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University of Toronto St. George
Kristie Dukewich

PSY280: Lecture 3 Notes 1. Explain receptive fields a. Retinal ganglion cells can receive information from multiple photoreceptors. b. The electrical activity of other neurons in the retina is transient, c. A neuron’s receptive field is the area on the receptor surface that, when stimulated, affects the firing rate of that neuron. 2. Determine relative firing rates for RGCs with centre-surround receptive fields, given a particular pattern of light on the receptive field a. Under uniform illumination, retinal ganglion cells fire between 20& 50 times per second 3. Define off-centre and on-centre RGCs, and their relationship to bipolar cells a. ON centre: these receptive fields can be excitatory-centre, inhibitory-surround. TThese receptive fields can be ON excitatory-centre, inhibitory- Ocenter excitatory-centre, inhibitory- center surround surround b. OFF centre: these receptive fields can be inhibitory-centre, excitatory-surround. OFF … or inhibitory-centre, excitatory- center OFF s… or inhibitory-centre, excitatory- center surround. 1 c. Whether a RGC has an excitatory or inhibitory centre depends on the bipolar cells that receives signals from i. ON bipolar cells: ON center 1. When a stimuli intensity is higher, the firing rate increases. ii. OFF bipolar cells: OFF center iii. Bipolar cells are not responsible for the surround, they just account for the centre fo the receptive field of the retinal ganglion cells (RGCs). 4. Explain the potentials of the 2 types of bipolar cells in response to light stimulation a. In the light, photoreceptors are hyperpolarized and release less glutamate. b. ON bipolar cells spontaneously depolarize in the absence of glutamate (under bright environment) c. OFF bipolar cells become hyperpolarized in the light, making RGCs less likely to fire. 5. Describe the role of horizontal and amacrine cells a. Signals are sent horizontally by horizontal cells and amacrine cells b. Horizontal cells: between receptors c. Amacrine cells: between bipolar and RGCs d. These cells contribute to the surround of the RGCs e. For ON ganglion cells, i. Signals produced by photoreceptors in the surround are passed onto the horizontal cells, which inhibit photoreceptors linked to the centre responses 6. Explain lateral inhibition a. Lateral inhibition describes the antagonistic neural interactions between adjacent regions of the retina. b. The lateral plexus is a sheet of branching axons that transmit lateral inhibition. c. The lateral inhibition is proportional to the intensity of the stimulation. d. The higher the intensity of the stumuli is, the more lateral inhibition it is. 7. Describe why the limulus crab is the model for lateral inhibition a. The change in stimulus intensity is a stepwise function, but our perception is not. This can be explained by lateral connection /lateral inhibition. 2 24 8. Use lateral inhibition to explain mach bands, the Hermann grid illusion, and simultaneous contrast the darker border a. Mach bands i. The lighter border 1. Brighter (higher intensity) receives 2 inhibitions, one from higher intensity and one from lower intensity, given same proportion of inhibition. ii. The darker border the light er border 1. Darker (lower intensity) receives 2 inhibitions, one from higher intensity and one from lower intensity, given same proportion of inhibition. 2. mor e int ens it y s t imult iona t t hphot or ecept or s , mor e la t einhibition 26 T E Q!!!!! e.g. t hepr oport ionis 10% of inhibition her e; dif f er ef ir inr a t e dut o dif f s t imuli int ens it y 100 100 100 20 20 20 -10 -10 -10 -2 f r omX f r omB f r oB f r omD T hey ar e ON bipolar cells becaus e t he lar gert hes t imulint ens it y , thigher t he f ir ir a t e. b. Hermann grid illusion i. Dark spots at the intersection 1. Lateral inhibition from the surround, in which the inhibition is proportional to the stimuli intensity. 2. 3 c. Simultaneous contrast i. Both central squares receive the same amount of stimulation, but different inhibitions. ii. The darker surround sends less inhibition to the center, thus the center appears lighter. iii. 9. Recognize the relationship between lateral inhibition and centre-surround receptive fields 10. Summarize the differences between parasol and midget RGCs 36 a. There are 2 kinds of RGCs, parasol and midget Parasol RGCs recept ive fields is positively related to the dendritic field6 b. Parasol ‣large RF Parasol RGCscoverage of peripheral i. Large receptive fields (positively related to the dendritic retina
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