Lecture 5: Visual Projection Pathways

Retinogeniculocortical pathway

The pathway responsible for transmitting visual information from the retina to the brain. Information remains segregated through the thalamus by the lateral geniculate nucleus. One side of the cortex views the opposite hemifield (i.e. retinal ganglion cells in the right eye capture information from the left visual field and transmits it into the right optic tract). 

Internuclear ophthalmoplegia

The neurological condition characterized by impaired horizontal eye movements that occurs due to a lesion in the medial longitudinal fasciculus (MLF). 

Retinotopy

The mapping of visual input from the retina to neurons within the visual stream, showing that neighboring neurons map to neighboring target regions. (i.e. neurons in the retina map to neighboring ones in the LGN, which then map onto the striate cortex (layer IVC)). 

Optic Chiasm 

The point where contralateral axons from each eye cross over. Lesions in this area illustrate the flow of visual information. 

Striate Cortex (V1, or Brodmann area 17)

The main area of the brain responsible located in the occipital lobe that is responsible for processing visual information about motion, shape, and color from the retina. Information from each eye is segregated into ocular dominance columns within the six cortical layers. 

Layer III

The layer of V1 where information from each eye begins to merge.

Layer IVC

The layer of V1 principal recipient of the thalamus where axons of the lateral geniculate nucleus terminate.

Cytochrome oxidase staining of V1

This technique reveals the presence of blobs in Layer II, III, V, and VI, signaling that these areas receive input from koniocellular layers of the lateral geniculate nucleus. 

Dorsal Stream

The stream involved in the analysis of visual motion and the visual control of action. From area V1: → V2 → V3 → MT → MST

Ventral Stream

The stream involved in the perception of the visual world and the recognition of objects. From area V1: → V2 → V3 → V4 → IT → Other areas. 

Akinetopsia

The inability to detect motion due to bilateral damage to areas such as the MT and MST in the dorsal stream. (i.e, 1983 Josef Zihl reported having difficulty pouring a cup of tea because the fluid appeared frozen. Also had trouble following conversations because lip movements were missed). 

Achromatopsia

The partial or complete loss of color vision due to damage to area V4 in the ventral stream. 

Prosopagnosia

The inability to recognize faces due to damage to area IT in the ventral stream. 

Visual perception

The ability to identify and assign meaning to objects. There is a hierarchy of receptive fields or light patterns a neuron responds to, but there is no specific to ‘grandmother cell’.