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Selective Attention (What factors can modulate it? (searching for a…
Selective Attention
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Neural Mechanisms
enhancing signal efficacy via synchrony among neurons encoding the attended stimulus -> high frequency gamma-band synchronization in the output from projection neurons
greater postsynaptic efficacy means increased firing rate & increased signal -> attention alters the transmission of signals across the visual system
reduced variability of responses to repeated stimuli -> reductions in firing rate variability increase information & diminish the noise in the neural signal (spiking regularity)
The neural response gets more specific for a certain stimulus (less neurons fire at the end, only those that are necessary)
Most of those mechanisms are related to top down processes, not bottom up
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Glutamatergic feedback from higher areas mediates attentional modulation (modulate neuronal responses in lower areas)
Basics of Visual System
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V2: similar function to V1, also illusory contours, determining depth
- Retina transduces image into electrical pulses using rods & cones
- The optic nerve carries these pulses through the optic canal
- They reach optic chiasm & nerve fibers decussate
- Most end in the LGN (first processing of perception)
- There is also a tectopulvinar pathway through superior colliculus for control of eye movements
- LGN forwards the pulses to V1 & also some fibers go to V2 & V3
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V4: recognizes simple shapes, color, strong input from V1
V5 & V6: motion analysis eg. self-motion, motion of objects relative to background
Layers of LGN:
- Magnocellular cells: large size, source from rods, rapid & transient response necessary for perception of movement & depth
- Parvocellular cells: small size, source from cones, slow & sustained response, necessary for perception of color & form
Cells in V1:
- Simple: respond to oriented edges & gratings, anything that goes into RF makes them active
- complex: only active when there is specific orientation & direction
- hypercomplex: when RF is hit by the light & there is specific motion
Dopamine System
role in attention:
- via its effects on PFC (more top down)
- via D1 receptors, DA can alter the strength & reliability of converging excitatory Glutamatergic synapses
- inverted U- shaped property: optimal DA levels lead to peak effects on synaptic efficacy
- manipulation of D1receptor-mediated FEF activity: increase in saccadic target selection, magnitude, selectivity, reliability of V4 visual responses
Pathways:
- Nigrostriatal/Mesostriatal: from SN to Striatum
- Mesolimbic: from VTA to nucleus accumbens
- Mesocortical: from VTA throughout the cerebral cortex
especially found in Substantia Nigra & VTA, but also in hypothalamus, olfactory bulb & even retina
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Receptors: G-protein coupled
- D1(stimulating, more abundant in PFC) & D2
Function: Movement, processing of rewarding experiences, memory, attention, sleep regulation, motivation, arousal
Study: Manipulating DA-mediated activity within FEF
- injections of D1 receptor antagonist -> increased saccadic target selection
- manipulation of D2 had no effect
- D1 manipulation elicited correlates of covert attention within extrastriate cortex
- injections of D2 agonist into FEF had same target selection effects as those of D1 antagonist
Acetylcholine System
role in attention:
- mainly bottom up & salience driven
- enhances selective visual attention
- Basal forebrain stimulation enhances sensory signals within the somatosensory, auditory & visual cortices -> ACh can increase perceptual performance globally
- processing of sensory signals within posterior areas might be influenced by the interaction PFC with ascending cholinergic projections (nicotinic receptors)
- within V1, gain control achieved by nicotinic receptors
- application of scopolamine (mACh receptor antagonist) reduces attentional modulation
- basal forebrain ACh interacts with amygdala to direct spatial attention according to motivational factors
Clusters:
- CH4 region of basal forebrain (Nucleus Basalis of Meynert), septal nucleus, nucleus of diagonal band (enhancing signal to noise ratios, vigilance, memory-encoding, working memory)
- Brainstem: penduculopopntine nucleus, laterodorsal tegmental nucleus (sleep-waking cycle)
in the periphery found in autonomic ganglia eg. heart, neuromuscular junction
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Receptor types:
- Nicotinic: in skeletal muscle (inotropic effect), ligand-gated ion channels, response is brief & fast, postsynaptic
- Muscarinic: in heart & smooth muscle (metabotropic effect), GP second messenger system, response is slow & prolonged, pre & post synaptic
Function:
- Muscles activation, transmits signals between motor nerves & skeletal muscles
- in CNS, primary neuromodulator: areas of the brain that control motivation, arousal, attention
- its deterioration is associated with Alzheimer's
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Biased competition:
In motion sensitive MT area:
- The dorsal (‘where’) stream is sensitive to motion (compared to the ventral ‘what’ stream)
- Pairing preferred and non- preferred directions leads to average response
- Attention to one direction in a pair leads to response matching the attended direction
in Neuroimaging:
- In some recordings the response goes up in others the response goes down -> depends on the tuning of the cell
in fMRI:
- In the simultaneous condition stimuli compete for neural representation
(Bold response decreased)
- The sequential condition, prevents competition between stimuli
- Presentation time Is too fast for BOLD to differentiate based on time
- neural representations of the different stimuli interact in a suppressive way
Why?
- Neurons become selective for more complex at higher cortical areas
- combining the output of many neurons in lower levels
- Multiple signals from lower areas project to each higher level neuron
- Salience and attention can bias the competition
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Brain regions
Bottom up: less understood, probably also involved in top down
- parietal cortex & PFC: computing/amplifying global salience
- superior colliculus: salience computation
Feature-based:
- V4: neurons respond more vigorously to RF stimuli that more closely match the feature that one searches for
- ventral PFC
- ventral prearcuate VPA, FEF, inferior temporal cortex: selective for the memorized visual image, active in visual search task
Top down:
- dorsal LGN
- FEF (directing eye movements, visually guided saccades)
- V4: projections from LGN, planning of saccadic movements to the RFs
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role of Norepinephrine in attention:
- more associated with behavioral arousal
- NE neurons within LC respond selectively to salient sensory stimuli
- activity depends on task relevance of stimuli --> LC neurons respond with phasic bursts to the presentation of learned targets but only weakly when non-targets are presented
How to measure spatial acuity? Landolt C or square test -> indicate which direction the gap is facing
How to measure contrast sensitivity? simple task with varying contrast
- Participants can perform the task at lower contrast at the cued site