Bettina Olk

Why are antisaccades slower than prosaccades? A novel finding using a new paradigm.

Eye movements away from a new object (antisaccades) are slower than towards it (prosaccades). This finding is assumed to reflect the fact that prosaccades to new objects are made reflexively, and that for antisaccades, reflexive eye movements have to be inhibited and antisaccades are generated volitionally. Experiment 1 investigated the relative contribution of saccade inhibition by comparing the latency difference between pro- and antisaccades obtained in the traditional blocked paradigm and in a new paradigm in which oculomotor inhibition across pro- and antisaccades was matched. When inhibition was placed on the oculomotor system, the latency difference between pro- and antisaccades was significantly reduced. Experiment 2 examined the contribution of volitional saccade programming and execution by requiring both pro- and antisaccades to be programmed volitionally. This manipulation did not decrease further the difference between pro- and antisaccades. It is thus concluded that oculomotor inhibition is the main factor leading to long antisaccade latency. The remaining difference is attributed to the reallocation of covert attention from the target location towards the opposite antisaccade location.

Manual responses and saccades in chronic and recovered hemispatial neglect: a study using visual search

Hemispatial neglect affects both the ability to respond to targets on the contralesional side of space and to programme saccades to such targets. In the current study, we looked in detail at saccade programming and manual reaction times (RTs) in a range of visual search tasks, in which task difficulty was systematically increased by changing the nature of the distractors. In condition 1, the target was presented with no distractors. In the other conditions, displays contained three distractors that were changed across conditions to manipulate similarity to the target and so task difficulty. We tested two neglect patients, one chronic, one recovered along with two RCVA control patients and 12 age-matched controls. Both neglect patients studied could successfully execute saccades into the neglected field when the target was presented alone. However, a dissociation emerged between the two patients when the target was presented with distractor items. Patient ERs first saccade to target performance in the three search conditions revealed clear effects of distractor type. In contrast for the recovered patient AF, the left/right difference was present for all search displays and appeared to be constant regardless of distractor type. This differential pattern of behaviour may reflect the different underlying neural causes of the neglect in these patients. In the current study, the measurement of saccades allowed the task to be fractionated, and thus, reveal the action of multiple mechanisms controlling saccades in search.

First Saccades Reveal Biases in Recovered Neglect

Hemispatial neglect affects the ability to explore space on the side opposite a brain lesion. This deficit is also mirrored in abnormal saccadic eye movement patterns. The present study investigated if the recovery of neglect is also reflected in saccadic eye movements. Patient AF, who displayed strong hemispatial neglect 1 month post-right thalamic stroke, had largely recovered 3 months later when tested on visual exploration tasks of the Behavioural Inattention Test. At this stage, AF was tested on a visual search task while his eye movements (direction, latencies and amplitudes of first saccades) and manual reaction times were recorded. The experimental conditions differed with respect to stimulus number and distracter type and increased in difficulty. AF correctly generated saccades into the neglected field when the target was presented alone. In contrast, a considerable left/right difference was present for all multiple-stimulus search displays. Although recovered from neglect in standardized assessment, AF showed a strong rightward bias resulting in highly asymmetric response times and eye movement behaviour. We conclude that eye movement patterns are far more susceptible to remaining spatial impairments and can thus provide a sensitive means to assess the extent of neglect recovery.

Involuntary but not voluntary orienting contributes to a disengage deficit in visual neglect.

Patients with left neglect are particularly slow to respond to visual targets on their left when attention is first engaged to their right. This deficit is known as the disengage deficit (DD). Studies investigating the DD typically employ nonpredictive peripheral onset cues to measure involuntary orienting and predictive central arrow cues to measure voluntary orienting. A DD has been observed with both cues, suggesting that a DD occurs for involuntary and for voluntary orienting. Recent evidence questions this conclusion because nonpredictive central arrow cues trigger involuntary orienting. This implies that predictive central arrows also involve involuntary orienting and do not measure only voluntary attention. This new knowledge suggests a new conceptualization of the DD. While it is undisputed that a DD occurs when attention is shifted involuntarily, it is uncertain whether a DD is produced by voluntary orienting because most previous cuing studies of the DD have involved shifts of involuntary attention. To address this critical question, we tested neglect and control patients with nonpredictive and predictive peripheral onset cues (Experiment 1), nonpredictive and predictive central arrow cues (Experiment 2), and predictive central number cues (Experiment 3). The experiments provide three lines of converging evidence that voluntary orienting does not contribute to a DD. First, the DD was the same whether attention was engaged involuntarily by nonpredictive peripheral cues or engaged involuntarily and voluntarily by predictive peripheral cues (Experiment 1), indicating that voluntary orienting does not modulate the DD. Second, the DD was the same whether attention was engaged involuntarily by nonpredictive central arrow cues or engaged involuntarily and voluntarily by predictive central arrow cues (Experiment 2), replicating the finding of Experiment 1 with very different cues. Third, the DD was not present when attention was only engaged voluntarily by central predictive number cues (Experiment 3).

Illusion processing in hemispatial neglect

Twelve patients with hemispatial neglect and two control groups were tested to examine the effects of the Müller-Lyer and Judd illusions on bisection behaviour. The studies were designed to investigate whether neglect patients were indeed unaware of the left sides of the illusory figures. In Experiment 1, participants were asked to describe the illusory figures prior to bisection, whereas in Experiment 2, they compared two illusions whose fins, in the critical condition, differed on the left and then performed the bisection. It was found that the illusions worked equally well in all three groups. Interestingly, apart from one exception, almost all neglect patients explicitly reported the left-sided fins in Experiment 1. Only five patients failed to do so but only on an average of 16% of trials. In Experiment 2, six patients made errors in the comparison task but four of these patients did not neglect any left-sided fins in Experiment 1 (with the exception of three overall trials for LC and EdR). This finding seems a good indication that the two tasks differ in their requirements. The comparison task may be perceived as harder as it requires discrimination rather than detection and thus lead to more neglect type errors than the bisection task. In one neglect patient, the illusions consistently failed to work. This patient presented with an occipito-temporal and basal ganglia lesion and the mechanisms responsible for the processing of simple visual features might have possibly been impaired in her case.

Enhanced orienting effects: Evidence for an interaction principle

Predictive arrow cues, as used in the classic “Posner paradigm”, that were long thought to engage and isolate voluntary attention, may in fact trigger a strong interaction between voluntary and involuntary attention (Ristic & Kingstone, 2006). This interaction produces an orienting effect that exceeds both the effects of involuntary and voluntary attention alone, and the additive combination of involuntary and voluntary orienting. The present study shows that nonpredictive peripheral cues—understood to engage and isolate involuntary attention—if made predictive, result in enhanced orienting effects similar to predictive arrows. The important contribution of these data is that they suggest an “interaction principle”: If attention cues can elicit reliable involuntary orienting, then when they are made spatially predictive, the resulting attention effect will be greater than the sum of involuntary and voluntary orienting alone.

Eye-movement patterns do not mediate size distortion effects in hemispatial neglect: looking without seeing

Over the last decade a range of studies have shown that some patients with hemispatial neglect subjectively underestimate the size of objects presented in their contralesional hemispace. Recently, it has been suggested that the effect is simply due to either hemianopia [Brain 124 (2001) 527], or the combination of neglect and hemianopia [Neurology 52 (1999) 1845]. In the current study we asked right hemisphere lesioned patients with and without neglect and hemianopia as well as healthy controls to judge either two horizontal or vertical lines presented simultaneously in right and left hemispace and monitored their eye movements. Three out of the six patients showed the predicted size distortion effect for horizontal lines. We found no evidence that the effect was mediated by eye movements. The two neglect patients who showed the strongest left side underestimation showed symmetrical (left, right) scanning of the lines both in terms of number of fixations and fixation time, yet they still failed to judge the relative size veridically. In addition, we did not find strong evidence for a link with hemianopia. We therefore propose that the effect reflects a computational/representational failure of processing for horizontal extent.

Perceiving press photography: a new integrative model, combining iconology with psychophysiological and eye-tracking methods

Any analysis of how mass-mediated visuals are perceived and interpreted in multimodal contexts should be informed by a scientific understanding of the biological constraints on visual processing, as well as a solid culturally aware visual communication approach. This article focuses on the interdisciplinary combination of three methods – iconology, a qualitative method of visual analysis targeted at the meanings of visuals and based in the humanities, and eye-tracking and psychophysiological reaction measurement, both based in experimental psychology. The authors propose a Visual Communication Process Model as an integrative means for connecting different facets of the communication processes involved in visual mass communication. The goal of this new model is to widen and sharpen the focus on explaining (a) meaning-attribution processes, (b) visual perception and attention processes, and (c) psychophysiological reactions to mass-mediated visuals, illustrated in this article with examples of press photography.

Non-lateralised deficits in anti-saccade performance in patients with hemispatial neglect

We tested patients suffering from hemispatial neglect on the anti-saccade paradigm to assess voluntary control of saccades. In this task participants are required to saccade away from an abrupt onset target. As has been previously reported, in the pro-saccade condition neglect patients showed increased latencies towards targets presented on the left and their accuracy was reduced as a result of greater undershoot. To our surprise though, in the anti-saccade condition, we found strong bilateral effects: the neglect patients produced large numbers of erroneous pro-saccades to both left and right stimuli. This deficit in voluntary control was present even in patients whose lesions spared the frontal lobes. These results suggest that the voluntary control of action is supported by an integrated network of cortical regions, including more posterior areas. Damage to one or more components within this network may result in impaired voluntary control.

A new look at aging and performance in the antisaccade task: The impact of response selection

Aged adults respond more slowly and less accurately in the antisaccade task, in which a saccade away from a visual stimulus is required. This decreased performance has been attributed to a decline in the ability to inhibit prepotent responses with age. Considering that antisaccades also involve response selection, the present experiment investigated the contribution of inhibition and response selection. Young and aged adults were compared between conditions that required varying percentages of prosaccades, antisaccades, and no-go trials. The comparison between no-go (inhibition of a prosaccade) and antisaccade trials (inhibition of a prosaccade and selection of an antisaccade) showed significantly worse performance in the antisaccade task, especially for the older group, suggesting that they failed to select the antisaccade in a situation in which a competing, prepotent response is available. The impact of this response selection failure was underlined by an equivalent ability of both groups to impose inhibition.