Biomedical subjects

A Allport

Publications and source records attributed to A Allport.

4 recordsLinked to original sources

PubMed1998

Independent reference frames in human spatial memory: body-centered and environment-centered coding in near and far space.

Our apparently seamless experience of the spatial environment seems to be derived from information coded across a variety of spatial reference frameworks, each tied to the metric of a different sensory or motor system. A fundamental distinction is that between body-centered and environment-centered reference frameworks. This study reports the first clear evidence of a behavioral dissociation between body-centered and environment-centered coding in human adults. Subjects, seated in a rotating chair with closed eyes, were required to point to remembered, auditorily presented target locations. The subjects were rotated between the presentation and recall of targets. Targets were held stationary with respect to either body-centered or environment-centered spatial coordinates. Prior to recall, subjects were required to point to a series of prelearned distractor locations, which also remained fixed with reference either to the subject's body or to the stationary environment. Memory for the target locations was selectively impaired when distractor locations were specified within the same spatial reference frame as the target, regardless of whether target and distractor locations were near to or distant from the subjects. In contrast, distractor locations specified in a different reference frame from that of the target had either little or no effect on memory for target locations.

Adult↗

PubMed1994

Shifting attention in a rapid visual search paradigm.

A method is introduced for studying shifts of attention in semantic space, testing 56 subjects in four experiments on a semantic monitoring task based on rapid, serial, visually presented (RSVP) word-sequences. Following a cue to shift attention, accuracy of semantic monitoring drops abruptly to a low level, then gradually recovers to reach preshift levels over successive stimuli in the RSVP sequence. Using this method, we compared two kinds of criterion-shifts, one requiring a set-reversal ('reversal shifts'), the other involving a shift between orthogonally defined categories ('orthogonal shifts'); no differences were found. We have also examined the difference in a shift between two different processing domains (semantic vs typographic) compared with a shift of criterion within the same processing domain. The results showed no differences for within- vs between-domain shifts. Finally, we studied the time-course of a semantic attention shift. Execution of a semantic shift did not follow an internally controlled time-course but was a direct function of the rate of stimulus presentation. No evidence was found for the operation of a 'supervisory attentional system' independent of external stimulus triggering.

Attention↗

PubMed1993

Study of selective reaching and grasping in a patient with unilateral parietal lesion. Dissociated effects of residual spatial neglect.

In the present study we investigated the possibility of a dissociation between the visual control of reaching and the visual control of grasping in a prehension task. To this purpose we studied the kinematics of prehension movements in a patient with a right parietal lesion and in six right-handed healthy control subjects. The task we used was one in which the subjects had to reach and grasp target objects in the presence or absence of a simultaneously presented distractor object. All stimuli were presented in the space ipsilateral to the lesion. The distractor could be either of the same or different size to the target object and was presented either to the right or to the left of the target. The following parameters of the prehension 'transport' component were analysed: wrist trajectory, transport time, tangential peak velocity, acceleration. Maximal finger aperture, time to maximal finger aperture, peak acceleration and time to peak acceleration of grip aperture were the parameters of the 'grasping' component analysed. The results showed that, although the patient had no misreaching, her hand trajectory deviated abnormally towards the distractor position when the distractor was to the right (ipsilateral) side of the target. In contrast, the grasp kinematics was not affected by the distractors, even when the size of the right distractor was different from the target. It appears, therefore, that the attentional shift towards the ipsilesional side, typical of neglect patients, determines a surprising dissociation in motor control. In the presence of a right distractor, the patient plans and partially executes a reaching movement towards that object and simultaneously performs a grasping movement towards a second object, i.e. the centrally located target. The presentation of distractors had no effects on the prehension kinematics of the control subjects.

Attention↗

PubMed1992

Levels of processing for visual stimuli in an "extinguished" field.

Volpe et al. (Nature 282, 722, 1979 [19]) described an experimental study of four patients with parietal tumours who were able to judge whether two simultaneous stimuli were identical or different, even when they were unable to name the stimulus contralateral to their brain injury. We report the case of another patient, E.M., in whom we have investigated this phenomenon further. E.M. had undergone a right temporal lobectomy to prevent recurrent seizures. She could correctly name photographs of objects presented in isolation to either the left or right visual field, at 150 msec exposure (although she was impaired for single objects on the left at 10 msec exposures). She was able to judge correctly whether two simultaneous objects on the left and right had the same or different names, even though she was often unable to name the object on the left. These judgements remained above chance when same-name pairs of stimuli showed the same object but seen from two different viewpoints, or even when they showed visually dissimilar exemplars of the same name category. This implies that the patient based her same-different judgements on categorical information about the pair of objects, even though she was often unable to name the contralateral object.

Adult↗