Lecture 4 – Vision 3 – The Retina All lecture material from the following two links: 1) http://hubel.med.harvard.edu/book/bcontex.htm 2) http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20- %20Kandel/gateway.ut.ovid.com/gw2/ovidweb.cgisidnjhkoalgmeho00dbookimagebookdb_7c_2fc~32.htm Raghav Rajan Bio 354 – Neurobiology 2 January 21st 2015 1 From the last two classes ...!! ● Is there a difference in which percept is predominant? (Harini) ● What happens to percepts in split-brain patients? (Sahana) ● What happens in induced synesthesia? ● Why do you see an after-image with opposite colours much like a photo negative? (Gaurav) ● Cone opsins – how do they work? ● Why 3 pairs of extra-ocular muscles? ● 2 Retinal processing ● Photoreceptors – graded potentials (no spikes) ● Photoreceptors are depolarised at rest – dark current ● Light hyperpolarizes the photoreceptors – cGMP dependent mechanism ● Output of the retina consists of ON-center, OFF-surround RGCs and OFF-center, ON-surround RGCs ● How does this arise? ● What are the responses of other cells? ● What processing goes on in the retina? 3 Receptive fields of RGCs – output of retina ● Stephen Kuffler was the first to find these responses in the cat ● Diffuse light throughout the field did not evoke any responses! (earlier research) ● Cat was a good choice because of no motion- selective cells, no color complications 4 Receptive fields ● Useful for predicting responses of neurons to stimulus ● Not obvious from the wiring diagram 5 Wiring diagram of retina 6 Receptive fields of neighbouring RGCs ● Neighbouring RGCs have considerable overlap in their receptive fields 7 Size of receptive fields ● Size measured by either size on retina or angle ● Primate retina, near fovea – ~10um or ~0.017degrees ● Further out, size of receptive field centers increase, size of receptor population connecting to RGCs (direct pathway) increases and visual acuity falls 8 Receptive fields of RGCs – output of retina ● Stephen Kuffler was the first to find these responses in the cat ● Diffuse light throughout the field did not evoke any responses! (earlier research) ● Cat was a good choice because of no motion- selective cells, no color complications 9 How do bipolar cells respond? 10 Bipolar cells also have center-surround receptive fields ● Two types – ON-center and OFF-center http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/bipolar-cell-pathways-in-the-vertebrate-retina/ 11 Direct pathway to ON and OFF center bipolar cells ● Cones/rods – hyperpolarized by light ● ON center bipolar cell – depolarized by light in center – Inhibitory connection from cones/rods ● OFF center bipolar cell – hyperpolarized by light in center – Excitatory connection from cones/rods 12 ON and OFF bipolar cells - mechanism ● Photoreceptors release only 1 neurotransmitter – glutamate ● OFF bipolars express ionotropic receptors ● ON bipolars express mGluR6, which links to TRPM1-L channels, non- selective cation channel 13 http://retina.anatomy.upenn.edu/~noga/noga.html Indirect pathway for SURROUND ● Lateral inhibition through horizontal cells ● GABA used as neurotransmitter ● Horizontal cells are thought to contribute to most of SURROUND responses 14 CENTER-SURROUND interactions ● Horizontal cells mediate surround interactions http://thebrain.mcgill.ca/flash/a/a_02/a_02_cl/a_02_cl_vis/a_02_cl_vis.html 15 Human retina has 4 major types of RGCs ● Midget (M) cells – 80% - project to parvocellular pathways – small cells – slow responses – colour, details ● Parasol (P) cells – 10% - project to magnocellular pathways – big cells – fast responses – movement ● Bistratified cells ● Light sensitive RGCs 16 http://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/morphology-and-circuitry-of-ganglion-cells/ Dorsal and ventral streams – M cell and P cell pathways BIO 354 - Neurobiology 2 17 http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20- %20Kandel/gateway.ut.ovid.com/gw2/ovidweb.cgisidnjhkoalgmeho00dbookimagebookdb_7c_2fc~31.htm Actually lots more cell types in retina ● Don't know what most of these cells are doing 18 http://wws.weizmann.ac.il/neurobiology/labs/ulanovsky/sites/neurobiology.labs.ulanovsky/files/uploads/schneidman_compulsory_reading_mastland_2001_gollisch _meister_2010.pdf More complicated processing too ● Retina has direction selective ganglion cells (rabbit, frogs) ● Color comparisons are done by RGCs 19 Starburst amacrine cells are direction selective – input to direction selective RGCs ● Selective for outward motion ● Timing of excitation and inhibition determines motion selectivity 20 http://www.sciencedirect.com/science/article/pii/S096098220602611X Distribution of S, M and L cones in the human retina ● Diagram of distribution based on data http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html 21 Colour comparison computations start in the retina ● a – Foveal Midget cell that does M–L ● b – Foveal Midget cell that only connects to L cones ● c – Peripheral midget cell Some bias in the connections of this cell ● d – Small bistratified ganglion cell does S – (M+L) http://neurosurgery.stanford.edu/research/chichilnisky/PDFs/Field2010-NV.pdf 22 Retinal processing and output ● Center-surround organisation of receptive fields – Basically codes for CONTRAST – compares illumination of center with surround ● Two different pathways for responding to light increases and light decreases – ON-center and OFF-center RGCs ● M and P cell pathways ● Lateral inhibition, convergence, etc. ● Direct pathway – photoreceptor --> Bipolar cell --> RGC ● Indirect pathway – through horizontal and amacrine cells – SURROUND ● More complex responses – motion sensitivity, colour comparisons, etc. in retina itself 23.