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Unit 2, Week 2 Spatial Frequency (end) & Object Recognition
Topics Covered • CSF Channels theory • Extrastriate cortex • IT cortex • The dilemma of object recognition • & Middle vision’s burden • Gestalt psychology • Texture segmentation Group rules • Committee analogy
Exam 2 is March 13th – Next Wednesday! • Professor Maloney wants to communicate that the exam will cover the 2nd half of chapter 3 (post retina), chapter 4 (recognition), and chapter 5 (color). • We will not get to chapter 6 (Depth perception) for this unit. • You are accountable for: Chapters 3-5, pages (pages 62-148). • Study questions posted, from textbook site. • Practice exam posted this weekend. • Answers posted Monday by midnight.
CSF Channels • We know that each neuron has the lowest threshold for the orientation it likes the most, and that its threshold changes for orientations as it deviates away from what it “likes” • Similarly, each neuron has a “window” of spatial frequencies it “listens” to
CSF Channels • What this means is neurons are “closed-minded”. They only see what they want to see, or something pretty darn near what they want to see, and are “blind” to all other stimuli. • The CSF channels embolden this theory, through data we gained from adaptation experiments, much like the tilt aftereffect experiment we discussed last week.
CSF Channel –Experimental Data Notice this says threshold ELEVATION, meaning around the adapted spatial frequency 7cycles/degree, the requirement for firing got HARDER to accomplish
Breaking it Down • Using Fourier analysis, which breaks down an image to its separate spatial frequency components, we found the following: • Low frequency spatial frequencies ONLY blurry images, with strong contrast • High frequency spatial frequencies ONLY sharp images, but faint 2 1
Why Do We Squint? • We are filtering out high spatial frequencies, in hopes for more contrast & better edge detection • Should be unsurprising that we squint so much in light – we don’t see much contrast at all. • REVIEW: What else explains why we squint in sunlight?
Why Do We Squint? • REVIEW: What else explains why we squint in sunlight? • Right! Just like when we reduce the size of the pupil when we get more light information, our eyelids also act as an “exterior” aperature, for when our pupil can’t get any smaller and the lightness information from the stimulus is still overwhelming, or we have a focusing failure (myopia/hyperopia) and everything is blurry. Cutting off rays coming in from higher eccentricity angles makes the focusing job of the lens much easier to create a crisp image on our retina, because it only has to reconcile and channel a more agreeable set of rays (rays coming in from a smaller range of angles). • Secondly, squinting changes the shape of the eye due to the compression it creates, EVER SO SLIGHTLY, so that focusing failures can be compensated for
Extrastriate Cortex • After-V1-Before-What/Where Pathways-Visual Processing, aka the Extrastriate Cortex • V2 (prestriate cortex), V3, V4, MT (sometimes called V5) AKA Brodmann’s areas 18, 19
What and Where Pathways Dorsal/Parietal Ventral/Temporal
IT Cortex Visual agnosia video: https://www.youtube.com/watch?v=ze8VVtBgK7A • Part of the “What” pathway • Function: Object recognition, feed forward process (doesn’t need feedback from later stages in pathway in order to process information) • We know this from lesion studies. People with lesions in IT developed visual agnosia (could see, but couldn’t name object) • Receptive fields cover almost half of our visual field (one neuron!). Fire best for objects neuron is specified for, not for dots or bars. • The notion of a grandmother cell, Jennifer Aniston cell, the letter A cell
Dilemma of Object Recognition • 2D versus 3D presentation – How do we know the house is the same object from different angles? It looks completely different, yet we know it’s the same.
Middle Vision • Responsible for contour judgments • Doesn’t answer the question “what is this?” or where is this?” Answers “is this inside or outside?” • Step-by-step: • First, it finds the edges • Second, it groups the edges
Gestalt Psychology • KNOW THESE, guys: Good continuation, Contour completion, Similarity, Proximity, Parallelism, Symmetry, Common region, Connectedness, Common fate, Synchrony • We won’t review these in recitation, but a GREAT way to review these is to go to this website and play their games: http://sites.sinauer.com/wolfe3e/chap4/gestaltF.htm
Figure-Ground • Gestalt figure–ground assignment principles: • Surroundedness: The surrounding region is • likely to be ground • Size: The smaller region is likely to be figure • Symmetry: A symmetrical region tends to be • seen as figure • Parallelism: Regions with parallel contours tend to be seen as figure
Committee Analogy • Committee rules: • Know how the world works (honor and use the laws of physics to inform judgments) • Know what’s probabilistic (understand statistics) • You must always make a decision (cannot hold more than one view/make more than one judgment for the perception of a stimulus)
For this demo… • I need 7 volunteers at the front of the classroom. Of these 7, choose 2 to report to 1 • 5 underlings need to make grid with 5x5 cells • The rows shall be A-E, and the columns 1-5. • Overlord just sits back and watches his/her classmates sweat. ;-DDD
REST OF THE CLASS • You will be giving your brave DAEMONS their data. Your knowledge will determine how noisy their sampling data are and will impact the final interpretation, so perk up! • Assemble into 4 rows. • Each row competes against the other for a confetti-sabotage at the end of the game (ahaha! YES.) • Starting with desk#1, you will get a MC question over material you are accountable for on exam 2, of varying difficulty. DO NOT READ OUTLOUD, because your middlemen are deaf to this information. He only hears your input (your answer). DEAL? Cheaters shall be punished with dirty looks ;-). • Each row has only 1 pass (where your confidence is so low that you don’t report your answer) before you are forced to guess, regardless of your confidence/”threshold information”.
OVERLORD Schematic Overlord Interprets Level 2-A Level 2-B Level 2 Picks Most Sensible One Level 1-A Level 1-B Level 1-C Level 1-D Level 1 Transcribes Grid ROW 1 ROW 2 ROW 3 ROW 4 Underlings sample data
Do we Understand the rules so far? • The last one is to HAVE FUN! This is supposed to be a fun learning experience. • YAY! Let’s do this biznasssss.
Here is How you will report: • If you know the answer, tell your middle man the direction and cell number to the right of your answer choice. EX: What is 2+2? • 5 : Horizontal line at B2 • 7 : Vertical line at C4 • 4 : 45 degree right angled line at E1 • 2 : 45 degree left angled line at C3
Overlord: What is your verdict? • What is your interpretation of what we are looking at? Don’t say it outloud. Show me and I will declare the final success. • If underlings failed, switch versions between rows and try again with new volunteers. Some rows may be better suited for material in other versions – and perhaps more trustworthy to their superiors for detecting “features”! All versions are different and lead to different paths.