* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Download Lecture 7A
Document related concepts
Prepare videos: • Current injection into a FFA of a patient: http://www.jneurosci.org/content/32/43/149 15.full • https://youtu.be/7s1VAVcM8s8 (start at 4.4min to 6 min) Sensory Neuroscience What realities of the world can we feel? • five traditionally recognized (Aristotle, 350BC): • sight (ophthalmoception, rods=B&W, cones=color vision), • hearing (audioception), • taste (gustaoception), • smell (olfacoception or olfacception), • touch (tactioception) Do our receptors cover the complete spectrum of informative radiation? Can other animals feel thing that humans cannot? • Ultraviolet radiation (birds and insects). • Ultrasound (bats) UV • Electrical sense (sharks) • Radioactivity (rodents). • Magnetoception (the ability to detect the direction one is facing based on the Earth's magnetic field birds) IR • Sensors: convert external signal (radiation, molecules, pressure) to electrical signal. • How do they encode the intensity of the signal? • All sensory receptors first transmit to thalamus, then to cortex (except olfaction) Thalamus • The active relay station of the brain. • Every sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area. • Located in the walls of the 3d ventricle Taste What is more important: sensory receptors or the cortical neurons? • Taste is responsible for evaluating food items. • Sweet and bitter are two of the most important sensory percepts for humans and other animals – Sweet taste allows the identification of energyrich nutrients – Bitter warns against the intake of potentially noxious chemicals. • information from taste receptor cells in the tongue is transmitted through multiple neural stations to the primary gustatory cortex. • In the primary gustatory cortex sweet and bitter are represented by neurons organized in a spatial map with each taste quality encoded by distinct cortical fields. • EXPERIMENT: activate the brain field representing bitter taste and give mouse sweet water - will mouse drink? • activate the brain field representing sweet taste and give mouse bitter water - will mouse drink? Perception is in the cortex! • The activity in the primary gustatory cortex directly controls an animal’s internal perception of sweet and bitter taste and drives behavioral actions. • Essentially, it does not matter what sensory receptors are reporting. You perceive only the cortical neurons activity. Vision Hearing Touch Thalamus - active relay station • Every sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area. • Connections run both ways: from thalamus to cortex and from cortex to thalamus. • This allows the thalamus to actively amplify relevant information. • Major role in regulating the level of awareness (e.g. meditation experiment). • Damage to the thalamus can lead to permanent coma. Olfaction • Smell receptors connect directly to the brain • That explains the ability of certain smells to trigger vivid memories (e.g. of a grandmother) • 5% of DNA in mammals is encoding smell receptors (only 40% of these genes are expressed in humans) • Olfactory bulb phylogenetically old structures in temporal lobe: • primary olfactory area in piriform cortex, • the entorhinal cortex • the amygdala • Orbitofrontal olfactory area in frontal lobe For all non-primate mammals olfaction is the main sense • The neocortex (Latin for "new bark"), also called the neopallium ("new mantle") can be viewed as a large outgrowth of neurons around the hippocampus. • It is the invention of mammals • A sheet of simple cortexlike structure in between hippocampus and olfactory lobe (piriform area) in the brain of the reptile is replaced in mammals by the 6-layered neocortex in mammals. Hearing • The vestibular system provides the sense of balance and spatial orientation. • The cochlea converts sound pressure patterns from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve. • In the spiralled cochlea, waves propagate from the base (near the middle ear and the oval window) to the apex (the top of the spiral). The stiffness of the basilar membrane determines the mechanical wave propagation properties (“The place– frequency map” = tonotopic organization): • High frequencies lead to maximum vibrations at the basal end of the cochlear coil, where the membrane is narrow and stiff, • Low frequencies lead to maximum vibrations at the apical end of the cochlear coil, where the membrane is wider and more compliant. •A core component of the cochlea is the Organ of Corti, the sensory organ of hearing • The hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane Basilar membrane Mechanically-gated channels (cochlea) • cochlear prosthesis are easy because of cochlear tonotopic organization Vision • About half of neocortex in humans is devoted to vision (Barton, 1998). • Photoreceptor, the protein sensitive to photons, evolved once. Different types of eyes evolved over 100 times in animals. • E.g. in mammals, receptors for light are in the back of the eye; In octopus receptors face light • A lot of computational processing happens in the neurons located right in an eye: • if every retinal receptor had its own axon going into the brain, the optic nerve would be over one inch thick red, green and blue • Rods and Cones • Fovea=only 2 degrees of visual field (sun=0.5 degrees) • Rods are extremely sensitive, and can be triggered by a single photon. At very low light levels, visual experience is based solely on the rod signal. • Cones require significantly brighter light (i.e., a larger numbers of photons) in order to produce a signal. IR actual Perceived • Most mammals, including dogs and cats, have only two different kinds of UV color receptors (cones). • Why? • A remote vertebrate ancestor of all mammals possessed 4 color receptors, but nocturnal mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. • Most fish, reptiles and birds still have 4 color receptors while all mammals, with the exception of some primates still have only 2 color receptors. • Some primates (including apes) have acquired the third color receptors. • Why? • Some people (2.4% of males) have two color receptors – cannot distinguish between red, orange, yellow and green. • Visual signal is fed into two neuronal pathways: • 1. Into neocortex via lateral geniculate nucleolus in the thalamus (Conscious) • 2. Into superior colliculus in the midbrain (Unconscious) • RECALL: Neurons in the cortex are organized territorially based on their function: • motor neurons are located in the motor cortex, • neurons sensitive to touch in the somatosensory cortex, • neurons responsible for language comprehension are concentrated in Wernicke’s area, • neurons responsible for language production are located in Broca’s area. • The visual system also consists of multiple departments: perception of motion, color and depth is handled by different departments. • The primary visual cortex, V1, is located in the occipital lobe. • It receives information from the retina via the lateral geniculate nucleus in the thalamus. • The primary visual cortex is the first cortical area that receives visual information. • Bilateral damage to the primary visual cortex results in blindness; this disorder is often referred to as cortical blindness, to distinguish it from retinal blindness. Visual System From the primary visual cortex, the visual information is passed in two directions, or streams: • In the first stream, the information flows from the primary visual cortex to the inferior temporal cortex. • This stream includes the departments that deal with object recognition • Due to the stream’s direction from the back of the brain towards the front of the brain (along “the brain’s belly”), it is referred to as the ventral pathway from venter, the Latin word for the abdomen. • The second where stream deals with an object’s position in space. • The departments along the where stream are concerned with an object’s motion, location, as well as controlling the eyes and arms in the specialized tasks of grasping nearby objects. • Due to the stream’s direction from the back of the brain towards the parietal lobe (along the “the brain’s back”), it is referred to as the dorsal pathway from dorsum, the Latin word for the back. The dorsal stream has been described as providing “vision for action.” • E.g. area V5/MT (middle temporal) is in the dorsal where stream: • Lesion: patient unable to see motion, seeing the world in a series of static "frames“ • Consider a 40-something woman is Switzerland who suffered a parietal lobe stroke in 1978: she become unable to see motion, seeing the world in a series of static "frames“; e.g. when pouring tea into a cup the moving liquid appeared frozen. She would then find it difficult to see when to stop pouring. She also found it difficult to cross the road because the cars appeared suddenly. • MT/V5 integrates local visual motion signals (from V1) into the global motion of complex objects Departments along the ventral “WHAT” stream: V1 • The departments become more specialized the farther the information moves along the visual pathway. • Neurons in the lateral geniculate nuclei can be activated by visual stimulation from either one eye or the other but not both eyes. They respond to any change in activity of the retinal neuron that they are connected to. • Neurons in V1 can be activated by either eye, are sensitive to specific attributes, such as the orientation of line segments and color. • Some neurons in V1 respond to differences in the position of stimuli in the images from the left and from the right eyes. This deference is called the binocular disparity. • Majority of neurons are sensitive to simple motion: some tuned to line moving up/down’ others are tuned to line moving to left/right V1 relies on topographical representation of spatial information Neurons in V1: •organized topographically •can be activated by either eye •sensitive to orientation of line segments •color •binocular disparity Adapted from Tootell, 1982 Device for blind by Paul Bach y Rita: Electrodes on the tong. • https://youtu.be/7s1VAVcM8s8 (start at 4.4min to 6 min) • Scan brain activity in visual area Neurologically this input indistinguishable from sight. • Psychologically too patients experience the tong data as vision. • Blind patient perceived objects as out there in space in front of them. They escaped from ball flanged at them and could sense when objects moved closer and farther. They even experienced waterfall illusion. • Similarly, Daniel Kish (see TED) who uses clicks to navigate the space shows strong activity in visual cortex while listening to reflected auditory clicks Departments along the ventral what stream: • The departments become more specialized the farther the information moves along the visual pathway. • Neurons in the lateral geniculate nuclei can be activated by visual stimulation from either one eye or the other but not both eyes. They respond to any change in activity of the retinal neuron that they are connected to. • Neurons in V1, which can usually be activated by either eye, are sensitive to specific attributes, such as the orientation of line segments, color, and binocular disparity. • Neurons in V2 are more specific. They • respond to: • short lines and corners, • contours, • • small spots of color within larger receptive fields • to synchronized movement of contrast borders and rows of spots against a background. Neurons in V4 are even more specific. They respond selectively to aspects of visual stimuli critical to shape identification. Neurons in the inferior temporal lobe are most specific. They respond only when an entire object (such as a face) is present within the visual field. Fusiform gyrus • Have you ever wondered why do we have photos of a face used for passport identifications? Not finger prints, not iris of the eye, not smell. • Because we have a lot real estate in the brain dedicated to processing of faces. • Fusiform face area is important for recognition of birds, cars, dogs, Chinese characters, i.e. whenever we need to parse a narrow class of nearly identical things. • Current injection into a FFA of a patient: http://www.jneurosci.org/content/32/43/14915.full • The surgeon can be heard in a video, talking to the patient, “Look at my face and tell me what happens when I do this.” • On the first trial, the physician pretends to inject current, and the patient just shakes his head and mutters, “Nothing.” • But when a four-milliampere current is sent through the electrodes, he says, “You just turned into someone else. Your face metamorphosed. Your nose got saggy and went to the left. You almost looked like somebody I'd seen before but somebody different. That was a trip.” How do we recognize an object? -- we guess! Self-organization of a neuronal ensemble Bottom-up activation Perception of the object • Recognition occurs based on Bayesian statistics: the brain evaluates the probability of a hypothesis, using some prior probability, • i.e. the brain is not calculating but finding the closest matching neuronal ensemble: neurons from multiple visual areas synchronize and that results in perception (conscious closure). • Neuronal ensemble self-organization is made possible by feedback axons. The number of feedback axons is an order of magnitude greater than feed-forward axons. Visual recognition is matching to the closest neuronal ensemble! • M. Gazzaniga explains Hawkins’s hypothesis: “Computer scientists have been modeling intelligence as if it were the result of computations—a one- way process. They think of the brain as if it, too, were a computer doing tons of computations. They attribute human intelligence to our massively parallel connections, all running at the same time and spitting out an answer. They reason that once computers can match the amount of parallel connections in the brain, they will have the equivalent of human intelligence. But Hawkins points out a fallacy in this reasoning, which he calls the hundred-step rule. He gives this example: When a human is shown a picture and asked to press a button if a cat is in the picture, it takes about a half second or less. This task is either very difficult or impossible for a computer to do. We already know that neurons are much slower than a computer, and in that half second, information entering the brain can traverse only a chain of one hundred neurons. You can come up with the answer with only one hundred steps. A digital computer would take billions of steps to come up with the answer. So how do we do it? • “The brain doesn’t ‘compute’ the answers to problems; it retrieves the answers from memory. In essence, the answers were stored in memory a long time ago. It only takes a few steps to retrieve something from memory. Slow neurons are not only fast enough [to] do this, but they constitute the memory themselves. The entire cortex is a memory system. It isn’t a computer at all.” It is matching! • Hawkins: “For many years most scientists ignored these feedback connections. If your understanding of the brain focused on how the cortex took input, processed it, and then acted on it, you didn’t need feedback. All you needed were feed forward connections leading from sensory to motor sections of the cortex. But when you begin to realize that the cortex’s core function is to make predictions, then you have to put feedback into the model: the brain has to send information flowing back toward the region that first receives the inputs. Prediction requires a comparison between what is happening and what you expect to happen. What is actually happening flows up, and what you expect to happen flows down “ It is matching! • M. Gazzaniga: “So back to the visual processing of the face that we started with: Inferior Temporal Lobe is firing away about identifying a face pattern, sending this info forward to the frontal lobes, but also back down the hierarchy. “I'm getting a face code, still there, still there, ahh . . . , OK, it's gone, I'm out.” But V4 had already put most of the info together, and while it sent it up to IT, it also yelled back down to V2, "I betcha that's a face. I got it almost I got it almost pieced together, and the last ninety-five out of one hundred times the pieces were like this, it was a face, so I betcha that's what we got now, too!” And V2 is yelling, “I knew it! It seemed so familiar. I was so guessing the same damn thing. I told V1 as soon as it started sending me stuff. Like I am so hot!” • Stop Brain comparisons Historically thinkers have always compared the brain to the technological marvels of the age: • Roman doctors likened it to aqueducts. • Descartes saw a cathedral organ. • Scientists of the Industrial revolution spoke of mills, looms, clocks. • Thinkers of early 1900s saw the telephone switchboard. • Nowadays the brain is most compared to a computer. • The brain is not a computer!