Re: How can color trigger emotional states ?

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