Flavia Filimon              

                                                107 B SECTION 4

 

                                           THE VISUAL SYSTEM

 

 

Retina –

-          photoreceptors don’t spike, only become hyperpolarized

-          bipolar cells also don’t spike – ON bipolars become depolarized, OFF bipolars become hyperpolarized (in response to light)

-          Ganglion cells: X (sustained), Y (transient), ON (e.g. X-ON, Y-ON) (respond to light being turned ON), OFF (respond to light being turned OFF) , lagged and nonlagged.

 

·        Y cells have larger cell bodies and receptive field sizes and are also known as magno

·        X cells have smaller cell bodies and receptive fields and are also known as parvo

 

 

YOU SHOULD BE ABLE TO DRAW THE RESPONSE PROPERTIES OF X AND Y ON AND OFF GANGLION CELLS

 

            * X on, X off, Y on, Y off graphs, as well as lagged and non-lagged

 

Note: BOTH THE RETINA AND THE LGN HAVE X AND Y GANGLION CELLS, BUT LAGGED/NONLAGGED GANGLION CELLS ARE ONLY PRESENT IN THE CAT LGN

 

LGN -

-          Know projections from retina to dLGN (left visual field à R dLGN)

-          In the LGN: massive expansion of the center of gaze representation (? 70%)

-          Mutant Belgian Sheep dog: the projections from the nasal hemiretinas do not cross (do not project contralaterally), instead they project to the ipsilateral eye just like the temporal hemiretinas. This means two points at different locations in your visual field are superimposed (overlapping) in the dLGN (be able to draw diagram). I.e. different coordinates are represented on top of each other in one dLGN.

-          Siamese Cats:  projections from both nasal and temporal hemiretinas cross completely and connect to the contralateral dLGN

-          Conformal map in the dLGN: from polar coordinates (retina) to Cartesian coordinates (LGN).

-          If we want to flatten /stretch out the retina, which has a higher density of cells at the center, we would get more distortion in the circumferential than the radial direction. Instead, a conformal map the same amount of stretching occurs in both the circumferential and the radial directions.

 

Receptive field = the part of the visual field that a neuron responds to, when you stimulate it with stimuli shown in that location

 

 

V1, V2- 

-          know how objects in the visual field are represented in V1 and V2!!!

-          Lower visual field is represented in the upper half of V1 (and V2), upper visual field is represented in the lower half of V1 (and V2)

-          V1 IS  ALSO AN APPROXIMATE CONFORMAL MAP, LIKE THE LGN