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Abstract
LeDoux (1996) has identified a sub-cortical neural circuit that mediates fear responses in rats. The existence of this neural circuit has been used to support the claim that emotion is a non-cognitive process. In this paper I argue that this sub-cortical circuit cannot have a role in the explanation of emotions in humans. This worry is raised by looking at the properties of this neural pathway, which does not have the capacity to respond to the types of stimuli that are generally taken to trigger emotion responses. In particular, the neurons in this pathway cannot represent the stimulus as a complete object or event, rather they represent the simple information that is encoded at the periphery. If it is assumed that an object or event in the world is what, even in simple cases, causes an emotion, then this sub-cortical pathway has limited use in a theory of emotion.
Acknowledgments
Versions of this paper were presented at the MidSouth Philosophy Conference in Memphis, Tennessee and to the Indiana University Cog-X group. For much appreciated assistance throughout the preparation of this paper I would like to thank Thomas Polger. Thanks also to Hilmi Demir, Jesse Prinz, Jennefer Robinson, and John Bickle who provided valuable feedback on earlier versions of this paper.
Notes
Gregory Johnson is an adjunct Assistant Professor in the Department of English and Philosophy at Drexel University.
Notes
[1] Lazarus and Scherer are responding to LeDoux (Citation1989), which covers the same material.
[2] What I am calling the emotion response includes the bodily response, for instance, the raised heart rate, trembling, sweaty palms, a particular facial expressions, and so on. The emotion response may have other components as well, one of which is presumably the individual's conscious awareness of the emotion.
[3] The distinction between language comprehension and the eye blink reflex is rather stark, but it seems to be consistent with the exclusive distinction with which the debate in emotion has been cast (i.e., cognitive versus non-cognitive).
[4] Robinson is using affective appraisal, non-cognitive affective appraisal, and non-cognitive appraisal interchangeably.
[5] The accounts of emotion as a cognitive process typically do not suggest that the process is conscious or under the individual's control. The term cognitive in this sense is used in the same way as it is for a cognitive task such as language comprehension. However, this is different than the way in which cognitive is used in some areas of philosophy, e.g., metaethics.
[6] A response that includes freezing, blood pressure changes, heart rate changes, and hormonal changes.
[7] There are two types of hair cells, inner hair cells and outer hair cells. The inner hair cells synapse on about 20 fibers of the auditory nerve, but each of those fibers will only be innervated by one or two inner hair cells. This preserves (redundantly) the frequency that the inner hair cell encodes. The other 5–10% of the nerve fibers receive inputs from about ten outer hair cells, which means that these fibers carry less specific information about frequency.
[8] This contrasts with convergent connections (many-to-one) and divergent connections (one-to-many) between neurons. Each of these types of connections—topographic, convergent, and divergent—transfers information in a different way (see Thivierge & Marcus, Citation2007).
[9] This includes temporal cortex, area 2 (Te2); temporal cortex, area 3 (Te3); and parts of the perirhinal cortex.
[10] In particular, cells in the PIN respond to a much wider range of frequencies (i.e., have much broader tuning curves) than cells in MGv.
[11] These three studies all used macaque monkeys.
[12] Visual information is also organized topographically in the LGN where the organization of the neurons preserves the pattern of stimulation that the retina receives.
[13] It seems to be the case that the lateral posterior nucleus (pulvinar) projects to the extrastriate cortex. This has been suggested as the pathway for blindsight in humans (Guzeldere, Flanagan, & Hardcastle, Citation2000).
[14] Note that Prinz also seems to be saying that projections to the amygdala are from the area in the thalamus that is part of the main visual pathway, which is not accurate.
[15] But keep in mind that if the emotion response is generated solely by the activity in the thalamo-amygdala pathway, then the process that generates the response is a modular one (ie., mandatory and encapsulated in Fodor's Citation1983 terms for a modular process). This means that it can only be one of the simple cues that causes the emotion response, and not one of these cues combined with other information, for instance, information about the context of the event. Intuitively, it is easy to see how a fear response might be generated by a sudden loud sound when an individual is walking alone through a dark alley. The stimulus, however, in this case would be much more than just the particular sound. The stimulus would include the dark alley and knowledge about such locations. Again intuitively, it seems hard to image that any of these simple cues that are encoded at the periphery do consistently cause emotion responses by themselves.
[16] Discussing the location of neurons that respond to whole objects is beyond the scope of this paper, but I have in mind the explanation of the visual system that is put forward by Van Essen and Gallant (Citation1994). In whatever way we might conceive of the function of the neurons in V4 and in the inferotemporal areas (in the cortex), which Van Essen and Gallant describe as responding to complex patterns and shapes, these neurons seem to be necessary parts of a mechanism that can respond to a spider qua spider.
[17] Note that removing the thalamo-amygdala pathway from the debate alters the resources available to the advocate of the non-cognitive position. One can suggest that a psychological process is not utilizing conceptual content, and then point to a sub-cortical pathway that could be carrying out the process to support this claim (e.g., see the quote from Prinz in Section 5). If, however, this sub-cortical pathway is not available, then this claim is much less compelling.
[18] Or, to be more precise, encephalization in humans, which is a function of the ratio of brain size to body weight, and this ratio compared to other animals (Jerison, Citation1976, Citation1977).
[19] “Pathway” in this context becomes a difficult term to pin down. Some neural connectivity between the thalamus and the amygdala is not what we are after. Rather, something like the same degree of connectivity (i.e., numbers of axons relative to brain size) and performing a similar function is perhaps closer to what seems to be in order.
[20] I.e., the six-layer cortex. The hippocampus is part of the archicortex (three layer cortex), which is the oldest part of the cortex.
[21] And of course, areas in the brainstem that drive the bodily changes.