Human Sensing: The eye and visual processing Physiology and Function Martin Jagersand.

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Presentation transcript:

Human Sensing: The eye and visual processing Physiology and Function Martin Jagersand

Visual Processing elements and Pathways Eye transforms light into nerve impulses Optic chiasm splits left and right visual fields LGN: Exact function unknown. May have to do with stereo. V1 (Striate cortex) performs spatial filtering / coordinate transforms

The Eye The Biological Camera Lens, cornea and fluids focus light. Six eye muscles orient the eye Iris adjusts light Retina captures images

Retina Converts light to nerve impulses Photoreceptor converts light Other cell layers perform image processing

Photoreceptors Rods and cones Rods: Night vision, but no color. 125E6, none in fovea, outnumber cones 20:1 Cones: Color sensitive, but poor light sensitivity 6.4E6, peak density in fovea

Photopigment Large molecule with two energy levels Cis retinal has low energy Trans slightly higher energy Incoming light photon adds energy => changes cis to trans state.

Interneurons and Ganglion cells Center-surround organization: 1. Light hyperpolarizes the rod and excites the bipolar cell below it 2. But inhibitory connections through horizontal cells suppress signals 3. Best response to localized “dot” 4. While stimulating surround only lowers firing rate What is this??? Convolution!!! Im*[-1 2 –1] + --

Disappearing figure? Focus steadily on first the left then the right black dot

LGN: Laterate Geniculate Nucleus “Switchboard” between retina and visual cortex. Exact function unknown Retinotopic. Axons from left and right eyes in alternate layers. Cells with same spatial angle but different eye adjacent

Visual Cortex: P and M cells

Simple Cells in V1 Direction sensitive “line finders”?? Responds best to edge segments of particular orientation Like convolving image with line filters?!?

V1 Simple cells What are they for? Spatially localized filters can be found with 1. Many (all needed) orientations. 2. Many scales. Perhaps performing localized spatial frequency analysis?

More cells in V1: Complex and Hypercomplex cells Complex cells: No specific retinotopic region. Responds to particular orientation or direction over a wide area. Hypercomplex cells:End stop. Only responds to a terminating line.

Dorsal and Ventral Pathways Where/What or Action/Perception?

Example responses Higher cells in the M pathway Follower neurons: Responds to particular object moving regardless of where in the visual field. Reach neurons: Responds when reaches to contact a specific 3D location. Attention neurons: Responds when gaze is directed to a specific object. Gaze neurons: Responds when gaze is directed to a specific 3D location.

Conclusions: Biological vision The visual system provides researchers with a window into the brain. A fair amount is known about the eye, retina and early retinotopic areas like striate cortex (V1). Little is known (for certain) about the function and purpose of higher areas.