TheVisualSystemFromEyetoCortex从眼睛到皮层的视觉系统课件

1、The Visual System:From Eye to CortexCh. 6第1页，共27页。OutlineThe EyeThe RetinaVisual Transduction by RhodopsinFrom Retina to Primary Visual Cortex第2页，共27页。The EyesStructure: review the major structures of the eye (diagram given in class)(note that the size of the pupils is a compromise between sensitivi

2、ty and acuity)第3页，共27页。The Eyesciliary muscles adjust the lenses to focus visual images sharply on each retina, regardless of the distance of the image from eyes; this focusing is called acommodation第4页，共27页。The EyesWhen we look at something near, the ciliary muscles contract, tension on the lenses

3、is reduced, and they become more cylindricalWhen we look at distant object, the muscles relax, tension on the lens is increased, and the lenses flatten第5页，共27页。The EyesBinocular Disparity: unlike most vertebrates, most mammals have two eyes on the front of their heads, rather than one on each side;

4、this cuts down the field of view, but it insures that most of what is seen is seen through both eyes第6页，共27页。The EyesBecause each eye sees things from a slightly different perspective, there is a difference in the two retinal images; this binocular disparity is greater for closer things; the degree

5、of binocular disparity associated with a particular visual stimulus helps the brain create a 3-D perception from two 2-D retinal images thus depth perception is partially achieved from this (remember, we can still perceive depth with one eye closed)第7页，共27页。The RetinaStructure: five layers receptor

6、layer; horizontal cell layer; bipolar layer; amacrine cell layer; and retinal ganglion cell layer(diagram in class)The receptor is the farthest from light; therefore, incoming light is distorted by four layers of neurons before reaching the receptors第8页，共27页。The RetinaThe fovea is a pit about 330 mi

7、crometers in diameter; it is the only part of the retina capable of mediating high-acuity vision. This is due, in part, to the fact that axons of retinal ganglion cells are thinnest over the fovea and light is distorted less before reaching the layer of receptors. The fovea is roughly in the center

8、of your visual field.第9页，共27页。The RetinaThe optic disk is where the axons of retinal ganglion cells penetrate the retina and exit the eye; the optic disk has no receptors thus creating a blind spot第10页，共27页。The RetinaWe all have a blind spot at the optic disk, due to the exit of axons from the retin

9、al ganglion cells We are normally unaware of our blind spots, even when looking through one stationary eye because of completion; the visual system is able to use visual information gathered from receptors around the optic disk to complete the visual imageCompletion illustrates the creativity of the

10、 visual system (demonstration in class)第11页，共27页。The RetinaCone and Rod Vision: in a sense, each of us has two visual systems; the photopic system functions in lighted conditions; the scoptopic system functions in dim light第12页，共27页。The RetinaCones mediate photopic vision; rods mediate scotopic visi

11、on; this is called the duplexity theory of visionThe photopic system mediates high-acuity vision; the scotopic system mediates low-acuity vision第13页，共27页。The RetinaThe photopic system has low sensitivity with few receptors information combined at the next cell level (low convergence)The scotopic sys

12、tem has high sensitivity with many receptors converging on ganglion cells (high convergence)Only cones in the fovea; rods predominate in the periphery第14页，共27页。The RetinaOur eyes are in continual motion; they make a series of fixations (about 3 per second) connected by saccades (rapid movements betw

13、een fixations)第15页，共27页。The RetinaEye movements keep the visual image in continual motion on the retina; the importance of this movement is illustrated by the fact that stabilized retinal images disappear; most visual system neurons respond to change, not to steady input第16页，共27页。The RetinaWhat we p

14、erceive at any instance is the sum of the input that has been received during the last few fixations; this temporal integration of the retinal images explains why our visual images are detailed, colored, and wide-angled, despite the small size of the fovea; it also explains why things dont disappear

15、 when we blink第17页，共27页。Visual Transductionby RhodopsinTranduction is the conversion of one form of energy to another; the first step in visual transduction is conversion of light to neural signals by the rods and cones第18页，共27页。Visual Transductionby RhodopsinTransduction by rods is well understood;

16、 there is pink substance in rods called rhodopsin; light is absorbed by rhodopsin, and this bleaches it (removes its color); this bleaching occurs because retinal and opsin, the two components of rhodopsin, separate in light第19页，共27页。Visual Transductionby RhodopsinThis bleaching reaction hyperpolari

17、zes the rods, thus the bleaching reaction tranduces light to a cascade of intracellular events resulting in neural signalWhen rhodopsin is totally bleached by intense light, rods lose their ability to absorb light and transduce; however, in the dark, retinal and opsin are reunited, and rhodopsin reg

18、ains its color and its ability to absorb light and transduce第20页，共27页。Visual Transductionby RhodopsinRhodopsin is a G-protein linked receptor that responds to light by initiating a cascade of intracellular chemical eventscGMP is an intracellular chemical that keeps Na+ channels partly open when rods

19、 are in darkness; rods are thus slightly depolarized第21页，共27页。Visual Transductionby RhodopsinBleaching rods via exposure to light results in an intracellular cascade of events that deactivates cGMP, closing Na+ channels, which hyperpolarizes the rod and reduces the release of glutamateRods thus tran

20、smit signals through the neural system via inhibition第22页，共27页。From Retina toPrimary Visual CortexThis is called the retina-geniculate-striate pathway (diagram in class)The primary visual cortex is striate cortex after the stripe seen in a Nissl-stained cross section of lower layer IV; the stripe is

21、 composed of the terminals of axons from the lateral geniculate nuclei第23页，共27页。From Retina toPrimary Visual CortexThe retinal ganglion cell axons from the nasal hemiretinas decussate via the optic chiasm (they are contralateral); those from the temporal hemiretinas remain ipsilateral providing redundancy for the system第24页，共27页。From Retina toPrimary Visual CortexThe most important feature of v