Chapter 10The Central Visual System

Introduction

• Neurons in the visual system– Neural processing results in perception

• Parallel pathway serving conscious visual perception originate in the retina– Progress to lateral geniculate nucleus, primary visual cortex &

higher order visual areas in temporal and parietal lobes

• Overlapping neuronal receptive fields – Sensitive to different facets of the visual input

The Retinofugal Projection

• The Optic Nerve, Optic Chiasm, and Optic Tract

The Retinofugal Projection

• Right and Left Visual Hemifields

– Left hemifield projects to right side of brain– Ganglion cell axons from nasal retina cross, temporal retinal

axons stay ipsilateral

• Visual deficits from lesions in the retinofugal projection

• Notice the specific types of loss shown in black as compared to the intact fields

The Retinofugal Projection

Nonthalamic Targets of the Optic Tract are areas that will use visual input but not in the sense of SEEING objects.

– Hypothalamus: Biological rhythms, including sleep and wakefulness

– Pretectum: Size of the pupil; certain types of eye movement

– Superior colliculus: Orients the eyes in response to new stimuli

The Lateral Geniculate Nucleus

(LGN)

The pathways occur in alternating layers.

Contralateral = on the other side

Ipsilateral = on the same side

Parvocellular Cells

• Parvocellular cells, also called P-cells, are neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus.

• "Parvus" means "small" in Latin, and the name "parvocellular" refers to the small size of the cell compared to the larger magnocellular cells.

• The parvocellular neurons are sensitive to color, and are more capable of discriminating fine details than their magnocellular counterparts. Parvocellular cells have greater spatial resolution, but lower temporal resolution, than the magnocellular cells.

Magnocellular Cells

• Magnocellular neurosecretory cells are large cells within the supraoptic nucleus and paraventricular nucleus of the hypothalamus.

• There are two types of magnocellular neurosecretory cells, oxytocin-producing cells and vasopressin-producing cells, but a small number can produce both hormones.

• These cells are neuroendocrine neurons, they are electrically excitable, and generate action potentials in response to afferent stimulation.

The Lateral Geniculate Nucleus (LGN)

Inputs Segregated by Eye and Ganglion Cell Type

P type = (also known as beta or midget ganglion cells) are believed to be responsible for detecting details in vision.

M type = (also known as alpha or parasol ganglion cells) are believed to be responsible for detecting motion.

nonM-nonP type =are a diverse group of cell types that make up the remaining 5% of RGCs. Their roles in vision are less understood than M- and P-type ganglion cells, but it is known that some non-M, non-P type cells are involved in color vision.

The Lateral Geniculate Nucleus (LGN)

Receptive Fields

– Receptive fields of LGN neurons: Identical to the ganglion cells that feed them

– Magnocellular LGN neurons: Large, monocular receptive fields with transient response

– Parvocellular LGN cells: Small,monocular receptive fields with sustained response

Anatomy of the Striate Cortex

The calcarine sulcus is where the primary visual cortex is concentrated. The central visual field is located in posterior portion of the calcarine sulcus and the peripheral visual field in the anterior portion.

Anatomy of the Striate Cortex

Retinotopy

– Map of the visual field onto a target structure (retina, LGN, superior colliculus, striate cortex)

– Central visual field overrepresented

– Discrete point of light: Activates many cells in the target structure due to overlapping receptive fields

– Perception: Based on the brain’s interpretation of distributed patterns of activity

Anatomy of the Striate Cortex

Retinotopy the spatial organization of the neuronal responses to visual stimuli. In many locations within the brain, adjacent neurons have receptive fields that include slightly different, but overlapping portions of the visual field. The position of the center of these receptive fields forms an orderly sampling mosaic that covers a portion of the visual field.

Human Anatomy and Physiology, 7eby Elaine Marieb & Katja Hoehn

Copyright © 2007 Pearson Education, Inc.,publishing as Benjamin Cummings.

(a) (b)

Pretectalnucleus

Right eye Left eye

Fixationpoint

Opticradiation

Superiorcolliculus(sectioned)

Lateralgeniculatebody

Optic tractOptic chiasma

Uncrossed (ipsilateral) fiberCrossed (contralateral) fiber

Optic nerve

Lateral geniculatebody of thalamus

Superior colliculus

Occipital lobe (visual cortex) Corpus callosum

Supra-chiasmaticnucleus

Anatomy of the Striate Cortex

Lamination of the Striate Cortex

– Layers I – VI

– Spiny stellate cells: Spine-covered dendrites; layer IVC

– Pyramidal cells: Spines; thick apical dendrite;

layers III, IV, V, VI

– Inhibitory neurons: Lack spines; All cortical layers; Form local connections

Anatomy of the Striate Cortex

Inputs to the Striate Cortex

– Magnocellular LGN neurons: Project to layer IVC

– Parvocellular LGN neurons: Project to layer IVC

– Koniocellular LGN axons: Bypasses layer IV to make synapses in layers II and III

Anatomy of the Striate Cortex

Ocular Dominance Columns

– Studied with transneuronal autoradiography from retina, to LGN, to striate cortex.

Anatomy of the Striate Cortex

Inputs to the Striate Cortex

– First binocular neurons found in striate cortex - most layer III neurons are binocular (but not layer IV)

Physiology of the Striate Cortex

Cortical Receptive Fields

– Orientation Selectivity

Physiology of the Striate Cortex

Cortical Receptive Fields

– Direction Selectivity

» Neuron fires action potentials in response to moving bar of light

Physiology of the Striate Cortex

Cortical Receptive Fields– Simple cells: Binocular; Orientation-selective; Elongated on-off

region with antagonistic flanks responds to optimally oriented bar of light

– Possibly composed of three LGN cell axons with center-surround receptive fields

Physiology of the Striate Cortex

Cortical Receptive Fields

– Complex cells: Binocular; Orientation-selective; ON and OFF responses to the bar of light but unlike simple cells, no distinct on-off regions