Descartes description of a reflex
Many historians credit Descartes with being the first to argue that the brain is a machine, but he believed very strongly (as do most people in the world today) that the mind is separate from the brain. What do you think? Class discussion...
Descartes description of a reflex
Descartes' description of a reflex includes some correct observations (e.g., nerves are involved, there are fluid-filled ventricles in the brain) but the details are way off.
At least, Descartes figured out that the brain was somehow involved. Over the millenia there have been various theories about where in the body thought processes occurred, and it hasn't been but for a few centuries that people were certain the brain was relevant. For example, although the ancient Egyptians took great care to preserve their dead (for the afterlife), they had very little respect for the brain which they removed with a spoon through the nose during the preservation process. Aristotle wrote that the brain simply serves to cool the blood.
Schematic showing neuron's parts
Microscope pictures neuron with microelectrode
Neurons represent and transmit information electrically. If you place an electrode close to a neuron's soma then you can measure and record these electrical signals.
Schematic action potential
This slide shows a schematic of a single action potential. Action potentials are often referred to as spikes or impulses. When its not stimulated, a typical neuron rests at a voltage of about -70mV. When stimulated and it fires an action potential, the voltage jumps way up to about +40mV, then comes back down again. It all happens very fast (note the time scale), all in a matter of a few thousandths of a second. The action potential starts at a neuron's soma and travels all the way along its axon. The shape of the action potential is basically the same anywhere along the length of the axon. Hodgkin and Huxley won the Nobel prize for work they did in the 1950s to understand what makes action potentials happen and how they propogate along axons.
Firing rate vs contrast
Raster plot of neural responses
Responses of a neuron in a visual area of the monkey brain to 210 presentations of an identical visual stimulus. Each tick mark corresponds to an action potential. Each row corresponds to a different trial (a separate repeat of the identical stimulus presentation). There are a different number of action potentials from one trial to the next and the individual action potentials occur at slightly different times from one trial to the next. The graph at the bottom shows the average response (in spikes/sec), averaged across all 210 stimulus presentations. I'll come back to this issue of noise in neural responses later when I lecture on psychophysical methods and signal detection theory.
Neurons in different parts of the brain are selective for different things. Just keeping to neurons involved with vision, there are neurons that are selective for all kinds of properties: stimulus position, color, pattern, direction and speed of motion. There are even neurons that respond selectively to faces.
Example: recordings are from a type of neuron (called a retinal ganglion cell) that was located in the retina (at the back of the eye) of a cat.
Action potential recordings
The bottom trace shows the time course of a flashed spot of light. The light was turned on, stayed on for a while (about 1 second), and then was turned off again. The top trace is the electrical voltage recorded by the electrode. While the light was on, the neuron was stimulated and its voltage jumped way above its resting level a number of times. The time scale here is very different from that in the previous graph; each of the three panels represents a couple of seconds. So the action potentials, which each last for only a few milliseconds, look like impulses or spikes in the electrical voltage recording.
The three panels correspond to lights flashed at different locations in the visual field. It turns out that many neurons are very picky. This particular neuron reponds when the light is on if it is flashed in one position. It responds after the light goes off if it is flashed at a nearby postion. And it responds to both on and off if the light is flashed in an intermediate position. It's not shown here but if the light is flashed just a little bit further away, the neuron would not respond at all. We say that the neuron is selective for the position of the light flash.
Another example: In 1959, Jerry Lettvin (an electrical engineer at
MIT) and his colleagues published a very influential research article
entitled "What the Frog's Eye Tells the Frog's Brain." They recorded
from individual fibers (axons) in the optic nerve of frogs and
discovered that many neurons were extremely selective for the kinds of
things that frogs ought to care about, small dark spots moving relative
to the background, i.e. they were "bug detectors". Some quotations
from their paper.
Schematic of a synapse
Electron microscope picture of a synapse
How do neurons communicate? The axon of one neuron comes very close to (about 20 nanometers; a nanometer is a billionth of a meter) but doesn't quite touch the dendrite of its target. The narrow space between the two neurons is called a synapse. The electrical signal from the (pre-synaptic neuron's) axon is not transmitted directly across the synapse. Rather, when the action potential comes along, it causes the release of certain chemicals called neurotransmitters. The neurotransmitters diffuse across the synapse and bind to receptors in the membrane of the (postsynaptic neuron's) dendrite. This, in turn, causes a change in the electrical properties of the postsynaptic neuron. Bernard Katz won a Nobel prize for figuring out how synapses work.
Why do neurons communicate by chemical synapses instead of by direct electrical connections? Discuss...
A given neuron may have thousands of synapses distributed on its dendritic tree. If there is enough combined excitation from all of these synapses then the postsynaptic neuron will fire an action potential. If there is even more excitation, then this neuron will fire a bunch of action potentials, and its firing rate will increase with dendritic excitation.
Schematic of brain, side view
Medial (split in half) view
Of the structures visible in this slide we'll be mainly concerned in this class with the auditory brain stem nuclei (small collections of neurons in the brain stem involved in hearing), retina, optic nerve, thalamus, and cerebral cortex.
Back of monkey brain
Cross-section of visual cortex
This tissue is stained so that the cell bodies appear purple. All the cell bodies are in a layer near the surface of the brain, called the gray matter of the brain because it has grayish color when its not stained. The longer axons reach out (below) the cortex into the central part of the brain, called the white matter of the brain because the axons altogther look white. The white matter is kind of like a massive tangle of wires - axons connecting from one part of the cortex to another.
Gray matter or the cortical sheet is about 3-4 mm thick, filled with
cell
bodies (somas), dendrites, and axon terminals (synapsing on the
dendrites). Gray matter is where all the interesting stuff happens; a
neuron
receives inputs (via synapses) from a whole bunch of other neurons, and
then it computes a new output. The axons in the white matter just
transmit that output to other
parts of the brain.
Treatment of Animals
The use of animals in experiments is controversial. The US government and individual universities have strict guidelines about the treatment of animals in experiments. In some of the experiments we'll talk about in this class, the animals were fully anesthetized. In other experiments, the animals were awake while electrodes are implanted to record neural activity. It is important to realize that there are no pain receptors in the brain so even when the animals are awake they do not feel anything when the electrodes are implanted. It is also important to realize that the scientists (faculty, grad students, etc.) depend on having healthy and cooperative animals in their experiments. Everyone is motivated to make sure that the animals are well taken care of. It is still an ethical issue as to whether or not animals should ever be used in science. What do you think? Class discussion... I will show you some examples later in the course of how sensory neuroscience is leading to treatments for deafness, blindness, and developmental disabilities like dyslexia.
Penfield patient and her brain during surgery
Microstimulation: In the course of surgical treatment of patients suffering from epilepsy, neurosurgions sometimes operate with the patient awake under only a local anesthetic, so that they can electrically stimulate the brain and make sure that they do not accidentally remove critical (e.g., language) brain centers. Wilder Penfield pioneered this surgical technique during the middle part of the century. In the process, he mapped out much of what we know about the organization of the human cortex. He localized specific regions of the brain involved in vision, hearing, touch, and motor control.
Penfield maps of motor and somatosensory cortex
By plotting the movements and sensations that are evoked by electrical stimulation along the central sulcus, Penfield revealed a homunculus, a representation of the body surface. Stimulating in front of the central sulcus (colored red) evokes twitching movements of various body parts. Stimulating at one position in front of the central sulcus evokes a twitching movement of one of the toes. Stimulating a nearby position evokes a movement of another toe, then the foot, ankle, etc. Likewise, stimulating just behind the central sulcus evokes touch sensations. Stimulating at one position evokes a sensation of the thumb being tickled, then nearby for each of the other fingers, then the hand, the arm, etc.
Penfield also localized a brain area that when stimulated evoked a kind of stream of consciousness. Some quotations from one of his patients.
Neuropsychology: Study patients with brain damage to specific brain areas (e.g., due to stroke, accident, or surgury). Examples: blind sight (residual ability to discriminate visual stimuli when forced to guess, even though the subject has no conscious experience of the stimuli), face agnosia (inability to recognize faces), object agnosia (inability to recognize objects), akinetopsia (inability to see motion), achromatopsia (inability to see color), etc.
Lesion in the occipital lobe
For example, this MRI picture shows a slice through the brain of a subject known in the research literature as GY. GY suffered a head injury during a bike accident as a child. The black region (near the left) is now filled with fluid instead of being filled with cortical brain tissue. This has left him effectively blind in part of his visual field, from which we know that this part of the brain is critical for vision. Interestingly, however, GY can to discriminate certain visual stimuli presented to the blind part of his visual field when forced to guess, even though he has no conscious experience of the stimuli. This has been called "blind sight" and we'll talk more about it later in the semester.