Timothy L. Hodgson

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.

Predictive Learning, Prediction Errors, and Attention: Evidence from Event-related Potentials and Eye Tracking

Prediction error (surprise) affects the rate of learning: We learn more rapidly about cues for which we initially make incorrect predictions than cues for which our initial predictions are correct. The current studies employ electrophysiological measures to reveal early attentional differentiation of events that differ in their previous involvement in errors of predictive judgment. Error-related events attract more attention, as evidenced by features of event-related scalp potentials previously implicated in selective visual attention (selection negativity, augmented anterior N1). The earliest differences detected occurred around 120 msec after stimulus onset, and distributed source localization (LORETA) indicated that the inferior temporal regions were one source of the earliest differences. In addition, stimuli associated with the production of prediction errors show higher dwell times in an eye-tracking procedure. Our data support the view that early attentional processes play a role in human associative learning.

The Strategic Control of Gaze Direction in the Tower of London Task

In this paper, we describe a novel approach to the study of problem solving involving the detailed analysis of natural scanning eye movements during the one-touch Tower-of-London (TOL) task. We showed subjects a series of pictures depicting two arrangements of colored balls in pockets within the uper and lower halves of a computer display. The task was to plan (but not to execute)the shortest movement sequence required to rearrange the balls in one half of the display (the workspace)to match the arrangement in the opposite half (the Goalspace)and indicate the minimum number of moves required for problem solution. We report that subjects are more likely to look towards the Goalspace in the initial period after picture presentation, but bias gaze towards the Workspace during the middle of trials. Towards the end of a trial, subjects are once again more likely to fixate the Goalspace. This pattern is found regardless of whether the subjects solve problems by rearranging the balls in the lower or uper visual fields, demonstrating that this strategy correlates with discreate phases in problem solving. A second experiment showed that efficient planners direct their gaze selectively towards the problem critical balls in the workspace. In contrast, Individuals who make errors spend more time looking at irrelevant items and are strongly influenced by the movement strategy needed to solve the preceding problem. We conclude that efficient solution of the TOL requires the capacity to generate and flexibly shift between control sets, including those underlying ocular scanning. The role of working memory and the prefrontal cerebral cortex in the task are discussed.

Eye movements and spatial working memory in Parkinson's disease.

Abstract Mechanisms of spatial working memory and eye movement control were investigatedin eight mild to moderate Parkinsons disease patients (PDs). Subjects were presented with asequence of four targets which had to be memorized and then recalled by moving their eyes tofixate the four locations in the correct order. Two variations on this procedure were used inwhich either a different sequence of lights was presented on each trial, or an identical sequenceof lights was repeated on each trial. In both conditions subjects made memory-guided eyemovements in the dark, without any visual cues to eye movement accuracy or the locations of thepreviously illuminated lights. Analysis of the amplitude of the primary eye movement and finaleye position for each step in the sequence showed that PDs made several discrete saccadic eyemovements of reduced amplitude before reaching the final eye position (multi-stepping). When anovel target sequence had to be memorized on each trial, the final eye position reached by PDsfor each location was also found to undershoot relative to controls. In contrast, when an identicalsequence of targets was repeated on each trial, PDs final eye position was found to be normal,although primary movement amplitudes were still reduced. PDs showed no multi-stepping andnormal final eye position gain under conditions for which the target lights in the sequence wereilluminated during movement execution. PDs also made an increased proportion of overt errorsin target sequence recall. Parallel neuropsychological testing in PDs and controls revealed thaterror rates in the sequential memory-guided saccade task were significantly correlated withperformance in a task thought to be sensitive to spatial working memory dysfunction. Thefindings suggest that short-term spatial memory representations are disrupted in the early stagesof PD.

Impaired reading in patients with right hemianopia.

A left occipital stroke may result in alexia for two reasons, which may coexist depending on the distribution of the lesion. A lesion of the left lateroventral prestriate cortex or its afferents impairs word recognition (“pure” alexia). If the left primary visual cortex or its afferents are destroyed, resulting in a complete right homonymous hemianopia, rightward saccades during text reading are disrupted (“hemianopic” alexia). By using functional imaging, we showed two separate but interdependent systems involved in reading. The first, subserving word recognition, involved the representation of foveal vision in the left and right primary visual cortex and the ventral prestriate cortex. The second system, responsible for the planning and execution of reading saccades, consisted of the representation of right parafoveal vision in the left visual cortex, the bilateral posterior parietal cortex (left > right), and the frontal eye fields (right > left). Disruption of this distributed neural system was demonstrated in patients with severe right homonymous hemianopia, commensurate with their inability to perform normal reading eye movements. Text reading, before processes involved in comprehension, requires the integration of perceptual and motor processes. We have demonstrated these distributed neural systems in normal readers and have shown how a right homonymous hemianopia disrupts the motor preparation of reading saccades during text reading. Ann Neurol 2000;47:171–178.

Suppression of pendular nystagmus by smoking cannabis in a patient with multiple sclerosis.

To the Editor: I read with interest the article by Schon et al.1 In the early 1960s, I observed an individual with congenital nystagmus whose nystagmus damped after smoking cannabis; the damping was obvious and it was evident to others. Unfortunately, the setting precluded ocular motor recording and the date preceded the development of accurate techniques to accomplish such recording. However, the subject was able to read small print on a poster across the room on the wall opposite to where he was seated, which was not possible before smoking the cannabis. Over the ensuing years, that observation has been supported by unsolicited, first-hand reports of similar effects by several patients with congenital nystagmus referred to our …

Identification of an oculomotor biomarker of preclinical Huntington disease

The authors examined oculomotor function to identify a biomarker of disease progression in genetically confirmed preclinical and early clinical Huntington disease (HD). Initiation deficits of voluntary-guided, but not reflexive, saccades were characteristic of preclinical HD. Saccadic slowing and delayed reflexive saccades were demonstrated in clinical but not preclinical HD. Saccadic measures provide biomarkers of disease progression in both preclinical and early clinical stages of HD.

The effects of acute exercise on attentional bias towards smoking‐related stimuli during temporary abstinence from smoking

RATIONALE Attentional bias towards smoking-related cues is increased during abstinence and can predict relapse after quitting. Exercise has been found to reduce cigarette cravings and desire to smoke during temporary abstinence and attenuate increased cravings in response to smoking cues. OBJECTIVE To assess the acute effects of exercise on attentional bias to smoking-related cues during temporary abstinence from smoking. METHOD In a randomized cross-over design, on separate days regular smokers (n = 20) undertook 15 minutes of exercise (moderate intensity stationary cycling) or passive seating following 15 hours of nicotine abstinence. Attentional bias was measured at baseline and post-treatment. The percentage of dwell time and direction of initial fixation was assessed during the passive viewing of a series of paired smoking and neutral images using an Eyelink II eye-tracking system. Self-reported desire to smoke was recorded at baseline, mid- and post-treatment and post-eye-tracking task. RESULTS There was a significant condition x time interaction for desire to smoke, F((1,18)) = 10.67, P = 0.004, eta(2) = 0.36, with significantly lower desire to smoke at mid- and post-treatment following the exercise condition. The percentage of dwell time and direction of initial fixations towards smoking images were also reduced significantly following the exercise condition compared with the passive control. CONCLUSION Findings support previous research that acute exercise reduces desire to smoke. This is the first study to show that exercise appears to also influence the salience and attentional biases towards cigarettes.

Imaging the impossible: an fMRI study of impossible causal relationships in magic tricks

Understanding causal relationships and violations of those relationships is fundamental to learning about the world around us. Over time some of these relationships become so firmly established that they form part of an implicit belief system about what is possible and impossible in the world. Previous studies investigating the neural correlates of violations of learned relationships have focused on relationships that were task-specific and probabilistic. In contrast, the present study uses magic-trick perception as a means of investigating violations of relationships that are long-established, deterministic, and that form part of the aforementioned belief system. Compared to situations in which expected causal relationships are observed, magic trick perception recruited dorso-lateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC), brain regions associated with the detection of conflict and the implementation of cognitive control. These activations were greater in the left hemisphere, supporting a role for this hemisphere in the interpretation of complex events. DLPFC is more greatly activated by magic tricks than by surprising events, but not more greatly activated by surprising than non surprising events, suggesting that this region plays a special role in causality processing. The results suggest a role for cognitive control regions in the left hemisphere in a neurobiology of disbelief.