Psyc 358 1st Edition Lecture 6 Outline of Last Lecture II. The brain basis of Cognition (Continued)III. Neural CircuitsIV. LocalizationV. Neural ImagingVI. The ForebrainOutline of Current Lecture II. The Nervous System related to perceptionIII. How Neurons enable visionIV. Different pathways in the brain that allow for visionCurrent LectureDay 6 (starting Chapter 3)QOTD:How do we explain the physical environment in our own minds?“Explaining vision”- Reflected light focused by cornea and lens on to retina (fig. 3.10)- Retina is a map of photoreceptors that respond to lighto Rods- low lights levels; colorblind; dominate peripheral visiono Cones- sensitive to color and fine detail- Greatest acuity at fovea due to lots of cones- Rods and cones eventually project to bundles… and then to the optic nerve- Most fibers go to the LGN in the thalamus…and then on to visual cortex (known as V1)How do neurons enable visual experience?- Computation occurs along the way in the neural circuitso An example is lateral inhibition that produces edge enhancement.Neuron 1   Neuron 2   Neuron 3- Each arrow represents an inhibitory (-1 message)3 3 3 3 3 0 0 0 0 Firing rates based on stimulus \ /…. - 2 -2 -2 -1 -1 0 0 ….Inhibition levels……1 1 1 2 -1 0 0…. Result is an enhanced edge|  |So our first row represents an edge, the 3 is a firing rate and the neighboring neurons aren’t firing, they are at a rate of 0. So the thing you were looking at ended. The highlighting represents an edge, a place where light was reflecting off somethingand now boom not there anymore. But the edge is subtle only 2 neurons coding for that edge. First row is the firing rate. Second row is how much inhibition is occurring. Third row is what are you going to see after the inhibition has occurred.So in the third row, the firing rates have altered. The part in yellow is the perceived edge.The edge is represented by saying there’s activation here, and different activation in the next node. How neurons enable vision?- ‘divide and conquer’ strategyo you go solve this problem for over here, you solve this problem over there and tell us what you find and then theres neurons whose jobs it is to integrate back high level things and pass it on to the next neural circuit. It’s a good strategy.- Parallel processing is fasto Allows interactivity and specializationo Challenge is coordinating the multiple processes leads to binding problem Binding problem is that how do we find out which problems in the visual field go together? So if I see a red shiny circle there might be some neural circuit figuring out red and some neural circuit finding out it’s a circle how do you do the job of binding that info saying all of that info is for the same thing? How do you know which things that are determined to be characteristics of the world are bound together to describe one component. o Solution: spatial location as frame of reference You keep track of oh the redness that’s there, and the circle is there and the shiny thing is there, so everything has a tag that says where its happening. - On the pathway from the retina to the optic nerve…o Receptive fields (Hubel and Wiesel) Different kinds of sensitivity- Dot detectors or center surround cells- Orientation- specific cells- Motion- sensitive cells Early evidence for plasticity. From the optic nerve to LGN in thalamus…- Magnocellular system receives from M cells big receptive fields, code for WHERE (motion, depth)- Parvocellular system receives from P cells smaller receptive fields, code for WHAT (form, color)But hard problems remain…- Parsing/ segmentation (fig 3.12, 3.13)o The process of figuring out which parts of a scene actually go together.How do you use the edges that exist to say oh that’s rock or bush or hose.- Recovering the third dimension, occlusion (fig 3.2)- Correcting for point of observationo Blurringo Is it big or near? Far away or small? (fig 3.6)o Viewpoint invariance (fig 3.9)- The role of unconscious inferences (Helmholtz) o Visual illusions E.g., occlusion heuristic (fig 3.14)- The role of cooperativityo E.g., shape from shading (fig 3.17)Gestalt approach- Laws of Perceptual Organization- Perception not a precursor to cognitiono It does cognitive work, but how?o The percept does not arise solely from the stimulus.- Heurists used to organize a holistic visual experience (where the whole is greater tan sum of the parts)Simplicity/ pragnanz (fig 3.19)- We experience things so that they are simple and straightforward, subconsciously and implicitly. Similarity (fig 3.20, 3.21)Good continuation (fig 3.18)What is the role of