J cell as a hardware preprocessor?

I saw the movie of the hangout and because of my lack of understanding of spoken English,  I'd have some questions to ask to better understand all the interesting things said yesterday.
1. It seemed to understand that some components of the retina behave as hardware preprocessors on the contrary of photoreceptors that act as pure sensors. In particular the J cell preprocesses the downward movement. The question is: does it regardless of the photoreceptors or in some way (which?) they take into account the signals from the photoreceptors?
2. From the point of view of evolution is there any reason why the photoreceptors in the retina are arranged behind the neurons?

Thanks in advance

PS I'm sorry not having been able to follow the live hangout due to technical problems with my computer but it is probably better to follow you at a later time so I got the possibility to review the discussion in order to better understand what was being said.
PPS in the discussion did you refer to a Wikipedia page. Which one?

  1. There are five broad classes of retinal neurons: photoreceptor, horizontal, bipolar, amacrine, and ganglion cells.  The photoreceptors sense light, and transmit signals to horizontal and bipolar cells.  Eventually signals get to the ganglion cells, which are the outputs of the retina.  The J is a kind of ganglion cell.  (For an exception to the rule that only photoreceptors sense light, see http://en.wikipedia.org/wiki/Photosensitive_ganglion_cell.)  So yes the J ultimately receives signals from the photoreceptors.
    2. Evolutionary explanations are just guesses.  One guess is that the outer segments of the photoreceptors are very metabolically active, so they are placed in the outer retina near vasculature.  See section 7 of http://webvision.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/ for more about retinal vasculature.

    This is the EyeWire wiki page on the J:  http://wiki.eyewire.org/en/JAM-B_Cell
thanks Sebastian

Another question:
since there are so specialized ganglion neurons (movement, direction, etc.), one may think that the axons of each of them are connected to specific circuits of neurons (for the movement, direction etc..) in the brain?


P.S. Sorry for my questions probably trivial but I'm really a beginner in the field of neuroscience
btw Sebastian,
in a message posted by you some time ago (I don't remember where), I read a quote taken from oration of Mark Antony in Shakespeare's Julius Caesar. 
Remembering my study at school (so many years ago!) of English literature and hoping that Shakespeare has nothing to say you might rewrite :-) that quote in searching eyewirer as follows:

Friends, romans, countrymen lend me your eyes and time!   :-)


Here’s a quote from the original paper on the J ganglion cell (Kim et al. 2008):

J-RGCs projected most
heavily to the superior colliculus (Fig. 4c–e), the major retinorecipient
structure in mice. Some J-RGCs also projected to the dorsal
lateral geniculate nucleus. In contrast, we detected no J-RGCs in the
pretectal nucleus, in the ventral nucleus of the lateral geniculate body
or in the accessory optic system (Fig. 4c and Supplementary Fig. 7).
The accessory optic system is a major site to which other direction selective
RGCs project.

This shows you are right about multiple circuits.  According to this finding, the J sends signals to the superior colliculus but not to the accessory optic system.  Some other types of direction selective (DS) ganglion cells (discovered decades before the J) send signals to the accessory optic system, which is involved in gaze-stabilizing eye movements.

I think you got the quote right!

How far is Bari from Rome?  :)
Bari is located approximately 450 Km South East from Rome.

Btw: Nice Thanksgiving to all US eyewirer


P.S. I hope to meet you in Bari or in  Italy :-) (food is very good) :-)
P.P.S. I'll try to read the paper of Kim et al. even if I'm trying to read (with my great difficulty) some of the articles of the book WebVision. From this point of view it would be much more comfortable your lectures on Edx (or similar). I think it could increase the number of eyewires.