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Biomedical subjects

N C Nettleton

Publications and source records attributed to N C Nettleton.

At least 19 recordsLinked to original sources
PubMed1994

Can task specific perceptual bias be distinguished from unilateral neglect?

The present study examined visuoperceptual bias in 12 right hemisphere damaged patients, eight of whom showed left unilateral neglect on standard clinical tests, and in 30 normal controls. In the chimeric faces task, subjects were required to judge which of a pair of faces appeared happier. Stimuli comprising each pair were mirror images, with the smiling half on the left of one face and on the right of the other. In the grey scales task, subjects were required to indicate which of two shaded rectangles appeared to be darker overall. Again, stimuli were mirror images, with the darker end appearing either on the left or on the right. Patients exhibited a significant rightward bias on both experimental tasks, in contrast to the significant leftward bias exhibited by controls. There was no significant correlation between patients' performances on standard clinical tests and the extent of bias on the two experimental tasks, suggesting that such patients exhibit distinct impairments of spatial cognition which are differentially indexed by the two types of task. Moreover, for both patients and controls, scores obtained on the two perceptual bias tasks were unrelated, suggesting that they may engage stimulus-specific processes which have different underlying patterns of asymmetrical processing. These data provide further support for models which propose that the heterogeneity of disorders of spatial cognition arise from disruption of distinct neural mechanisms.

Adult↗
PubMed1994

Recovery from directional hypokinesia and bradykinesia in unilateral neglect.

Patients with acute right hemisphere (RH) damage and left unilateral neglect may exhibit slowness in both the initiation and execution of goal-directed movements toward the contralesional side (respectively, directional hypokinesia and directional bradykinesia). We retested 13 patients with RH damage, who on initial testing 1 year earlier had shown these impairments on a visually cued sequential movement task. Initiation and execution times were measured separately for leftward and rightward movement sequences, performed in either hemispace and across the body midline. Patients with anterior/subcortical lesions exhibited directional hypokinesia and bradykinesia on initial testing, but no longer showed either after 12 months. It is suggested that the recovery exhibited by these patients was determined conjointly by residual functioning of flexible anterior/subcortical structures involved in response production, in addition to the resolution of transient secondary dysfunction (i.e., diaschisis) of posterior (parietal) areas responsible for attentional orienting and target selection. In contrast, patients with posterior (parietal) lesions continued to exhibit directional hypokinesia after 12 months, probably as a result of permanent damage to a region whose dedicated or specialised functions cannot readily be compensated by other brain areas. These results support the notion that spatial attention and goal-directed movement are subserved by a distributed network involving separate but interconnected brain regions. Distinct functional losses may be predicted, in both the acute and chronic stages postinjury, on the basis of temporary or permanent dysfunction of discrete components of this network.

Adult↗
PubMed1994

Residual rightward attentional bias after apparent recovery from right hemisphere damage: implications for a multicomponent model of neglect.

Unilateral neglect may be a multicomponent attentional disorder consisting of an initial automatic orienting of attention toward the ipsilesional side and a subsequent impairment in contralesionally reorienting attention, both of which are superimposed on a generalised reduction in attention resources. It has been hypothesised that patients' ability to reorient attention contralesionally may recover relatively quickly, but that the ipsilesional attention bias may be relatively persistent. This hypothesis was tested by consecutively examining 13 patients who had had a right hemisphere stroke, and who had left unilateral neglect. They were examined once shortly after the stroke and again 12 months later, using a battery of standard clinical and experimental tasks. Patients initially showed a strong and consistent rightward attentional bias in addition to a failure to reorient their attention leftward. After 12 months patients continued to show an abnormal ipsilesional attentional bias, though most were now able to fully reorient their attention toward the contralesional side. These results suggest that restitution of the capacity to reorient attention contralesionally may underlie the apparent recovery from clinical signs of unilateral neglect. The presence of a residual ipsilesional attentional bias in most patients, however, may, at least in part, account for the poor functional outcomes in some apparently "recovered" patients.

Adult↗
PubMed1990

Abduction, adduction and hand differences in simple and serial movements.

Abductive or adductive movements were made either towards single targets left or right of "home", or sequentially from target to target with various levels of advance information. In the former situation the preferred hand completed responses (movement time, MT) faster than the non-preferred, while the non-preferred hand initiated them faster (reaction time, RT); these effects were in both cases stronger with harder (knob turn) than with easier (touch) responses. Abductive responses (MTs, not RTs) were faster than adductive, especially with the preferred right hand. However in the sequential task adductive responses were the faster, consistently so by MTs, while with respect to time spent motionless at each target (down time, DT) more so with the non-preferred hand, and under conditions of maximal advance information. Findings were discussed in the contexts of movement complexity, hemispatial representation, and how advance information may be utilized in the resolution of directional uncertainty. There may be an evolutionary advantage in making complex manipulative responses adductively, close to the body, while reaches are usually made abductively, to the periphery of circumcorporeal space.

Adult↗
PubMed1989

Direction and location of movement in kinesthetic judgements of extent.

In unimanual kinesthetic judgements, larger ipsilateral extents felt equal to smaller contralateral ones. This was more likely to be due to a limb's general "intentional" tendencies towards its own ipsilateral hemispace than to e.g. differential attentional tendencies or the effects of muscular tension when crossing the midline. However hand asymmetries in this respect cannot account for the "pseudoneglect" phenomenon of setting the subjective midpoint of a felt rod slightly too far to the left. This phenomenon may instead be due to an asymmetric deployment of attentional and processing resources.

Adult↗
PubMed1988

Movement initiation and control: abduction, adduction and locus of limb.

Dextrals were timed in moving a peg from hole to hole, when the board containing the 13 holes was placed horizontally either across the midline or entirely to one or other side of the body. Either hand made movements in either direction. Both movement times (between holes) and resting times (within holes) were measured. Resting times (motor programming?) were better performance indices than the actual movement times. The superiority of the preferred (right) hand was greater when it operated on its own (ipsilateral) side of the body. Adductive movements were faster than abductive, which were easier to program when a limb operated ipsilaterally.

Adult↗
PubMed1988

Crossing the midline by four to eight year old children.

In Experiment 1 children (Kinder, 4-5 yr; Prep, 5-6 yr; Grade 1, 6-7 yr; Grade 2, 7-8 yr) bisected horizontal lines placed to the left, right or across the midline. The youngest groups displayed symmetrical neglect, erring to the left with the left hand and to the right with the right, the adult pattern of leftwards error not appearing until about Grade 2. However, while Prep, Grade 1 and Grade 2 sinistrals showed bigger between-hand differences than dextrals, this was not, unlike an earlier study, true of the youngest Kinder group, and symmetrical neglect did not appear to be peculiar to young sinistrals. A timed peg-moving task in Experiment II showed that performance did not slow when crossing the midline; nor did young sinistrals perform better with centrifugal abductive movement. These and other findings were incompatible with the idea of callosal immaturity in young sinistrals.

Analysis of Variance↗
PubMed1988

Vibrotactile reaction times in unilateral neglect. The effects of hand location, rehabilitation and eyes open/closed.

Patients with left hemispatial neglect resulting from right hemisphere damage performed a vibrotactile reaction time (RT) task with the stimulated and responding (right) limb placed either ipsilaterally in right hemispace, or across the midline in contralateral (left) hemispace. In 8 patients who had undergone little or no rehabilitative training, responses made on the left were very much slower (by an average of 45 ms) than responses made on the right. Four out of 5 rehabilitated patients who had been taught compensatory strategies to overcome their difficulty in attending to events on the left, and who were tested with eyes open, were faster on the left, a pattern opposite to the small (9 ms) but significant right-side advantage shown by normal subjects. However, when 4 of the rehabilitated patients were tested with their eyes closed, their performance reverted to that shown before rehabilitation, that is, a substantial left-side disadvantage. Overt and covert attentional strategies and spatial asymmetries in normal subjects and patients with unilateral attention deficits are discussed. The vibrotactile RT task provides a promising method for quantifying disability during the various phases of the neglect syndrome.

Adult↗
PubMed1987

Why is there a left side underestimation in rod bisection?

Subjects set the extremities of a horizontal rod to appear equidistant from a central reference point, with or without central fixation. On either side, the contrast (salience) of the rod against the background was high or low. Extents to the left were set smaller than those to the right, an effect (LSU) which was stronger with central fixation, indicating that both hemispatial and anatomical pathway factors contribute. Reduced salience on the left increased rather than decreased the LSU, indicating the importance of attentional factors.

Adult↗
PubMed1987

Line bisection by left-handed preschoolers: a phenomenon of symmetrical neglect.

When a normal dextral attempts to bisect a horizontal line placed across his/her midline, there is a tendency for the transection to be placed slightly to the left of true center, a tendency which may be a little more pronounced when the left hand is used. However, left-handed children at or younger than 5 years of age err substantially to the left with the left hand and to the right with the right hand. This symmetrical neglect of extents contralateral to the hand currently in use, which may possibly reflect relatively greater commissural immaturity in left-handers, was not seen in dextral children of comparable ages, whose performance closely followed adult patterns.

Child Development↗
PubMed1986

Dichotic and dichhaptic techniques.

Auditory and tactual asymmetries frequently demonstrate right side advantages for verbal stimuli and the opposite effects for nonverbal stimuli. Such asymmetries may, however, reflect the perceived position of events in space, rather than the role of anatomical afferent pathways with suppression of ipsilateral by contralateral routes. Consequently, despite popular tradition, simultaneous competitive stimulation (dichotic or dichhaptic) may be unnecessary. We review the reliability and the relatively low validity of such techniques for predicting hemispheric specialization in various clinical and normal populations. We discuss the role of stimulus factors, the various indices and measures, the effect of order and mode of report, division of attention, task difficulty and memory factors.

Attention↗
PubMed1986

Tactual-kinesthetic matching of horizontal extents by the long-term blind: absence or reversal of normal left-side underestimation.

Ten adults, who were blind from infancy or childhood, laterally slid a horizontal rod within a short length of pipe located at the body midline, until the rod extremities were tactually-kinesthetically judged to be equidistant from the ends of the pipe. Whereas sighted subjects normally set the left extremity of the rod closer to the midline than the right irrespective of arm posture, crossed or uncrossed, blind subjects, especially those blind from birth showed trends in the opposite direction. The role of vision is discussed with respect to the development and expression of asymmetries relating to the coding of extracorporeal space.

Adult↗
PubMed1986

Leftwards error in bisecting the gap between two points: stimulus quality and hand effects.

Subjects attempted to bisect the gap between two LEDs; these differed from each other in brightness, stimulus duration, onset/offset (a) synchronies, and in continuous or flashing mode of presentation. There was a systematic bias to the left, especially with the left hand, indicating a motor contribution to this perceptual task. A brief, faint or flashing LED attracted the subjective midpoint, perhaps because of increased processing demands; indeed the general leftwards bias was attributed to a greater visuospatial processing capacity in the right hemisphere. Leftwards bias was rather greater when central fixation was maintained, indicating that hemispatial mechanisms may be more important than anatomical connectivities. The technique may be useful for understanding and treating unilateral neglect.

Attention↗
PubMed1986

Lateralized recognition of incomplete figures in a prolonged display controlled by eye movements.

A left field superiority is typically reported for processing a variety of simple and complex non-verbal and patterned visual stimuli, including pictures, though stimulus clarity, degradation and exposure duration may be a significant determinant. Clinical studies indicate deficits in the ability to recognize incomplete figures with right hemisphere damage. However when normal subjects attempted to recognize 80 series of progressively-less-incomplete common objects (each series graded in 10 steps) presented for prolonged inspection to one or other visual field via a display driven by an infrared eyemovement transducer, either field (hemisphere) proved to be equally competent. We review evidence to suggest that such "ecologically-more-valid" techniques may not in fact be intrinsically able to tap minor hemisphere function.

Dominance, Cerebral↗
PubMed1985

Hand and hemispace effects in tactual tasks in children.

In Experiment I, 3-and 5-yr-old dextrals matched textures better by either hand when it operated in left hemispace. Girls and 3-yr olds were the more disadvantaged by non-alignment of hand and hemispace. In Experiment II, 8-yr-olds reproduced finger sequences; dextrals demonstrated a right-hand and sinistrals a right-hemispace superiority. In Experiment III, both a left-hand and a left-hemispace superiority appeared when 5-yr-old dextrals reproduced a static configuration of finger spacing. Asymmetries were generally stronger for the side of presentation than for the side of response. Our findings are consistent with the operation of two semi-independent systems, one involving hand-hemisphere connections, and the other mapping of extracorporeal space by the hemispheres.

Age Factors↗
PubMed1985

Bisecting rods and lines: effects of horizontal and vertical posture on left-side underestimation by normal subjects.

In Experiment I normal subjects performed the classical clinical line-bisection task, and demonstrated a left-side underestimation. In Experiment II subjects maintained fixation upon a central position and adjusted a rod passing through this fixation point so that both extremities were judged equal. The left-side underestimation was very much greater under these conditions, but was considerably reduced when retinal and gravitational coordinates were dissociated by making the subjects lie horizontally on one or other side. Subjects then demonstrated greater left-side and top-half underestimation when lying on the left than on the right side. Gravitational coordinates and the apparent locus of events in extrapersonal space are determinants of perceptual asymmetries at least as important as anatomical connectivities.

Adult↗