Discrimination of natural scenes in central and peripheral vision.

We conducted suprathreshold discrimination experiments to compare how natural-scene information is processed in central and peripheral vision (16° eccentricity). Observers' ratings of the perceived magnitude of changes in naturalistic scenes were lower for peripheral than for foveal viewing, and peripheral orientation changes were rated less than peripheral colour changes. A V1-based Visual Difference Predictor model of the magnitudes of perceived foveal change was adapted to match the sinusoidal grating sensitivities of peripheral vision, but it could not explain why the ratings for changes in peripheral stimuli were so reduced. Perceived magnitude ratings for peripheral stimuli were further reduced by simultaneous presentation of flanking patches of naturalistic images, a phenomenon that could not be replicated foveally, even after M-scaling the foveal stimuli to reduce their size and the distances from the flankers. The effects of the peripheral flankers are very reminiscent of crowding phenomena demonstrated with letters or Gabor patches.

Around the clock surveillance: simple graphic disturbance in patients with hemispatial neglect carries implications for the clock drawing task.

BACKGROUND: Drawing, and the clock drawing task in particular, is widely used as a diagnostic tool in the study of hemispatial neglect. It is generally assumed that the errors in graphic production, such as the misplacement of numbers, reflect a visuospatial deficit, and that drawing production itself (for example, producing the circle) is unimpaired. OBJECTIVES: To test this assumption by examining whether the production of simple circles is affected by neglect. METHODS: 16 right hemisphere stroke patients copied circles of various sizes and their drawings were measured for size accuracy. RESULTS: Patients with more severe neglect produced greater scaling errors, consistently drawing the circle smaller than the original. Errors were not in the horizontal axis alone--shrinkage occurred equally in both height and width axes. CONCLUSIONS: Neglect can co-occur with constructional difficulties that serve to exacerbate the symptoms presented. This should be taken into account in the assessment of even apparently simple drawing tasks.

Are the perceptual biases found in chimeric face processing reflected in eye-movement patterns?

Studies of patients with focal brain lesions and neuroimaging indicate that face processing is predominantly based on right hemisphere function. Additionally, experiments using chimeric faces, where the left and the right-hand side of the face are different, have shown that observers tend to bias their responses toward the information on the left. Here, we monitored eye-movements during a gender identification task using blended face images for both whole and chimeric (half female, half male) faces [Neuropsychologia 35 (1997) 685]. As expected, we found a left perceptual bias: subjects based their gender decision significantly more frequently on the left side of the chimeric faces. Analysis of the first saccade showed a significantly greater number of left fixations independent of perceptual bias presumably reflecting the tendency to first inspect the side of the face better suited to face analysis (left side of face/right hemisphere). On top of this though, there was a relationship between response and fixation pattern. On trials where participants showed a left perceptual bias they produced significantly more left saccades and fixated for longer on the left. In contrast, for trials where participants showed a right perceptual bias there was no reliable difference between the number, or total fixation duration, on the left or the right. These results demonstrate that on a trial-by-trial basis subtle differences in the extent of left or right side scanning are related to the perceptual response of the participant, although an overall initial fixation bias to the left occurs irrespective of response bias.

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.

Visual search in Alzheimer's disease: a deficiency in processing conjunctions of features.

Human vision often needs to encode multiple characteristics of many elements of the visual field, for example their lightness and orientation. The paradigm of visual search allows a quantitative assessment of the function of the underlying mechanisms. It measures the ability to detect a target element among a set of distractor elements. We asked whether Alzheimer's disease (AD) patients are particularly affected in one type of search, where the target is defined by a conjunction of features (orientation and lightness) and where performance depends on some shifting of attention. Two non-conjunction control conditions were employed. The first was a pre-attentive, single-feature, "pop-out" task, detecting a vertical target among horizontal distractors. The second was a single-feature, partly attentive task in which the target element was slightly larger than the distractors-a "size" task. This was chosen to have a similar level of attentional load as the conjunction task (for the control group), but lacked the conjunction of two features. In an experiment, 15 AD patients were compared to age-matched controls. The results suggested that AD patients have a particular impairment in the conjunction task but not in the single-feature size or pre-attentive tasks. This may imply that AD particularly affects those mechanisms which compare across more than one feature type, and spares the other systems and is not therefore simply an 'attention-related' impairment. Additionally, these findings show a double dissociation with previous data on visual search in Parkinson's disease (PD), suggesting a different effect of these diseases on the visual pathway.

Impaired size processing for single objects after hemispatial neglect.

Some patients with hemispatial neglect show deficits in horizontal size perception. Most previous studies investigating this effect required the relative comparison of two horizontal stimuli. We examined whether the effect would also be present for single stimuli which would reflect an impairment in the computation of absolute horizontal length. Ten neglect patients, five with and five without hemianopia, and two control groups were asked to verbally judge the length of horizontal lines varying in length (3, 3.5, 4 inches) and spatial location (extreme left to extreme right). Three of the ten neglect patients judged a single object on the left as significantly smaller than a single object on the right, thus demonstrating a size processing deficit. This deficit was unrelated to hemianopia.

Saccade target selection in visual search: the effect of information from the previous fixation.

This paper reports an analysis of saccades made during a task of visual search for a colour shape conjunction. The analysis concentrates on the saccade following the first saccade, thus complementing an earlier paper where the first saccades were analysed. The further analysis addresses the issue of what information might be held in trans-saccadic memory. As with the first saccade, incorrect second saccades tend to fall on distractors sharing one feature with the target. The proximity of the target to the fixation location immediately prior to the saccade is a very significant determinant of whether the saccade will reach the target. The results lead to the conclusion that in the majority of cases, choice of saccade destination is made afresh during each fixation with no carry-over from the previous fixation. However, in a small number of cases, second saccades are made after extremely brief fixation intervals. Although these saccades show a similar probability of reaching the target as those following longer fixations, it is argued that this sub-set of saccades are pre-programmed at the time of the preceding saccade.

Is visual search really like foraging?

The visual-search paradigm provides a controlled and easy to implement experimental situation in which to study the search process. However, little work has been carried out in humans to investigate the extent to which traditional visual-search tasks are similar to more general search or foraging. Here we report results from a task in which search involves walking around a room and leaning down to inspect individual locations. Consistent with more traditional search tasks, search time increases linearly with display size, and the target-present to target-absent search slope is 1:2. However, although rechecking of locations did occur, compared to more traditional search it was relatively rare, suggesting an increased role for memory.

Refixation frequency and memory mechanisms in visual search.

Visual search-looking for a target object in the presence of a number of distractor items-is an everyday activity for humans (for example, finding the car in a busy car park) and animals (for example, foraging for food). Our understanding of visual search has been enriched by an interdisciplinary effort using a wide range of research techniques including behavioural studies in humans [1], single-cell electrophysiology [2], transcranial magnetic stimulation [3], event-related potentials [4] and studies of patients with focal brain injury [5]. A central question is what kind of information controls the search process. Visual search is typically accompanied by a series of eye movements, and investigating the nature and location of fixations helps to identify the kind of information that might control the search process. It has already been demonstrated that objects are fixated if they are visually similar to the target [6]. Also, if an item has been fixated, it is less likely to be returned to on the subsequent saccade. This automatic process is referred to as inhibition of return (IOR [7,8]). Here, we investigated the role of memory for which items had been fixated previously. We found that, during search, subjects often refixated items that had been previously fixated. Although there were fewer return saccades than would be expected by chance, the number of refixations indicated limited functional memory, indeed the memory effects that were present may primarily be a result of IOR.

Acute stress, memory, attention and cortisol.

An investigation was conducted to explore the relationship between acute changes in cortisol and memory and attention in the context of an acute naturalistic stressor, namely, examination stress. Sixty students (36 male, 24 female) participated in an assessment of self-reported levels of stress, salivary cortisol, short term memory, selective and divided attention and auditory verbal working memory. Assessments were conducted during a non-exam and exam period. The results revealed that the exam period was associated with an increase in perceived levels of stress, but also a significant reduction in levels of salivary cortisol, compared with the non-exam period. This reduction in cortisol was associated with enhanced short-term memory (as measured by the total number of words recalled in a free recall task), impaired attention and an impairment in the primacy effect (a hippocampal-specific index of short term memory), but no significant effects on auditory verbal working memory. It was concluded that the results support the view that cortisol can modulate cognitive processes and that the effects of corticosteroids on cognitive function are selective.

Saccade selection in visual search: evidence for spatial frequency specific between-item interactions.

We present two experiments in which subjects were required to make a saccade to a target amongst distractors. Targets were oriented Gabor patches. Analysis of errors, when subjects fail to make a saccade to the target, showed two interesting features. First, most error saccades were directed towards a distractor and not to the blank space between distractors. This suggests that although the location of the target may not be encoded correctly, the locations of the items in the display are encoded. Second, when the display items were all of the same spatial frequency, a long-range effect occurred whereby the likelihood of an error saccade in a specific direction decreased systematically as the distance from the target increases. This systematic influence of the target location extended over practically the whole display. The long-range effect appeared whenever all display items had the same spatial frequency and showed little dependence on the spatial frequency of the display items. However, when the items had different spatial frequencies the long-range effects were absent.

Using the eye-movement system to control the head.

We tested the hypothesis that A.I., a subject who has total ophthalmoplegia, resulting in a lack of eye movements, used her head to orientate in a qualitatively similar way to eye-based orientating of control subjects. We used four classic eye-movement paradigms and measured A.I.'s head movements while she performed the tasks. These paradigms were (i) the gap paradigm, (ii) the remote-distractor effect, (iii) the anti-saccade paradigm, and (iv) tests of saccadic suppression. In all cases, A.I.'s head saccades were qualitatively similar to previously reported eye-movement data. We conclude that A.I.'s head movements are probably controlled by the same neural mechanisms that control eye movements in unimpaired subjects.

Luminance and edge information in grouping: a study using visual search.

Preattentive grouping is supported by 2 systems, a brightness system that is contrast polarity sensitive and an edge system that is relatively insensitive to contrast polarity. Search was spatially parallel for pairs of same contrast polarity vertically aligned circles, among horizontal pairs, and serial for pairs of circles that had the opposite contrast polarity (Experiments 1-3). By replacing the circles with squares, the authors investigated the effect of adding collinear edge information. When collinear edges were present, the polarity difference between paired items did not disrupt grouping (Experiments 4-6). These results support models of grouping in which brightness and edge information are processed separately (e.g., S. Grossberg & E. Mingolla, 1985) and models of visual search in which complex relations between stimuli can be computed in parallel across the display.

Spatial scale and saccade programming.

The global effect in eye orienting occurs when saccades land at the 'centre of gravity' of a target stimulus configuration. Short-latency saccades are particularly prone to this effect whereas longer-latency saccades may show more influence of fine detail. Alternative explanations of these effects are considered and data are presented from an experiment in which the influence of different stimulus features on the global effect in a search task was examined. The effect shows a substantially different time course for target-distractor combinations differing in contrast polarity (black vs white) than for combinations differing in shape (circle vs square). It is concluded that the global effect cannot be explained either as a high-level strategic effect or as an effect of automatic fast processing of low-spatial-frequency information in early sensory channels. Instead it is suggested that the visual-spatial-integration characteristic of the global effect is an integral and unavoidable part of the process of selection of saccadic response.