Neural Representations - PSYC211 Past Paper PDF

PSYC211 – Machado – 3 Neural Representations Synaptic Transmission Note that each neuron can receive input from many other neurons and can output to many other neurons. Sufficient input to the postsynap...

PSYC211 – Machado – 3 Neural Representations Synaptic Transmission Note that each neuron can receive input from many other neurons and can output to many other neurons. Sufficient input to the postsynaptic neuron can trigger an action potential, causing the electrical signal to be conducted down the axon. Action potentials trigger synaptic transmission. Chemical transmission of the signal from the pre-synaptic neuron leads to the continuation of the signal through the system of neurons that comprise a neuronal circuit. Synaptic Transmission = Neuronal Communication Action Potentials An action potential is a rapid change in the voltage of the cell’s membrane. When an action potential occurs, the neuron is said to have fired. Terminology: The neuron fired at a rate of 10 spikes/sec (i.e., 10 action potentials/sec) Action potentials can be elicited artificially… (Image from Neuroscience: Exploring the Brain, p. 72, Figure 4.2) Left panel: One electrode is stimulating the neuron with electrical current and the other electrode is recording the voltage of the neuron. Right panel: When electrical current is injected into the neuron (top trace), the neuron fires action potentials (bottom trace). What normally causes action potentials? Sufficient input can trigger an action potential. For example, moving a whisker on a rat can cause somatosensory neurons to fire rapidly. Single Cell Recording Single cell recording involves recording action potentials from an individual neuron. Using single cell recording, we can measure the activity of an individual cell while different stimuli are viewed. Which types of stimuli cause a neuron to fire action potentials?  PSYC211 – Machado – 3 Receptive Field All visually sensitive cells only respond to stimuli in a limited region of space. This region of space is referred to as that cell’s receptive field (demarcated by the dashed circle in the figure). (Images adapted from Cognitive Neuroscience: The Biology of the Mind, page 107, Figure 4.8) Left figure: While the activity of an individual neuron is monitored, the monkey is required to maintain fixation, and stimuli are presented at various positions in the visual field. Right figure: The vertical lines correspond to individual action potentials. The stimulus position that causes the neuron to fire the fastest defines the neuron’s receptive field. Now that we understand the activity of individual neurons, let’s consider the activity of populations of neurons… Maps of Neural Activity (Images adapted from Cognitive Neuroscience: The Biology of the Mind, page 110, Figure 4.9) While viewing the stimulus on the left, the monkey was injected with a radioactive agent. Metabolically active cells in primary visual cortex absorbed the agent, revealing that the organization of the cells in this area of the cortex represents the visual field (image on right).  PSYC211 – Machado – 3 How many neurons participate in the representation of a single visual image? Functional imaging techniques, such as fMRI, provide an opportunity to explore this issue because they allow the simultaneous detection of the entire neuronal population responding to each stimulus. fMRI is an adaptation of MRI that records changes related to metabolic activity in order to produce a functional view of the brain. Because the signal from fMRI is approximately proportional to neuronal activity, this method can be used to estimate the number of neurons involved in a specific cognitive process. Researchers recently scanned visual cortex while the participant viewed an image (e.g., a picture of a house or a face). Using a complicated calculation, the researchers determined that at least two hundred million neurons in the visual cortex are involved in the representation of a single image. Maps of Neural Activity The Biology of the Mind, page 110, Figure 4.9) (Image adapted from Cognitive Neuroscience: In primary auditory cortex, the frequency tuning properties of the cells define a tonotopic map. The activity of many neurons in the olfactory bulb was recorded. Different scents evoked different spatial patterns of neural activation in the olfactory bulb. The different patterns of activity can account for the ability to discriminate these three scents. Consider two distinct scents that cannot be distinguished.  PSYC211 – Machado – 3 Topographic maps are also seen in primary motor cortex and primary somatosensory cortex. (Image adapted from Cognitive Neuroscience: The Biology of the Mind, page 65, Panel C) Reference List Bear, M. F., Connors, B. W., & Paradiso, M. A. (1996). Neuroscience: Exploring the Brain. Baltimore, MD: Williams & Wilkins. ISBN 0-683-00488-3 (Figure 4.2 from page 72) Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. (2002). Cognitive Neuroscience: The Biology of the Mind (Second ed.). New York, NY: W. W. Norton & Company. ISBN 0-393-97777-3 (Figure 4.8 from page 107, Figure 4.9 from page 110, Panel C from page 65)