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Human (ERP and imaging) and monkey (cell recording) data together 1. Modality specific extrastriate cortex is modulated by attention (V4, IT, MT). 2. V1.

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Presentation on theme: "Human (ERP and imaging) and monkey (cell recording) data together 1. Modality specific extrastriate cortex is modulated by attention (V4, IT, MT). 2. V1."— Presentation transcript:

1 Human (ERP and imaging) and monkey (cell recording) data together 1. Modality specific extrastriate cortex is modulated by attention (V4, IT, MT). 2. V1 is modulated when task conditions are demanding in cell studies, but disagreement between ERP and fMRI for V1 may reflect both initial (ERP) effects and later (fMRI) effects. Have to see it before you attend to it? 3. Attention Mechanisms include both enhancement and inhibition. Increased ‘neuronal gain’

2 Modeling Selective Visual Attention Robert Desimone and John Duncan(1995) – Biased Competition Model – Hypothesis: Multiple stimuli in the visual field activate populations of neurons that automatically engage in competitive interaction, which are assumed to be through intracortical connections. When attention is directed to a stimulus, this is thought to be accompanied by feedback signals generated within areas outside the classical visual system. These signals bias the competition. As a result, neurons responding to attended stimulus remain active while suppressing neurons responding to unattended stimulus.

3 Duncan and Desimone proposal for attention selection Competition results in few or one stimulus actively represented at a time in distributed representations (lateral inhibition, winner take all effect). This prevents cross talk or interference problems. Pattern completion highlights commonalities. Attending to a color will bring up all stimuli that have that color. Winner determined both by bottom up effects (intensity and novelty) and top down activation from higher areas.

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6 One theory that brings together all of the reviewed attention effects (top-down biases, gain modulation, enhancement and suppression) is Desimone and Duncan’s ‘biased competition’model of attention. The theory rests on three assumptions. First, given the limits on our ability to process several stimuli at once, visual objects compete for representational resources, and only one or a small number of stimuli can be represented at one time. As the neural representations of visual stimuli are highly distributed, competitive processing occurs in many of the brain areas sensitive to visual input. Second, the competition is integrated across several areas, such that the neural populations that represent different aspects of a single object interact in a mutually facilitatory fashion. The gain in response to the selected object is accompanied by suppressed processing in the neural populations representing features of different objects. Therefore, as a ‘winner’ emerges in one system, the same object becomes dominant across the distributed network. Last, the competition can be biased not only by bottom-up factors (for example, stimulus intensity), but also by top-down influences that are based on current task demands. Top-down bias is reflected in neural priming (enhanced processing) of populations representing the relevant object attributes, resulting in a competitive advantage for the relevant stimulus. An important challenge for this theory (and other theories of attention) is to explain precisely how the distributed neural populations responding to a single object ‘know’ that they are representing the same object and so should enhance each other while suppressing the neural representations of other objects (the binding problem). More about biased competition

7 Modeling Selective Visual Attention Figure from Neural Mechanisms of Selective Visual Attention by Robert Desimone and John Duncan

8 Controlling attention: The top down Prefrontal cortex Prefrontal cortex is called the executive system of brain and has major role in working memory (maintaining representations while we need to keep thinking about them) consistent with a control of attention role. Dorsolateral prefrontal cells maintain activity during active attention to a location in absence of stimulus, and level of activity is related to level of attention so it could bias earlier areas. Ventromedial areas maintain activity during active attention to objects. Bloodflow studies provide best evidence, but need good designs to be convincing. Best control is exogenous (nonpredictive) cue like at bright box at some location versus endogenous pointing cue at fixation. Right dorsolateral prefrontal bloodflow is unique to endogenous (top down) condition.

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10 More top down: The posterior parietal cortex Neurons in posterior parietal cortex increase firing for attended stimuli and locations. Attention to spatial locations increases bloodflow in posterior parietal cortex. Visual neglect or extinction occurs with right posterior parietal damage.

11 Why do we need posterior parietal cortex? Need to mediate between ‘high level representations of objects in space that guide our topdown allocation’ and retinotopically mapped modality specific representations of visual stimuli that are subject to competition effects’ We don’t want attention to jump when we move our eyes, we also want to attend to other aspects besides visual. Unlike attention experiments discussed so far we don’t keep our eyes on a constant fixation point. We move around so correspondence between retinal location and location in the world is constantly shifting. We need to translate between world and retinal coordinates.

12 Posterior parietal cortex neurons encode for intention to move Egocentric space is represented. Eye movements, head movements, limb movements that will get you to what you are interested in. Salient (important, attended to stimuli) are represented. So interface between what we want and how we get to it.L6Action.swfL6Action.swf

13 Parietal cortex translates between world and retinotopic co-ordinates Parietal neurons modulate firing to receptive field stimuli depending on fixation or eye position. This isn’t all you need to figure out where the stimulus is in head- centered space but if you have a population of these (distributed representation) then you can pinpoint one location

14 Parietal cortex also has nonretinotopic fields (LIP) Cells encode the memory of the location of the field and shift with eye movement (before the eyes get there!).

15 A planned shift in the visual world is seen by parietal neurons before it happens

16 Supramodel attentional control Need to co-index attention for visual, auditory and tactile qualities.Need integration of multiple sensory and motor representations. See this in VIP. Hemispatial neglect is all about loss of supramodal attentional control (p )

17 Desimone 2005: Parallel searches then serial searches so not a single spotlight model and not a simple binding by location model.

18 New parallel and serial model Throughout the period of searching, neurons gave enhanced responses and synchronized their activity in the gamma range whenever a preferred stimulus in their receptive field matched a feature of the target, as predicted by parallel models. Neurons also gave enhanced responses to candidate targets that were selected for saccades, or foveation, reflecting a serial component of visual search. Thus, serial and parallel mechanisms of response enhancement and neural synchrony work together to identify objects in a scene.

19 Features (parallel) The feature-related enhancement we observed is likely the result of a combination of feature-selective responses in the visual cortex, including V4, and top-down feedback from structures involved in working memory and executive control, such as the prefrontal cortex and possibly the parietal cortex. Such feedback must be capable of targeting neurons with the appropriate feature preferences throughout the visual field map. Location (serial) The saccade-related enhancement, on the other hand, likely originates from feedback to V4 neurons with RFs at particular locations, originating from structures with spatial attention and oculomotor functions such as the frontal eye field and the lateral intraparietal area. These areas are thought to represent a salience map in which stimuli are represented according to their behavioral relevance independent of their features

20 More top down: The posterior parietal cortex Neurons in posterior parietal cortex increase firing for attended stimuli and locations. Attention to spatial locations increases bloodflow in posterior parietal cortex. Visual neglect or extinction occurs with right posterior parietal damage.

21 Hemispatial neglect

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23 “Right” “Left”“Right” Extinguished Left Visual Extinction


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