10/13/2010. Brain Mechanisms in Vision Attention. Search Task. The visual system is limited in its capacity to process information,

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1 Brain Mechanisms in Vision Attention Given The visual system is limited in its capacity to process information, Michael J. Earley, OD, PhD Professor of Clinical Optometry The Ohio State University College of Optometry therefore, complex visual scenes must be broken down into parts and each part must be processed one at a time. We are, however, typically fooled into believing that when we look at a scene, we are processing the entire scene at one time. Search Task Which is Different? Which is Different? 1

2 Visual system seems to process entire scene at one time in both examples. Which is Different? But what about this. To accomplish this task, you have to move from one object to the next and process individually. Visual search has no memory Horowitz and Wolfe, Nature, 1998 Visual search is random and redundant, therefore does not use computationally intensive memory You likely used a very random search strategy Vision Attention Allows us to increase efficiency of this search E.g. Finding the target is much faster if I tell you where to look (spatial attention) or what to look for (feature or object attention). Find the green vertical bar in the next slide. Which is Different? Modified from Reitan and Wolfson 2

3 Neural Mechanisms of Attention Experiments with animals have revealed direct neural correlates to vision attention selection. Neural Mechanisms of Attention Single cell recording results show: Cells respond to stimuli within their receptive fields If multiple stimuli fall within the receptive field of a single cell, then the response of the cell is determined by the average to all stimuli, unless.. One of stimuli is ATTENDED! Neural Mechanisms of Attention Why is this feature important? What if less salient stimulus for the cell is still the targeted stimulus of a specific task (i.e. is behaviorally relevant)? e.g. Find a bug camouflaged on a tree for food We must show/tell/allow time to learn what is visually relevant for our patients! How many objects can fit into one receptive field? Obviously depends on size of field? Cone near foveola: 0.5 minutes of arc (resolves 60 cycles/ degree or 20/10 acuity) Cells in V1: about 1 degree Cells in higher cortical areas: 20 degrees!! Gattass et all, 1988 A lot of stimuli can fall in a 20 degree field so we must have a way to deal with this efficiently! Neural Correlate of Attention in V4 Biased Competition Model Moran and Desimone, 1985 Determined preferred stimuli for specific cells in V4 of ventral stream elicited a strong response when presented alone Also determined a poor stimulus for same cell elicited a weak response when presented alone 3

4 Neural Correlate of Attention in V4 If monkey attends away from receptive field (e.g. attend centrally and record from cell in periphery) then cell response to two stimuli is about average of two individual responses. Non Attend paradigm Weak probe Intermediate probe Strong probe Neural Correlate of Attention in V4 If, however, animal is trained to attend to one stimulus vs. other (i.e. make behaviorally relevant), then the response is determined primarily by the attended stimuli Is no longer an average i.e. total response is biased toward attended stimuli Attended Paradigm When don t attend response equal to average of two When attend response equal to JUST strong reference (i.eweak probe no effect) Response equal to WEAK reference i.e. strong probe no effect Biased Competition Model Biased Competition Model Bottom Up Salience Biasing Competition If one stimulus is constantly attended and this attention has positive outcome, then that synapse can become heavily weighted. Pay attention to Hebbian Learning and neuroplasticity tomorrow!!! If attend toward the poor then you can counteract the effect of the more salient stimulus Suppression therapy in an amblyopia Current research is showing binocular cells DO exist in event constant strabismics, they just have very high suppression so act like monocular cells 4

5 e.g. can alter processing at x Directing attention can alter sensory processing over time Biased Competition Attention can have affect by: 1. altering efficiency of synapse at higher area e.g. W 1 and W 2 2. altering activity of lower neurons feeding into higher receptive fields (top down) e.g. X 1 and X 2 Higher Cortical Areas Many labs have demonstrated attention effects in higher cortical areas Parietal Lobe Previous studies have shown many spatial maps of environment (eye centered, head-centered, etc) whose responses are effected by attention Frontal Lobe Prefrontal cortex shown to respond to object category and response modulated by attention V4 Featured based attention and location based attentional modulation Etc, etc. Frontal Lobe Attentional mechanisms not well understood Prefrontal lesions can lead to unwanted behavior in contralesional field indicating normal inhibitory function that is lost. Prefrontal lesions can cause contralesional neglect or extinction indicating loss of normal excitatory function Activity in frontal lobe is vary similar for a vast variety of functional tasks Adaptability of Frontal Lobe 5

6 Frontal Lobe Maybe resolution of measurement not precise enough to visualize specialization, or Frontal lobe neurons are dynamic and may freely adapt to support cognitive distinctions relevant to current task. Reitan and Wolfson did an extensive review of neuropychological literature on frontal lobe damage to elicit function. Found a bewildering array of deficits attribute to frontal lobe damage. V1: Primary Visual Cortex Logical location for attentional modulation given Is bottleneck for information to higher cortical areas Retinotopic map of V1 is very precise so can accurately focus attention However, single cell studies could not confirm attentional effects at V1 V1: Primary Visual Cortex Attentional effects have been demonstrated by fmri studies Haeger, Gandhi, Huk and Boynton Correlated fmri recordings from V1 and changes in behavioral performance V1 Attentional Mechanisms Subject viewed concentric circles moving radially inward or outward from center Alternated between passive viewing and performing speed discrimination task More V1 Evidence Subject was instructed to attend to either right or left window and perform speed discrimination task More V1 Evidence Recorded fmri from V1 and found that activity level varied as anatomy would predict (e.g. attend to right side and activity in left V1 increased). 6

7 Results of V1 Attention Experiments Attentional effects of V1 depend on attentional demand for given subject based on difficulty of behavioral task for THAT subject. Effect is NOT same for all subjects for a task Effect might only be measured when task has effect on behavior. My Proposed Experiment for V1 Attention Record fmri from two groups while perform SAME task However, choose subjects and segregate into groups so that task is difficult for one group and much easier for second group E.g. judging type of pitch for novice vs. excellent hitter Would you expect differences in V1 activity given what you know about V1 attention effects? Answer should be Yes, by the way.. Group good at task Possible results Group poor at task Shaywitz et al. (PNAS, 1998) Recorded fmri to series of tasks of ascending phonemic demands (reading tasks) from 17 regions of interest (ROI) Compared activity in normal readers vs dyslexics Group poor at task certainly not attending to details as well! Maybe doesn t know what details to attend?? Found decreased activity in posterior language center Pediatrics, July 28, 2009 Learning Disabilities, Dyslexia and Vision What about Attention effect below even V1 Dyslexia is a LANGUAGE problem Why process at V1 if going to disregard anyway? Children with dyslexia, on the other hand, use different areas of their brain when reading. People with dyslexia demonstrate a dysfunction in the left-hemisphere posterior reading systems and show compensatory use of the inferior frontal gyri of both hemispheres.. 7

8 Thalamus Processing through LGN Lateral Geniculate Nucleus Classical (e.g. Hubel and Livingstone, NOVA, popular press): Retina to LGN to Occipital Lobe Type 1 and 2 to Parvo layers Type 3 to Magno layer Relay station to reach cortex classical thalamic nucleus Receptive field center surround from retina modified by increasing strength of surround therefore removes DC level of scene Why we are poor light meters Function of LGN Only10% of synapses in LGN are received from retina, 90% of synapses are received from cortex and brainstem (vast majority from layer 6 of cortex). In fact MORE fibers go from cortex to LGN than from LGN to cortex! Many inputs go through thalamic reticular nucleus and then to LGN Visual Attention and the LGN Attention Modulates Responses in the Human Lateral Geniculate Nucleus O Connor et al Nature Neuroscience 2002 Previous research only demonstrated attentive changes in V1 and higher. No attentive changes found in LGN. Used fmri to record activity level in LGN and cortex and document changes due to attention 8

9 Visual Attention in LGN Enhances neural responses to attended stimuli Increase signal (e.g. increases W 1 ) Attenuates neural responses to unattended stimuli Decrease noise (e.g. decreases W 2 ) Increases baseline activity in absence of visual stimuli Primes the system (increase resting potential of y) O Connor et al O Connor et al, 2003 Results Function of LGN Attentional Enhancement LGN is greater than V1 Effect is greater in higher cortical areas Attentional Suppression LGN effect is greater than most higher cortical areas Retinogeniculate Driver Corticogeniculate Modulator Baseline Increase LGN is equal to higher cortical areas 9

10 LGN lies directly lateral to mesencephalic flexure which is major locus of axonal shearing in acceleration /deceleration injury. Can damage in this area partially explain tremendous loss of attentional ability in MANY traumatic brain injury patients? New Theory of Vision Attention Much more than simple signal detection What and Where: A Bayesian Inference Theory of Attention Chikkerer, Serre, Tan and Poggio Vision Research, 2010 Thanks Len! What is Function of Visual Recognition? Lots of research that helps explain how vision solves What question Lots of research that helps explain how vision solves the Where problem Visual recognition, however, needs to solve the What is Where problem Marr, 1982 Well established from research and clinical observation that the visual pathway can be described as two parallel pathways Ventral Stream involved in solving what question Dorsal Stream solving where problem What is Where Well established that separate features of a stimulus are processed by highly specialized regions of the brain Dorsal stream orientation, location, movement Ventral stream: object identification Clinical observation can lead to false conclusion that two pathways are functional independent as can have selective damage to one that seems to spare the other. Optic ataxia Constructional apraxia 10

11 This despite evidence that the brain is not wired as parallel streams Optic Ataxia Deficit in visual guided motion despite normal acuity and fields More common in superior parietal lobe lesions Optic Ataxia Can visual identify orientation but motor action is inaccurate. Apperceptive Agnosia Damage to the early parts of the ventral stream (occipito-temporal region) due to deafferentation of the Infero-Temporal cortex due to gross, bilateral damage to the occipital lobes. commonly following anoxia e.g. carbon monoxide poisoning e.g. ventricular fibrillation x 8 minutes e.g. massive internal bleeding. Case DF, reported by Dr. Melvyn Goodale, of the University of Western Ontario. pilot light on water heater went out during shower, which lead to CO poisoning. severe damage to occipital lobes and pre-striate cortex (V2, V3, V4) resulting in deafferentation of the ventral steam.. D.F.: things she can t recognize CAN T identify grating orientation (vertical vs. horizontal) using verbal response. CAN T recognize letters and numerals visually. CAN T recognize line drawings of common objects, including those she has drawn herself. 11

12 D.F.: apperceptive agnosia In spite of her severe impairment in recognizing and drawing stimuli she sees, she has dramatic abilities to interact with the world using her intact dorsal stream. Grasping experiments Posting a letter experiments Despite these clinical entities, experience in everyday life shows us that an object s identification and location MUST be unified. Visual attention is the process that has the computational goal of inferring simultaneously the form and location of objects in the visual world. Chikkerur et al, Vision Research, 2010 Bayesian Inference of Attention Conclusion: Visual Attention solves problem of What is Where by: Uses Spatial Attention to reduce uncertainty in shape information (e.g. reduces all possibilities of what it is). Something flies over your head What is it?? Bayesian Inference Different from other forms of probability as it allows you to apply probability to truth or falsehood of events that are not randomly distributed, but rather which is simply unknown. Uses Feature-Based Attention to reduce uncertainty in spatial information (reduces all possibilities of where it is). find your shoes. QUEST and ZEST Bayesian adaptive psychophysical procedures used to determine thresholds in Matrix and FDT. P(H/E)= P(H)*P(E/H) / P(E) H is hypothesis E is evidence observed P(H) is prior probability of H before evidence is observed (can be subjectively biased!) P(E/H) is likelihood function of E being observed given H is true P(E) probability of witnessing E given ALL possible hypotheses P(H/E) posterior probability of H given E (new estimate of H being true after E occurred. This will become P(H) for next observation! Therefore P (H/E) is more accurate as evidence is accumulated. 12

13 P(H) Prior Probability Proponents say that even if biased in error then repeated E s will correct towards correct probability Opponents say if P(H) is significantly off then even repeated E s will not bring P(H/E) to correct value E.g. extreme prejudice P(H) Prior Probability In visual system there are separate P(H) for Features P(F) Location P(L) An important premise is that these two priors are INDEPENDENT Also for attention to work in this matter we must assume that the visual system selects and localizes objects, one object at a time (Attentive Spotlight) and then iterates this process over entire scene as needed. From Priors to Salience Maps Given you have priors of P(F) and P(L) and you observe an image I You then calculate the posteriors of P(F/I) and P(L/I) which are location and feature SALIENCE MAPS Model Applied to Different Stimuli P(F/L) can be thought of as attention spotlight in space Model Applied to Different Stimuli Attention is NOT just the spotlight. Attention is the FEATURE / LOCATION LOCKING system. 13

14 Bayesian Model Dueling Theories of Attention Bottleneck Theory of Attention: Attention prioritizes the visual scene toward relevant regions given we have limited visual processing resources. Attention is needed to reduce computational complexity of recognition. Bayesian Theory Attention is needed to reduce the sample complexity of learning the relevant probabilities of all objects, features and locations. This would take too long UNLESS prior assumptions are made (i.e. P(F) and P(L)) What if Priors of P(F) or P(L) are dysfunctional Overwhelmed when enter a cluttered space E.g. I hate Wal-Mart Feel unsteady in unfamiliar surroundings Cannot concentrate on one person (or listen to one person if apply to auditory) Don t know where to start a task Don t recognize certain critical features of objects (orientation critical for letter identification) What if object/location locking mechanism of attention is dysfunctional Words seem to move or float on the page Sequential memory deficits Visual closure deficits?? Ventral Simultanagnosia Can shift attention to all parts of scene but cannot integrate parts of scene into whole E.g. Great cookie theft test from Boston Diagnostic Aphasia Examination 14

15 Thank You 15