|MadSci Network: Neuroscience|
Dear Bernard, It is not entirely clear that sensory information must enter the amygdala to have emotional content extracted. There is a large amount of data on the necessity of the amygdala for the acquisition and expression of conditioned fear. In this paradigm, an animal is subjected to a noxius perturbation that is preceded by a neutral stimulus. After a time, it responds to the stimulus with a set of behaviors characteristic of fear. An animal that has had its amygdala lesioned does not learn this response to the stimulus, and lesioning the amygdala in a trained animal abolishes the association. However, it would be hard to argue that the amygdala is the seat of emotion simply because it is necessary for conditioned fear. Emotional responses may arise from activity distributed across a large number of cortical and subcortical regions. If this is the case, you need not find an anatomical route from cones to the amygdala to in order to speculate on how color elicits emotion. That said, there may be some pathways for color information to reach the amygdala. For instance, you can follow information through the visual hierarchy into regions of association cortex, and some connections between association cortex and the lateral nucleus of the amygdala have been identified. There are also connections between sensory thalamus and the lateral nucleus of the amygdala (these have mainly been studied for the medial geniculate nucleus of the auditory system, but an analogue for the visual system appears to exist). As you stated, information from the superior colliculus is unlikely to play any role. Are these pathways fast enough? It depends on how quickly emotions need to be pulled from color vision, and I do not know what this constraint is. Finally, let me state that we have volumes of information left to uncover in regard to brain anatomy. One way to study to circuit for color vision is to inject a dye into the parvocellular layers of the LGN. It will get taken up into parvocells and transported into primary visual cortex, and you can examine where it shows up there to identify areas that process color. If you’re lucky, the dye will spread through additional synapses, and you can start to look in extrastriate areas. This process has many technical limitations. Even if the dye was perfect and spread as many synapses as you desired, how do you take the brain circuitry you find and assign function to it? The current means is to record from the various labeled regions in an awake, behaving animal and try to correlate the physiology of those areas to behavior. If this correlation is good, it still remains an open question how you interpret it in terms of cognitive processes. If you want to understand emotion, you can imagine how difficult it is to induce and then gauge emotion reliably in an animal model. I hope this helps. If you are interested in more details on the relationship of color vision to emotion, I suggest you consult the scientific literature directly. Your knowledge of visual pathways appears sophisticated, so I would expect that you would glean more information from the database than from my brief answer. You can find the PubMed search engine at www.nlm.nih.gov. Some of the information in this answer came from Fundamental Neuroscience, edited by Zigmond, Bloom, Landis, Roberts, and Squire (Academic Press, 1999). An excellent treatment of how one correlates physiology and behavior is given by Parker and Newsome, “Sense and the Single Neuron: Probing the Physiology of Perception,” Annual Review of Neuroscience, 1998, 21:227-277. Best, Michael
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