Divided Attention And The Frontal Cortex

14.3.1 Functional Imaging Studies

Brain imaging studies involving human participants consistently show differential activation of various frontal cortical areas in tasks involving behavioral manipulations of attention. Pardo et al. (1990) observed that the anterior cingulate, left premotor cortex, and supplementary motor cortex show differential activity on congruent and incongruent trials of the Stroop task (Pardo et al., 1990). Using a more direct assessment of divided attention (Corbetta et al. 1997; Vandenberghe et al., 1997) have reported differential activation of the anterior cingulate and premotor cortex, as well as the dorsolateral prefrontal cortex, when participants attended to a single object dimension, compared to when they attended to multiple object dimensions.

14.3.2 Effects of Brain Damage

Consistent with these imaging results, damage of the frontal lobe impairs the ability to divide attention between two stimuli or two stimulus dimensions in nontiming tasks (Godefroy et al., 1996; Godefroy and Rousseaux, 1996) or to divide attention between a temporal and a nontemporal task (Casini and Ivry, 1999). Overall, there is an accumulating body of evidence that supports a general role for frontal cortical areas in attentional processes, including divided attention. Based on these studies, the implication for timing is that frontal cortical areas should be differentially activated on simple and compound trials of the STP task.

No previous imaging or brain damage studies with human participants have directly investigated divided attention during STP for durations in the seconds-to-minutes range. However, several variants of the STP task have been used with rats. Although normal rats can accurately time up to three stimuli presented concurrently, as evidenced by similar peak times on simple and compound trials (Meck, 1987; Meck and Williams, 1997; Olton et al., 1988), rats with damage to the frontal cortex show a pattern of results that support the involvement of the frontal cortex in divided attention (Olton et al., 1988). In the STP procedure, rats with frontal cortex damage produce peak times for the compound long stimulus that overestimate the actual duration of the long fixed interval by an amount approximately equal to the duration of the short fixed interval (Olton et al., 1988). This contrasts with the relatively accurate timing by the same rats for short and long fixed intervals on simple trials. Thus, during divided attention conditions, it appears that rats with frontal cortex damage exhibit a complete breakdown of divided attention, timing the two stimulus durations in a serial fashion, rather than in parallel.

The results from rats with frontal cortex damage support the attentional switch hypothesis in the following way. If frontal cortex lesions impair divided, but not focused attention, then timing will be accurate on simple trials of the STP task, but not on compound trials. On compound trials, rats will initially attend to and time the long stimulus. At the onset of the short stimulus, normal animals are able to divide attention between short and long stimuli. However, rats with damage of the frontal cortex are unable to divide attention between stimuli. Instead, these rats stop attending to the long stimulus, which stops the timing of the long stimulus, and start attending to and timing the short stimulus. At the end of the fixed interval for the short stimulus, rats with lesions stop attending to (and timing) the short stimulus and switch attention back to the long stimulus. The switch of attention back to the long stimulus resumes timing of the long stimulus. Thus, lesions of the frontal cortex limit attention to only one stimulus at any instant, and pacemaker pulses can only be accumulated for the attended stimulus, resulting in two concurrent stimuli being timed serially. This interpretation is consistent with the view that attention is necessary for transfer of pacemaker pulses to an accumulator, and that the frontal cortex allows animals to attend to multiple stimuli and accumulate pacemaker pulses simultaneously for both short and long stimuli.

14.3.3 Summary

Overall, functional imaging of normal human participants and behavioral assessment of humans and animals with brain damage have provided evidence that the frontal cortex is a critical component of divided attention. Information-processing models of timing that incorporate an attentional switch help to explain how attention and the frontal cortex might influence short-interval timing. However, the exact role of the various areas of the frontal cortex in divided attention is not well understood. Single-cell recording can help to identify functions that various areas of the frontal cortex contribute to divided attention and timing.

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