Vision: CNS 2017
Pathway from the eye to the cortex
Themes of this lecture Visual information is analyzed in more complicated ways than in the retina. One major pathway from the eye leads to the striate cortex and from there to many other visual centers. Cortical lesions can lead to difficulty in determining what an object is or where it is.
Optic Nerve one of the major targets of Multiple Sclerosis The optic nerve is an outgrowth of the diencephalon and is surrounded by meninges. Optic axons become myelinated by oligodendrocytes after they leave the retina.
Optic Nerve one of the major targets of Multiple Sclerosis The myelin of the optic nerve is one of the commonest locations of damage in multiple sclerosis.
Optic nerve and tract
Optic nerve and tract Inversion of the visual field; the left half of each eye sees the right half of the visual field. The right half of the visual field is seen by the left half of the brain. fixation point
Optic nerve and tract Nasal vs temporal hemiretina The fovea is the dividing point. Axons leave the eye at the optic disc (optic papilla). Half of the optic axons cross to the opposite side of the brain at the chiasm. temporal nasal optic nerve chiasm optic tract
Optic nerve and tract At the chiasm, axons sort out: Axons that look at the left half of the visual field project to the right half of the brain. Nasal axons from the left eye cross at the chiasm, and temporal axons from the right stay on the same side. temporal nasal optic nerve chiasm optic tract
The lateral geniculate nucleus (LGN) About 90% of optic axons project to the lateral geniculate nucleus. Optic axons go to about 40 other nuclei, too. The receptive field properties of cells in the geniculate are almost the same as the properties of retinal ganglion cells.
Lateral geniculate nucleus 12 Layers What s different about these layers? Different kinds of ganglion cells project to different layers. For example, noncolor, transient cells project to different layers than red-green color-selective cells. Parvo cellular Konio cellular Magno cellular
Lateral geniculate nucleus activity Imaging studies indicate that the lateral geniculate plays a role in selective attention. The activation comes from the cortex as well as the brainstem and thalamic reticular formation During sleep and periods of inattentiveness, transmission from the eye through the lateral geniculate is reduced, and cells fire spontaneous bursts.
Fibers from cells in the LGN enter the optic radiations, sweep around the lateral side of the lateral ventricle and pass to the occipital lobe. The visual fibers are also called the geniculocalcarine tract. The optic radiations
Primary Visual Cortex (Area 17, Striate cortex, V1) Located in the pole of the occipital lobe above and below the calcarine sulcus on the medial aspect of the hemisphere. The projections form a retinotopic map. calcarine sulcus
Retinotopic map: the largest area is devoted to the fovea Visual field!
Cortical circuitry Axons from lateral geniculate terminate in layer 4. pia 1 2 3 4 5 6 white matter
Cortical circuitry Axons from lateral geniculate terminate in layer 4. Information is then passed to upper layers and deeper layers. pia 1 2 3 4 5 6 white matter
Receptive fields in striate cortex Most cells in layer 4 have circular receptive fields with center-surround organization. The vast majority of cells in other layers respond best to straight bars or edges. Some respond best to dark bars or lines. Some respond best to light bars or lines.
Typical cell outside of layer 4 stimulus response orientation selectivity
Direction-selectivity stimulus response Different columns have cells that respond to different orientations.
Orientation columns Cortex is organized into columns (30-100 microns in diameter). In striate cortex, the orientation column is the basic unit. In each column, all of the cells outside of layer 4 will respond best to the same orientation. Some will be direction selective, others not. Other columns have cells that respond to other orientations.
Output of striate cortex: more than 40 other regions (directly or indirectly)!- "where" (parietal) "What" (temporal)
The where stream Cells in some of these regions respond almost exclusively to moving objects. Some respond to circular or spiraling movement. Some respond to visual flow. Some respond best to approaching or receding objects.
Lesions of the where stream loss of speed and motion perception loss of ability to use visual information to grasp objects visual neglect in peripersonal space Neglect syndromes tend to be transient after trauma but may be permanent in conditions such as Alzheimer s disease or major brain lesions.
Case study You are a neurologist with an 87-year female patient with a history of small strokes (transient ischemic attacks). You and the previous 3 neurologists who have seen this patient have examined the MRIs of her brain and found lesions in the right occipitoparietal region. Why is it extremely dangerous that all four of you have only told her that she has spots on the brain?
The what stream (temporal cortex) Some areas have many cells that are colorselective. Lesions lead to achromatopsia, the inability to perceive colors. Some areas have cells that respond to complex shapes. Objects such as houses, tools Body parts, such as hands, feet, legs
The what stream (temporal cortex) Some areas (e.g., fusiform face area) are particularly important for aspects of face perception. Expression Identity Direction of gaze
Face blindness Prosopagnosia Occipital (early visual stages) lesions: people can t make any sense of faces or even distinguish whether pictures of 2 people are the same or different. Anterior temporal lesions: people understand the characteristics of the faces, but can t identify who they belong to. Generally, right-side lesions lead to these problems (in right-handed people).