Heirarchy of the Visual System
Our visual system functions as a hierarchy of information, starting with tiny pieces of visual information about light in the retina, to specific types of objects and scenes in the cortex. Seeing seems so effortless for us, but what's really happening is an almost unbelievably complex transformation of light to a visual scene. It begins in the retina, which sends information to LGN, which relays the signal to the primary visual cortex, which then sends information to the “higher” visual association cortex. (Note that the primary visual cortex is called other things too; it’s also known as the striate cortex, visual area 1 (V1 for short), and Brodmann’s Area 17.) Beyond the primary visual cortex, the visual processing stream splits into two pathways: the ventral (what?) pathway and the dorsal (where and how?) pathway. So here we go....
Anatomy of The Eye
If you want to get up close and personal with the inside of an eye, you could try dissecting a cow’s eye. You can get a real eye if you have a large meat processing plant nearby; if not (or if you’re perfectly fine not cutting open a cow’s eye), see this online dissection. It’s a little gross but informative). See these websites here and here for good views of eye anatomy.
Transduction in the Retina: From Light to Neural Signal
The retina is actually part of your brain, and is where the sensory receptor neurons reside. In your eye, these receptor neurons are called photoreceptors, which include rods and cones. Transduction is the process of converting light into a neural signal by changing the photoreceptors’ membrane potential. Does this sound familiar? It’s the same general process as any other neuron. Instead of being neurotransmitter-gated, the ion channels on the photoreceptor are “gated” by the receptors’ reaction to light. The photoreceptors, rods and cones, contain pigments that absorb light. (The cells in the retina are generally transparent, which allows light to get through the layers of cells in the retina.) The absorption of even one quantum of light—one photon—is enough to let a rod photoreceptor generate a signal. It starts when light breaks a photopigment molecule like rhodopsin into two parts, retinal and opsin, which is called bleaching. As a result, the photoreceptor experiences changes in the membrane potential which affects it’s release of neurotransmitter (glutamate) to the bipolar cell, but in a backwards way…..
In the dark our photoreceptors are always “on”. They are in a depolarized state with the sodium channels open, and constantly release a neurotransmitter that inhibits bipolar cells and makes them inactive. When a rod or cone absorbs light it closes the sodium channels in the membrane, which make it stop releasing its neurotransmitter, which disinhibits the bipolar cells that the photoreceptor is connected to. Then the bipolar cells can excite the ganglion cells "normally". The release of neurotransmitter in the dark is called the “dark current”. You can get the whole story here.
Need to “see” it? If so, here’s a video on what goes on in the retina: YouTube - Vision - Light and Neuronal Activity
On the Road to the LGN
The sensory information received by the photoreceptors has a long way to travel (well, from their point of view) to get from the front of the head to the back of the head. After information about the visual scene is received by the photoreceptors, it is passed along by bipolar and other cells to the ganglion neurons within the retina. The axons of the ganglion neurons collect at the optic chiasm (which is why you have a blind spot) and leave the eye. Some of the axons cross over at the optic chiasm. It’s important to remember that not all axons cross over. It’s common to think that the left hemisphere gets information from the right eye ……wrong! The left hemisphere receives information from the right visual field, which is the visual scene in the environment, not in the eye. Remember that the lens completely inverts the image, so information from the right visual field lands on the left half of our retina.
First Stop: LGN
The lateral geniculate nucleus (LGN) of the thalamus is where the gangion axons from the retina end up. There is one LGN on each side of the thalamus. The thalamus acts as a relay station to the cortex.
In the LGN, visual information is mapped into six orderly layers that retains the visual information detected in the retina. The magnocellular, parvocellular, and koniocellular layers of the (LGN) are part of the “M”, “P”, and “K” pathways or channels, named after magnocellular, parvocellular, and koniocellular pathways that begin with ganglion cells in the retina. The M layers get most of their input from the rods, and are important for detection of movement. The P and K layers get most of their input from the cones, and are important for detecting detail and color. The P pathway carries information from red and green cones, while the K channel carries information from blue cones. Information from each layer is then transferred to the cortex.
Next Stop: The Primary Visual Cortex
The visual cortex integrates information about spots of light and puts them together into larger features that represent our visual environment. Nobel prize winners Hubel and Weisel found that neurons in the retina and LGN fired in response to spots of light, but neurons in the primary visual cortex did not fire. By accident they discovered that the primary visual cortex neurons respond to lines in specific orientations, which result in our ability to detect edges that we see. You can see an example Hubel and Wiesel’s cat experiments here: YouTube - Hubel and Wiesel Cat Experiment. Hubel is also in this short video about vision research here: YouTube - Hubel's research .
Within the primary visual cortex there is a retinotopic map that represents the neighborhoods of photoreceptors found in the retina. This map is so specific that corresponding points from the two retinas project to adjacent modules (also called hypercolumns) in the visual cortex.
Hubel and Weisel pioneered the discovery of edge detectors in the primary visual cortex. These days, after additional discoveries by DeValois and others, vision researchers use the concept of spatial frequency, which is the number of light-dark transitions that occur within a single degree of visual angle.
Visual angle is used a unit, because perceived detail of an object changes with distance. A set of light and dark bands will look thinner (higher spatial frequency) farther away, and larger (lower spatial frequency) closer up. Using the visual angle makes the distance between the bars constant, because it’s based on the inside of the eye. Think of the eye as a globe that can be subdivided into 360 degrees of curvature. Distance across the retina can be described in degrees and minutes of retinal arc. A single degree of visual angle is generally close to the finest detail we can discriminate. If you look at your thumb after stretching your arm in front of you, the retinal image of your thumb will cover about two degrees of arc, or two degrees of visual angle.
The number of light-dark contrasts within each degree of visual angle is the spatial frequency of an image. Fine details in an image have a high spatial frequency because there are many such transitions per degree; crude features have low spatial frequency. A typical scene will contain many spatial frequencies. Axons in the P pathway carry information about high spatial frequencies, or fine detail. Axons in the M pathway carry information about lower spatial frequencies.
Look at the following image. Who do you see close up? Move a little farther away.... Who do you see now?
Details that are too coarse to recognize up close become finer (higher spatial frequency) from a distance. We see whatever face is presented by a medium grain, because we are most sensitive to those spatial frequencies. That's the mystery behind Mona Lisa’s smile and whether you perceive a smile or a frown in these pictures.
What And Where?
The primary visual cortex creates a map of our visual field, but this basic information is transferred to the “higher” visual association areas described on p. 255 in the Vision chapter -- with areas labeled V2, V3, V4, V5, MT and so on. Here, information from the primary visual cortex is integrated and interpreted in ways that are meaningful to us, by providing a context for the features we see. Each area performs a separate function, as can be seen by lesions to these areas that result in specific impairments in vision and behavior.
Also in the “higher” processing areas, the visual information splits into two streams: a dorsal stream or pathway that identifies where an object is, and a ventral stream that tells us what the object is (p. 255). These two streams function in parallel to each other.
The dorsal pathway receives input mostly from the M (magnocellular) pathway. (To help you remember, think “M” for “movement”.) The dorsal pathway analyzes visual information in ways that enable us to act on it. Knowing the location and movement of what we see is needed for us to react to it in some way (grab it, run away from it, pounce on it). Damage to this area results in akinetopsia. The dorsal stream can play a role in detecting objects, as seen in the ability to detect form from motion. There are some good motion after effect illusions here, and here.
The ventral pathway receives input from the M pathway and also the P and K pathways. The P and K input provide the ventral stream color information. Area V4 in the ventral stream processes color, not as in blue versus green, but the context in a color occurs. This gives rise to the color constancy illusion (more here). Lesions to specific areas in the ventral stream reveal the functional organization of the brain: different areas of the inferior temporal (IT) cortex correspond to analysis of unique categories of objects. For example, the Extrastriate Body Area (EBA) might be involved in the visual distortions of anorexia nervosa.
At some point the parallel dorsal and ventral pathways must be combined so that we know “what” is “where”, especially if we are to act upon the objects we see. Children may not have fully developed this connection between the dorsal and ventral pathways, which is why children may show “scale errors” (see this experiment with children , and also here -- so cute). Can you think of a time when you’ve seen a child demonstrate this lack of connection?
Effects of Brain Damage
So what happens if our visual cortex is damaged? Are we blind? Well, yes, and no. See this video on “blindsight”, a phenomenon in which brain-damaged people can’t see consciously but can see motion unconsciously. The retina is also connected to pathways to lower brain areas. Thinking back to Week 1, what do you think this tells us about consciousness?
Damage to the fusiform face area (FFA, see p. 255) can result in prosopagnosia, or face-blindness, Prosopagnosia and visual agnosia may reflect different aspects of perception, related to discriminating objects based on fine-grained detail or global information. How do you think a visual agnosia patient versus a prosopagnosia patient might view Arcimbaldo’s paintings?
Besides visual agnosia and prosopagnosia, there are other bizarre syndromes of visual distortion, such as Capgras Syndrome (scroll down for cool illusion), or Charles Bonnet syndrome. By the way, there’s a really good novel about Capgras called “The Echo Maker” by Richard Powers.
Illustration credits: Einstein-Monroe: http://www.moillusions.com/2007/03/einstein-monroe-hybrid-image.html
Here are the questions (please answer all 3; a few sentences each should be enough):
- On p. 255 of the Vision chapter, the authors describe specialized brain areas for recognizing different types of objects and information in our environment. What might happen if one of these areas becomes damaged? Using links provided above (in the Effects of Brain Damage section), or your own Google/internet search, see if you can find a webpage or website on the internet that describes what happens when a specific brain area is damaged. (Make sure the information is credible!). For this question, name and describe the area that is damaged, and the visual impairment it causes. Describe how the brain damage in this area leads to the impairment that it does. Don’t forget to share the link you used with your classmates.
- How do you know that the color red is red? What makes this color red? Do you experience red the same way as everyone else? If we traveled to another planet where the environment was composed of a very difficult chemical composition, would we see new colors?
- What do you not understand, or are confused about, from the Vision chapter? Or, was there anything that you did not know or was surprising about vision before reading the chapter and viewing the links above?
please answer the questions very detailed. use APA source if needed.
|Due By (Pacific Time)
||09/05/2015 09:19 pm