Attentional selection and the processing of task-irrelevant information: insights from fMRI examinations of the Stroop task.

In this chapter, we discuss our research that reveals how attentional mechanisms can modulate activity of posterior brain regions responsible for processing the unattended attribute of a stimulus. To do so, we utilized fMRI to reveal patterns of regional brain activity for variants of the Stroop task that differ in the nature of the task-irrelevant stimulus attribute. In all variants, individuals had to identify the ink color in which an item was presented. To vary attentional demands, we manipulated whether or not the task-irrelevant information contained conflicting color information. The variants differed in whether the conflicting color information was contained in a word naming a color (e.g. the word 'red' in blue ink), a word naming an object highly associated with a specific color (e.g. the word 'frog' in red ink), or a line drawing of an object highly associated with a specific color (e.g. a drawing of a frog in red ink). When the unattended stimulus attribute contained color information that conflicted with an item's ink color, increased activity was observed in the posterior brain region that processes the aspect of the task-irrelevant attribute related to color. Increased activity was observed in the left precuneus and left superior parietal cortex when the conflicting information arose from a color word; in the middle temporal gyrus and insular cortex when the word named an object highly associated with a specific color, and included extensive regions of early portions of the ventral visual processing stream when a line drawing was highly associated with a specific color. These areas have been implicated in word processing, semantic processing, and visual processing, respectively. Our results suggest that attentional selection can occur by: (1) increasing the gain on all posterior regions responsible for processing information related to the task demands, regardless of whether that information is contained in the task-relevant or task-irrelevant dimension; (2) limiting the processing of task-irrelevant information in order to reduce interference; and (3) modulating the processing of representations varying from those of a low-level perceptual nature up through those of a higher-order semantic nature.

The relative involvement of anterior cingulate and prefrontal cortex in attentional control depends on nature of conflict.

While numerous studies have implicated both anterior cingulate and prefrontal cortex in attentional control, the nature of their involvement remains a source of debate. Here we determine the extent to which their relative involvement in attentional control depends upon the levels of processing at which the conflict occurs (e.g., response, non-response). Using a combination of blocked and rapid presentation event-related functional magnetic resonance imaging techniques, we compared neural activity during incongruent Stroop trial types that produce conflict at different levels of processing. Our data suggest that the involvement of anterior cingulate and right prefrontal cortex in attentional control is primarily limited to situations of response conflict, while the involvement of left prefrontal cortex extends to the occurrence of conflict at non-response levels.

General and task-specific frontal lobe recruitment in older adults during executive processes: a fMRI investigation of task-switching.

Performance deteriorates when subjects must shift between two different tasks relative to performing either task separately. This switching cost is thought to result from executive processes that are not inherent to the component operations of either task when performed alone. Medial and dorsolateral frontal cortices are theorized to subserve these executive processes. Here we show that larger areas of activation were seen in dorsolateral and medial frontal cortex in both younger and older adults during switching than repeating conditions, confirming the role of these frontal brain regions in executive processes. Younger subjects activated these medial and dorsolateral frontal cortices only when switching between tasks; in contrast, older subjects recruited similar frontal regions while performing the tasks in isolation as well as alternating between them. Older adults recruit medial and dorsolateral frontal areas, and the processes computed by these areas, even when no such demands are intrinsic to the current task conditions. This neural recruitment may be useful in offsetting the declines in cognitive function associated with ageing.

Prefrontal regions play a predominant role in imposing an attentional 'set': evidence from fMRI.

fMRI was used to determine whether prefrontal regions play a predominant role in imposing an attentional 'set' that drives selection of task-relevant information. While monitoring for an atypical item, individuals viewed Stroop stimuli that were either colored words or colored objects. Attentional demands were varied, being greater when the stimuli contained two distinct and incongruent sources of information about the task-relevant attribute (e.g., when attending to color, seeing the word 'blue' in red ink) as compared to only one source (e.g., seeing the word 'late' in red ink). Prefrontal but not anterior cingulate regions exhibited greater activation on incongruent than neutral trials, suggesting that prefrontal cortex has a major role in imposing an attentional 'set'. In addition, we found that prefrontal activation is most likely to occur when that attentional set is difficult to impose.

fMri studies of Stroop tasks reveal unique roles of anterior and posterior brain systems in attentional selection.

The brain's attentional system identifies and selects information that is task-relevant while ignoring information that is task-irrelevant. In two experiments using functional magnetic resonance imaging, we examined the effects of varying task-relevant information compared to task-irrelevant information. In the first experiment, we compared patterns of activation as attentional demands were increased for two Stroop tasks that differed in the task-relevant information, but not the task-irrelevant information: a color-word task and a spatial-word task. Distinct subdivisions of dorsolateral prefrontal cortex and the precuneus became activated for each task, indicating differential sensitivity of these regions to task-relevant information (e.g., spatial information vs. color). In the second experiment, we compared patterns of activation with increased attentional demands for two Stroop tasks that differed in task-irrelevant information, but not task-relevant information: a color-word task and color-object task. Little differentiation in activation for dorsolateral prefrontal and precuneus regions was observed, indicating a relative insensitivity of these regions to task-irrelevant information. However, we observed a differentiation in the pattern of activity for posterior regions. There were unique areas of activation in parietal regions for the color-word task and in occipitotemporal regions for the color-object task. No increase in activation was observed in regions responsible for processing the perceptual attribute of color. The results of this second experiment indicate that attentional selection in tasks such as the Stroop task, which contain multiple potential sources of relevant information (e.g., the word vs. its ink color), acts more by modulating the processing of task-irrelevant information than by modulating processing of task-relevant information.

Hippocampal system and declarative (relational) memory: summarizing the data from functional neuroimaging studies.

In the last several years there have been impressive strides in the ability to explore the nature of hippocampal system functioning in humans by employing functional neuroimaging methods, permitting such methods to be used in conjunction with neuropsychological methods to better understand the role of the hippocampal system in memory. In this paper, we review the literature on functional imaging studies of the hippocampal system, summarizing the data and testing these data against a number of theories or explanatory accounts of hippocampal function. We consider five alternative explanatory accounts of, or ideas about, hippocampal function- some from already existing work, for which the functional imaging data can provide a new test, and others that have emerged directly from the functional imaging work, and that have yet to be tested for their fit of data from neuropsychological methods. We conclude that the relational (declarative) memory account, in which it is proposed that the hippocampal system plays a critical role in binding together multiple inputs to permit representations of the relations among the constituent elements of scenes or events, can better accommodate the full range of imaging (and other existing) data than any other explanatory account of hippocampal function.

Age-dependent changes in olfactory-mediated behavioral investigations in the male rat.

In the present experiment male Sprague-Dawley rats at three different ages (young, middle-aged, and old) were tested in a series of olfactory-mediated behavioral tests. The amount of ano-genital investigation directed to receptive females by old rats was significantly reduced compared to that of young and middle-aged animals. In contrast, general body investigation of receptive females was similar among the three age groups. The amount of anogenital investigation directed to nonreceptive females or immature male rats indicated significant differences only between young and old rats, with values from middle-aged rats being intermediate and not significantly different from either of the two extreme groups. General body investigation of nonreceptive females and immature males did not differ among the three age groups. Old rats showed the best performance in a location of buried food test with latencies to locate food approximately one-half that obtained from young and middle-aged rats. Measures of general locomotor activity indicated significantly greater values from young compared to middle-aged and old rats, with the latter two showing very similar levels. These results demonstrate clear discriminatory differences in olfactory-mediated behavioral performance of the male rat as a function of the specific test and age of the animal. Overall, the results suggest that olfactory-mediated behaviors related to reproduction appear more sensitive to age-related declines, while behavior related to food procurement is more minimally affected.