Selective Attention

Basics of Visual System

Neural Mechanisms

Brain regions

What factors can modulate it?

How could one study neural correlates of focused attention?

Relationship between LGN & TRN

Acetylcholine System

Dopamine 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

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

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

What ACh & DA have in common?

V1: edge detection to understand spatial organization

V2: similar function to V1, also illusory contours, determining depth

  1. Retina transduces image into electrical pulses using rods & cones
  2. The optic nerve carries these pulses through the optic canal
  3. They reach optic chiasm & nerve fibers decussate
  4. Most end in the LGN (first processing of perception)
  5. There is also a tectopulvinar pathway through superior colliculus for control of eye movements
  6. LGN forwards the pulses to V1 & also some fibers go to V2 & V3

V3: global motion (direction & speed)

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

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

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

searching for a matching feature affects V4 neurons

Presaccadic visual enhancement (V4, inferior temporal cortex)

  • reemergence of stimulus selectivity to targeted RF stimuli, selective encoding of stimulus features
  • when saccades are prepared to non-RF locations, the pre-saccadic activity & selectivity of V4 neurons are reduced

responses to repeated stimuli: more specific neural response, reduced firing rate variability, enhanced synchrony

planning of saccadic eye movements to the RFs of V4 neurons is sufficient to modulate visually-driven responses (top down)

Over Attention: V4 neurons more responsive to stimuli that are closer to the target when making eye movements instead of stable fixation -> interdependence of spatial attention & eye movement

  • focus attention on a location in the visual field
  • enhancing perception in one part of the visual field takes place at the expense of other areas

pop out stimuli, unexpected/sudden stimuli (bottom up): features that differ from surrounding distractors -> more salient & more easily located during visual search

Rewards

Conjunction search: when you need to combine two or more features eg. color & orientation

  • in feature search you just search for one feature and you dont need as much attention
  • different cortical areas are required for different features & attention is needed to combine them -> if wrongly combined, them we have illusory conjuctions

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

fMRI: There is more oxygenated blood in driven areas during attention

length tuning altered

Increased Sensitivity of V4 Neurons

What happens when RFs are crowded?

Receptive fields in higher visual areas are large and typically contain multiple objects at one time -> problems for neurons encoding specific features

when visual scenes are composed of multiple objects

Neurons with big RFs might have a ‘good’ stimulus and a ‘bad’ stimulus

  • The response to both combined looks like an average of the response to both stimuli alone

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

Glutamatergic feedback from higher areas mediates attentional modulation (modulate neuronal responses in lower areas)

Drugs: Scopolamine: induces sleepiness & blurred vision as side effects, lower spikes, lower attention, blocks ACh effects (blocks muscrinic receptors) -> suppresses attentional modulation & response

levels of ACh enhance attentional modulation

Both top-down and bottom-up biasing mechanisms influence the competition

Stimulation evoked potentials to look at coherence among the spikes

Study by McAlonan et al.:

  • Central cue directed monkeys to attend to one of two peripheral stimuli
    (horizontal or vertical light bar)
  • On each trial, while monkey fixated on the central spot, a cue appeared at the fixation point matching one of the upcoming stimuli
  • After 250ms, the two peripheral stimuli appeared, one in the RF of the neuron (always the same stimulus) & the other some distance from the RF
  • each stimulus could dim in luminance & the monkey indicated if the stimulus matching the cue dimmed (making a saccade)
  • if the matching stimulus didn't dim, the monkey remained fixating
    --> comparing neuronal responses when the cue matched the stimulus in the RF (AttIn) with responses when it matched the remote stimulus (AttOut)

the exact center of RF & its extent could be determined quantitatively

to localize LGN & other brain structures -> MRI

single neuron recordings, recordings of eye positions, EEG, ERPs, BOLD signal

attention decreases neuronal responses in TRN (thalamic reticular nucleus)

The visual sector of TRN receives excitatory inputs from LGN, but projects modulatory inhibitory input back to LGN (negative feedback system)

  • during attention, TRN reduces the inhibitory influence on LGN

attention modulates visual signals before they reach the cortex by increasing responses of parvocellular & magnocellular neurons in LGN

both can modulate selective attention

they are released primarily by neurons with specific brainstem or midbrain nuclei

their neuromodulating subcortical neurons project broadly to many subcortical & cortical structures & receive projections from PFC

  • projections to the cortex include: posterior sensory areas, where correlates of selective attention are, & PFC where the control of selective attention is

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

Pathways: wide-spread projections

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

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

Tyrosine -> LDopa -> Dopamine

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