Visual Attention9 We use our visual system to provide us with information to accomplish some behavioral goal.9 How does the brain represent information about the visual scene AND its behavioral context?“Every one knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things in order to deal effectively with others, and is a condition which has a real opposite in the confused, dazed, scatterbrained state …”Our attention is sensitive to, and captured by, "..strange things, moving things, wild animals, bright things, pretty things, metallic things, words, blows, blood, etc". William James (1890), The Principles of PsychologyVisual Attention9 Is guided through voluntary and involuntary mechanisms (aka endogenous, exogenous; top-down, bottom-up)9 Can be overt or covert. Integral to oculomotor control9 Sensory benefits: Increased sensitivity, more accurate, and respond more rapidly to attended items.9 Has limited capacity. Difficult to attend to multiple items simultaneously. 9 Attention acts as a gatekeeper to memory and cognitive control.Change Blindness (Rensink et al., 2000)Neurophysiological measures of attentionV1PG CortexRostral STSDPVIPLIP7aPPMSTcMSTpFSTPOV3AMTTPOPGaIPaTEaTEmSTPV1 V2V3V4TEOVTFTETG36,35MTpPGTE468451211,13LIP,VIP,DP,7aV2,V3,V4,MT,MSTTEO,TETE,TGPrefrontal Cortex36,35,28FRONTALOCCIPITALTEMPORAL - "WHAT"PARIETAL - "WHERE"35Response enhancement in the superior colliculus (Wurtz et al., 1982)Enhanced response to stimulus only when it was the target of an eye movement.Does not indicate that neurons in superior colliculus represent a pure attentional signal.Response enhancement in parietal cortex (Wurtz et al., 1982)Effect of stimulus salience in area LIP (Gottlieb et al, 1998)Summary of studies in parietal cortex• Enhanced response prior to saccade to RF stimulus• No enhancement to stimulus outside RF• Enhanced response to RF stimulus when task is lever press• Enhanced response when stimulus has recent onset, prior to eye movementSpatial representation of stimulus salienceObservations from behavioral studies of neurological patientsNeglect• A failure to acknowledge, explore, or respond to stimuli located towards the contralesional side of space.• Typically the result of right parietal damage• No sensory of motor damage• Have anosognosiaExtinction• Can attend to stimulus in contralateral visual field provided there are no stimuli in ipsilateral hemifield.• The presence of another stimulus in ipsilateral hemifield draws attention and patient is no longer aware of stimulus in contralateral field. • Thought to be a problem with disengaging attentionSimultagnosia• Bilateral superior parietal damage, intact visual fields, unable to see multiple objects simultaneouslyCopying test for neglectThree drawings on the right were made from models on the left by patients with unilateral visual neglect (from Kolb and Whishaw, 1990)Self-portraits of German artist Anton Raederscheidt at progressive time points after his stroke• Attention in the ventral stream (V1, V2, V4, IT)• Limited Capacity• Multiple stimuli in the receptive fieldV1PG CortexRostral STSDPVIPLIP7aPPMSTcMSTpFSTPOV3AMTTPOPGaIPaTEaTEmSTPV1 V2V3V4TEOVTFTETG36,35MTpPGTE468451211,13LIP,VIP,DP,7aV2,V3,V4,MT,MSTTEO,TETE,TGPrefrontal Cortex36,35,28FRONTALOCCIPITALTEMPORAL - "WHAT"PARIETAL - "WHERE"35Moran and Desimone (1985)Cued Attention ParadigmFixation spot cued the color of the valid gratingColor cueV4 receptive fieldTime (msec)0 200 400 600 800Attend awayPref Stimulus in RF0 200 400 600 800Attend awayNon-Pref Stimulus in RFAttend toPref Stimulus in RFAttend awayBoth stimuli in RF25 spikes/secOD122301s003Summary of V4 results• Suppression of the response to the unattended stimulus• Enhanced response to the attended stimulus if it was a preferred stimulus for that neuron• Response of the V4 neuron is captured by the attended stimulus• Similar findings in areas V2 and ITBiased competition theory of attention (Desimone and Duncan, 1995)Assumptions:• Given the limits on our ability to process several stimuli at once, visual objects compete for representational resources. Because the neural representations of objects are highly distributed, competitive processing occurs in many brain areas sensitive to visual input.• Competition is integrated across several areas, such that neural populations that represent different aspects of a single object interact in a mutually facilitatory manner.• The competition can be biased not only by bottom-up factors, but also by top-down influences that are based on current task demands.Biased Competition: Desimone and Duncan, 1995Top-down Feedback Mechanismsfronto-parietal attentional networkBottom-upsensoryIntermediate Representationcompetition among stimuli or associationsOutputmotor memoryQuestions:• How do the distributed neural populations ‘know’ that they are representing the attended object and, thus, enhance each other while suppressing the representation of unattended objects?• What is the source of top-down signals? • Are there dedicated neural systems for the direction of attention?Top-down control of visual attentionDorsal frontoparietal network for top-down control of visual attentionStimulus-related activity is modulated by attention in occipital regionsFrontal and parietal regions exhibit sustained activity during cue period in absence of stimulus arrayThe N2pc component Courtesy of Geoff Woodman-3µV+3µV100 300200Time (ms)Left HemisphereLVF TargetRV F Ta rg etLVF TargetRV F Ta rg etRight HemisphereContralateral TargetIpsilateral