week 7 Flashcards
(27 cards)
What are the primary functions of eye movements?
They allow us to fixate on objects, track moving targets, shift the high-resolution foveal region across a scene, converge on objects at different distances, and stabilize the retinal image despite self or scene motion.
What common visual disorders are linked to abnormal eye movements?
Abnormal eye movements can cause diplopia (double vision), nystagmus (drift), and may contribute to reading impairments such as dyslexia.
How do eye movements during reading illustrate automatic, context-dependent control?
During reading, the eyes fixate on individual words—often skipping common words—demonstrating that gaze paths are both automatic and influenced by context, as originally shown by Yarbus (1967).
How are the muscles controlling the eyes organized?
The intraocular muscles adjust pupil diameter, while the extraocular muscles (innervated by specific cranial nerves) move the eyeball; the eye itself behaves like a “spring,” naturally pointing forward without active input.
What are the two major categories of eye movement mechanisms?
Gaze stabilization mechanisms (e.g., optokinetic reflex and vestibular-ocular reflex) and gaze shifting mechanisms (e.g., vergence, smooth pursuit, and saccades), along with active fixation between movements.
What is the optokinetic reflex (OKR) and how does it function?
The OKR maintains image stability by using whole-field visual motion; it minimizes retinal slip through a slow drift phase followed by a corrective fast saccade.
How does the vestibular-ocular reflex (VOR) rapidly stabilize our gaze?
The VOR detects head movements via the semicircular canals, relays signals through the vestibular nucleus, and uses a three-neuron pathway to drive ocular motor neurons for rapid eye adjustments (~14 ms response).
What are vergence movements and why are they important?
Vergence movements involve both eyes moving in opposite directions to adjust focus on objects at different distances, ensuring clear vision.
Describe the smooth pursuit system in eye movement control.
Smooth pursuit allows slow, coordinated movements of both eyes to track moving objects; it requires the suppression of the OKR and is driven by motion signals from cortical areas (MT and MST).
What characterizes saccadic eye movements?
Saccades are rapid, high-velocity movements that quickly shift fixation points (up to 600°/sec) and help minimize the period during which vision is degraded.
What role does the superior colliculus play in saccades?
It contains a retinotopic map of the visual scene, transforms sensory inputs into motor commands, and triggers saccades—often by releasing inhibition from omnipause neurons.
What are the two complementary systems for spatial processing in the brain?
The hippocampus, which forms allocentric (object-to-object) maps, and the posterior parietal cortex (PPC), which forms egocentric (self-to-object) maps.
How do place cells in the hippocampus contribute to spatial memory?
Place cells fire when an animal is in a specific location, forming a cognitive map that underlies episodic and relational memory.
What evidence supports the hippocampus’s role in navigation?
Studies such as those on London taxi drivers have shown increased right hippocampal activity and volume with extensive navigation experience.
What is hippocampal neurogenesis and its significance?
In the human hippocampus, approximately 700 new neurons are generated daily (with an annual turnover rate of 1.75%), contributing to the brain’s capacity to form new spatial and episodic memories.
What are place cells and why are they important?
Place cells are hippocampal neurons that produce “place fields” by firing at specific locations, essential for forming dynamic representations (cognitive maps) of the environment.
How do place cells respond to changes in behavioral context?
They can “remap” either by changing firing rates (rate remapping) or by altering which cells are active (global remapping) when the environmental context shifts.
What are grid cells and how do they support navigation?
Located in the entorhinal cortex, grid cells fire in a regular, grid-like pattern and combine signals at different scales to help activate unique place cells for precise spatial localization.
Is there evidence for grid-like coding in the human brain?
Yes, virtual reality experiments using intracranial recordings or fMRI have observed grid-like activity in the human entorhinal cortex.
What is the primary role of the posterior parietal cortex (PPC) in spatial processing?
The PPC processes and encodes object positions relative to the body, enabling egocentric spatial representations that are critical for action planning.
What spatial disorders are linked to parietal lobe damage?
Right parietal damage can lead to spatial neglect (ignoring one side of the visual field), while left parietal damage is often associated with apraxia, affecting coordinated movement sequences.
What specialized regions exist within the PPC and what do they encode?
Ventral VIP: Multimodal encoding of near-space and motion.
Anterior AIP: Encoding of touch and grasp.
Medial MIP: Reaching and pointing information.
Lateral LIP: Intention for eye movements.
What is meant by “prospective coding” in the hippocampus?
It refers to the ability of some place cells to represent future goal locations rather than just current positions, supporting navigational planning.
What types of experimental evidence support prospective coding in rodents?
Recordings show that during navigation, a subset of place cells tunes to future goal locations, and their spatial firing remaps based on reward locations.
