Visual Processing - Hardware and Mechanisms

Visual Processing

Systems for the processing of visual information extract features from the visual field at multiple hierarchical levels. The photoreceptor cells in the retina detect points of light (image). The retinal ganglion cells respond to point contrast (form and color). The lateral geniculate nucleus provides a first rapid analysis of high-contrast, black and white features. The primary visual cortex provides a slower but more in-depth analysis with an emphasis on linear contrast, color, depth, object vs. background motion.

• Retinal receptor cells (i.e., cones for color and rods for black and white) feed into bipolar on or off cells in the retina
• Retinal ganglion neurons have center-surround characteristics and thus respond to spots of light of a particular size and in specific places of the visual field. Receptive fields are circular and monocular. Many axons cross to the opposite side as they project towards the lateral geniculate nucleus (LGN) and the primary visual cortex (V1).
• The LGN: center-surround, circular, monocular
• The visual cortex is arranged in multiple horizontal layers with outer layers focused on the processing of simpler features. Most neurons respond best to oriented bar stimulus, sensitive to motion, monocular or binocular. As the information proceeds further into the structure, features at increasing levels of abstraction are analyzed. Vertical columns across these layers primarily respond to input from one specific eye and respond to a particular feature orientation. Simple cells respond best to bars of given orientation at given location within receptive field. These oriented edge detection neurons feed their output into motion-sensitive neurons. Complex cells are less sensitive to stimulus position within the receptive field and more sensitive to stimulus motion. Hypercomplex cells respond like complex cells, but feature in addition an inhibitory region at one end. Some neurons at the highest-level receptive fields are quite specific i.e. a neuron that only responds to faces, faces with particular expressions, or belonging to one particular individual (i.e., grandmother neuron).
• Flicker-fusion frequency, the lateral geniculate body and dyslexia