Sabira K. Mannan

The relationship between the locations of spatial features and those of fixations made during visual examination of briefly presented images.

The locations of features such as extremes of contrast or luminance, high spatial frequency content and edge density in a set of images have been determined, and the locations of fixations made by a group of eighteen human observers who examined the images during brief (3 s) presentations were also measured. The similarity between the locations of the eye movements and those of each stimulus feature was determined by means of a least squares index IS. For averages taken over data for all observers, the similarity determined in this way is much lower than values for pairs of fixation locations made by different observers. It is concluded that the pattern of fixations made to a given image which is highly conserved between different observers, cannot be associated with any one of the local features examined by us. It is shown further that the distribution of fixation locations over the images is non-uniform, with a marked bias to central areas, whereas the image features are more uniformly distributed. Weighting the distributions of feature locations to take account of the non-uniform distribution of fixations produces much higher IS values, but the dominant contribution to these high values is the weighting function itself. Only in the case of edge density is there significant similarity between the locations of eye movements and those of the image features.

Automatic control of saccadic eye movements made in visual inspection of briefly presented 2-D images

This paper describes an investigation of eye movements made by eighteen observers with normal spatial vision in response to eleven images of natural scenes each of which was presented in three versions, unfiltered filtered by low-pass and filtered by high-pass spatial frequency filters. The ability of observers to identify the different images was determined after each set of measurements. An index of similarity, calculated in terms of the sum of the squares of the distances between each fixation point and its nearest neighbour in the other set, was developed in order to provide a basis for the comparison of two sets of eye movements, each made by a different observer or both made on different occasions by the same observer. Application of this index shows that brief (1.5-s) presentation, there exists a high degree of similarity between fixations made by different observers in response to the same image. For longer (3-s) periods, however, the similarity for inter-observer comparisons is reduced. The data demonstrate that filtering of the image has little effect on the pattern of fixations made to a given image, but the duration of fixations is greatest for low-pass filtered images and least for unfiltered images, whereas the amplitude of saccades is greatest for unfiltered images and least for low-pass filtered images. It is proposed that for brief presentations, eye movements made during examination of an unfamiliar image are performed automatically in response to the spatial features of the image.

Revisiting Previously Searched Locations in Visual Neglect: Role of Right Parietal and Frontal Lesions in Misjudging Old Locations as New

Right-hemisphere patients with left neglect often demonstrate abnormal visual search, re-examining stimuli to the right while ignoring those to the left. But re-fixations alone do not reveal if patients misjudge whether they have searched a location before. Here, we not only tracked the eye movements of 16 neglect patients during search, but also asked them to click a response button only when they judged they were fixating a target for the very first time. Re-clicking on previously found targets would indicate that patients erroneously respond to these as new discoveries. Lesions were mapped with high-resolution MRI. Neglect patients with damage involving the right intraparietal sulcus or right inferior frontal lobe re-clicked on previously found targets on the right at a pathological rate, whereas those with medial occipito-temporal lesions did not. For the intraparietal sulcus patients, the probability of erroneous re-clicks on an old target increased with time since first discovering it; whereas for frontal patients it was independent of search time, suggesting different underlying mechanisms in these two types of patient. Re-click deficits correlated with degree of leftward neglect, mainly due to both being severe in intraparietal cases. These results demonstrate that misjudging previously searched locations for new ones can contribute to pathological search in neglect, with potentially different mechanisms being involved in intraparietal versus inferior frontal patients. When combined with a spatial bias to the right, such deficits might explain why many neglect patients often re-examine rightward locations, at the expense of items to their left.

Saccadic visual search training: a treatment for patients with homonymous hemianopia.

Objectives: We describe a novel rehabilitation tool for patients with homonymous hemianopia based on a visual search (VS) paradigm that is portable, inexpensive, and easy to deploy. We hypothesised that by training patients to improve the efficiency of eye movements made in their blind field their disability would be alleviated. Methods: Twenty nine patients with homonymous visual field defects (HVFD) without neglect practised VS paradigms in 20 daily sessions over one month. Search fields comprising randomly positioned target and distracter elements, differing by a single feature, were displayed for three seconds on a dedicated television monitor in the patients’ homes. Improvements were assessed by examining response time (RT), error rates in VS, perimetric visual fields (VFs) and visual search fields (VSFs), before and after treatment. Functional improvements were measured using objective visual tasks which represented activities of daily living (ADL) and a subjective questionnaire. Results: As a group the patients had significantly shorter mean RT in VS after training (p<0.001) and demonstrated a variety of mechanisms to account for this. Improvements were confined to the training period and maintained at follow up. Three patients had significantly longer RT after training. They had high initial error rates which improved with training. Patients performed ADL tasks significantly faster after training and reported significant subjective improvements. There was no concomitant enlargement of the VF, but there was a small but significant enlargement of the VSF. Conclusion: Patients can improve VS with practice. This usually involves shorter RTs, but occasionally a longer RT in a complex speed–accuracy trade-off. These changes translate to improved overall visual function, assessed objectively and subjectively, suggesting that they represent robust training effects. The underlying mechanism may involve the adoption of compensatory eye movement strategies.

Scanning the visual world: a study of patients with homonymous hemianopia.

OBJECTIVES This study examined the scanpaths of patients with homonymous hemianopia while viewing naturalistic pictures in their original and also spatially filtered forms. Features of their scanpaths with respect to various saccade and fixation parameters were examined to determine whether they develop compensatory eye movement strategies. The effects of various lesion parameters including location, size, and age on the evolution of such strategies were considered. METHODS Eye movements of eight patients with homonymous hemianopia (four left, four right), but lacking neglect, were recorded while they viewed 22 images of real scenes, and they were compared with the eye movements of eight age matched controls. Subjects viewed each image for 3 seconds, initially in a spatially filtered form in which much of the semantic content had been removed, and then in their unfiltered, original form. RESULTS Patients differed significantly from controls in various fixation and saccade parameters. For fixation parameters patients with hemianopia fixated different spatial positions from controls, made more fixations which were more widely distributed and of shorter duration than controls, and spent a greater proportion of their total fixation time in the area corresponding to their blind hemifield. They did not make significantly more refixations than controls. For saccade parameters patients made more saccades into their blind hemifield, these saccades having shorter latencies and shorter amplitudes than those made into their seeing field, and had longer scanpaths than control subjects. The amplitude of their first saccade was longer than that of controls although its direction did not correlate simply with the side of the field defect. Their mean saccade amplitude was similar to that of controls. Filtering out high spatial frequencies within images seemed to accentuate the described differences between eye movement characteristics of hemianopes and controls. Scanpath differences correlated with increasing age but not location or size of lesions causing the hemianopia. CONCLUSION Various features of scanpaths produced by hemianopes were different from normal subjects. These differences correlated with lesion age and may reflect the evolution of a compensatory eye movement strategy.

Fixation Patterns Made during Brief Examination of Two-Dimensional Images

Measurements were carried out of saccadic eye movements made during brief (3 s) examination of images which the observer was asked to identify. Each image was identified in three forms: low-pass filtered, high-pass filtered, and unfiltered. The analysis of the eye-movement patterns was based on the locations of fixations made during examination of the images, for which purpose a least-squares measure of similarity between two sets of locations was introduced. It is shown that there is a high degree of similarity between fixations made by the same observer to the different versions of a given image and that for a given image there is a high degree of similarity between fixations made by the eighteen observers who participated in the experiments. The similarities are greater for the initial 1.5 s than for the full viewing period of 3 s. The similarity between the locations of fixations and those of selected image features such as local contrast, high-spatial-frequency content, and edge density was also examined. It is shown that there is only weak similarity between the locations of fixations and those of any given local image feature, and the tendency of observers to fixate centrally on the image is identified as the principal reason for the low similarity values. It is shown that if the nonuniform distribution of eye movements is taken into account, significant similarities are found between the locations of fixations and those of certain image features, such as edge density.

Fixation sequences made during visual examination of briefly presented 2D images.

Eye movements made by eighteen observers in response to brief (3 s) presentations of eleven different images, each in three forms (unfiltered, high-pass filtered and low-pass filtered), have been analysed in order to identify both repeated sequences of fixations and image locations which attract re-fixations. It is shown that eye-movement traces made by different observers in response to the same image have few common temporal sequences involving the same fixation locations, even for sequences of only two fixations. There is a greater incidence of such sequences in eye-movement traces made by the same observer in response to two presentations of the same image, but average numbers are still low. Conserved sequences involving more than two identical locations occur at a much lower frequency, and the incidence of repeated sequences is not increased if consideration is restricted to regions of the image which attract large numbers of fixations. It is concluded that the temporal sequence in which fixations are made is not a significant factor in the analysis of the eye-movement data considered in this report. Calculations based on a least squares index of similarity are consistent with this conclusion. The analysis shows a relatively high incidence of re-fixation on certain locations in the images and there is evidence that such re-fixations are a significant factor in the high similarity between fixation locations established by different observers when viewing the same image.

Impaired spatial working memory: one component of the visual neglect syndrome?

Both impaired spatial working memory (SWM) and unilateral neglect may follow damage to the right parietal lobe. We propose that impaired SWM can exacerbate visual neglect, due to failures in remembering locations that have already been searched. When combined with an attentional bias to the ipsilesional right side, such a SWM impairment should induce recursive search of ipsilesional locations. Here we studied a left neglect patient with a right temporoparietal haemorrhage. On a nonlateralised, purely vertical SWM task, he was impaired in retaining spatial locations. In a visual search task, his eye position was monitored while his spatial memory was probed. He recursively searched through right stimuli, re-fixating previously inspected items, and critically treated them as if they were new discoveries, consistent with the SWM deficit. When his recovery was tracked over several months, his SWM deficit and left neglect showed concurrent improvements. We argue that impaired SWM may be one important component of the visual neglect syndrome.

The oculomotor distractor effect in normal and hemianopic vision.

The present study investigated the inhibitory effect of visual distractors on the latency of saccades made by hemianopic and normal human subjects. The latency of saccades made by hemianopic subjects to stimuli in their intact visual field was not affected by visual distractors presented within their hemianopic field. In contrast, the latency of saccades made by normal subjects was increased significantly under distractor conditions. The latency increase was larger for temporal than nasal distractors. The results are inconsistent with previous proposals that the crossed retinotectal pathway from the nasal hemiretina to the superior colliculus may mediate a blindsight inhibitory effect when distractors appear within a hemianopic temporal visual field. Instead, the distractor effect appears to reflect the normal processes involved in saccade target selection which may be mediated by a circuit involving both cortical and subcortical structures.

Involvement of prefrontal cortex in visual search

Visual search for target items embedded within a set of distracting items has consistently been shown to engage regions of occipital and parietal cortex, but the contribution of different regions of prefrontal cortex remains unclear. Here, we used fMRI to compare brain activity in 12 healthy participants performing efficient and inefficient search tasks in which target discriminability and the number of distractor items were manipulated. Matched baseline conditions were incorporated to control for visual and motor components of the tasks, allowing cortical activity associated with each type of search to be isolated. Region of interest analysis was applied to critical regions of prefrontal cortex to determine whether their involvement was common to both efficient and inefficient search, or unique to inefficient search alone. We found regions of the inferior and middle frontal cortex were only active during inefficient search, whereas an area in the superior frontal cortex (in the region of FEF) was active for both efficient and inefficient search. Thus, regions of ventral as well as dorsal prefrontal cortex are recruited during inefficient search, and we propose that this activity is related to processes that guide, control and monitor the allocation of selective attention.