Module 2: Cortical Maps and Organisation; Sensory Transduction and Neural Coding; Modular Processing in Vision

60 flashcards covering direct vs indirect neural measurements, transduction mechanisms, neural coding, and modular/binding concepts in visual cortex.

What are direct measures of neuronal activity?

Direct measures of neuronal activity involve recording electrical or optical signals directly from the neurons themselves, reflecting their immediate physiological state. They offer superior temporal and spatial resolution for neural events.

Provide examples of direct measures of neuronal activity.

Examples of direct measures include intracellular microelectrode recordings (measuring membrane potential, EPSPs, IPSPs) and optical imaging with voltage-sensitive dyes (detecting membrane potential changes optically).

What are indirect measures of neuronal activity?

Indirect measures infer neuronal activity by monitoring downstream effects or metabolic consequences, rather than the electrical activity itself. They generally have lower temporal and spatial resolution but are often less invasive.

Provide examples of indirect measures of neuronal activity.

Examples of indirect measures include changes in blood flow (fMRI/BOLD), glucose metabolism (PET), electrical activity of target muscles (EMG), or autonomic responses like heart rate and skin conductance.

Compare the temporal and spatial resolution of direct vs. indirect measures of neuronal activity.

Direct measures (e.g., microelectrodes) offer very high temporal (microseconds-milliseconds) and spatial (single neuron) resolution, while indirect measures (e.g., fMRI) typically have lower temporal (seconds) and coarser spatial (millimeters-centimeters) resolution.

What defines a real-time neural measurement?

A real-time neural measurement acquires data on neuronal activity as it unfolds, providing immediate or near-immediate insights into brain function with high temporal fidelity (e.g., at millisecond scales).

Describe Intracellular Microelectrode Recording as a real-time neural measurement.

Intracellular Microelectrode Recording involves inserting a fine electrode directly into a neuron.Temporal resolution: Very high (microseconds to milliseconds).Spatial resolution: Very high (single neuron).

Describe Patch-Clamp Recording as a real-time neural measurement.

Patch-Clamp Recording creates a tight seal between the electrode and cell membrane to measure ionic currents.Temporal resolution: Extremely high (microseconds).Spatial resolution: Molecular/sub-cellular (single ion channel to small membrane patch).

Describe Voltage-Sensitive Dye (VSD) Imaging as a real-time neural measurement.

Voltage-Sensitive Dye (VSD) Imaging uses dyes that change fluorescence with membrane potential.Temporal resolution: Good (milliseconds).Spatial resolution: Good, depends on optics (tens to hundreds of micrometers, across many neurons).

Under what circumstances is microelectrode recording appropriate for studying neural coding?

Microelectrode recording is appropriate for investigating neural coding by observing precise single-unit activity in awake, behaving animals to understand how specific stimulus features are encoded in firing patterns.

When is microelectrode recording appropriate for investigating synaptic physiology?

Microelectrode recording is appropriate for studying synaptic physiology (EPSPs, IPSPs, dendritic integration), often in in vitro slice preparations, to understand biophysical mechanisms of neuronal excitability.

Name two clinical or BCI circumstances where microelectrode recording is appropriate.

Chronic microelectrode arrays (e.g., Utah array) are appropriate for Brain-Computer Interfaces (BCIs) to decode movement intentions from motor cortex activity, and for precise clinical localization of seizure foci in epilepsy.

What is a main limitation of microelectrode recording?

The primary limitation of microelectrode recording is its invasiveness, requiring surgical implantation, which generally restricts its use to animal models or specific clinical applications.

Under what circumstances is EEG recording appropriate for studying global brain states and rhythms?

EEG is appropriate for studying global brain states and rhythms (e.g., sleep stages, arousal) due to its high temporal resolution in capturing synchronized cortical activity, providing insights into the dynamics of brain networks.

When is EEG recording appropriate for investigating cognitive processes using ERPs?

EEG is appropriate for Event-Related Potentials (ERPs), where signals time-locked to events are averaged to reveal the precise timing of neural processing underlying cognitive functions (e.g., attention, language, memory).

How is EEG recording used in diagnosing neurological disorders?

EEG is appropriate for diagnosing neurological disorders like epilepsy (seizure detection) or sleep disorders by identifying abnormal brain activity patterns or characteristic brain waves.

What is a major limitation of EEG recording?

The main limitation of EEG is its poor spatial resolution, due to volume conduction and signal attenuation by the skull, making it difficult to precisely pinpoint the source of activity in deep brain structures or small cortical regions.

Describe receptor-channels (ionotropic receptors) in sensory transduction.

Receptor-channels (ionotropic receptors) are integral membrane proteins that combine receptor and ion channel functions. Stimulus binding directly opens or closes the channel, causing rapid ion flow and membrane potential change.

Provide examples of receptor-channels.

Examples of receptor-channels include mechanically-gated ion channels in touch receptors and auditory hair cells, and ligand-gated ion channels at chemical synapses.

Describe G-protein Coupled Receptors (GPCRs) in sensory transduction.

G-protein Coupled Receptors (GPCRs / metabotropic receptors) do not contain an ion channel. Stimulus binding activates an intracellular G-protein, initiating a cascade (often with second messengers) that indirectly modulates ion channels or other cellular processes.

Provide a key example of a GPCR in visual transduction and other senses.

A prominent example of a GPCR is rhodopsin (opsin) in photoreceptors, where light triggers an intracellular cascade to alter membrane potential. Other examples include receptors for many odors, tastes, and neurotransmitters.

Compare the speed and amplification mechanisms of receptor-channels vs. GPCRs.

Receptor-channels cause rapid, direct changes in membrane potential (milliseconds), while GPCRs typically lead to slower but often amplified and prolonged changes due to their indirect signaling cascades.

Describe the physiological state of a photoreceptor in the dark.

In the dark, photoreceptors are depolarized due to high cGMP keeping cGMP-gated Na^+ channels open (the 'dark current'), leading to continuous glutamate release.

What is the initial event when a photon is captured by rhodopsin in a photoreceptor?

When a photon strikes, it is absorbed by rhodopsin (or cone opsins) in the photoreceptor, causing rhodopsin to undergo a conformational change and become activated.

What is the immediate next step after rhodopsin activation in phototransduction?

Activated rhodopsin then activates its associated G-protein, transducin, an intracellular protein involved in transducing the light signal.

What is the role of transducin in the phototransduction cascade?

Activated transducin then activates the enzyme phosphodiesterase (PDE).

What is the specific role of phosphodiesterase (PDE) in phototransduction?

PDE rapidly hydrolyzes cyclic GMP (cGMP) to GMP, leading to a significant decrease in intracellular cGMP concentration.

How does the change in cGMP concentration affect ion channels in the photoreceptor during light exposure?

The decrease in cGMP causes the cGMP-gated Na^+ channels (which were open in the dark) to close, reducing the influx of Na^+ ions into the photoreceptor.

What is the final effect of light on the photoreceptor's membrane potential and neurotransmitter release?

The closing of Na^+ channels leads to the hyperpolarization of the photoreceptor membrane (it becomes more negative), which, in turn, decreases the rate of glutamate neurotransmitter release onto downstream bipolar cells.

What kind of animal models are typically used in neural coding experiments and why?

Experiments often use awake, behaving animals (e.g., rodents, primates) to allow for naturalistic processing, controlled behavioral responses, and stable, chronic neural recordings.

How is precise stimulus control ensured in an experiment designed to study neural coding?

Precise stimulus control involves presenting well-defined, repeatable stimuli (e.g., visual gratings of varying orientations, specific tactile textures via whisker deflections) whose parameters can be systematically varied.

What real-time neural recording techniques are suitable for neural coding experiments?

High temporal resolution techniques like extracellular microelectrode arrays (e.g., Utah array) are used to record action potentials from many individual neurons in real-time. Intracellular recordings offer more detailed subthreshold activity.

Why are behavioral readouts important in experiments studying neural coding?

Simultaneously recording behavioral readouts (e.g., eye movements, task choices) allows researchers to correlate neural activity directly with perception or action, distinguishing encoding from motor execution.

How is neural activity data analyzed in neural coding experiments?

Data analysis involves identifying patterns in spike trains, such as changes in firing rates (rate coding), precise spike timing (temporal coding), or correlated activity across populations, correlating them with specific stimulus parameters or behaviors.

What typically defines the receptive field and stimulus property response of a lower-level neuron?

Lower-level neurons (e.g., in V1) have simpler receptive fields, responding to basic elements like oriented line segments, specific frequencies, or points of touch.

How does information typically flow from lower-level to higher-level neurons in a hierarchical system?

Information flows from lower to higher-level neurons through convergence: multiple lower-level neurons project to and integrate their inputs onto a single higher-level neuron.

How can a higher-level neuron encode a stimulus property not represented by any single lower-level neuron?

A higher-level neuron encodes an emergent property by integrating convergent inputs from many lower-level neurons, building a more complex receptive field selective for a conjunction of features not present in any single input.

Give a specific example of an emergent property encoded by a higher-level neuron in the visual system.

A classic example is a neuron in the Inferotemporal (IT) cortex selectively responding to a specific face, or complex objects, formed by integrating simpler features (like edges, colors) processed by many V1-like neurons.

How does the concept of emergent properties relate to the 'grandmother neuron' idea?

This concept disproves the 'grandmother neuron' idea of a single neuron exclusively coding for a complex concept, favoring a more distributed representation where complex properties emerge from network integration.

What does binocular rivalry illustrate regarding emergent properties in cortical processing?

Binocular rivalry, where perception alternates between two distinct images seen by each eye, illustrates intra-cortical competition and the emergence of a single perceived representation from competing lower-level visual inputs.

How is a brain area generally defined?

A brain area is defined by its distinct anatomical location (e.g., Brodmann Area 17), unique cellular organization (cytoarchitecture, e.g., cortical layers), specific input/output connections, and specialized functional roles.

How does cytoarchitecture contribute to defining a brain area?

Brain areas are often characterized by their cytoarchitecture, notably the six distinct layers of the cerebral cortex, which have specific neuronal types and densities (e.g., Layer IV is typically the primary recipient of thalamic input).

How do connectivity patterns help define a brain area?

Brain areas are defined by their unique patterns of afferent (input) and efferent (output) projections, establishing specialized functional networks (e.g., V1 receives direct input from the LGN).

What is a topographic map in the cortex, and how does it define a brain area?

Contiguous regions of cortex that systematically map sensory space (e.g., retinotopic maps in vision, tonotopic maps in audition, somatotopic maps in touch) are called topographic maps, indicating functional areas.

What do 'topographic reversals' signify in the definition of a brain area?

Topographic reversals, where the mapping direction on the cortical surface changes, are key indicators of a boundary between distinct functional brain areas, marking the beginning of a new map.

Define hierarchical organization in the brain.

Hierarchical organization refers to information processing in successive stages, where lower-level areas process fundamental features and project to higher-level areas that integrate these inputs to encode increasingly complex, emergent properties.

What type of features do neurons in lower-level brain areas typically process?

Lower-level areas (e.g., V1) process simpler, more fundamental features like oriented edges or spatial points, forming the basic building blocks for higher-level representations.

What type of information do neurons in higher-level brain areas process?

Higher-level areas (e.g., Inferotemporal cortex) integrate information from lower-level areas to process increasingly complex, abstract, or Gestalt qualities such as object identity, facial recognition, or scene understanding.

What is the retinal origin and characteristic input of the M (Magnocellular) pathway?

M (Magnocellular) pathway originates from large retinal ganglion cells, primarily in the peripheral retina, which receive convergent input from many photoreceptors.

What type of visual information does the M pathway primarily process?

The M pathway is specialized for processing motion, flicker, low-contrast stimuli, and gross spatial features. It detects rapid changes but is poor at fine detail or color.

Describe the receptive field, temporal response, acuity, and color sensitivity of the M pathway.

M pathway receptive fields are large; they show fast, transient responses; have low spatial acuity; and are insensitive to color. They project to LGN layers 1 and 2, then to V1's layer 4C{\alpha}.

What is the retinal origin and characteristic input of the P (Parvocellular) pathway?

P (Parvocellular) pathway originates from small retinal ganglion cells, primarily from the central retina (fovea), where there's often a 1:1 cone-to-ganglion cell ratio for high acuity.

What type of visual information does the P pathway primarily process?

The P pathway is specialized for processing fine spatial detail (high visual acuity), form, and color. It is crucial for object recognition and detailed discrimination.

Describe the receptive field, temporal response, acuity, and color sensitivity of the P pathway.

P pathway receptive fields are small; they show slow, sustained responses; have high spatial acuity; and are color-sensitive. They project to LGN layers 3-6, then to V1's layer 4C{\beta}.

How is the segregation of M and P pathways maintained from the retina to the cortex?

The M and P pathways maintain their segregation as they project through distinct LGN layers and then to different sublayers of V1 (4C{\alpha} for M, 4C{\beta} for P), ensuring parallel processing streams.

What is the primary function and general cortical location of the Dorsal Stream?

The Dorsal Stream (or 'Where'/ 'How' pathway) extends from V1 through the parietal lobe and is responsible for processing spatial location, motion, and guiding visually-guided actions.

Provide an example of an area in the Dorsal Stream and specify its function.

Area MT (Middle Temporal Area / V5) is a key region in the dorsal stream. It is crucial for motion discrimination, with neurons highly selective for direction and speed of moving visual stimuli.

What is the primary function and general cortical location of the Ventral Stream?

The Ventral Stream (or 'What' pathway) extends from V1 into the inferior temporal lobe and is primarily involved in processing object identity, form recognition, and visual memory.

Provide an example area in the Ventral Stream and its specific function.

The Fusiform Face Area (FFA), located in the fusiform gyrus of the inferior temporal lobe, is a highly specialized region in humans for face recognition.

Give another example of an area in the Ventral Stream and its specific function.

The Parahippocampal Place Area (PPA), also in the ventral stream, is specialized for recognizing scenes and places.

What is a retinotopic map in V1?

V1 contains a retinotopic map, a topographic map where neighboring points in the visual field are represented by neighboring neurons in the cortex, systematically unfolding visual space onto the cortical surface.

How do receptive field sizes vary in V1's retinotopic map, and what is its significance?

Receptive field sizes in V1's retinotopic map are smaller for cortical representations of the fovea (central vision), enabling high acuity, and progressively larger for peripheral representations.

Describe ocular dominance columns in V1.

Ocular dominance columns are alternating, stripe-like regions in V1 (approx. 0.5 mm wide) where neurons primarily receive input from either the left or the right eye, segregating binocular input for depth perception.

Describe orientation columns in V1.

Orientation columns are vertical arrangements of neurons within V1 that are tuned to prefer a specific orientation of an edge or bar. As one moves horizontally across the cortex, the preferred orientation shifts systematically.

What are 'blobs' in V1 and what information do they primarily process?

'Blobs' in V1 are clusters of neurons (visible through cytochrome oxidase staining) that are particularly sensitive to color and generally lack orientation selectivity. They receive primary input from the P pathway.

What are 'inter-blob' zones in V1 and what information do they process?

'Inter-blob' zones are regions between the blobs in V1 that are generally not color-selective and instead primarily process information about orientation, motion, and form, receiving more input from the M pathway.

What is a cortical module in V1, in terms of overlaid maps?

A cortical module in V1 is an approximately 1mm^2 cube of cortex that contains a full set of ocular dominance columns, orientation columns, and blobs/inter-blobs, allowing for local, comprehensive processing of visual features.

Define activity-dependent plasticity in the context of cortical map building.

Activity-dependent plasticity is the process by which a neuron's activity, or lack thereof, shapes the strength of its synaptic connections and the organization of cortical maps, particularly during critical developmental periods.This plasticity involves the strengthening or weakening of synapses based on neural activity patterns, influencing how sensory information is processed and represented in the cortex.

How does synaptic competition contribute to the shaping of cortical maps?

Synaptic competition occurs as competing inputs vie for synaptic space and influence within cortical areas. More active or correlated inputs establish stronger connections and claim more cortical territory, while inactive inputs weaken.This process is essential for refining cortical maps, as it helps to prioritize sensory inputs that are most relevant, allowing for an efficient neural representation of the sensory environment.

What happens to ocular dominance columns if one eye is deprived of visual input during a critical period?

If one eye is deprived of patterned visual input during a critical period, the cortical territory representing that eye shrinks, while the area for the active eye expands, due to competitive synaptic weakening. This results in a shift in ocular dominance towards the active eye, illustrating the effects of activity-dependent plasticity on synaptic connections.

What is amblyopia, and what are its common causes?

Amblyopia is a developmental cortical blindness caused by abnormal visual development, often due to strabismus (misaligned eyes) or refractive errors (e.g., severe uncorrected myopia in one eye) during a critical period, leading to suppression in the brain.

How does specific sensory experience contribute to shaping cortical maps?

Specific sensory experiences can reshape cortical maps; for example, prolonged exposure to particular orientations can alter orientation maps in V1, and whisker usage patterns can refine the barrel cortex somatosensory map for texture processing.

What are critical periods in the context of cortical map development?

Critical periods are specific developmental windows during which the brain is highly susceptible to experience-dependent plasticity, meaning sensory input is crucial for the normal and precise formation of cortical maps.During these times, sensory stimulation can lead to significant and long-lasting changes in the organization of neural circuits.

What is the primary role of the Superior Colliculus in sensory processing?

The Superior Colliculus (SC) is a midbrain structure crucial for multisensory integration, containing superimposed topographic maps for visual, auditory, and somatosensory stimuli.

How are topographic maps organized for different senses within the Superior Colliculus?

The SC contains separate topographic maps for visual space, auditory space, and somatosensory space, all of which are precisely overlaid and in spatial registration with each other.

Why is spatial registration important for multisensory integration in the Superior Colliculus?

Spatial registration in the SC means that a visual stimulus, an auditory cue, and a touch sensation originating from the same physical location activate overlapping neural populations, enabling coherent spatial representation of multisensory information and enhancing the brain's ability to perceive and respond to complex environmental stimuli.

How does the Superior Colliculus contribute to orienting behaviors and attention?

The SC guides rapid orienting responses (e.g., eye and head movements, often via inputs to the Frontal Eye Fields, FEF) and directs attention towards salient multisensory stimuli, and is implicated in blindsight.

Define the 'binding problem' in neuroscience.

The 'binding problem' refers to how the brain integrates separately processed features of an object or event (e.g., color, shape, motion, sound) into a single, cohesive, unified percept.

Explain 'overlaying maps' as a proposed solution to the binding problem.

One solution involves 'overlaying maps,' where spatially aligned maps for different features or senses (e.g., in V1 for color/orientation or in the SC for multiple senses) help link related information belonging to the same object or location

Explain 'temporal synchronization' as a proposed solution to the binding problem.

Another solution is 'temporal synchronization,' where neurons encoding different features of the same object fire in synchronized patterns (e.g., 40 Hz gamma oscillations), acting as a temporal tag to bind them together into a coherent percept.

What does 'convergence of cone inputs' onto ganglion cells imply for receptive field size and visual acuity?

Synaptic convergence implies that multiple cone inputs onto ganglion cells in the peripheral retina result in larger receptive fields, leading to lower visual acuity in the periphery compared to the fovea.