G15 G1

Percent G.G5

Signal G Change

-5 0 5 10 15 20 seconds (relative to stimulus onset)

Left Inferior Frontal

-5 0 5 10 15 20 seconds (relative to stimulus onset)

Response to Words

-5 0 5 10 15 20 seconds (relative to stimulus onset)


Happy/Positive Neutral Sad/Negative

Q Stimulus ON (face/word) 0 Stimulus OFF (fixation cross)

Percent Signal 0 Change

-5 0 5 10 15 20 seconds (relative to stimulus onset)

Figure 6 Strength of affect versus difficulty of processing. As described in Section II.A, detection of the hemodynamic responses to different types of events can reveal unintuitive effects. The stimuli and timing for this experiment are indicated in the top row. The stimuli were "positive," "negative," and "neutral" faces and words. One of these was presented for 2 sec every 8 sec in random order. Subjects were required to categorize each stimulus presentation as beingpositive, negative, or neutral. Two of the resulting activation foci are presented. A region of the brain known to respond strongly to face stimuli (the fusiform gyrus on the right side of the brain) and a region known to respond strongly to words (the inferior frontal region on the left side of the brain) were detected, and their locations are indicated by the crosshairs in the anatomical images shown on the left. The graphs on the right indicate the hemodynamic responses to the three classes of stimuli (positive, negative, and neutral). For words in the frontal region and for pictures of faces in the fusiform region, the most activity was elicited not by the most emotionally salient and affective stimuli but by the neutral stimuli. Apparently, categorizing happy or sad stimuli places a smaller processing load on these areas than categorizing a neutral stimulus (data and analysis courtesy of Patricia Deldin and David Cox).

whole story. In the case of face stimuli, it was the negative (rather than neutral) faces that had the longest reaction times and lowest percentage correct. Face stimuli were generally classified more quickly than words, but neutral faces may still evoke more processing because the observer tries harder to find positive or negative nuances (albeit quickly). However, these are speculations.

The point of recounting this study is to emphasize both the complexity of interpreting fMRI data and their potential nonintuitiveness. To an experimenter, it can naturally seem that positive and negative stimuli will, in one sense, evoke stronger responses than neutral stimuli. For some brain areas, this is no doubt true. However, the constraints of a classification task are different. In that context, the fact that neutral stimuli elicit more activity from their respective processing areas (for words and faces) can be plausibly interpreted as an indicator of greater processing effort.

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