1 Perception and VR MONT 104S, Spring 2008 Lecture 3 Central Visual Pathways

2 The Eye

3 Receptive Fields The receptive field of a neuron is the region of the visual field to which a cell responds. Receptive fields can be mapped for neurons in the primate visual cortex by recording electrical signals from the cell while showing the animal a visual stimulus (e.g. a bar of light). The cell will respond by increasing or decreasing the rate of action potentials when the light is in the cell's receptive field. Show Movie!

4 Response of Retinal Ganglion Cells Retinal Ganglion cells and bipolar cells have receptive fields that exhibit a center-surround structure. Discussion Question: What is the center- surround structure useful for?

5 After-images When photoreceptors absorb light, they are temporarily "bleached", and rendered less sensitive to light. If we are exposed to a bright light, such as a camera flash, we will see a dark after-image in the location of the bleaching. The image appears to drift as we move our eyes. If we bleach the region of the fovea, it becomes difficult to read or perform tasks that require high visual acuity.

6 Central Visual Pathway Overview Processing of visual information in primates is accomplished in stages along the visual pathway: Retina Lateral Geniculate Nucleus Striate Cortex Extrastriate cortex

7 Projections from the Retina Axons from the retina project to three areas: 1)Pretectal area: a midbrain area that controls pupillary reflexes. 2)Superior colliculus: controls saccadic (rapid) eye movements. 3)Lateral geniculate nucleus (LGN) of the thalamus: This is the principal pathway, which projects to visual cortex. 4)Some of the axons from the retinal ganglion cells cross over to the opposite side of the brain before reaching the LGN (at the optic chiasm).

8 Projection from the Retina

9 The Optic Chiasm

10 The Lateral Geniculate Nucleus 6 layers of cells 4 parvocellular 2 magnocellular Input from the two eyes largely separate. Projection from retina is retinotopic.

11 Primary Visual Cortex Primary Visual Cortex (also known as Striate Cortex or V1) is the first cortical area in the visual pathway. Hubel and Wiesel (1950's and 60's) were the first to describe properties of V1 cells. They described 3 types: Simple cells: Elongated Receptive fields. Orientation selective. Defined regions of excitation and inhibition. Complex cells: Also orientation selective. No well defined regions of excitation and inhibition. Hypercomplex cells: End-stopped.

12 Columns in the cortex The cortex is a folded sheet of cells, about 2 mm thick. The cells form layers (6 layers in primary visual cortex). If move perpendicular to the surface of the cortex, cells will respond primarily to input from one eye (ocular dominance). The pattern of responses forms columns of ocular dominance.

13 Ocular Dominance Columns

14 Orientation Columns Preference for a given orientation also has a columnar structure:

15 Retinotopic organization The visual field maps onto visual cortex in a systematic way. More of V1 is devoted to processing the central visual field than to the periphery.

16 Mapping of Visual Areas This map shows a flattened cortex with the known visual areas mapped onto it. There are a large number of distinct visual areas (probably at least 20). Each area appears to have a specific function. The areas show a roughly hierarchical organization (although most areas have reciprocal connections).

17 Two Major Processing Streams There appear to be 2 major processing streams (although there are cross connections between them): 1.The Dorsal Stream: Includes areas MT, MST, VIP, 7a, etc. Processes motion, stereo, spatial relationships The "where" pathway. 2.The Ventral Stream: Includes areas V4, IT, etc. Processes color, form, objects. The "what" pathway.