in this video we will look at the anatomical structures within the visual system as well as the pathways through which visual information received by the eyes is sent to the central nervous system remember that each sensory receptor is specialized to detect a particular type of energy in the visual system the photo receptors found in the retina of our eyes respond only to light energy the light waves that are detectable by The receptors in our eyes vary along three different dimensions the First Dimension is the light's color or hue color is determined by the wavelength of light or more precisely by the distance between the peaks of two subsequent waves the second dimension of light is its brightness which is determined by the height or amplitude of the light wave the last dimension of light is the light sat atation which represents how pure the light is a highly saturated light is considered more pure because it's not mixed with other colors in this picture you can see the range of light wavelengths detectable by the human eye which Falls somewhere in between 400 and 750 nanom wavelengths outside that range cannot be detected by the human visual receptors the distance between the peaks of two subsequent waves determines the color of the light shorter wavelengths are perceived as bluish light while longer wavelengths create the perception of reddish lights on the other hand the amplitude of the light wave determines the brightness of light brighter lights have higher amplitudes while dimmer lights have shorter amplitudes this image shows the structure of your eye you are not expected to know the anatomical details but you do need to pay attention to the part that you need to remember most which is the retina because this is where our photo receptors which enable us to sense light are located the retina itself is a multi-layered light sensitive surface located at the farthest back part of the eye the retina is able to detect the electromagnetic energy known as light and converts it to neural impulses processed by the brain the retina consists of about 126 million photo receptor cells which can be further divided into two types namely the rods and the cones rods have several distinct characteristics first rods are more numerous than cones with humans having an estimated 120 million rods rods also respond to dim light which means they function well in environments with low illumination Rod cells are more densely packed at the peripheries of the retina and they become less dense towards the center of the retina and because rods are more active in dim light they are unable to discern the colors and details of a visual object cones have different characteristics compared to rods our eyes contain much less cones than rods humans have around 6 million cones in their eyes cones are also more abundant at the center of the retina and less abundant in the periphery of the retina cones work best in brightly lit environments and so they are able to help us see the details and colors of a visual object with great acurity as their name suggest rods are called so because they're shaped like rods while cones get their name from their shape which resembles a cone feel free to pause your video at this moment so you can better observe the differences between rods and cones remember that the retina is a multi-layered Surface in addition to photo receptor cells the retina also consists of layers that are comprised of gangon cells bipolar cells which form synapses with gangon and photo receptor cells Amrine cells which form synapses with ganglion cells and horizontal cells which form synapses with photo receptors the bipolar cells colored blue in this image and ganglion cells colored yellow are lined in front of the retina and their primary function is to send information received by the photo receptors to the brain the axonal fibers of ganglion cells bundle together to form the optic nerve which is the primary pathway through which visual information is transmitted from the eye to the brain at the retina there exists an area that has no photo receptors at all because it happens to be where the optic nerve and blood vessels leave the eye since it contains no photo receptors it follows that light that falls on this area cannot be perceived that is why this area is called the blind spot as a consequence of the blind spot theoretically speaking there should be a blank spot in your field of vision every time light information is received by the eye fortunately under normal circumstances you wouldn't notice the Blank Spot in your visual field that results from light falling on your blind spot this is because your brain using top- down processing compensates by filling in the blank for you in addition our eyes and head are usually constantly moving such that light will generally fall on the part of the retina that does contain photo receptors this is true unless we deliberately set let the stimulus to fall in the blind spot area you can demonstrate where your blind spot is by trying the blind spot activity in your king and colot textbooks in the most central part of the retina is a special region known as the fobia which has an extremely high population of cones occupying it because there are more cones than rods in this area light that falls on the fobia is usually perceived in much greater detail than light that falls on the peripheries of the retina the phobia plays an extremely important role in daily activities and tasks that involve visual information when you read a text printed on a page for example you will only be able to see Sharp details of the text when you place the page right in front of your eyes because doing so helps ensure that the text you're reading falls on your eyes phobia this table Compares fobal vision and peripheral vision fovial Vision refers to the vision that results from light waves falling exactly on the fobia that's densely packed with cones while peripheral vision is what we see when light reflected off a visual stimulus Falls outside the FIA in the peripheries of the retina you may pause this video to better examine the differences between these two types of vision to summarize here you can see a magnified image of the back of the eye where you can see the optic nerve leaving the eye towards the brain to convey visual information this picture also allows you to better visualize the row of photo receptors the row of bipolar cells and the row of ganglion cells you can also see how as mentioned before the fibers of the ganglion cells form the optic nerve that carries visual information to the brain another part of the eye that I want you to take note here is is the blind spot which is essentially a hole through which the optic nerve leaves the eye it's important to note that although gangon and bipolar cell layers are located in front of the photo receptors light simply passes through them without being processed instead initial processing of the light that enters the eyes is directly done by photo receptors because ganglion and bipolar cells are transparent light can pass through them without being distorted after being processed by the photo receptors the visual information is sent to the bipolar cells then from the bipolar cells to the gangon cells before it's finally conveyed to the brain by the optic nerve since light moves in a straight line then stimulus from the left visual field falls on the right half of the retina and both eyes this is repr represented by the green lines in the picture on the other hand the right visual field falls on the left half of the retina in both eyes as represented by the purple lines similarly light from the top portion of the World falls on the bottom half of the retina in both eyes and light from the bottom part of the World falls on the top half of the retina in both eyes after stimula from each visual field fall on the retina in both eyes axons of ganglion cells that make up the optic nerve will transmit the information to the brain now please take note of the green and purple lines in this image half of the axons of the optic nerve of each eye will cross in the optic kaym and travel to the opposite or contralateral side of the brain while the remaining half of the optic nerve does not cross and sends information to the same or IPS lateral side of the brain instead as such stimulus from the right visual field that falls on the left side of each retina depicted in this image in purple eventually gets processed by the occipital lobe in the left hemosphere of the brain conversely stimulus from the left visual field which falls on the right side of each retina eventually arrives and is processed by the occipital lobe in the right hemisphere of the brain please pay attention to these two images visual information received by the retina arrives at the visual cortex in the occipital lobe upside down this is due to the nature of the lens in the eyes that will always reverse the visual stimulus resulting in a reversal of the visual stimulus that falls on the retina this is not a problem though because the brain does not simply duplicate the visual stimulus received by the retina but rather performs coding based on the activity of neurons of the optic nerve after passing through the optic kaym most of the optic nerve fibers will go to the lateral geniculate nucleus located in the thalamus before the information is passed on to the visual cortex in the previous chapter we've briefly discussed that almost all sensory information will pass through the thalamus which functions as a relay station of sensory information and only olfactory information does not pass through the thalamus a rather small portion of the optic nerve fibers go to the superior calculus in the tectum before it travels to the pulverer nucleus in the thalamus and eventually to the visual cortex axons of optic nerve fibers that transmit information to the visual cortex through the lateral geniculate nucleus in the thalamus is known as the geniculostriate system it is named the geniculostriate system because information is transmitted through the lateral geniculate nucleus to the striate cortex which is another name for the primary visual cortex meanwhile the remaining axons of optic nerve fibers transmit information to the visual cortex through the superior calculus and therefore belongs to the tectopulvinar system it is named tectopulvinar because information is transmitted through the superior calculus located in the tectum to the puler nucleus in the thalamus after being processed by the thalamus visual information is conveyed to the primary visual cortex or V1 or striate cortex located in the occipital lobe the primary visual cortex is referred to as the primary area because it is where the initial stage of visual processing occurs the cells in V1 are able to respond to simple visual stimula such as straight lines wavy lines and edges of an object to further process the visual information area V1 sends visual information to the secondary visual area or V2 which is located anteriorly from V1 in V2 where visual information is further processed the cells have a wider receptive field and the cells in V2 are also able to respond to angles textures and complex shapes of a visual stimulus from area V2 some processed visual information is either sent back to V1 or transmitted to various other visual areas such as V3 V4 and V5 some cells in V2 and V3 are sensitive to to color While others are sensitive to information that's important for perceiving depth cells in the V4 area are found to respond mostly to the color of a visual stimulus while the V5 area are also known as Mt due to its location around the middle temporal cortex plays an important role in seeing movements as well as distinguishing whether a movement is produced by eye movements or movement of the object object the cells in V5 are found to respond specifically to objects that are moving in a certain direction and at a certain speed for more specialized processing of visual information the information that's already processed in V1 is sent to various visual areas in the cortex through two different Pathways the first is the dorsal pathway which is also called the how or where pathway this path is also known known as the action pathway this naming is consistent with the role of this path in generating movements that are Guided by sight such as reaching for a visual object this dorsal path begins at the occipital lobe and ends at the parial lobe the second path is a vental path which is also called the what pathway or perception pathway this naming is in line with the importance of this path in identifying and recognizing objects this ventral path begins at the occipital lobe and ends at the temporal lobe parallel processing of visual information allows visual information to be transmitted and processed quickly to help us perform daily activities this parallel processing also allows us to process different features of a visual stimulus such as its color shape movement and location separately but simultaneously in addition to parallel processing there's also the process of binding or integration of the results of visual information processing done by different areas of the cortex this binding process enables us to perceive a visual stimulus as a whole this marks the end of the explanation on the structure of the visual system and the transmission Pathways of visual information from the eyes to the brain