NEUROBIOLOGY B8

• Saccadic eye movements

  Shift the fovea rapidly to a new visual target
• Saccadic eye movements

  • Short and rapid movements of a pre-calculated trajectory
  • Firing rate corresponds to position
• Higher-order context

  Can drive saccades
• Higher-order context

  • Varies depending on the behavioural task
  • Can tell us about material circumstances, ages, actions, clothing, positions, and time of visitor
• Transient and sustained components are established by complex circuitry in the brainstem nuclei
• Transient and sustained components

  Combined and conducted via a network of neurons
• Burst neurons

  Initiate saccades (rapid eye movements)
• Omnipause neurons

  • Inhibit the activity of burst neurons
  • When a saccade is needed, their inhibition pauses, allowing the burst neurons to activate
• Tonic neurons

  • Help maintain the gaze position
  • Provide a steady firing rate that corresponds to the angle at which the eye is held
• Integrator

  • Too much or little recurrent excitation will either explode or squash response activity
  • Just the right amount of integration will elicit a persistent change in firing rate
  • Persistency in neuronal responsiveness can be generated by a simple model of excitatory feedback
• Neuronal pathways underlying voluntary and reflex saccades

  1. Neurons in frontal eye field initiate voluntary saccades (suppress via substantia nigra), compute direction and amplitude (pathways head to SC)

  2. Superior colliculus receives input from retina (inferior colliculus from auditory)

  3. Parietal cortex (e.g. visual association) updates success of saccade (visual cues)

  4. Voluntary and memory saccades from the frontal eye field

  5. Reflex orienting saccades initiated from the superior colliculus (likely from V1)

  6. In cerebellum there is a calibration of innervation for saccade accuracy
• Superior colliculus

  • Paired with the inferior colliculi, these layered structures are a major component of the midbrain
  • Superficial layers respond to visual stimuli (inputs from retina, V1 and higher, FEF)
  • Deep layers respond to other modalities
  • Even deeper layers have motor neurons eliciting eye movements
  • Layers have topographic maps (e.g. retinotopic in superficial)
  • Activation of neurons tend to generate behavioural responses in the corresponding body space (eye movements, arm reaching and shifts in attention)
  • For saccades, neurons activated to the mapped 'location' of where the saccade ends
  • This 'place code' translated into the oculomotor rate code
• Blindsight

  • Patients with lesions in specific areas of the visual cortex can still navigate around obstacles and guess about locations and orientations of stimuli in the blind region
  • After damage to V1, the LGN experiences significant degeneration however remains functional to a degree
  • It can still direct saccades through a direct pathway to the extrastriate cortex, bypassing the damaged V1
  • The superior colliculus and pulvinar (part of thalamus) form an alternative pathway that becomes more important after V1 damage
  • This pathway is upregulated and directly connects to the extrastriate and/or posterior parietal cortex
  • This suggests a compensatory mechanism allowing the brain to process visual information despite the loss of V1
• Gain-fields

  • With each eye movement, receptive fields are displaced from their formed locations
  • The position of the eye in the orbit 'modulates the gain' of parietal visual neuron responses with retinotopic receptive fields
  • From this representation, the position of an object in head-centred coordinates can be calculated
• Efferent copies

  • Some visual neurons (in the parietal cortex) are 'remapped' to account for the upcoming saccade
  • An efference copy (or corollary discharge) of the motor command must have been sent to update the sensory information
  • Efferent copies are duplicate signals sent by the brain to predict and adjust for the outcomes of movement
• Smooth pursuit

  • Keeps the moving target on the fovea
  • Faster than saccade (starts to travel back in same direction)
  • Smooth pursuit movements are controlled eye movements that allow us to smoothly track moving objects, keeping them in sharp focus
• Vergence

  • Vergence movements are much slower than saccades
  • The fovea of each eye is aligned with targets at varying distances from the observer
  • Cooperative movements that either converge or diverge both eyes to be fixed on the target
• Overt attention

  Physical direction of the sense organ (e.g. eyes) to a stimulus
• Covert attention

  Mental 'focus' to a spatial region
• Attention

  Prioritises limited neuronal resources
• Arrow cues

  Caused an observer to shift attention to the side where the arrow pointed (valid when arrow match to side), even though the eyes do not move
• A valid cue decreases reaction times
• With fixation maintain, covert attention is focused at the different red sectors (further and further to the periphery)
• fMRI reveals greater neuronal activity when attending to the region compared to when not attending
• Spatial attention mechanisms may be as early as the LGN
• Attention enhances sensitivity and reaction times, and helps ignore distracting stimuli while maintaining fixation
• Neurons in V4 are responsive to an 'effective' stimuli, and responses to the effective stimuli are stronger when they are attended
• Primate can be trained to attend to different regions
• Feature attention

  • Slower than spatial attention (~150ms)
  • Likely to several underlying mechanisms (shape, colour, etc)
  • Attention, both voluntary and involuntary, shortens reaction time and increases sensitivity
  • Increases sensitivity permits detection at lower contrast and to ignore distracters close to an attended object
• Saccades can move an object into the receptive field of a neuron, and whether the object is task-relevant or task irrelevant will determine whether the responses are strong
• Motor Prediction

  • The brain sends an efferent copy to the sensory areas before the actual movement happens, this copy is used to predict the expected sensory feedback
• Sensory Comparision

  • When the movement occurs, the actual sensory feedback is compared to the predicted feedback from the efferent copy
• Error Correction

  • If there is a mismatch between the expected and actual sensory feedback, the brain uses this discrepancy to adjust future movement