notes - NPB12 Lecture 12 direction of motion color Depth...

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Unformatted text preview: NPB12 Lecture 12 direction of motion color Depth Perception retinal disparity When you look directly at something, the object falls on the corresponding points on the retina. All other objects that are not at the same distance from the eye fall on correspondingly different points on the retina, a condition known as retinal disparity Monocluar Cues: Size: bigger is closer If something that you know is bigger than something else, but it appears smaller, then it is probably behind it. That is how a dog can appear larger than a car, for example. Occlusion borders: Things occlude what is behind them A second cue is occlusion borders. If you see that one object completely blocks parts of another object, then the blocking object is in front of the occluded object. Motion Parallax + + In a stationary scene, objects that are not in the plane of focus will be projected onto different parts of the retina. When you move your head to the right, objects that are closer and on the left will move to the left, but objects that are farther will move to the right. So things close move in one direction and things far move in the opposite direction Aperature Problem: bars must move orthoganol to their orientation V1 V2 V3 MT V4 MST Temporal Lobe Parietal Lobe MT selectively responds to direction of motion and velocity of motion + + MST responds to optic flow stimuli Parietal Lobe: Parietal reach region Parietal lobe strokes in humans: V1 V2 V3 MT V4 MST Temporal Lobe Parietal Lobe V2 receptive fields: similar to V1 V4: colors and shapes + + IT cortex responds to faces fusiform face area Parietal + MST + MT + V1 + V2 + IT + V4 + Delay match to sample Loss of spatial awareness Delay match to non-sample Loss of object awareness Plasticity in the visual cortex left eye right eye 6 5 primary visual cortex 4 + 1 23 7 6 1 8 receptive fields 4 2 3 7 8 5 Plasticity in the visual cortex A number of lesion studies have indicated that these two major anatomical pathways are actually processing two different types of visual information: what a particular object is, and where that object is (or is moving toward). This has been done using a procedure called ablation-behavior experiments left eye right eye 6 5 5 3 primary visual cortex 4 + 1 1 23 2 receptive fields 7 6 7 8 8 1 4 2 3 7 8 5 how do you get these things back together? FEATURE BINDING Eye Movements Two types Saccades Saccades are when you rapidly move your eyes from one location to another. This is like going to the start of the next line while you are reading. Smooth pursuit is when you look at something that is moving, like a flying bird. Smooth Pursuit Saccades: F or the saccade system, there is a structure that receives inputs from both the retina and V1 called the superior colliculus. There are actually two superior colliculi, one on the left and one on the right, and each one controls saccades for the contralateral side of the world. The superior colliculus has several layers of neurons, and each layer has a slightly different function Superior colliculus and visual cortex the colliculus is layered, with the top layer having visual receptive fields, and if you stimulate directly underneath them, you make a saccade to that same spot If you then turn your recording electrode into a stimulating electrode, and move it downward to the second layer of neurons, you find that the eyes will move toward the spot of light in the receptive field of the neurons directly above it. This "motor map" is in register with the visual map, so in essence the cells of the first layer are responsive to the location in space of a particular object, and the cells directly underneath will move the eyes to that spot. Smooth Pursuit Visual cortex area MT Cerebellum (saccade system) the smooth pursuit movement starts before the saccade Smooth pursuit eye movements use a different approach, and they can be thought of as a combination of both the saccadic system and a different, pursuit system. Take for example looking at a particular spot in the world, and suddenly you see a moving object out of the corner of your eye, and you think it might be important so you want to look at it. If you just made a saccade to the spot you first saw it, when your eyes got there the object would not be there anymore, unless it was moving really slowly.Somehow your brain is able to compute the direction and velocity of the motion, calculate how and where to move the eye, and execute the movement. left eye position right target time Lesions of MT prevent the system from calculating the velocity, so the eye movement is delayed left eye position right area of visual field affected by MT lesion target time Vestibulo-Ocular Reflex (VOR) A classic demonstration of the VOR is to look at your finger, and then move your head back and forth while still looking at your finger. As you move your head, your eyes move in an equal and opposite direction, keeping your eyes still in the world although your head is moving. This keeps the image of your finger stationary on your retina Balance: The vestibular apparatus. T he vestibular apparatus, as it is called, is located in the temporal bone in a complex that also houses the sensory endings for hearing. T he balance/hearing apparatus is contained within the temporal bone, right opposite the ear. It is composed of the cochlea (hearing) the semi-circular canals, the saccule an the utricle. In each of these structures there are a series of receptor cells called hair cells. They are called hair cells because they have small processes out of the top that look like hairs. They all have a longest one, called the kinocilium, and many shorter ones, called stereocilia. In all the hair cells, there is a membrane separating the cilia from the rest of the neuron, called the reticular lamina. This effectively makes a barrier dividing the outside of the neuron into two different compartments. The outside of the cell next to the hairs is called endolymph. This has actually a very high concentration of potassium (K +), much higher even than the inside of the cell. The other compartment has the same old ionic concentration that we have seen before. What is important is that ions cannot pass from the endolymph to the other compartment, they are absolutely distinct. equilibrium potential for K+ is +20 mV up here very high K+ concentration endolymph reticular lamina high K+ low K+ equilibrium potential for K+ is -85 mV down here What ions pass through the channel? Potassium and calcium. Since the K+ outside is so much higher than inside the cell, the equilibrium potential for K+ on this side of the reticular lamina is actually positive, so the K+ flows into the neuron. The Ca++ moves into the cell as well, also due to the positive equilibrium potential for K+ Ca++. This causes the membrane potential to depolarize, which then opened voltage-gated Ca+ + channels in the hair cell on the other side of the reticular lamina, which causes even further depolarization. This depolarization causes the hair cell to release neurotransmitter (glutamate) to the post-synaptic neuron, the spiral ganglion cell, which then will fire an action potential to the brain. This open voltage-gated Ca++ channels in this part of the membrane. Ca++ when these channels open, K+ flows into the cell to bring the membrane potential K+ up here to a more positive value. Ca++ this causes the synaptic vesicles to fuse with the membrane and release neurotransmitter The neurotransmitter causes ligand-gated Na+ channels to open in the spiral ganglion cells, which then fire action potentials to send the signal to the brain. The hair cell does not have action potentials. The spiral ganglion neurons are the first ones to have action potentials There are about 20 spiral ganglion cells for each hair cell. Nystagmus and the VOR 1) The vestibular system does ACCELERATION, not velocity 2) Rotating with you eyes closed is the same as being stationary to the vestibular and visual systems. 3) When you stop rotating, the vestibular system signals the change in velocity (it thinks you are rotating in the opposite direction). 4) The VOR then moves your eyes in the opposite direction to compensate. 5) The visual system sees that the world is moving across the retina, so you make a saccade back to straight ahead. 6) The visual system sees that the world is moving across the retina, so you make a saccade back to straight ahead. YOU DO IT UNTIL THE DECELERATION STOPS 7) repeat steps 4 – 6 until the deceleration stops. So the vestibular system is trying to drive a smooth pursuit movement, and the visual system recognizes that is wrong and drives a saccadic movement to get you back on target. This is called nystagmus, and is a key indicator that something is wrong with the vestibular system if you get it for no good reason. Nystagmus is also what you get as you look at a picket fence as you drive by. Your eyes will try to follow an individual picket, and once it gets too far over you will saccade back to the front and follow another picket. left smooth pursuit horizontal eye position right saccade time The visual system dominates the percept of the vestibular system IMAX theaters Ian Howard, York University ...
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