on these tests also demonstrated an optimal or pivotal stimulus value, i.e., a particular size center estimation test or number of lines on cancellation that either minimized error magnitude relative to other size stimuli (optimal) or marked the boundary between normal and abnormal performance (pivotal). Patients with right hemisphere lesions made increasingly greater errors on the center estimation test as stimuli were both larger and smaller than the optimal value, whereas those with left hemisphere lesions made greater errors as stimuli were smaller than a pivotal value. In normal subjects, the direction of errors on center estimation stimuli shifted from the right of true center to the left as stimuli decreased in size (i.e., the crossover effect). Right hemisphere lesions exaggerated this effect, whereas left hemisphere lesions diminished and possibly reversed the direction of crossover. Error direction did not change as a function of stimulus value on cancellation tests. The demonstration of optimal and pivotal stimulus values indicates that performances on center estimation and cancellation tests in neglect are only relative to the stimuli used. In light of other studies, our findings indicate that patients with spatial neglect grossly overestimate the size of small stimuli and underestimate the size of large stimuli, that crossover represents an "apparent" shift in error direction that actually results from normally occurring errors in size perception, and that the left hemisphere is specialized for one aspect of size estimation, whereas the right performs dual roles.

Aged↗
PubMed1998

Response channel activation and the temporoparietal junction.

When we learn to make one motor response to one visual stimulus and a different motor response to another, representations of these stimulus-response associations must be maintained to efficiently transduce perception into action. When an irrelevant distractor is presented adjacent to a target stimulus, interference is observed when the two stimuli are associated with conflicting responses, presumably due to response channel activation by the incompatible information. We have explored the neural bases of these interference effects. In a previous study, patients with hemispatial neglect showed normal interference from contralesional flankers. In another study, patients with lesions of the lateral prefrontal cortex were found not to show interference from distractors presented in the contralesional hemifield. The current study provided a more anatomically detailed investigation of the effects of posterior association cortex lesions on flanker interference. Patients with chronic, unilateral lesions involving the temporoparietal junction (TPJ), two of whom had hemispatial neglect, were compared with patients with lesions of the posterior association cortex not involving the TPJ. All patients performed a color discrimination task at fixation while a congruent or incongruent colored flanker was briefly presented (16.7 ms) in the adjacent contralesional or ipsilesional hemifield. Patients with TPJ lesions showed no interference effects from the contralesional flankers. These results suggest that the TPJ, in combination with the dorsolateral prefrontal cortex, is involved in transducing perception into action.

Aged↗
PubMed1998

Dissociable coordinate frames of unilateral spatial neglect: "viewer-centered" neglect.

Detailed analyses of reading and nonlexical tasks by three patients with unilateral spatial neglect (USN) secondary to stroke indicate that the USN in each of these patients affects the left side (contralateral to brain damage) of the viewer, with respect to the viewer's head, mid-sagittal plane of the body, or line of sight. In one case, the neglect was further specified as concerning the left side of the viewer's line of sight (the left half of her residual visual field). Thus, the frame of reference of USN in these three cases appears to have viewer-centered (in at least one case, specifically retinotopic) coordinates. The performance of these patients is contrasted to that of other patients in the literature whose USN appears to have a frame of reference with stimulus-centered or object-centered coordinates. These results are interpreted within a model of visual processing (adapted from Marr, 1980 and others) with at least three coordinate frames. It is argued that USN can affect one or more of these coordinate frames independently.

Aged↗
PubMed1999

Sex differences in cognition: the role of testosterone and sexual orientation.

The performance of both heterosexual and homosexual males and females was compared on four cognitive tasks which have been shown to reveal evidence of sexual dimorphism. In one spatial and one verbal task, significant sex and orientation effects were found. Significant relationships were also found between salivary free-testosterone levels and performance on both spatial tasks, but no significant associations were found for performance on the two verbal tasks. The present study revealed both within- and between-sex differences in cognition and indicates that these differences may be partly accounted for by the activational effects of free testosterone.

Adolescent↗
PubMed1999

Visual field differences in spatial frequency discrimination.

Subjects discriminated between sine-wave gratings that differed by either +/-0.125 octaves (small difference) or +/-1.0 octaves (large difference). Baseline stimuli consisted of either 1.0 or 4.0 cycles per degree gratings. A left visual field advantage was obtained for the small difference in frequency, with no visual field advantages for the large difference in frequency. Similarly, moderate support for right versus left visual field advantages in processing high versus low spatial frequencies was found, although these interactions were not statistically significant. The results are discussed in light of Kosslyn's (1987) categorical and coordinate framework.

Cognition↗
PubMed2000

The role of the dynamic body schema in praxis: evidence from primary progressive apraxia.

On an influential model of limb praxis, ideomotor apraxia results from damage to stored gesture representations or disconnection of representations from sensory input or motor output (Heilman & Gonzalez Rothi, 1993; Gonzalez Rothi et al., 1991). We report data from a patient with progressive ideomotor limb apraxia which cannot be readily accommodated by this model. The patient, BG, is profoundly impaired in gesturing to command, to sight of object, and to imitation, but gestures nearly normally with tool in hand and recognizes gestures relatively well. In addition, performance is profoundly impaired on imitation of meaningless gestures and on tasks requiring spatiomotor transformations of body-position information. We provide evidence that BG's apraxia is largely attributable to impairments external to the stored gesture system in procedures coding the dynamic positions of the body parts of self and others; that is, the body schema. We propose a model of a dynamic, interactive praxis system subserved by posterior parietal cortex in which stored representational elements, when present, provide "top-down" support to spatiomotor procedures computed on-line. In addition to accounting for BG's performance, this model accommodates a common pattern of ideomotor apraxia more readily than competing accounts.

Aged↗
PubMed2000

Perceiving human locomotion: priming effects in direction discrimination.

During the perception of biological motion, the available stimulus information is confined to a small number of lights attached to the major joints of a moving actor. Despite this drastic impoverishment of the stimulus, the human visual apparatus organizes the swarm of moving dots in a vivid percept of a human figure. In addition, observers effortlessly identify the action the figure is involved in. After a historical introduction and a short walk through the literature, data from a priming experiment are presented. In a serial two-choice reaction-time task, participants were presented with a point-light walker, facing either to the right or to the left and walking either forward or backward on a treadmill. Subjects had to identify the direction of articulatory movements. Reliable priming effects were established in consecutive trials, but these effects were tempered by the relation between priming and primed walker. The reaction time to a walker was shorter when the walker in the preceding trial moved in the same direction and was facing in the same direction. The findings are discussed in relation to recent data from neuropsychological case studies, neuroimaging, and single-cell recording.

Humans↗
PubMed1994

Spatial learning and noradrenaline content in the brain and periphery of young and aged rats.

The present experiment studied whether a dysfunction of the noradrenergic neurons is related to spatial learning impairment by investigating the levels of noradrenaline in the brain and periphery as well as the acquisition of water maze task in saline-pretreated young rats, in noradrenergic neurotoxin (DSP-4)-pretreated young rats and in saline-pretreated aged rats. Aged rats, which had an increased escape latency onto the hidden platform, revealed a decreased noradrenaline content in the heart, but not in the hippocampus, striatum, or hypothalamus, whereas DSP-4-pretreated rats had decreased noradrenaline content in the brain; the acquisition of water maze task was not impaired. These results suggest that the peripheral noradrenergic system can show age-related changes different from those in the central noradrenergic system, and they failed to provide support for the hypothesis that decreased activity of the central noradrenergic nerves is related to impairment in the acquisition of the water maze task.

Aging↗
PubMed1996

Characterization of olfactory deficits in the rat following administration of 2,6-dichlorobenzonitrile (dichlobenil), 3,3'-iminodipropionitrile, or methimazole.

The histopathology of the olfactory mucosal lesion associated with ip administration of 2,6-dichlorobenzonitrile (dichlobenil) and 3,3'-iminodipropionitrile (IDPN) has been well documented. Whether there is an olfactory deficit associated with the partial loss of the olfactory mucosa (localized around the dorsal medial meatus of the nasal cavity) has yet to be determined. Dichlobenil (100 mg/kg) or IDPN (200 mg/kg) was administered ip to adult male Long-Evans rats previously trained in an olfactory task to find a food pellet buried in approximately 7.5 cm of bedding in a 0.61 x 1.2 x 0.61-m Plexiglass chamber. As a positive control, another group received 300 mg/kg ip of 1-methyl-2-mercaptoimidazole (methimazole), a dosing regimen which destroys nearly all of the olfactory mucosa. All three compounds caused a transient increase in the mean latency to find the pellet, with the magnitude of the effect positively correlated with the extent of the olfactory lesion. In order to determine whether these deficits resulted from olfactory dysfunction or impaired cognitive function (a deficit previously attributed to IDPN exposure), another group of rats was dosed as above and tested in another spatial memory task, the Morris water maze (MWM), which is less dependent upon olfactory function. No performance deficit was detected in the MWM. These data suggest that the transient olfactory deficit in the dichlobenil-, IDPN-, and methimazole-treated rats is attributable to defective olfactory function.

Animals↗
PubMed1996

The development and temporal dynamics of spatial orienting in infants.

Shifts of visual attention can be studied in adults by cueing a particular spatial location and assessing the speed of detection of targets presented in the cued location as compared to other locations. A number of studies have used spatial cueing paradigms in an attempt to study attention shifts in infants. However, these studies have employed different procedures, time courses, measures, and age groups, making comparison between them difficult. The present experiments were designed to investigate the effects of varying the cue-target time interval on the speed and direction of orienting in 2-, 4-, and 6-month-old infants. The results of Experiment 1 indicated that, while 2-month-old infants showed only weak effects of the cue, 4-month-old infants show facilitation to a cued location when a target appears 200 ms after cue onset, and inhibition of responses to the same location when the target appears 700 ms after the cue onset. Six-month-old infants showed evidence of inhibition, but not facilitation. One account of this pattern of data is that 6-month-olds shift attention faster than do 4-month-olds. This hypothesis was tested in Experiment 2 in which four different cue-target intervals were used with a group of 7-month-old infants. The results obtained were consistent with the hypothesis that infants get faster to shift attention to a spatial location with age.

Age Factors↗
PubMed2002

Spatiotemporal dynamics of HIV propagation.

Although viral propagation is a localized process, mathematical models of viral replication kinetics have been limited to systems of ordinary differential equations describing spatially averaged behavior. In this paper, we introduce a cellular automaton model of viral propagation based on the known biophysical properties of HIV. In particular, we include the competition between viral lability and Brownian motion. The model predicts three testable effects not present in previous descriptions. First, we find a profound dependence of viral infectivity on cell concentration; virion instability decreases infectivity more than 100-fold under typical experimental conditions, resulting in misleading estimates of the number of infectious particles. Second, we find that, in a large parameter regime, infection extinguishes itself due to insufficient target cell replenishment. Finally, we find that propagation is limited by viral stability at low cell density and by geometry at high cell density. The geometry-limited regime can be modulated by downregulation of CD4. These different properties are analysed quantitatively and compared with previous experimental results.

Animals↗
PubMed1996

Spatial learning and memory in calpastatin-deficient rats.

The present experiments were concerned with the examination of the hypothesis that a deficiency in calpastatin, the endogenous inhibitor of calpain, enhances learning and memory performance. In the first experiment we used rats with an altered calpain/calpastatin balance (Milan hypertensive strain, MHS, low calpastatin) to investigate the learning and memory of a spatial task in the Morris water maze in comparison with control rats with a normal calpain/calpastatin balance (Milan normotensive strain, MNS). Since the two strains also differ in blood pressure, a third strain of rats was included to assess the role of hypertension (spontaneously hypertensive rats, SHR). Although the acquisition rate of the spatial task was better in the low-calpastatin MHS rats than in their normal-calpastatin MNS controls, their performance was similar to that of the SHR rats, thus thwarting the conclusion that differences were due to the low level of calpastatin. The availability of another mutant strain, low-calpastatin level and normotensive (MH.NE), allowed a further examination of the hypothesis. In the second experiment rats of the MH.NE strain acquired the spatial task as well as their normotensive controls, but their memory retrieval was clearly less than that of their normal-calpastatin controls. This deficiency was not due to impaired visual function or a slower swimming speed. The conclusion is that an inbalanced calpain/calpastatin ratio, although favoring calpain activity, is disadvantageous for remembering a spatial task. This disadvantage is clearly overruled when this inbalance is accompanied by hypertension.

Animals↗
PubMed1998

Septal lesions impair the acquisition of a cued place navigation task: attentional or memory deficit?

These experiments were designed to analyze how medial septal lesions reducing the cholinergic innervation in the hippocampus might affect place learning. Rats with quisqualic lesions of the medial septal area (MS) were trained in a water maze and on a homing table where the escape position was located at a spatially fixed position and further indicated by a salient cue suspended above it. The lesioned rats were significantly impaired in reaching the cued escape platform during training. In addition rats, did not show any discrimination of the training sector during a probe trial in which no platform or cue was present. This impairment remained significant during further training in the absence of the cue. When the cued escape platform was located at an unpredictable spatial location, the MS-lesioned rats showed no deficit and spent more time under the cue than control rats during the probe trial. On the homing board, with a salient object in close proximity to the escape hole, the MS rats showed no deficit in escape latencies, although a significant reduction in spatial memory was observed. However, this was overcome by additional training in the absence of the cue. Under these conditions, rats with septal lesions were prone to develop a pure guidance strategy, whereas normal rats combined a guidance strategy with a memory of the escape position relative to more distant landmarks. The presence of a salient cue appeared to decrease attention to environmental landmarks, thus reducing spatial memory. These data confirm the general hypothesis that MS lesions reduce the capacity to rely on a representation of the relation between several landmarks with different salience.

Analysis of Variance↗
PubMed1999

Involvement of the posteroventral caudate-putamen in memory consolidation in the Morris water maze.

Male Sprague-Dawley rats implanted with bilateral intracerebral guide cannulae were trained in the standard hidden platform version of the Morris water maze and given immediate posttraining infusions of the D2 dopamine receptor antagonist sulpiride (10.0 or 100.0 ng/side) or saline vehicle into the posteroventral caudate-putamen. Retention was tested 2 days later with a probe trial. Sulpiride-treated rats spent less time swimming near the trained platform location and more time in the periphery of the maze than controls, although their latency to reach the trained platform location was not significantly affected. The pattern of results suggests that whereas the posteroventral caudate-putamen seems to be involved in consolidation of memory in the Morris water maze, it may be involved in memory for procedural aspects of the task in a manner distinct from that of other brain regions such as the hippocampus.

Animals↗