Elsevier

NeuroImage

Volume 22, Issue 4, August 2004, Pages 1466-1479
NeuroImage

The functional anatomy of inspection time: an event-related fMRI study

https://doi.org/10.1016/j.neuroimage.2004.03.047Get rights and content

Abstract

Twenty healthy young adults underwent functional magnetic resonance imaging (fMRI) of the brain while performing a visual inspection time task. Inspection time is a forced-choice, two-alternative visual backward-masking task in which the subject is briefly shown two parallel vertical lines of markedly different lengths and must decide which is longer. As stimulus duration decreases, performance declines to chance levels. Individual differences in inspection time correlate with higher cognitive functions. An event-related design was used. The hemodynamic (blood oxygenation level-dependent; BOLD) response was computed as both a function of the eight levels of stimulus duration, from 6 ms (where performance is almost at chance) to 150 ms (where performance is nearly perfect), and a function of the behavioral responses. Random effects analysis showed that the difficulty of the visual discrimination was related to bilateral activation in the inferior fronto-opercular cortex, superior/medial frontal gyrus, and anterior cingulate gyrus, and bilateral deactivation in the posterior cingulate gyrus and precuneus. Examination of the time courses of BOLD responses showed that activation was related specifically to the more difficult, briefer stimuli and that deactivation was found across most stimulus levels. Functional connectivity suggested the existence of two networks. One comprised the fronto-opercular area, intrasylvian area, medial frontal gyrus, and the anterior cingulate cortex (ACC), possibly associated with processing of visually degraded percepts. A posterior network of sensory-related and associative regions might subserve processing of a visual discrimination task that has high processing demands and combines several fundamental cognitive domains. fMRI can thus reveal information about the neural correlates of mental events which occur over very short durations.

Introduction

Inspection time is a procedure that examines the early stages of visual information processing Deary and Stough, 1996, Vickers et al., 1972. Participants are briefly shown a simple line stimulus and are asked to make a two-alternative, forced-choice discrimination (Fig. 1). The duration of the stimulus varies from trial to trial. Stimuli are backward-masked to limit processing of the stimulus after its offset. Responses are not speeded. The participant's performance is described by plotting the accuracy of the discrimination versus the duration of the stimulus. Performance increases from near-to-chance at very brief durations to near-perfect responding at longer durations, with a psychometric function in between that is a cumulative normal ogive (Deary et al., 1993).

Inspection time has attracted much research interest for three reasons. First, it might conceptualize and test a fundamental limitation of visual information processing that shows interindividual differences (Burns et al., 1998). Second, inspection time differences correlate moderately with differences in higher cognitive abilities, including psychometric intelligence tests, and share genetic influences with them Deary, 2000, Grudnik and Kranzler, 2001, Luciano et al., 2001. Third, the capability tested by inspection time and related backward-masking procedures is disrupted by certain clinical conditions, such as dementia, Parkinson disease, multiple sclerosis, mania, schizophrenia, and hypoglycemia Deary and Stough, 1996, Ewing et al., 1998, McCrimmon et al., 1996.

Relatively little is known about the cerebral correlates of inspection time performance. There is evidence for a cholinergic contribution to both efficient inspection time performance and higher psychometric intelligence (Stough et al., 2001). Hypoglycemia disrupts both inspection time and higher cognitive functions but does not affect peripheral nerve conduction velocity (Strachan et al., 2001). Inspection time mediates much of the effect of aging on higher mental functions (Nettelbeck and Rabbitt, 1992). Studies using event-related potentials indicate that the topography of the brain potentials between 100 and 200 ms after stimulus onset is related to inspection time ability (Caryl, 1994), a time window in which visual selective nonspatial attention, feature encoding, and pattern analysis take place (Hillyard et al., 1998). People with more efficient visual information processing have a steeper gradient from the negative wave at about 140 ms to the positive wave at about 200 ms Caryl, 1994, Morris and Alcorn, 1995.

A pilot functional magnetic resonance imaging (fMRI) study, with a block design, investigated the functional anatomy of inspection time (Deary et al., 2001). Participants viewed either easy (200 ms), hard (40 ms), or control (mask only) inspection time stimuli. During the more difficult discrimination, there was a greater blood oxygenation level-dependent (BOLD) signal in the cingulate gyrus and some frontal and parietal lobe areas and a lower BOLD signal in some frontal, temporal, and parietal lobe areas. There were several limitations in the pilot study (Deary et al., 2001): only seven participants were tested, subjects were prewarned whether they would receive blocks of hard, easy, or control stimuli, there was limited familiarization with the inspection time test before imaging, and participants' responses to inspection time stimuli were not collected during imaging. These limitations were corrected in the present study which included 20 subjects, an event-related design, detailed familiarization and pretesting, and the collection of performance data during imaging.

A number of cognitive processes are assumed to be involved in performing the inspection time task, including sensory encoding, discrimination, and decision making Chaiken, 1993, White, 1996, and the pilot study that indicated a number of disparate brain regions might show altered BOLD signals (Deary et al., 2001). It is unrealistic, therefore, to propose the existence of one particular brain area in the cerebral cortex which is committed to the inspection time task. The present study has three aims. We describe those areas of the brain in which the hemodynamic response (as estimated using the BOLD signal) is correlated with the duration of the stimulus in a visual inspection time task. We describe the time course of the BOLD response in key brain loci for each stimulus duration in the inspection time task. We undertake a functional connectivity analysis using, as seeds, areas that show increased and decreased hemodynamic responses as a function of stimulus duration. By combining these aims, we demonstrate that the cerebral processing network underlying the performance of the inspection time task alters as a function of task difficulty, here controlled by duration of stimulus presentation.

Section snippets

Participants

Twenty healthy adults (12 women, 8 men) participated; none of whom took part in the prior pilot study (Deary et al., 2001). Their ages ranged from 21 to 34 years (mean = 24.6, SD = 4.1). All were undergraduate or postgraduate students, or research staff at the University of Edinburgh. All structural MRI scans were checked for normality by a consultant clinical neuroradiologist. Subjects had normal visual acuity, either unaided or corrected. Where required, subjects wore correcting lenses,

Behavioral data

The mean psychometric curve for the baseline inspection time test session is shown in Fig. 2. This shows almost chance responding at the shortest duration with a steady increase in accuracy to almost perfect responding above about 60 ms. The mean psychometric curves for the two inspection time tests performed during brain imaging test sessions are very similar (Fig. 3). In addition to the similarity of group-level means between imaging inspection time sessions 1 and 2, individual differences in

Discussion

The present study investigated the neural correlates of performing the inspection time task. The patterns of activation and deactivation were congruent with those found in the pilot study (Deary et al., 2001). Additionally, we examined the extent to which the cerebral network is sensitive to a decrease in stimulus duration. A novel finding of this study is the dissociation of the local patterns of cortical activity that varied as a function of task difficulty.

In turn, we discuss specific

Conclusions

In summary, the present study finds consistent activity in lateral and medial frontal brain regions—areas that do not subserve primary sensory or motor functions—as a function of shorter presentation times in the inspection time procedure. Interestingly, the pattern of brain responses is significant in some left and right hemisphere sites (e.g., ventral portion of anterior insula, anterior cingulate gyrus, and medial frontal gyrus) that have not previously been described as devoted to

Acknowledgements

We thank Susan Flett and Rayna Azuma for help with programming and running some of the fMRI sessions. ES was supported by a Research Development Grant from Scottish Higher Education Funding Council (SHEFC), which also provided some of the scanner time and fMRI experimental equipment through the Centre for Functional Imaging Studies. This research was conducted at the SHEFC Brain Imaging Research Centre for Scotland which is supported with a Joint Research Equipment Initiative grant from the

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