USING PET TOWARD A NATURALIZED MODEL OF HUMAN LANGUAGE PROCESSING

Robert S. Stufflebeam

Philosophy-Neuroscience-Psychology Program
Washington University
Campus Box 1073, One Brookings Dr.
St. Louis, MO 63130-4899
rob@twinearth.wustl.edu

Abstract. My purpose for this paper is to show that there are 'facts' about brains revealed by Positron Emission Tomography [PET] that preclude serial models of human language processing [HLP] from being implementable. For this end, I first explain how PET works. I also identify some of its limitations. I then apply the data from some recent language-related PET studies to evaluate the realizability of the Wernicke-Geschwind model [W-G model]. I focus on the W-G model because it is the most pervasive neurological model of HLP; it's also a serial model. I show that the PET data is inconsistent with what ought to be the case if the W-G model were accurate: (1) language is not processed exclusively in a serial, feed-forward manner, but also along parallel pathways; (2) visual language stimuli are not transformed into an auditory code, and neither are visual stimuli [always] fed-forward via Wernicke's area; and (3) Wernicke's area cannot be the only area where meanings are stored. Additional research is required. Nevertheless, I show that HLP depends on a much richer set of functional areas and interconnections than the W-G model [or any serial model] identifies.

1. Introduction

Few cognitive scientists refer to the brain or its organization in their models of human language processing [HLP], this 'decade of the brain' notwithstanding. A commitment to naturalism makes such references essential: the model ought to be implementable, and its realizability depends on whether it is consistent with a host of facts, including those about brains. But given the disparate emphasizes of the various disciplines engaged in modeling HLP, it is unlikely that any single discipline will be informed about all the relevant facts. Hence, an interdisciplinary effort is required, both to constrain one's portion of the overall model and to avoid the pitfalls discovered outside one's domain. Toward the interdisciplinary end of a naturalized -- and thus implementable -- model of HLP, the neurofunctional imaging of Positron Emission Tomography [PET] has already made significant contributions. I aim to reveal how.

My purpose for this paper is to show that there are 'facts' about brains revealed by PET that preclude serial models of HLP from being implementable. But for all its potential, I also intend to show that PET isn't a panacea. Toward these rather modest ends, after briefly showing how PET works, I appeal to data from recent language-related PET studies to evaluate the realizability of the Wernicke-Geschwind model [W-G model]. I focus on the W-G model because: (1) it is clinically useful and (2) it is the paradigmatic serial, neurological model of HLP. I show that the W-G model isn't accurate, for among other failings, it makes predications that are inconsistent with the PET data.

2. What is PET?

The performance of any cognitive task supervenes on discrete portions of the brain, functional areas dedicated [though not necessarily exclusively] to specific types of information processing. For example, visual perception would not be possible without an antecedent increase in the neuronal activity of the primary and secondary visual processing areas of the occipital lobe. And as with any brain-mediated task, increases in neuronal activity alters local blood flow [and metabolism].

Enter PET. The PET scanner is designed to detect the distribution of gamma rays within its oval aperture. Because two gamma rays are emitted whenever a positron collides with an electron, if a positron-emitting isotope tracer is administered into a subject's blood, the scanner can detect the emission of paired gamma rays as blood flows through the brain. Thus, the functional image computed by PET directly measure changes in the local blood flow [or metabolism] of discrete functional areas of the brain, and indirectly measure changes in neuronal activity [see Figure 1].



To image brain blood flow during a language-related task, the subject's head is stabilized and then inserted into the scanner aperture. A radioactive isotope [such as 11C, 13N, 18F, or 15O] is then administered. Most activation studies in normal subjects use 15O, which is incorporated into a saline solution and then injected into the blood stream. Given its short half-life of 122.3 seconds, an additional dose of 15O-labeled water must be administered before each scan. Each scan lasts approximately 40 seconds; a total of 5 to 10 scans are performed during a session. The resolution of current PET-produced images is between 2-5 mm.

Because multiple scans may be made during any one session, each subject for a given study will normally perform a specific task for each of the scans. In so doing, it is possible to identify changes in local blood flow [and indirectly changes in neuronal activity] by comparing images from two sets of task conditions, a control task and an active task. That is, to isolate changes in local blood flow, the image generated during performance of a control task is subtracted from the image generated during the performance of an active task.1 This subtracted image is called the difference image. To increase the signal-to-noise ratio, the difference images across subjects performing the same task can then be averaged, thus increasing the likelihood of identifying the functional significance of discrete brain regions.

2.1 Limitations

Before presenting any actual PET studies, it is worth noting that PET is not a panacea. Like any methodology, PET too has limitations. For example, (1) the actual imaging site is once removed from the site of positron formation, and blood flow measurement is itself once removed from the neurons being activated. As such, the resolution of PET images employing current techniques and positron-emitting isotopes will [likely] never approximate the anatomical imaging of magnetic resonance imaging [MRI]. And (2) given a scanning time of even 40 seconds, much useful information may not be imaged, due either to a transient increase in neuronal activity, or a functional area being maintained at or below threshold.2

No single discipline or methodology [including PET] will likely be informed about all the relevant language-related facts. Hence, given that no intertheoretically consistent model of HLP has yet to be devised, an interdisciplinary approach is required. As such, we can progress toward an intertheoretically consistent model of HLP by evaluating the implementability of the most common neurological model of HLP; viz., the W-G model.

3. The W-G Model

Much of our knowledge of the brain structures [or areas] involved in HLP comes from the study of aphasia. The W-G model is an abstraction from that data. As such, in addition to being the most pervasive neurological model of HLP, the W-G model remains diagnostically and conceptually useful (Kolb & Whishaw, 1990, p. 581; Mayeux & Kandel, 1991, p. 843).

According to the W-G model, initial visual and auditory language stimuli are processed in their respective primary and secondary cortical sensory areas of the left hemisphere: (l) Broadmann's areas 17, 18, and 19 of the occipital lobe for vision; (2) areas 41 and 42 of the temporal lobe for audition. The sensory code is then conveyed to (3) the angular gyrus, an area of association cortex specialized for the integration of visual, auditory, and tactile information into a phonetic or auditory code important for both speech and writing. This integrated representation of the initial stimulus is then fed forward to (4) Wernicke's area, where the code is not merely registered as language [i.e., not mere noise], but also associated with meaning. On this model, Wernicke's area is the [only] site of the stored meanings of words. Should it be damaged, the ability to comprehend language is lost, a disorder called 'Wernicke's aphasia'. Once associated with meaning, the modified neural code for the initial stimulus is relayed via (5) the arcuate fasiculus to (6) Broca's area. The arcuate fasiculus is a fiber tract connecting Wernicke's area in superior temporal cortex with Broca's area in prefrontal cortex. A lesion here will result in 'conduction aphasia' -- a language disorder characterized by impaired repetition and naming, as well as paraphasic errors. Broca's area, the site where the memory of word articulation is stored, lies near the facial region of (7) motor cortex. Hence, Broca's area sends the signals that the motor cortex transforms into commands that cause the mouth and tongue to form words. It is in Broca's area, therefore, where the code [with attending meaning] is transformed from a sensory representation to a grammatical motor representation that can be used as spoken or written speech. If damaged, a language production deficit called 'Broca's aphasia' will obtain. I summarize this serial pathway in Figure 2.


3.1 Predictions

If the W-G model accurately depicts how language processing occurs, then each of the following predictions about HLP should be consistent with the PET data:

  1. Because visual linguistic stimuli should be processed along the same pathway as auditory linguistic stimuli, reading should be processed in a serial, feed-forward manner;

  2. visual linguistic stimuli should be transformed into an auditory representation, which is then fed forward to Wernicke's area; and

  3. words not processed through Wernicke's area should have no meaning.

If the evidence weighs strongly against these predictions, then the greater the unlikelihood that the W-G model is an accurate model of HLP. Since time and space considerations preclude a thorough analysis, I assume that what follows is nevertheless sufficient.

4. Using PET Data to Evaluate the Predictions

Petersen et al. (1988, 1983) were among the first to use PET to study lexical processing -- the processing of single words. Their results do not bode well for serial models, especially the W-G model.

Subjects were asked to perform three tasks. First, common English nouns were passively presented visually [for one set of scans] and auditorily [for another set of scans]. The sensory task was simply to process the word. In the second task, an output task, subjects were asked to repeat aloud all the presented nouns. For the third task, an association task, subjects were asked to generate aloud a verb corresponding to the presented noun; e.g., one could say 'bake' or 'eat' when presented with the noun 'cake'. Difference images were obtained by subtracting the image generated from the lower-level active task from the higher-level active task; i.e., the initial control task was subtracted from the sensory task; the sensory task was subtracted from the output task; et cetera. They found that (1) visual and auditory stimuli are processed in modality-specific processing areas during the passive sensory task. (2) During the output task, there was bilateral activation of the motor and sensory face areas, as well as activations in the cerebellum. And (3) during the association task, there was bilateral activation of the anterior cingulate, left anterior inferior prefrontal cortex, and the right inferior lateral cerebellum (Petersen & Fiez, 1993, 515-517).

These results [having been duplicated and expanded upon in subsequent PET studies] falsify each of the predictions. Let me explain.

4.1 Analysis

If visual stimuli are fed-forward to be rendered into a phonetic or auditory code, then there should have been some overlap in visual and auditory processing reflected in the difference images. There was not [see Figure 3]. Instead, there wasn't any significant temporal lobe activation during any level of lexical processing upon visually presented words (Petersen & Fiez, 1993, 522). This finding is significant. First, given that Wernicke's area was altogether bypassed during even the verb generation [association] task, contrary to the W-G model, some other area(s) must be involved in word comprehension. The surprising finding was that the frontal lobe [rather than the posterior ones] seems to play a key role in the associative aspects of lexical processing.



Second, because visually presented words can be processed without having been modified in Wernicke's area, visual linguistic stimuli need not be processed along the same pathway as auditory linguistic stimuli.

And third, even low-level linguistic processing need not be fed-forward in a serial manner. Thus, HLP is not exclusively a serial process. As such, despite its utility, the W-G model isn't consistent with the 'facts', including: (1) the cerebellum and some subcortical structures -- viz., the left thalamus and the left caudate nucleus -- have also been implicated in language processing. And (2) PET studies have revealed that because visual linguistic stimuli are not transformed into an auditory representation, visual and auditory linguistic stimuli are processed independently by modality-specific pathways that have independent access to Broca's area. Moreover, (3) because the linguistic processing of visual stimuli can bypass Wernicke's area altogether, other brain regions must be involved with storing the meaning of words (Mayeux & Kandel, 1991,p. 845; also see Kolb & Whishaw, 1990, pp. 582-583).

Thus, not only do there seem to be separate -- parallel -- pathways for processing the phonological and semantic aspects of language, language processing clearly involves a larger number of areas and a more complex set of interconnections than just those identified by the W-G model (Mayeux & Kandel, 1991, p. 845). Indeed, the PET studies support the notion that language production and comprehension involve processing along multiple routes, not just one:

No one area of the brain is devoted to a very complex function, such as 'syntax' or 'semantics'. Rather, any task or function utilizes a set of brain areas that form an interconnected, parallel, and distributed hierarchy. Each area within the hierarchy makes a specific contribution to the performance of the task. (Fiez & Petersen, 1993, 287)

[Hence, modular models ought to go by the boards, for there is no one area of the brain devoted to reading, or any other language-related cognitive ability.] Simply put, it is the parallel, distributed feature of HLP that precludes any serial, feed-forward, hierarchical model from being implementable.

5. Conclusion

My purpose for this paper was to show that there are 'facts' about brains revealed by PET that preclude serial models of HLP from being implementable. For this end, I first focused on the nature of PET and its limitations. I then applied the data from recent language-related PET studies to evaluate the realizability of the W-G Model. I have shown how the PET data is inconsistent with what ought to be the case if the W-G model [or if serial models in general] were an accurate account of HLP. Among the failings of such models include: (1) language is not processed exclusively in a serial, feed-forward manner; rather, it is processed along parallel pathways; (2) visual language stimuli are not [always] transformed into an auditory code; (3) visual linguistic stimuli are not [always] fed-forward via Wernicke's area; and (4) Wernicke's area cannot be the only area where meanings are stored.

Further research is clearly required -- to not only increase the pool of tested subjects over which to generalize, but to corroborate and improve upon the PET findings. It is just as clear, however, that HLP depends on a much richer set of functional areas and interconnections than the W-G model identities. As such, although clinically useful, the W-G model isn't implementable. The task now ought to be evaluating the implementability of the host of parallel, multiple-route models (see Pollatsek & Rayner, 1989). Agreed. But that is another paper.

Notes

1. The labels 'control task' and 'active task' are relative, for the image generated from an active task during one scan may be the control task in another. For a thorough account of how PET works, see Posner & Raichle (1994).

2. Some limitations' of PET are more apparent than actual. For instance, PET can't answer all the questions. But neither can any other methodology or discipline. Indeed, this 'limitation' serves to promote the need for an interdisciplinary approach to modeling HLP, an appeal PET researchers themselves have made on several occasions (see Petersen & Fiez, 1993). And while it's true that no two brains are exactly alike, neural variability across individuals is small, and the averaging techniques are themselves very effective (Petersen & Fiez, 1993, 514). So, to demand exacting specificity is to set one's standard unattainably high.

References

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Stufflebeam, R. S. (1995). The brain, language, and PET: Revealing constraints on a naturalized model of language processing. 10th International Congress of Logic, Methodology, and Philosophy of Science, Florence, Italy (Aug. 19-25). Volume of abstracts: 556.

Stufflebeam, R. S. & Bechtel, W. (forthcoming). PET: Exploring the myth and the method. Philosophy of Science.