Emotional Affect versus Cognitive Processing Load

A unique strength of functional brain imaging is the ability to test various intuitions and hypotheses about our mental activities by virtue of the quantitative nature of the MR signal changes. Sometimes the most salient aspect of a stimulus (such as its emotional valence) may be less cognitively engaging than the lack of that cue. Specifically, although the localization of function repeatedly found in studies of the low-level aspects of sensory processing appears to have analogs in other cognitive and emotional tasks, the tasks and stimuli that most effectively activate those areas may be counterintuitive. Higher contrast in visual stimuli generally evokes stronger modulation of early visual processing areas in the brain, but high contrast in an emotional domain does not always evoke the strongest variation in the brain areas associated with processing those stimuli. The data and experiment presented in Fig. 6 exemplify this idea.

Subjects were required to classify stimuli along an emotional scale as "neutral," "positive," or "negative." The stimuli were individual words (e.g., "calm," "delighted," and "insecure") intermixed with individual pictures of human faces (Fig. 6, top). There is ample evidence, both from the domain of human functional neuroimaging and from the literature of human brain lesions, that words and faces are exceptionally good stimuli for the activation of specific brain regions. Pictures of faces are good stimuli to activate the fusiform gyrus, especially on the right side. Single words, when associated with a semantic (rather than a purely perceptual) task, are effective stimuli for activating the inferior frontal region, almost always most strongly on the left side. One might reasonably expect that variation along the important dimension of emotional valence would modulate the strength of activation in these two areas. Given the demonstrated power of emotional stimuli to activate areas of the brain associated with general arousal, one might also predict that the most emotionally powerful stimuli (the positive and negative stimuli) would elicit greater neural activity than the neutral stimuli.

The stimuli were presented for 2 sec each, with 8 sec between the onset of each stimulus. This design, as one might deduce from Section II.A, is less than optimal because it does not take advantage of the power of rapid single trials, nor does it completely isolate the hemodynamic responses from the different stimulations. However, it was sufficient to address the preceding question, and it is useful in illustrating the way in which event-related fMRI data are often reported-via overlapping plots of the time course of the hemodynamic responses to the different stimulus types.

The stimuli were effective in eliciting emotional responses. Not surprisingly, the emotional faces (happy, in particular, and, to a lesser extent, sad) evoked stronger amygdala responses than neutral faces, with a weaker finding (but in the same relative order of strength) for words. Also not surprisingly, faces of any type elicited minimal response in left inferior frontal cortex and words of any type elicited minimal response in right fusiform cortex.

However, in the context of this categorization task, neutral stimuli (both faces and words) were far more effective activators of the regions known to be especially responsive to such stimuli (right fusiform and left inferior frontal, respectively) than were the emotionally powerful stimuli. The graphs in Fig. 6 show the time course of activity for each stimulus type in the relevant regions of the brain. The amplitude of the hemodynamic responses to the neutral stimuli was approximately twice as large as that to the positive stimuli, and it exceeded the response to the negative stimuli even more dramatically.

Interpretation of this result is not trivial. Behavioral measures of reaction time and accuracy for the words suggest one possible explanation. In the case of the word stimuli, the reaction times were longest, and percentage correct was lowest, for the neutral words. The fact that it was faster and easier to categorize positive and negative words could mean that the relevant brain area (left inferior frontal) was active for a shorter period of time during the emotional words, resulting in less time for the hemodynamic signal to grow. However, this kind of explanation cannot be the

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