Please enable JavaScript.
Coggle requires JavaScript to display documents.
Task 4: Selective attention. Neural mechanisms and Neurotransmitters…
Task 4:
Selective attention. Neural mechanisms and Neurotransmitters
Visual system
Pathways within brain
Dorsal pathway:
upper, where and how, unconscious, action guiding, spatial awareness (top-down attention)
V1 - V2 - V6 - V5
Ventral pathway:
lower, what, conscious, object recognition, form representation (bottom up)
V1 - V2 - V4 - inferior temporal cortex
Normal pathway:
Eye > retina > optic nerve > optic chiasm > optic tract (left visual field = right tract) > LGN (thalamus) > optic radiation > V1
Within eye:
light passes through
cornea
, enters the eye through the
pupil
, iris controls how much light enters by changing shape, light rays then pass through the
lens
, which bends the light so it converges on the
retina
, here it hits the
photoreceptors
: cones (for colour) and rods (for light)
From optic nerve to LGN
: process for reconstructing the world (most information, 90%)
From optic nerve to Superior Colliculus
: to perform eye movement and motor responses (10%)
From SC to pulvinar
: for saccade and visual attention
Amygdala:
Achetylcholine interacts with amygdala to direct spatial attention, according to motivational factors. Non-conscious process processing of affective visual stimuli (superior colliculus and pulvinar sends information)
Areas of visual cortex
V1/Primary Visual Area: respond to visual stimuli; raw data
V2: pass on information and respond to complex shapes
V3: register angles and symmetry, combining motion and direction
V4: respond to colour, orientation, form and movement
V5: respond to movement
V6: detecting motion in periphery or visual field
V7: involved in perception of symmetry
V8: probably involved in processing of colour
Neuromodulators
Dopamine
Receptors
:
D1 family
(D1 and D5) for excitation or inhibition (bilaminar pattern, so two layers); and
D2 family
(D2, D3 and D4) normally for inhibition (primarily in infragranular layer, so one layer)
Changes in D2 receptors in FEF activity
ONLY
affects target selection, not visual cortical activity
Dopamine effects when acting via D1 receptors: 1) altering strength and reliability of excitatory synapses 2) modulatory influence can exhibit an inverted-U shaped property wherein optimal DA levels lead to peak effects on synaptic efficacy
Manipulation of D1R-mediated FEF activity increases not only saccadic target selection, but
also
1) magnitude 2) selectivity 3) reliability (variability) of V4 visual response
Important for
top-down attention
Major pathways:
Nigrostriatal
: mot function, reward-related cognition, associative learning
Tuberinfundibular
: inhibition of release of prolactin
Mescorticolimbic
Mesolimbic
: reward-related cognition, aversion-related cognition
Mesocortical
: executive functions
Most abundant in PFC, suggesting a prominent role in regulating the cognitive functions. DA changes > PFC changes > selective attention changes (indirect effect)
Dopaminergic modulation of activity in PFC is achieved by the influence of D1Rs on recurrent connections between FEF - PFC
Possible role of DA: control the extent of FEF gain modulation --> optimum DA levels would translate into larger differences between attended and unattended stimuli
Synthesized and released by midbrain nuclei, substantia nigra pars compacta and ventral tegmental area
FEF sends map signals to other areas, and dopamine can modulate this kind of activation. Dopamine can regulate what is more important and what is less important. =
Spatial representation
Higher level representation are more stable vs Bottom-up control (related to retino topic representations-
Acetylcholine
Receptors:
nAchR
: nicotinic acetylcholine receptors = ionotropic acetylcholine receptors
mAchR
: muscarinic acetylcholine receptors = metabotrophic acetylcholine receptrs
Important for
bottom-up attention
Synthesized and released by nucleus basalis of Meynert
Application of Ach augments the attentional modulation of V1 responses
Neuromodulators:
compose a class of NT that influence synaptic transmission broadly within neural circuits, often altering the postsynaptic effect of other transmitters. Main ones in attentional control = acetylcholine and dopamine
(1) They are all released primarily by neurons within specific brainstem or midbrain nuclei
(2) These neuromodulating subcortical neurons project broadly to many (sub)cortical structures, projection to posterior sensory areas (selective attention) and PFC (control of attention)
(3) Each of the neuromodulatory nuclei also receives projections from PFC (thus PFC controls neuromodulators)
What are the main differences in the DA system in reward and attention
? Ach is more in bottom-up attention (orienting) and in physical salience of stimuli, DA more in top-down attention and relevance of stimuli
There is involvement of
cholinergic and dopaminergic systems
in selective attention, but there is still
no understanding
of their roles
Pre-motor theory of attention:
planning a saccade is sufficient to facilitate visual perception at targeted locations. => a single mechanism drives both the selection of objects for perceptual processing and the preparation needed to produce an appropriate motor response. If you block the eye movement, the area of the attention you are planning to look at is still active (evidence).
The influence of selective attention on psycho- physical performance (attended and unattended stimuli): attention
increases
reponses in the magnocellular and parvocellular neurons in the LGN, and
decreases
in the thalamic reticular nucleus TNR)
The TRN inhibits the LGN
= if the TRN receives excitatory input from the LGN, still gives inhibitory input back to the LGN => TRN should decrease with attention response, since you reduce the inibitory influence od TRN on LGN and therefore, you cause an increase in LGN responses
Attention already has an impact from the very early stages of the visual/perception process. It shows that 30 about miliseconds after performing the task, selective visual attention, has an early effect.
Attended and unattended stimuli
=> selective attention increases LGN, but decreases responses in the TRN
Neural mechanisms of selective attention (Moore)
3 dichotomies of selective attention
Spatial attention vs feature based attention
Feature-based
: enhances the representation of image characteristics throughout the visual field, a mechanism that is particularly useful when searching for a specific stimulus feature. Neurons within the primate visual system identified, beginning within the middle temporal visual area
(V5)
and
V4
and continuing in later cortical areas. Paradigm = stroop task
Stroop: distinguish the word from the colour
Spatial attention
: Involves directing attention to a location in space. the modulation of visually driven activity depends critically on the relationship between a given RF stimulus and the attended location. paradigm = Posner task
overt attention vs covert attention
Overt attention
: selective processing of stimuli in the
absence
of orienting movements: e.g. looking at the eyes
the perceptual benefits of attention can be achieved at the
fovea
(small depression in the retina where visual acuity is highest) overt attention = you pay attention tothe central visual field
Covert:
selective processing of stimuli which occur in conjunction with orienting movements: e.g.
covert attention = paying attention to the peripheral visual field, you dissociate the gaze from the attention
neurons in the
ventral stream
visual area are instead actively involved in guiding oculomotor commands according to those attributes. Pre-saccadic enhancement of activity in
V4
Top-down vs bottom-up selective attention
Top down
: selective visual processing due to an
endogenously
generated signal (e.g. representation of a rule, strategy or motivational style).
selective visual attention
increases
visually driven
firing rates of neurons
encoding the attending stimulus; this modulation is present as early as the
dorsal lateral geniculate nucleus
(dLGN)
and
increases in magnitude
at subsequent stages of visual processing. You perceive better the signal because there is
less noise
. => you can predict the probability of the target appearing in one location.
Top-down: high firing rate ==> dLGN strenghtens signal, and leads to a greater difference between signal and noise, which means more selective attention
Attention reduces the variability and covariability leading to more relevant information. 1) reduced the variability of responses to repeated stimuly => increase if the information available in stimulus-driven activity by effectively diminishing the noise in neural signal. 2) reduces redundancy => reduced covariability of neuronal response to repeated stimuli in simultaneously recorded neurons- / 3) potentially enhances signal efficacy via synchrony among neurons encoding the attended stimulus. => Clearer information and stronger information (improvement in performance)
Bottom up:
physical saliency of stimus (e.g. bright or moving objects);
exogenous
. visual targets composed of features that differ from surrounding distracters are more salient and thus more easily located during visual search:
pop outs
=>
V4 neurons
Abrupt onset stimuli
also produce involuntary shift of attention towards the location of the onset. => correlating enhancements in the activity of
V1 neurons
following such abrupt onset.