Andrea Tales

Mismatch negativity in the visual modality

In the auditory system, the automatic detection of stimulus change provides a mechanism for switching attention to biologically significant events. It gives rise to the mismatch negativity (MMN) event related potential. It is unclear whether a similar mechanism exists in vision. To investigate this issue, evoked potentials were recorded to target stimuli in the centre of the visual field, and to frequent standard and infrequent deviant stimuli presented outside the focus of attention, in the peripheral field. Deviants evoked a more negative potential than standards 250-400 ms after the stimulus. The negativity, distributed over supplementary visual areas of occipital and posterior temporal cortex, was associated with the rarity of the deviants and not the physical features which distinguished them from standards. This negativity shares a number of characteristics with auditory MMN.

Visual mismatch negativity: the detection of stimulus change.

Mismatch negativity is an event related potential generated by a mechanism which detects stimulus change. Such a mechanism is important to enable attention to be switched to important changes in the environment. The effect has been extensively studied in the auditory modality. The present investigation was designed to establish whether the enhanced negativity in the visual event related potential evoked by deviant stimuli presented infrequently among a sequence of repeated standard stimuli is really associated with the detection of stimulus change. The experiment set out to distinguish effects associated with stimulus change from those related to the physical attributes of the stimuli or to differences in the refractory state of receptors or neurons. The findings support the hypothesis that deviance-related negativity reflects the operation of a change detection mechanism and not the refractory state of elements of the visual system.

Abnormal visual search in mild cognitive impairment and Alzheimer's disease

Our aim was to further characterize the clinical concept of mild cognitive impairment (MCI). We examined the status of visual attention-related processing in such patients in relation to healthy older adults and patients with Alzheimer’s disease (AD) by measuring performance on a computer-based visual search task. We tested 20 older adult control participants, 13 patients with amnestic mild cognitive impairment and 12 patients with AD. Patients with AD and with MCI exhibited a significant detriment in visual search performance compared to the older adult controls. The deficit in visual search was greater for the patients with AD than the patients with MCI. The pattern of results displayed by the MCI group indicates that patients who appear clinically to suffer only from a deficit in memory also display a deficit in visual attention-related processing, which although not as severe as those with AD, represents a significant detriment in such performance compared to that seen in healthy ageing. The authors would like to thank all the participants for taking part and Mr A. Hughes for statistical support. Sara Nelson is now at The Department of Experimental Psychology, University of Bristol, Bristol, UK

Spatial shifts in visual attention in normal ageing and dementia of the Alzheimer type.

Using a spatial-cueing paradigm, we assessed the ability of Alzheimers disease patients, age-matched controls and younger participants to use cues to guide attention to the location indicated by the cue. In separate experiments, we attempted to isolate cues that attract attention automatically (exogenous cueing) and those that require the wilful movement of attention (endogenous cues). We found significant cueing effects for all three groups of participants for both types of cue. However, the group with Alzheimers disease showed far greater cueing effects when using an exogenous cue, whilst no difference between groups ability to use the cue was found for the endogenous cue. No differences in cueing were found for either cue type as a function of normal ageing. We further tested whether the differences in cueing found in the group with Alzheimers disease was due to a generalised slowing of function. After transforming the data to take account of the overall slowing of all responses in this group, we still found significant differences between this group and the control groups. We conclude that patients with Alzheimers disease have an abnormality in automatic, but not controlled visuospatial attention.

Abnormal spatial and non-spatial cueing effects in mild cognitive impairment and Alzheimer's disease

Our aim was to further characterize the clinical concept of mild cognitive impairment (MCI). We examined visual attention-related processing in 12 patients with amnestic MCI, 16 healthy older adults and 16 patients with Alzheimer’s disease (AD) by measuring performance on computer-based tests of attentional disengagement, alerting ability, and inhibition of return. Unlike the healthy older controls, the patients with AD and the patients with amnestic MCI exhibited a significant detriment in both the ability to disengage attention from an incorrectly cued location and the ability to use a visual cue to produce an alerting effect. The pattern of results displayed by the MCI group indicates that patients who only appear clinically to suffer from a deficit in memory also display a deficit in specific aspects of visual attention-related processing, which closely resemble the magnitude seen in AD. The authors would like to thank the Bristol Research Into Alzheimer’s and Care of the Elderly (BRACE) Charity [Registered Charity Number 297965] for financial support. The authors would also like to thank all the participants taking part and Mr A. Hughes for statistical support.

What makes cast shadows hard to see

Visual search is slowed for cast shadows lit from above, as compared to the same search items inverted and so not interpreted as shadows (R. A. Rensink & P. Cavanagh, 2004). The underlying mechanisms for such impaired shadow processing are still not understood. Here we investigated the processing levels at which this shadow-related slowing might operate, by examining its interaction with a range of different phenomena including eye movements, perceptual learning, and stimulus presentation context. The data demonstrated that the shadow mechanism affects the number of saccades during the search rather than the duration until first saccade onset and can be overridden by prolonged training, which then transfers from one type of shadow stimulus to another. Shadow-related slowing did not differ for peripheral and central search items but was reduced when participants searched unilateral displays as compared to bilateral ones. Together our findings suggest that difficulties with perceiving shadows are due to visual processes linked to object recognition, rather than to shadow-specific identification and suppression mechanisms in low-level sensory visual areas. Findings are discussed in the context of the need for the visual system to distinguish between illumination and material.

Age-related changes in the preattentional detection of visual change

&NA; The ability to detect changes in the environment which occur outside the focus of current awareness is essential if the individual is to be able to divert attention to biologically salient stimuli. The preattentional mechanism underlying the automatic detection of stimulus change in the auditory modality has been extensively studied by recording an event‐related potential known as the mismatch negativity. Recently a homologous response from the visual cortex has also been described. Ageing has been shown to affect the efficiency of preattentional processing in the auditory modality, a factor which may contribute to cognitive changes in the elderly. It is unclear whether a similar effect occurs in the visual system. To investigate this issue the visual mismatch negativity was recorded from 12 older adults and 24 younger adults. Whereas the younger adults displayed a robust visual MMN, that evoked in the older adults was significantly reduced in amplitude. The results are indicative of age‐related deficits in automatic visual processing. Neuro Report 13: 969‐972

The effects of saliency and task difficulty on visual search performance in ageing and Alzheimer's disease

We asked whether the poor performance on visual search tasks typical of patients with Alzheimers disease (AD) is the result of a selective deficit in the ability to shift attention from item to item, or the consequence of an inefficient processing of each item within the search set. We attempted to manipulate the ease of attention shifting and item processing in a visual search task by manipulating target salience and task difficulty, respectively. Significant effects of both target saliency and task difficulty for both AD patients and age-matched controls were obtained, with the AD group displaying greater effects of both of these manipulations than the controls. This interaction remained even when the reaction time data were log-transformed to account for the overall slower reaction times of the AD group. We conclude that inefficiency in visual search tasks in AD probably represents the product of both attention shifting and target processing factors.

Visual mismatch negativity (vMMN): A review and meta-analysis of studies in psychiatric and neurological disorders

The visual mismatch negativity (vMMN) response is an event-related potential (ERP) component, which is automatically elicited by events that violate predictions based on prior events. VMMN experiments use visual stimulus repetition to induce predictions, and vMMN is obtained by subtracting the response to rare unpredicted stimuli from those to frequent stimuli. One increasingly popular interpretation of the mismatch response postulates that vMMN, similar to its auditory counterpart (aMMN), represents a prediction error response generated by cortical mechanisms forming probabilistic representations of sensory signals. Here we discuss the physiological and theoretical basis of vMMN and review thirty-three studies from the emerging field of its clinical applications, presenting a meta-analysis of findings in schizophrenia, mood disorders, substance abuse, neurodegenerative disorders, developmental disorders, deafness, panic disorder and hypertension. Furthermore, we include reports on aging and maturation as they bear upon many clinically relevant conditions. Surveying the literature we found that vMMN is altered in several clinical populations which is in line with aMMN findings. An important potential advantage of vMMN however is that it allows the investigation of deficits in predictive processing in cognitive domains which rely primarily on visual information; a principal sensory modality and thus of vital importance in environmental information processing and response, and a modality which arguably may be more sensitive to some pathological changes. However, due to the relative infancy of research in vMMN compared to aMMN in clinical populations its potential for clinical application is not yet fully appreciated. The aim of this review and meta-analysis therefore is to present, in a detailed systematic manner, the findings from clinically-based vMMN studies, to discuss their potential impact and application, to raise awareness of this measure and to improve our understanding of disease upon fundamental aspects of visual information processing.

New insights into feature and conjunction search: I. Evidence from pupil size, eye movements and ageing

Differences in the processing mechanisms underlying visual feature and conjunction search are still under debate, one problem being a common emphasis on performance measures (speed and accuracy) which do not necessarily provide insights to the underlying processing principles. Here, eye movements and pupil dilation were used to investigate sampling strategy and processing load during performance of a conjunction and two feature-search tasks, with younger (18-27 years) and healthy older (61-83 years) age groups compared for evidence of differential age-related changes. The tasks involved equivalent processing time per item, were controlled in terms of target-distractor similarity, and did not allow perceptual grouping. Close matching of the key tasks was confirmed by patterns of fixation duration and an equal number of saccades required to find a target. Moreover, moment-to-moment pupillary dilation was indistinguishable across the tasks for both age groups, suggesting that all required the same total amount of effort or resources. Despite matching, subtle differences in eye movement patterns occurred between tasks: the conjunction task required more saccades to reach a target-absent decision and involved shorter saccade amplitudes than the feature tasks. General age-related changes were manifested in an increased number of saccades and longer fixation durations in older than younger participants. In addition, older people showed disproportionately longer and more variable fixation durations for the conjunction task specifically. These results suggest a fundamental difference between conjunction and feature search: accurate target identification in the conjunction context requires more conservative eye movement patterns, with these further adjusted in healthy ageing. The data also highlight the independence of eye movement and pupillometry measures and stress the importance of saccades and strategy for understanding the processing mechanisms driving different types of visual search.