A.D. Milner

To halve and to halve not: An analysis of line bisection judgements in normal subjects

Normal adults were tested in a series of three experiments to examine the influences of spatial location and cueing upon line bisection judgements. Judgements in all three experiments were strongly influenced by cueing with a letter at one or other end of the line. The spatial location of the line (in left, central or right body space) also had a minor effect in Experiments 1 and 2, where evidence was additionally found for a small constant error when lines were presented centrally. It is argued from the results of Experiments 2 and 3, where no explicit bisection response was required, that perceptual/attentional factors, rather than an orienting bias, play the major role in mediating the cueing effect. It is concluded that there is a substantial attentional effect upon judgements of extent, whereby paying less attention decreases perceived relative line length. However the constant error and the effect of spatial location, whilst mainly perceptual in nature, may also be partly determined by premotor orienting biases caused by differential hemispheric activation.

Line bisection errors in visual neglect: misguided action or size distortion?

The rightward line bisection errors made by patients with visuospatial neglect can be explained as due to a spatially misdirected response, which would be predicted on either of two accounts. An alternative view, however, is that such patients actually misperceive the left half of a horizontal line as being shorter than the right half. We have tested this possibility directly in three neglect patients, by giving them prebisected lines: they were found to judge a central transection mark as lying nearer to the left end of the lines. We were also able to test one of the patients on a series of size comparisons using computer-generated patterns. She was found to judge horizontal lines as shorter in the left half of visual space than in the right. This was also true for comparisons of the areas of nonsense figures. However she did not make such constant errors when comparing the lengths of vertical lines. It is suggested that an attentional deficit in left hemispace may result in the underestimation of horizontal extent. This would act in combination with misdirected reaching to determine the magnitude of line bisection errors.

Modulation of visual event-related potentials by spatial and non-spatial visual selective attention☆

Event-related potentials (ERPs) were recorded in two experiments involving selective visual processing. In Experiment 1, subjects attended to light flashes emanating from one visual field, in order to detect occasional slightly deviant targets, while ignoring equiprobable stimuli from the opposite field. ERPs elicited by stimuli in an attended field were characterised by larger posteriorly distributed P120 and N170 components, and a larger anteriorly distributed N145 component. In addition, these ERPs were, in comparison to those elicited by unattended stimuli, more negative-going in the latency region of approx. 200-400 msec. This late effect had a marked fronto-central distribution. In Experiment 2 subjects attended to either horizontal or vertical bars, displayed equiprobably in the same spatial location. No enhancement of early components was observed as a function of attention but, as in Experiment 1, a late, sustained, fronto-centrally distributed negative shift was observed in ERPs elicited by attended compared to unattended stimuli. It was concluded that the enhancement of P120 (P1) observed in Experiment 1 reflects the engagement of attentional mechanisms specific to the selection of stimuli on the basis of spatial cues. The later sustained negative shift seen in both experiments was considered to reflect a feature of within-channel processing common to both spatial and non-spatial selective tasks.

Visual orientation in the rat: a dissociation of deficits following cortical and collicular lesions.

SummaryRats with either bilateral ablations of superior colliculus, bilateral ablations of visual cortex, or sham operations were trained to run across a large arena towards a small illuminated target which varied in location from trial to trial. An impairment in this visually-guided running was apparent in the cortical group, but not in the collicular group. When, in a second experiment, the spatial relationships within the apparatus were changed by extending the entry-tunnel some distance into the arena, the running of the cortical group became even more impaired, while the collicular animals continued to run towards the targets under efficient visual control. In a third experiment, the effect of introducing a novel flashing light in various locations around the perimeter of the arena was investigated. It was found that unlike the other two groups, the collicular animals showed no orienting reflex to the novel stimulus when it was presented outside a broad central area of the visual field.

Visuomotor sensitivity for shape and orientation in a patient with visual form agnosia

We have previously demonstrated that a patient with visual form agnosia (DF), who is unable to report the orientation or size of visual targets, can nevertheless use these same visual attributes to control motor acts. In the first of three new experiments, we found that DF is able to grasp everyday tools and utensils proficiently (i.e. with a well-formed hand posture) but has difficulty in visually selecting the correct part of the object to grasp (e.g. the handle) for subsequent use of that object. A second experiment revealed that DFs visuomotor system is able to adjust concurrently to variations in both the size and orientation of target objects; when these visual attributes were both varied, she adjusted both her grip aperture and the orientation of her hand well in advance of target contact. These spared visuomotor abilities do not seem to extend to shape processing per se, however. In the final experiment we found that DF was insensitive to changes in the orientation of a cross-shaped object, where no single principal axis could be extracted to control orientation of the grasp. These observations extend our knowledge of DFs residual visuomotor abilities, and suggest limitations on the visual processing capacities of the human dorsal stream.

Cerebral correlates of visual awareness

While it may be a long time before we can specify the mechanisms through which a brain process achieves awareness, it may be possible to determine as a first step whether awareness is limited to the products of certain kinds of processing. In the domain of vision, for example, perceptual awareness might only be attainable in association with object-centred coding, configural representations of space, and other such forms of abstracted (non-retinocentric) coding. It appears that these forms of visual coding are anatomically restricted to telencephalic structures, and indeed it has been argued that they may be peculiar to, or at least visually dependent upon, the ventral stream of visual areas with the cortex. It is suggested here that such a brain process would still not be able to enter visual awareness unless it was selectively amplified through neuronal gating of the kind that has been shown to be correlated with selective spatial attention. The present paper explores the extent to which this putative dual requirement for visual consciousness might form a basis for understanding the various phenomena of covert vision seen in patients suffering from hemianopia, apperceptive agnosia, and unilateral spatial neglect.

Visuo-spatial performance following posterior parietal and lateral frontal lesions in stumptail macaques.

Three groups of four monkeys were trained to negotiate a small hand (stylus) maze, and to use a pointer to guide response in a two-choice position discrimination task. One group was given bilateral lesions of posterior parietal cortex, and the second bilateral lesions of lateral frontal cortex. Postoperative impairment on the maze was evident in three of the four parietal animals, and both operated groups showed impairment on the pointer task. Since maze performance was positively associated with speed of stylus recovery in a control version of the task but was not correlated with pointer task performance, it is suggested that the parietal deficit resulted primarily from a sensory-motor dyscoordination rather than a spatial perceptual disorder. The latter could however be the cause of parietal impairment on the pointer task. Error analysis suggests an interpretation of the frontal deficit on the pointer task in terms of perseveration on position.

Go-left go-right discrimination performance and distractibility following lesions of prefrontal cortex or superior colliculus in stumptail macaques

Abstract Monkeys were trained to respond to left or right according to the colour of a brief central light. They were tested for retention of this discrimination task following bilateral lesions of either the frontal eye-field, dorsolateral frontal cortex, or the superior colliculus. Impairment was found after frontal eye-field lesions only. Subsequently tests were given in which an irrelevant white light was flashed peripherally at the start of a trial. These indicated a reduced distractibility following both collicular lesions and frontal eye-field lesions, though in the latter case this look the form only of a reduction in the duration of distraction, rather than a reduction in its probability of occurrence.

The effect of stimulus intensity on visual evoked potential estimates of interhemispheric transmission time

SummaryVisual evoked potentials (VEPs) to bright or dim lateralised light flashes were recorded from homologous occipital and central sites. In a GO/NOGO reaction time task (Experiment 1) the latency of the N160 component of the VEP was found to be shorter from the contralateral hemisphere by approximately 16 ms at occipital sites, but only 3 ms centrally. In addition, there was a trend for the occipital contralateral latency advantage to increase with decreasing stimulus brightness. In Experiment 2 a wider intensity range and a simple visual reaction time task were employed. Contralateral N160 latency advantages were again found to be larger occipitally (approx 13 ms) than centrally (3 ms). Furthermore the occipital contralateral latency advantage was significantly increased at the lower stimulus intensity, while that from central sites remained constant. These data suggest that two types of interhemispheric relay can be distinguished — a sensory one recorded occipitally and a non-sensory one recorded from central sites.

Visual evoked potentials to lateralised stimuli in two cases of callosal agenesis.

Visual evoked potentials (VEPs) to lateralised light flashes were recorded from two acallosal patients. In one patient, these recordings were made while he performed a choice-reaction time task, and in the other patient the VEPs were obtained during a simple reaction time task. In both cases the patients VEPs from electrode sites contralateral to the visual field of stimulus delivery resembled those of controls. Their VEPs from ipsilateral sites were aberrant, however, in that while control subjects showed a smaller and slightly delayed ipsilateral N160 component, this was not discernible in the patients data. It is concluded that the ipsilateral N160 relies for its generation on the transcallosal transfer of information processed initially by the contralateral hemisphere.