Module 3 Notes
5.2 Pathways from the Retina to the Brain
Visual information exits the eye via the optic nerve and terminates in two main locations: the superior colliculus (tectum) and the lateral geniculate nucleus (LGN) of the thalamus (which then extends to the primary visual cortex).
Minor projections also go to other regions (e.g., retinal ganglion cells project to the suprachiasmatic nucleus of the hypothalamus to entrain bodily rhythms to daily light/dark cycles).
The two main destinations demarcate the beginning of two processing streams:
Tectopulvinar pathway (retina → superior colliculus → pulvinar → cortical motion areas, etc.)
Geniculostriate pathway (retina → LGN → primary visual cortex, i.e., striate cortex)
The geniculostriate pathway is the route associated with conscious vision, color perception, and fine-grained details.
Mapping of visual fields across the two eyes (binocular processing): information from the left visual world is transmitted via both eyes to the right LGN and right striate cortex, while information from the right visual world is transmitted via both eyes to the left LGN and left striate cortex.
Crossing at the optic chiasm: information from the nasal halves of each retina crosses to the opposite LGN, whereas information from the temporal halves travels ipsilaterally.
The geniculostriate path enables conscious awareness and detailed analysis; the tectopulvinar path enables rapid orienting and response to peripheral motion and novelty.
Color constancy and V4: human color processing involves ventral visual cortex; V4 is retinotopically organized in some regions (V4) but is not a single, uniform color center. Color processing in humans involves multiple ventral extrastriate subregions; debates about homology with monkey V4 and exact functional roles persist.
Color and motion processing in humans: color discriminations tend to activate ventral occipital regions; motion processing engages dorsal areas (MT/V5) and related regions.
Blindsight and the visual pathways illustrate how different pathways contribute to vision when the primary visual cortex is damaged (see section 5.5.3).
5.2.1 The Tectopulvinar Pathway
Purpose: rapid, automatic orientation to important visual information.
Example: a dark shape appears in peripheral vision; you orient your head and eyes toward it even before recognizing what it is (e.g., bat).
Pathway anatomy and properties:
Retina → superior colliculus (SC) in the midbrain (tectum); inputs are biased toward motion and coarse patterns; input largely from M ganglion cells (which code coarse patterns and motion).
SC is fast-acting and specialized for motion detection and novelty in the periphery.
From SC, information travels upstream to the pulvinar nucleus of the thalamus and to cortical areas that process visual motion.
SC also sends projections to motor regions controlling eye and head movements, enabling rapid orienting.
Multisensory integration in SC:
Deep-layer neurons can show multisensory convergence (auditory + visual) with synergistic responses exceeding sums of unimodal responses.
This multisensory integration is particularly adaptive for orienting to combined audiovisual events (e.g., a loud sound with a flash).
Role in attention and action:
The tectopulvinar pathway contributes to quick orienting of eyes toward peripheral stimuli, enabling rapid acquisition of central vision for further analysis via the geniculostriate pathway.
Upstream connections extend to pulvinar and cortical areas that process motion; SC also projects to motor control regions for eye/head movements.
Functional significance: fast, motion-sensitive, peripheral detection supports rapid orientation to potential events in the environment.
5.2.2 The Geniculostriate (LGN) Pathway
In primates, about of optic nerve fibers project to the geniculostriate pathway; this pathway enables conscious visual experience.
Route:
Retina → LGN (lateral geniculate nucleus) of the thalamus → primary visual cortex (V1, striate cortex) → further processing in ventral visual stream.
Geniculostriate pathway name and function:
Named for projecting from the LGN to the geniculostrate (striate) cortex; supports conscious perception, color, and fine-grained features necessary for object recognition.
Visual field mapping and crossing:
Information from each visual field projects to the contralateral LGN (and to contralateral striate cortex) due to optic chiasm crossing.
Color processing and V4:
V4 is a color-sensitive region; human color perception involves ventral extrastriate subregions and is not solely localized to a single color center.
Human neuroimaging shows color discrimination tasks activating ventral occipital regions; lesion studies link color processing to areas around V4, but the exact organization and homology with monkey V4 remain debated.
Color constancy and higher-order properties:
Research suggests color processing involves computations beyond pure color – e.g., line orientation, depth, and motion in V4 and nearby areas.
Studies indicate V4 contains clusters of color-sensitive cells intermingled with non-color-sensitive cells, indicating functional heterogeneity within V4.
Human color processing and debates:
Although V4 is implicated in color processing, color perception relies on distributed ventral pathways; simplistic one-region color-center views are inadequate.
Bottom line: Geniculostriate pathway supports conscious vision with detailed feature processing and color perception, while the tectopulvinar pathway supports rapid orienting to potentially important peripheral events.
5.5.3 Blindsight & the Visual Pathways
Cortical blindness results from extensive damage to the striate cortex (V1). Some patients show blindsight: the ability to perform rudimentary visual discriminations without conscious visual experience.
Consistent preserved functions in blindsight (common findings across patients):
Localize spots or bars of light by pointing or eye movements; distinguish line orientations with about (i.e., ~10 degrees);
Detect whether a target is moving vs stationary;
Make basic color judgments (e.g., red vs gray);
Some form knowledge of visual form (e.g., hand shaping for grasping) in the blind field.
Interpretations and mechanisms:
Two non-mutually-exclusive pathways may support blindsight:
1) Tectopulvinar pathway: intact superior colliculus with damaged geniculostriate pathway; residual vision may rely on tectopulvinar processing and subcortical-to-cortical pathways.
2) LGN-to-extrastriate pathway bypassing V1: koniocellular layers of LGN project directly to motion-sensitive extrastriate areas (e.g., MT/V5); diffusion tensor imaging shows intact LGN-to-MT pathways in blindsight patients with striate damage, enabling some residual motion processing.
Evidence and variability:
The two explanations are not mutually exclusive; blindsight is rare and each patient may have a different pattern of brain damage and residual perceptual experiences.
Residual motion sensitivity and localization can depend on preserved pathways involving the tectopulvinar system or LGN-to-MT connections.
10. Attention
10.1 Types of Attention
Attention is a selective process that allows the brain to manage limited processing capacity by prioritizing certain information for processing.
Four broad categories of attention:
Alertness and arousal: basic levels of wakefulness; necessary for extracting information and selecting responses; extremely disrupted in coma.
Vigilance (sustained attention): maintaining alertness over time; crucial for continuous tasks (e.g., listening to a lecture for a full period).
Selective attention: prioritizing task-relevant information via a filtering process; can be applied to incoming sensory information, information in working memory, or potential responses.
Divided attention: splitting attention across tasks or modalities (multitasking); involves resource limitations and potential bottlenecks.
10.3 Vigilance
Neurotransmitter systems and arousal:
Cholinergic system and the RAS: projections to the basal forebrain; acetylcholine supports sustained attention; destruction of cholinergic pathways reduces sustained attention; stimulating these pathways or increasing acetylcholine enhances sustained attention.
Noradrenergic system: supports alerting and readiness to process upcoming information; modulating norepinephrine affects cue utilization and vigilance.
Thalamus and arousal:
Midline thalamic nuclei receive noradrenergic input and are important for vigilance; thalamus is central to arousal.
Cortical involvement and right-hemisphere dominance:
Cortical regions contribute to arousal and vigilance; right hemisphere shows a predominant role in vigilance tasks, with damage to the right hemisphere causing greater performance decrements and disrupted autonomic responses to warning cues.
Overall: Vigilance relies on multiple neurotransmitter systems and subcortical and cortical structures, including thalamic and right-hemisphere networks.
10.4 Selective Attention
Concept: selection or prioritization of information for processing, occurring across time and multiple brain regions; not a single filter but a series of filters acting across time.
10.4.1 The Time Course of Attentional Selection
Attentional selection can occur early or late in processing; ERP/MEG studies show automatic gating very soon after stimulus (early, modality-nonspecific) and effects of voluntary attention a bit later.
Early sensory gating: P50 component occurs at approximately after stimulus presentation.
Attentional effects in ERP emerge around after stimulus; later components reflect more task-related processing.
N2pc: ~; reflects focusing attention on potential target items in a display to prioritize processing over distractors.
P300: occurs at ≥; indexed to task relevance and updating of working memory.
Attention effects are not restricted to a single region or a single time window; selection engages multiple regions across time.
10.4.2 Brain Regions Mediating Selective Attention
Superior Colliculus: automatic orienting and eye movements; saccades; two types:
Express saccades (~) are reflexive and linked to SC bursts.
Regular saccades (~) can be driven by salient stimuli or be voluntary; frontal eye fields (FEF) contribute to voluntary movement; damage to FEF impairs voluntary programming of eye movements without external stimuli.
Thalamus: Lateral Geniculate Nucleus (LGN) and pulvinar in attention gating.
LGN acts as a gatekeeper to the cortex: attention modulates contralateral LGN activity when attending to a visual field.
Pulvinar filters distractions; damage impairs target detection in the presence of distractors; pulvinar activity correlates with the degree of filtering required.
Pulvinar also coordinates information transmission between cortical regions and can synchronize activity (e.g., 8–15 Hz alpha band) across regions involved in attention.
Parietal Lobe: key roles in both top-down and bottom-up attention; right hemisphere especially important; right parietal damage often leads to hemineglect.
Superior Parietal Lobe (SPL): top-down control of where attention will be directed and how it will be shifted; attention can act as a spatial spotlight; expanded spotlight increases SPL activity.
Inferior Parietal Lobe (IPL): bottom-up aspects; reorienting attention to salient, unattended stimuli; TPJ involvement; linked to hit rate of detecting unattended targets and to awareness of stimuli.
Intraparietal Sulcus (IPS): integrates top-down and bottom-up information to form a priority map guiding attention and action (e.g., eye movements).
Salience/Spotlight Integration: intraparietal sulcus computes salience maps that weigh top-down goals against bottom-up stimulus features to prioritize locations.
Anterior Cingulate Cortex and Supplementary Motor Area (SMA): response-related selection; motor planning and selection when decisions are difficult or conflict-heavy (e.g., Stroop tasks).
Frontal regions and posterior sites: prefrontal control (sources) bias posterior sensory regions (sites) via top-down signals; fMRI shows prefrontal activity (e.g., FEF) increases before stimulus on attentional cue; posterior regions (V4) show stronger responses after stimulus presentation when attended.
Space-based versus object-based attention:
Space-based attention modulates contralateral extrastriate activity; ERP P1 component enhanced for attended visual fields.
Object-based attention engages ventral stream regions when attending to object attributes (e.g., color, form) or object identity.
Mechanisms of feature-based attention and object-based attention show similarities in timing (color-based attention can be as early as ~) but can vary by feature and task.
10.4.3 Sources & Sites of Attentional Control
Distinguishing between sources (frontal/parietal areas that bias processing) and sites (posterior sensory regions actively involved in selection at the moment).
Classic demonstration: cueing studies show frontal and superior parietal regions as sources that bias subsequent processing, while posterior regions (e.g., V4) act as sites that show strong modulation after cue and target onset (Fig. 10.8).
Prefrontal regions drive top-down control; posterior regions carry out the actual selection and processing when the target appears.
Time-course data indicate a sequence: cue → sources activate to bias processing → sites engage more strongly during target processing.
10.4.4 Biased Competition
Core idea: attention biases ongoing neural activity to favor task-relevant information; competition among stimuli within a receptive field is resolved in favor of attended items.
Mechanisms observed:
Enhancing processing of attended items (biased competition) and suppressing distracting items;
Spatial attention reduces the influence of non-attended items within a receptive field (as shown in single-neuron recordings and human imaging).
Alpha versus gamma rhythms:
Gamma-band synchronization (≈40–60 Hz) between frontal control regions and posterior sensory areas supports enhanced processing of attended stimuli; the frontal regions often lead the synchronization with the phase of oscillations preceding posterior regions (~8–13 ms lead of FEF over V4).
Alpha-band synchronization (≈8–12 Hz) between prefrontal/parietal regions and early visual regions is thought to support inhibitory processing and cortical excitability modulation to suppress unattended information.
Computational models: winner-takes-all networks illustrate how attention boosts the attended item's representation and suppresses competitors; Stroop task models show how prefrontal biasing can alter processing tendencies (e.g., ink color vs word reading) to produce expected performance patterns.
Interactions across brain regions: biased competition can propagate across networks, with attentional selection in one region enhancing representation in others and feeding back to lower levels to fine-tune feature sensitivity (e.g., faces vs houses studies showing dorsal/ventral network coupling).
10.5 Neural Bases of Divided Attention
Divided attention refers to multitasking, either across same-modality tasks or across different modalities.
The neural basis is debated and not fully agreed upon.
Some evidence suggests increased activity in dorsolateral prefrontal regions when attention is divided across tasks in different modalities; rTMS over dorsolateral PFC can disrupt divided attention between auditory and visual tasks, indicating a possible critical role for these regions.
Some findings show training-related improvements in multitasking are accompanied by changes in lateral prefrontal activation, not in sensory regions.
Competing theories include a central bottleneck in the lateral prefrontal cortex (or other networks) that limits simultaneous processing, versus a more distributed view where overlapping networks must be reorganized to reduce interference.
Overall, the degree to which two tasks can be performed concurrently depends on the overlap of the brain networks they recruit; tasks sharing more same-region processing tend to interfere more.
Real-world implications include driving while conversing; dual-task performance often relies on executive control load and recruitment of lateral PFC networks; highly skilled “supertaskers” show less prefrontal activation during multitasking, suggesting different network strategies.
10.6 Attentional Control
10.6.1 Brain Regions Involved in Attentional Control
Proposes a diffuse, overlapping network for directed attention with specialized but not exclusive roles for:
Reticular activating system (RAS): vigilance/arousal
Cingulate cortex: motivational significance and value attribution
Posterior parietal region: sensory mapping for directing attention to locations or objects
Frontal regions: motor programs for exploration, scanning, reaching, and fixation
The network requires three representations of extrapersonal space: sensory map (posterior parietal), spatial-distribution schema (frontal), and motivational map (cingulate).
10.6.2 Executive Attention
Three attentional networks:
Alerting network (green): maintaining high sensitivity to incoming stimuli; linked to norepinephrine; involves locus coeruleus, thalamic regions, and right-hemisphere arousal network.
Orienting network (purple): aligning attention to the source of a sensory signal; involves superior colliculus, parietal areas, frontal eye fields; cholinergic system involvement.
Executive attention network (red): directing attention according to goals/desires and resolving conflicts; involves basal ganglia, lateral ventral prefrontal regions, anterior cingulate; dopaminergic involvement.
Lesion studies show selective deficits in different components (alerting, orienting, executive), but there is considerable interaction among networks, suggesting overlapping and integrated systems.
10.6.3 Top-down vs Bottom-up Processes
Proposes dorsal (top-down) and ventral (bottom-up) attention networks:
Dorsal attention network: includes intraparietal cortex, superior frontal cortex, and frontal eye fields; supports goal-directed selection.
Ventral attention network: includes temporoparietal cortex, inferior frontal cortex, and anterior insula; right-lateralized; specialized for detection of behaviorally relevant stimuli; acts as a circuit breaker to reset the dorsal system when new salient information appears.
Locus coeruleus may drive the ventral system, implicating noradrenaline in resetting attention.
The ventral system helps prevent tunnel vision by redirecting attention to salient events not aligned with current goals.
The dorsal system can suppress the ventral system when goal-directed focus should be maintained.
10.6.3 Salience Network
Some researchers refer to the ventral system as the salience network or cingulo-opercular network, highlighting its role in detecting salient stimuli and maintaining task-set across time.
10.7 Hemineglect
10.7.1 Clinical Features
Hemineglect (unilateral neglect) is a disruption of attention where individuals ignore information on the contralesional side of space (usually left space after right-hemisphere damage).
Manifestations can be spatial neglect (ignoring left side of space), allocentric neglect (left side of objects), or personal neglect (left side of the body).
Line bisection and cancellation tasks are classic measures; neglect is typically severe early after injury and may dissipate over weeks to months but can re-emerge with competing information (double simultaneous stimulation, hemi-extinction).
Right-hemisphere lesions produce more severe neglect than left-hemisphere lesions due to right-hemisphere dominance for overall attention/arousal and broad attentional resources.
Damage often involves a right-hemisphere network including the inferior parietal lobe, temporoparietal junction, posterior temporal regions, and ventrolateral prefrontal cortex; white matter connections (superior longitudinal fasciculus, arcuate fasciculus, inferior occipitofrontal fasciculus) are critical.
10.7.2 Underlying Deficit Theories
Representational neglect: patients may fail to represent the neglected side in mental imagery, suggesting an internal cognitive representation deficit.
Interhemispheric competition: hemispheres compete for control of attention; after a lesion, the intact hemisphere dominates, pulling attention toward the ipsilesional side; gradient differences across hemispheres can explain asymmetries.
Evidence: cueing paradigms showing disengagement deficits, double-stimulation tasks, TMS studies reversing neglect by altering interhemispheric balance.
10.7.3 Treatments
Goals: rebalance activity across hemispheres and/or improve top-down/bottom-up attention.
Brain stimulation (TMS, tDCS, theta-burst stimulation): can reduce neglect with repeated sessions; effects may last days to weeks.
Bottom-up approaches: sensory orientation manipulations (neck proprioceptive stimulation, optokinetic stimulation) to bias attention to left space.
Prism adaptation: wearing prisms shifting visual world to the right; after removal, patients point more toward midline, implying leftward reorientation learning.
Visual scanning training: therapist-assisted strategies to prompt leftward scanning; verbal cues and motivational strategies used.
Overall: multiple approaches with varying efficacy; treatment often requires repeated sessions and individualized plans.
10.8 Hemineglect
10.8.1 Object-based Attention
Object-based neglect: neglect can occur for the left half of objects regardless of object position in space; object-based neglect often involves temporo-occipital regions; spatial neglect involves dorsal parietal networks.
Tests show dissociations: some patients show space-based neglect, others object-based neglect, and some show both depending on task demands;
These findings support the idea that attention can be directed to objects, not only space.
10.8.2 Hemispheric Differences in Attentional Control
Right hemisphere contributes to overall attention/arousal and is particularly involved in sustained attention. Left hemisphere tends to have a stronger bias for the rightward portion of space.
Studies show that right-hemisphere damage leads to more severe, widespread neglect and greater loss of global attentional resources than left-hemisphere damage.
TMS studies show that manipulating activity in one hemisphere can bias attention to contralateral space, supporting interhemispheric competition and differential hemispheric roles.
10.8.3 Processing of Unattended Stimuli
Unattended stimuli can still influence performance (priming) even when not consciously perceived.
In neglect patients, left-field information can prime processing in the right field, indicating residual processing without conscious awareness.
fMRI/ERP studies show unattended stimuli can activate lower-level visual areas but fail to engage higher-order or conscious processing when neglect is present.
Early processing can occur (e.g., up to ≈ in some cases) without conscious perception, whereas later components (e.g., visual awareness negativity around ≈ ) differentiate conscious perception.
These findings underscore that attention modulates processing: attended information is processed more fully, while unattended information is processed to a lesser degree, potentially unconsciously.