Frontal injury: Impairments of fundamental processes lead to functional consequences

Frontal functions, or impairments, have achieved the status of pornography: everyone knows them when they see them, but there is little agreement on their exact defining properties. Patients with frontal lobe lesions have impairments in planning, monitoring, sequencing, and inhibiting responses. They cannot organize complex behaviors. They are somehow simultaneously unaware and distractible, irritable and apathetic, violent and passive, impulsive and perseverative. They lack empathy, self-awareness, and emotional regulation. Patients often are incapacitated by frontal impairments that paradoxically are not at all obvious to most observers. To understand the effects of frontal lesions, science has sacrificed countless rats and nonhuman primates. Neuropsychology and Neurology have sanctified a few tests as the tests of frontal function, and then spent money, time, and the mental health of countless graduate students and research assistants trying to tease out of those tests some basic knowledge of frontal functions. Experimental Psychology has brought novel constructs to the problem of defining frontal functions, and modern neuroimaging has merged the traditional clinical and the novel experimental with imaging technology. (A quick PubMed search of “fMRI and frontal lobes” on October 12, 2005 retrieved 4,646 articles, the most recent, “An fMRI study of the Trail Making Test”.) Yet, confronted with a patient with a large right prefrontal lesion who cannot hold a job or even be relied upon to buy the groceries he set out to buy, we have precious few tools to define his problem and almost nothing to offer to fix it.

The Effect of Cingulate Cortex Lesions on Task Switching and Working Memory

Anatomic interconnections between the prefrontal and anterior cingulate cortices suggest that these areas may have similar functions. Here we report the effect of anterior cingulate removal on task switching, error monitoring, and working memory. Neuroimaging studies have implicated the cingulate cortex in all these processes. Six macaques were taught task switching (TS) and delayed alternation (DA) paradigms. TS required switching between two conditional response tasks with mutually incompatible response selection rules. DA required alternation between two identically covered food-well positions. In the first set of experiments, anterior cingulate lesions did not consistently impair TS or DA performance. One animal performed worst on both TS and DA and in this animal the cingulate sulcus lesion was most complete. In the second set of experiments, we confirmed that larger anterior cingulate lesions, which included the sulcus, consistently impaired TS but only led to a mild and equivocal impairment of DA. The TS error pattern, however, did not suggest an impairment of TS per se. The consequence of a cingulate lesion is, therefore, distinct to that of a prefrontal lesion. TS error distribution analyses provided some support for a cingulate role in monitoring responses for errors and subsequent correction but the pattern of reaction time change in TS was also indicative of a failure to sustain attention to the task and the responses being made.