Gillian Porter

Effort during visual search and counting: Insights from pupillometry

We investigated the processing effort during visual search and counting tasks using a pupil dilation measure. Search difficulty was manipulated by varying the number of distractors as well as the heterogeneity of the distractors. More difficult visual search resulted in more pupil dilation than did less difficult search. These results confirm a link between effort and increased pupil dilation. The pupil dilated more during the counting task than during target-absent search, even though the displays were identical, and the two tasks were matched for reaction time. The moment-to-moment dilation pattern during search suggests little effort in the early stages, but increasingly more effort towards response, whereas the counting task involved an increased initial effort, which was sustained throughout the trial. These patterns can be interpreted in terms of the differential memory load for item locations in each task. In an additional experiment, increasing the spatial memory requirements of the search evoked a corresponding increase in pupil dilation. These results support the view that search tasks involve some, but limited, memory for item locations, and the effort associated with this memory load increases during the trials. In contrast, counting involves a heavy locational memory component from the start.

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.

Turning the other cheek: the viewpoint dependence of facial expression after-effects

How do we visually encode facial expressions? Is this done by viewpoint-dependent mechanisms representing facial expressions as two-dimensional templates or do we build more complex viewpoint independent three-dimensional representations? Recent facial adaptation techniques offer a powerful way to address these questions. Prolonged viewing of a stimulus (adaptation) changes the perception of subsequently viewed stimuli (an after-effect). Adaptation to a particular attribute is believed to target those neural mechanisms encoding that attribute. We gathered images of facial expressions taken simultaneously from five different viewpoints evenly spread from the three-quarter leftward to the three-quarter rightward facing view. We measured the strength of expression after-effects as a function of the difference between adaptation and test viewpoints. Our data show that, although there is a decrease in after-effect over test viewpoint, there remains a substantial after-effect when adapt and test are at differing three-quarter views. We take these results to indicate that neural systems encoding facial expressions contain a mixture of viewpoint-dependent and viewpoint-independent elements. This accords with evidence from single cell recording studies in macaque and is consonant with a view in which viewpoint-independent expression encoding arises from a combination of view-dependent expression-sensitive responses.

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.

Intra-Individual Reaction Time Variability in Amnestic Mild Cognitive Impairment: A Precursor to Dementia?

We used an exogenous target detection cueing paradigm to examine whether intra-individual reaction time variability (IIV) or phasic alerting varied significantly between patients with amnestic mild cognitive impairment (aMCI) (n = 45) and healthy older adult controls (n = 31) or between those with aMCI who, within a 2.5 year follow-up period, developed dementia (n = 13) and those who did not (n = 26). Neither IIV, nor simple reaction time, differentiated aMCI from healthy aging, indicating that raised IIV and overall response slowing are not general characteristics of aMCI. However, within the aMCI group, IIV did differentiate between those who converted to dementia and those who remained with a diagnosis of aMCI (non-converters), being significantly more variable in those who later developed dementia. Furthermore, there was no difference in IIV between non-converters and healthy controls. High IIV appears related to an increased probability that an individual with aMCI will become demented within 2.5 years, rather than to amnestic dysfunction per se. In contrast, phasic alerting performance significantly differentiated aMCI from healthy aging, but failed to discriminate those with aMCI who developed dementia from those who did not. In addition, those patients with aMCI who did not develop dementia still showed a significantly poorer phasic alerting effect compared to healthy aging. The phasic alerting abnormality in aMCI compared to healthy aging does not appear specifically related to the performance of those patients for whom aMCI represents the prodromal stages of dementia.

Females, but Not Males, Show Greater Pupillary Response to Direct-Than Deviated-Gaze Faces:

Under suitable conditions, pupillary dilation is a reliable index of processing activity. Pupil size was tracked in male and female observers following the presentation of face stimuli for an age-judgment task. The eyes on the faces were either directed towards the observer or deviated to the side. Pupil dilation accompanied processing of the faces, but female observers showed significantly more sustained pupil dilation when viewing direct- than deviated-gaze faces over the period 3 to 7 s after stimulus onset, regardless of stimulus sex. In contrast, male observers did not show a consistent pattern in response to either the gaze or sex of the face stimuli. These findings indicate a sex difference in the processing of gaze direction and suggest that females, but not males, apply increased effort to processing socially relevant (direct-gaze) than irrelevant (deviated-gaze) faces. They also demonstrate that pupillary measurement can potentially provide new insights into the processing of even visual input, provided reflexes are sufficiently controlled.

Exogenous phasic alerting and spatial orienting in mild cognitive impairment compared to healthy ageing: Study outcome is related to target response ☆

Whether or not attentional mechanisms such as phasic alerting, spatial cueing and inhibition of return (IOR) remain intact in adults with Alzheimers disease (AD) and mild cognitive impairment (MCI) remains a matter of debate. This is possibly the result of inter-study outcome variation caused by the adoption of different methodological components by different research groups. Here we investigated the influence of methodological factors upon study outcome, using a Posner-type exogenous cueing paradigm with amnestic MCI patients and healthy older controls. Specifically, we compared results when the required response involved target discrimination with results for a simple target detection response, using cue-to-target intervals (CTIs) of 200msec and 800msec in each case and with the same participants completing all conditions. For both groups, the presence or absence of both alerting and spatial cue-related effects depended upon the combination of target response requirement and CTI. Moreover, differences between the groups were specific to certain task conditions. The MCI group showed the same alerting effects as healthy people with a discrimination response, but the alerting effect shown by controls with a 200msec CTI and target detection was absent in MCI. Patients and controls showed similar spatial cue validity effects at 200msec CTI, but group differences emerged at 800msec CTI: target discrimination evoked a validity effect in the MCI group only, while target detection evoked an IOR effect in the healthy group only. These data indicate that detection and discrimination responses may each activate different attentional mechanisms, which are themselves differentially vulnerable in MCI. Thus a seemingly arbitrary choice of response may directly influence whether attentional processing appears preserved or disrupted in MCI. Furthermore, these data provide further evidence in support of the existence of significant visual attention-related functional abnormalities in amnestic MCI.

Different trajectories of decline for global form and global motion processing in aging, mild cognitive impairment and Alzheimer's disease

The visual processing of complex motion is impaired in Alzheimers disease (AD). However, it is unclear whether these impairments are biased toward the motion stream or part of a general disruption of global visual processing, given some reports of impaired static form processing in AD. Here, for the first time, we directly compared the relative preservation of motion and form systems in AD, mild cognitive impairment, and healthy aging, by measuring coherence thresholds for well-established global rotational motion and static form stimuli known to be of equivalent complexity. Our data confirm a marked motion-processing deficit specific to some AD patients, and greater than any form-processing deficit for this group. In parallel, we identified a more gradual decline in static form recognition, with thresholds raised in mild cognitive impairment patients and slightly further in the AD group compared with controls. We conclude that complex motion processing is more vulnerable to decline in dementia than complex form processing, perhaps owing to greater reliance on long-range neural connections heavily targeted by AD pathology.

Abnormal inhibition of return in mild cognitive impairment: is it specific to the presence of prodromal dementia?

Although there is some evidence that amnestic mild cognitive impairment (aMCI) can be characterized by significant deficits in visuospatial function, the cross-sectional design of the majority of these studies renders it impossible to determine whether such deficits occur in aMCI as a result of, or accompany, amnestic dysfunction per se or whether they are the result of disproportionately poorer performance in a sub-group of patients for whom aMCI represents prodromal dementia. Similarly, whether the absence of aMCI-related functional deficit stems from the masking of dementia-specific abnormality by the preserved performance of those with a different cause of aMCI cannot be ascertained. Here we report the outcome of a cross-sectional and 2.5-year longitudinal evaluation follow-up, computer-based study of visuospatial attention, specifically attentional disengagement and inhibition of return and the mean (RTSPEED) and intra-individual variability (IIVRT) of their component reaction times, in 45 patients with aMCI and 31 cognitively healthy older adults. Reduced inhibition of return (p = 0.01 and p = 0.037 in response to 400 and 800 ms cue to target interval conditions), slowed RTSPEED (p = 0.038 and p = 0.03 in response to 400 and 800 ms cue), and raised IIVRT at baseline testing (p = 0.003, p = 0.026, p = 0.013 in response to 200, 400 and 800 ms cue) were associated with the development of dementia within the 2.5-year follow-up period, whereas the performance of patients with aMCI who did not develop dementia did not differ significantly from that of the cognitively healthy controls. Attentional disengagement appeared insensitive to the presence of prodromal dementia or amnestic dysfunction per se. The results indicate that those patients for whom aMCI represents prodromal dementia may experience, in addition to amnestic dysfunction, a decline in the functional integrity of some fundamental aspects of visual information processing, an effect potentially capable of increasing disease burden and reducing quality of life.

Stimulus onsets and distraction in younger and older adults.

Changes in task performance that accompany healthy aging are often attributed to age-impaired inhibitory control. For example, Maylor and Lavie (1998) demonstrated greater interference in older than younger people for response-incompatible visual distractors presented peripherally to a central low-load task. Here we explore the possible contribution of age-related changes in bottom-up visual processing in this task, and specifically the effect of the abrupt visual onsets associated with the distractors. In Experiment 1, with distractors presented as abrupt onsets, we replicated Maylor and Lavies (1998) effect. In Experiment 2, when placeholders preceded the stimuli to eliminate the abrupt onsets, response-incompatible distractors had a markedly reduced effect relative to neutral distractors, for older participants in particular. Stimuli presented as abrupt visual onsets, therefore, capture attention differentially depending upon the stimulus identity in combination with the age of the individual, with the greatest effects here for response-incompatible distractors in older people. We conclude that age-related differences in basic bottom-up processes may contribute to many purported declines in higher-level functioning in older people. More generally, this study provides further evidence for the interaction, and nonadditivity, of stimulus-driven and goal-driven influences in determining processing priorities across the age span.