Several methods developed recently which help observe brain activities in healthy, awake subjects have led to considerable enthusiasm in cognitive neuroscience. These methods, commonly referred to as neuromaging techniques provide a reliable way to map brain activities in determining areas getting activated during various tasks. The two most common techniques are the positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Both rely on the notion of increased blood flow in active areas. While the former measures the increase in blood flow (Raichel, 1987), the latter techniques relies on the change in oxygen concentration in the blood to identify activated areas (De Yoe et al., 1994).
These new techniques, nevertheless, have several limitations. The first limitation arises from the fact that all areas in the brain have baseline values that are always present, which has to be taken into consideration in any study. This necessitates a comparison task in the determination process. In addition, two assumptions are made during the task comparisons; that the processing demands of the tasks are well understood, and that the processing demands remain uniform in the specific task context. The last limitation relates to the sophistication level of present technology. The PET and fMRI processes are considerably slower than the pace of brain activities, thus leading to averaging the outcome of several steps and processes.
The beneficial use of neuroimaging techniques could be seen in the areas of speech production and memory. One good example is its use in the characterization of the left prefrontal cortex"s role in speech production. Petersen et al"s (1988) study involving passively viewed words, overt speech and word generation led to new ideas about the role of prefrontal cortex. There seemed to be a difference in the activated area depending on whether the task involved practiced/automated speech as compared to unpracticed generation, leading to the hypothesizing of separate speech pathways for different kinds of speech production. Other experiments suggest that the prefrontal cortex is related to processes involving accessing and/or representing verbal information without a necessary link to overt speech output. Subsequent hypotheses regarding the role of prefrontal cortex has led to the idea of multiple areas within it underlying distinct processing functions. Buckner et al. (1995) have been able to demonstrate at least two separate prefrontal regions in speech production tasks. There are however many areas outside the prefrontal region which has been shown to be active concurrently with prefrontal cortex, strengthening the notion of multiple brain area recruitment and distributed processing. Neuroimaging techniques have also been useful in memory tasks (encoding, storage and retrieval). It has been shown that areas within the prefrontal cortex are activated across a wide range of memory retrieval tasks (contrary to previous understanding). In recall experiments PET scans have been useful in isolating the right prefrontal cortex as the consistently active area, thus providing novel insights. Further, neuroimaging techniques have been useful in discovering patterns associated with the activation related to various types of memory tasks.
Neuroimaging techniques, in conjunction with methods in other fields provide a powerful tool to study the complexities involved with human cognition.