160 report

The Visual System…

Let there be Light….

Light: waves of electromagnetic energy that are between 380 and 760 nanometers.

Wavelength: distance between one peak of a light wave and the next peak. Plays an important role in the perception of colour

Intensity: plays a role in the perception of brightness.

Let there be Light…

Irises: donut shaped bands of contractile tissue that regulates the amount of light that reach the retinas. Gives our eyes their characteristic colour.

Retina: a light sensitive tissue lining the inner surface of the eye.

Pupil: Hole in the iris. This is where light enters. Adjustment of pupil size in response to changes in illumination represents a compromise between sensitivity and acuity.


Sensitivity: Ability to detect presence of dimly lit objects.

Acuity: Ability to see details of objects. Pupils constrict in high illumination because

sensitivity is not important. When constricted, the image falling on each retina is sharper and there is a greater depth of focus.


Pupils dilate in low illumination to let in more light, sacrificing acuity and depth of focus.

Lens: located behind ach pupil. Focuses light on the retina.

Ciliary muscles: eye muscles that control the shape of the lenses.

Accommodation: process of adjusting the configuration of the lenses to bring images into focus on the retinas.


When looking at an object that is near, the tension on the ligaments holding each lens is adjusted by the ciliary muscles, and the lens assumes its natural cylindrical shape. This increases the ability o the lens to refract light.

When focusing on distant objects, the lens is flattened.

Eye Position

Vertebrates have 2 eyes because they have two sides. The left and the right…

Predators: have front facing eyes because this enables them to accurately perceive how far away a prey is.

Preys: have side facing eyes because it gives them a wider field of vision, thus enabling them to see approaching predators.

Eye Position

Front facing eyes- an arrangement that is an important basis for our visual system’s ability to create three-dimensional retinal images.

Binocular Disparity

It is the difference in the position of the retinal image of the same object on the two retinas.

Greater for close objects than for distant objects.

Visual system can use the degree of binocular disparity to construct one three-dimensional perception from two two-dimensional retinal images.

The Retina and Translation of Light into Neural signals…or the Boring Part

The retina converts light to neural signals, conducts them toward the CNS, and participates in the processing of the signals.

It is composed of 5 layers of different kinds of neurons. Each of the five types of retinal neurons comes in a variety of subtypes. Over 50 different kinds have been identified.


Receptors: cells that are specialized to receive, mechanical, or radiant signals from the environment; also proteins that contain binding sites for particular neurotransmitters.

Horizontal cells: type of retinal neurons whose specialized function is lateral communication.


Bipolar cells: bipolar neurons that form the middle layer of the retina.

Amacrine cells: type of retinal neurons whose specialized function is lateral communication.

Retinal ganglion cells: retinal neurons whose axons leave the eyeball and form the optic nerve.


Blind spot: the area on the retina where the bundle of axons of the retinal ganglion cells penetrate the receptor layer and leave the eye as the optic nerve.

Fovea: Cone rich and central indentation of the retina, which is specialized for high acuity vision.


Retinal neurons communicate chemically through synapses and electrically through gap junctions.

Completion: the visual system’s automatic use of information obtained from receptors around the blind spot, or scotoma, to create a perception of the missing portion of the retinal image.


Surface Interpolation: process by which the visual system perceives large surfaces, by extracting information about edges and from it, inferring the appearance of adjacent surfaces.

Cone and Rod Vision

Cone: visual receptors in the retina that mediate high acuity colour vision in good lighting.

Rods: visual receptors in the retina that mediate achromatic, low-acuity vision under dim light.

Duplexity theory: theory that cones and rods mediate photopic and scotopic vision, respectively.

Cone and Rod Vision

Photopic Vision: cone mediated, predominates when lighting is good.

Scotopic: rod mediated, predominates in dim light. Difference between phopic and scotopic vision is

how they are “wired” In scotopic vision, the output of several hundred rods

converge on a single retinal ganglion cell. In the photopic system only a few cones converge

on each retinal ganglion cell to receive input from only a few cones.

Spectral Sensitivity

Photoptic spectral sensitivity curve: graph of the sensitivity of cone-mediated vision to different wavelengths of light.

Scoptic spectral sensitivity curve: graph sensitivity of rod-mediated vision to different wavelengths of light.

Purkinje effect: in intense light, red and yellow wavelengths look brighter than blue or green wavelengths of equal intensity; in dim light, blue and green wavelengths look brighter than red and yellow wavelengths of equal intensity.

Eye Movement

Fixational eye movements: involuntary movements of the eyes that occur when a person tries to fix their gaze on a point.

Tremor Drifts Saccades: rapid eye movement of the eyes

between fixations

Visual Transduction: Conversion of Light to Neural ignals

Transduction: conversion of vone form of energy to another.

Visual Transduction: conversion of light to neural signals by the visual receptors.

Rhodopsin: photopigment of rods. Loses colour when exposed to light.

Primary Visual Cortex

Retina-geniculate-striate pathway: major visual pathway from each retina to the striate cortex [PVC] via the lateral geniculate nuclei of the thalamus.

Primary Visual Cortex: area of the cortex that receives direct input from the lateral geniculate nuclei

Lateral geniculate nuclei: 6 layered thalamic structures that receive input from the retinas and transmit their output to the primary visual cortex.


To simplify things, all signals from the left visual field reach the right primary visual cortex either ipsilaterally from the temporal hemiretina of the right eye or contralaterally (cia the optic chiasm) from the nasal hemiretina of the left eyes.


Retinotopic: organized according to the map of the retina [retina-geniculate-striate]

Parvocellular layers: layers of the lateral geniculate nuclei that are composed of neurons with small cell bodies; the top four layer.Particularly responsive to colour, fine pattern details, and to stationary or slow moving objects. Cones provide the majority of input.


Magnocellular layers: layers of the lateral geniculate nuclei that are composed of neurons with large cell bodies; the bottom two layers. Particularly responsive to movements. Rods provide the majority of input.

Seeing Edges

Story of the horse shoe crab and machbands from 135

Lateral Inhibition: inhibition of adjacent neurons or receptors in a topographic array.

Receptive field: the area of the visual field within which it is possible for the appropriate stimulus to influence the firing of a visual neuron.

Seeing Edges

Simple cells: neurons in the visual cortex that respond maximally to straight-edge stimuli in a certain position and orientation.

Complex cells: neurons in the visual cortex that respond optimally to straight-edge stimuli in a certain orientation in any part of their receptive field.

Seeing Colour

Component Theory (trichromatic) Proposed by Thomas Young in 1802 and

refined by Hermann von Helmholtz in 1852 The relative amount of activity produced in

three different classes of cones by light determine its perceived colour

Seeing Colour

Opponent process Theory Ewald Herring 1878 A visual receptor or a neuron signals one colour

when it responds in one way and signals the complementary colour when it responds in the opposite way.

Complementary colours: colours that produce white or gray when combined in equal measure.

Seeing Colour

Colour Constancy: tendency of an object to appear the same colour even when the wavelengths of light that it reflects changes.

Retinex theory: colour of an object is determined by its reflectance, which the visual system calculates by comparing the ability of adjacent surfaces to reflect short , medium and long wavelengths.

Seeing Colour

Cytochrome Oxidase: an enzyme present in particularly high concentrations in the mitochondria of dual-opponent color cells of the visual cortex.

Cortical Mechanisms of Vision And Conscious Awareness….


Secondary visual cortex: areas of cerebral cortex that receive most of their input from primary visual cortex

Visual association cortex: areas of cerebral cortex that receive input from areas of secondary visual cortex as well as from secondary cortex of other sensory systems.


Prestriate cortex: band of tissue in the occipital lobe that surrounds the primary visual cortex

Inferotemporal cortex: cortex of inferior temporal lobe.

Posterior parietal complex: area of association cortex that receives input from the visual, auditory, and somatosensory systems and is involved in the perception of spatial location and guidance of voluntary behaviour.


Scotoma: an area of blindness produced by damage to, or disruption of, an area of the visual system.

Perimetry Test: procedure used to map scotomas.

Hemianospic: scomota covering half of the visual field


Blindsight: ability of some patients who are blind as a consequence of cortical damage to unconsciously see some aspects of their visual environments [outlines/foggy images/etc]


Dorsal stream: group of visual pathways that flows from the primary visual cortex to the dorsal prestriate cortex to the posterior parietal cortex; according to one theory, its function is the control of visually guided behaviour.

Ventral stream: group of visual pathways that flows from the primary visual cortex to the ventral prestriate cortex to the inferotemporal cortex; according to one theory, its function is conscious visual perception


Where vs What theory: Dorsal S specializes in visual spatial perception; Ventral S specializes in visual pattern recognition

Damage to DS disrupts visual spatial perception

Damage to VS disrupts visual pattern recognition


Control of Behaviour vs Conscious Perception Theory

DS specializes in visually guided behaviour VS specializes in conscious visual perception

Prosopagnosia

Visual agnosia for faces Agnosia: inability to consciously recognize

sensory stimuli of a particular class that is not attributable to a sensory deficit or to verbal or intellectual impairment

Visual agnosia: failure to recognize visual stimuli that is not attributable to sensory, verbal, or intellectual impairment

Akinetopsia

Deficiency in the ability to perceive motion, which often results from damage to dorsal visual pathway.

Science

160 report