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3.6 - Form Perception 2.pdf

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McMaster University
Joe Kim

Arnav Agarwal 2011 Form Perception 2 Module 1: Feature Detectors Introduction to Feature Detectors - Brain uses a division of labor, with each region along visual pathway processing relatively specific info, then passing it on; form recognition follows similar pattern Magno and Parvo Cells [ganglion cells] - They transduce the light stimulus into a neural impulse - Magno: o found mainly in periphery of retina o used mainly for brightness change detection, motion, depth - Parvo: o found throughout retina o used mainly for color, pattern, form detection - These ganglion cells have small receptive fields - Axons of ganglion cells: retina -> optic nerve -> LGN -> primary visual cortex (occipital lobe) - Cells in the primary visual cortex are very particular about what makes them fire; these cells are called “feature detectors” Arnav Agarwal 2011 Hodgkin and Huxley - 1952: Hodgkin and Huxley recorded electrical activity in an individual squid neuron - Technique used from then to see how individual neurons respond to specific stimuli Arnav Agarwal 2011 Lettvin et al - 1959: discovered neuron in optic nerve of frog that responded only to moving black dots -> “bug receptors” Hubel and Wiesel - Extended their work in their studies of cells in the visual cortex of cats and monkeys - 1981: won Nobel Prize Arnav Agarwal 2011 - 1962: Hubel and Wiesel began their exploration of the visual cortex o Tried to learn what type of stimuli individual cortical cells responded to - Used microelectrodes on cat’s cortex to record electrical activity of individual neurons while presenting different visual stimuli types (eg: flashes of light) o No response received for majority of visual stimuli - Presented cat with a slide that had a crack -> light projected from crack moved across cat’s visual field -> neuron started to fire - Conclusion: neurons respond to stimuli more complex than diffuse flashes of light - Lines of different orientations and thickness that moved in different directions presented o Finding: each neuron is very specific about what will make it fire the most o Cells fire maximally to stimuli of a certain shape, size, position and movement; this defines the receptive field for that cell Simple Cells - Responds maximally to a bar of a certain wavelength and orientation in a particular region of the retina - eg: simple cell below responds most to horizontal bar, but if same bar is moved outside region or changes orientation, cell will be inhibited -> will fire less than baseline Arnav Agarwal 2011 - Conclusion: receptive field is organized in an opponent fashion, making it sensitive to the location of the bar in the sensitive field Complex Cell - Similar to simple cell that it cares about length and orientation of bar - Responds maximally to a bar of a certain length and orientation, regardless of where the bar is located within the receptive field - Difference: does not care about where in its receptive field the bar is located; it will even continue to fire if the bar is moving within the receptive field - Some complex cells do care about direction of movement (eg: cell in figure below fires most when stimulus oriented at a certain angle and moving in particular direction) Arnav Agarwal 2011 Hypercomplex Cells - Respond maximally to bars of a particular orientation that end at specific points within the receptive field - eg: (below) fires most to a horizontal bar of light that appears anywhere in the “on” region of the receptive field; gives only a weak response if bar touches “off” region o therefore, these cells have an inhibitory region at the end of the bar, making them sensitive to length of the bar Arnav Agarwal 2011 Topographic Organization - Layout of visual scene is preserved in visual cortex; neighboring objects in field processed by neighboring brain areas - Mapping from visual field to brain is not exact: large amount of cortex is devoted to processing info from the central part of the visual field, which projects onto the fovea - Each region of the cortex receives some input from small piece of visual field; there are cells within each region that analyze specific features of the scene - For a particular part of visual field, there are neurons that fire maximally if there is something in the scene that has a line of certain orientation, length and movement - Other neurons respond maximally if there is something in that tiny portion of the visual scene that is a specific color - Other neurons respond most when there is a line that moves ina certain direction - Cluster of cells in the region of the cortex right beside this region are doing the same analysis for the neighboring part of the visual scene. - Important benefit of parallel processing strategy: speed Concept Check: Arnav Agarwal 2011 1) Which of the following are true regarding the magno cells? a. They are found throughout the retina b. They are found mainly in the periphery of the retina c. They detect changes in brightness, motion and depth d. They detect color, patter and form. 2) Please match the following terms and definitions: a. Responds maximally to a bar of a certain length and orientation i. Simple cell b. Responds maximally to a bar of a certain length and orientation, regardless of location i. Complex cell c. Responds maximally to a bar of a particular orientation that ends at specific points i. Hypercomplex cell Module 2: Ventral Stream Combining Information in the Extrastriate Cortex - specific cells in the primary visual cortex respond to relatively specific features from a small portion of the visual field - information is combined to form a meaningful whole Arnav Agarwal 2011 Subregions in Extrastriate Cortex - Combination begins in extrastriate cortex (ie: visual association cortex), which surrounds primary visual cortex - Has multiple subregions that each receive a different type of info from the primary visual cortex about the visual scene o eg: one deals with scene colors, one with movement in the scene, another with line orientations - Info gets segregated into two streams according to type of info being processed (in the extrastriate): o Dorsal (“where”) -> processes where objects are located, how they are moving  Info: primary visual cortex -> parietal lobe (spatial info processing) o Ventral (“what”) -> processes what the object is (form, color, etc.)  Info: primary visual cortex -> temporal lobe (pieces of feature information come together) Columns in the Temporal Cortex - Arranged in vertical columns oriented perpendicularly to the cortex surface o Each column has five layers of neurons -> each layer responds to complex stimuli coming from same category  Each layer responds to slightly different features within the category (eg: column response: to apples; layer 1: red apples; layer 2: maximal firing to green apples; layer 3: to big apples; etc.) Arnav Agarwal 2011 - Here, neurons respond to very specific stimuli -> much more complex than stimuli primary visual cortex responds to o eg: images like hands, faces, apples, chairs - Note: objects aren’t coded by a specific neuron o They are represented across many cells in several different brain areas o The individual cells described are just one component of the overall representation o Even individual cells respond to a range of stimuli Concept Check: 1) What is the collective name for the visual areas outside of V1? a. extrastriate 2) To what do the columns in temporal cortex preferentially respond? a. Categories of objects b. Specific objects c. Objects of the same shape d. Objects of the same color Module 3: Development of Pattern/Object/Face Recognition Infant Pattern Recognition Arnav Agarwal 2011 - Perception is not just being able to sense incoming stimuli; infants might have the “equipment” to see an object, but may not be able to perceive patterns, objects and faces like we do - eg: does a 9-month-old recognize shadows and forms as a mother’s face? is she even able to distinguish it from the background? -> is the ability to detect patterns/objects innate or learned? Preferential Looking Method - Determines what kinds of patterns infants can perceive by measuring which of two patterns the infant looks at most - Results: o infants prefer looking at patterns over plain stimuli o infants prefer patterns that have high contrast with sharp boundaries between light/dark regions o infants prefer looking at the most complex stimuli they are able to perceive Arnav Agarwal 2011 - eg: checkerboard o newborn prefers looking at bigger squares in checkerboard than smaller; poor visual acuity makes small square appear uniform grey (not interesting to see) o 2 months of age: improved acuity; preference to smaller-squared one (more complex) - poor visual acuity in infants: influences object perception as well - infants under 2 months: poor acuity -> may not be able to perceive whole forms at all Young Infants May Not Perceive Whole Objects - newborns: tend to stare at one corner of an object o attracted to certain features (eg: angles, edges) of stimulus, but not object as a whole - 2 months of age: begin to focus on entire object Arnav Agarwal 2011 - 3 months of age: can perceive whole form when given only parts of the form o eg: seeing square in the middle of four circles (below)  habituate them to a real square, then show four-circle square; they will stare less at the four-circle square than at a different four-circle shape The Trouble With Overlapping Objects - Hard to distinguish two touching/overlapping -> requires use of cues like pattern and colors to identify which components are the object, what belongs together, what is separate Arnav Agarwal 2011 - Infants cannot use cues like color or texture to tell two objects apart until almost 5 months, especially if objects are still or moving together objects - Independent movement of the objects is a much better way for young infants (ev
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