Lecture 7: Stimulus Localisation

Core Principles and Objectives of Stimulus Localisation

Primary Aims: The study of stimulus localisation in mammals provides a set of general principles regarding how organisms identify the position and movement of objects within their environment.
Educational Objectives:
- Identify the critical brain regions dedicated to stimulus localisation.
- Analyze the functional concept of retinotopic maps.
- Explain the physiological and behavioural mechanisms behind the orienting reflex.

Functional Examples of Object Localisation in the Visual System

Orienting Reflex: This involves the physical orientation of the head and eyes to align a salient or novel stimulus precisely on the fovea, the part of the retina responsible for high-acuity vision.
Smooth Pursuit: The neurological and muscular process of following a moving object through space with continuous eye movement.
Motion Anticipation (Prey Capture): The ability of the visual system to predict the future position of a moving object. This is critical for survival behaviors, such as a predator capturing prey.
Saccadic Movements during Object Inspection:
- When inspecting an image or object, eye movements are not recorded as random scans.
- Instead, the visual system focuses on specific, information-rich areas rather than scanning the entire image randomly.

The Orienting Reflex and the Superior Colliculus

Mechanism of Orientation: When a novel stimulus appears, animals typically turn their heads and eyes to a position that allows for detailed inspection.
Neuroanatomical Basis of the Reflex:
- The Optic Tectum (referred to as the Superior Colliculus in higher vertebrates/mammals) is the primary structure governing this reflex.
- Ablation Studies: Research shows that the ablation (removal or destruction) of the optic tectum/superior colliculus leads to the total disappearance of the orienting reflex. Cats with ablated superior colliculi, for instance, cannot orient themselves toward a stimulus.
Developmental Timeline in Humans:
- The orienting response is present in newborns and one-month-old infants.
- It disappears around $2$ to $3$ months of age.
- It reappears later, at approximately $10$ months of age.

Temporal Constraints in Visual Processing: The Necessity of Motion Anticipation

The Problem of Neural Lag: Visual perception is not instantaneous. We technically see the "recent past" due to the time required for phototransduction and neural processing.
The Phototransduction Cascade:
- A single photon is absorbed by $1$ opsin molecule.
- This triggers $800$ transducin molecules.
- These activate $800$ PDE (phosphodiesterase) enzymes.
- This results in $4800$ cGMP molecules being converted to GMP.
- Consequently, $200$ cGMP-sensitive ion channels close.
- This leads to hyperpolarisation of the cell and a decrease in glutamate release.
Processing Statistics:
- This entire visual processing sequence takes approximately $60,ms$ (roughly $1/16_{th}$ of a second).
- While $60,ms$ seems negligible, an object moving at high speed travels a significant distance during this processing lag.
Real-World Implications of Processing Delay:
- A tennis ball moving at high speed travels approximately $3.8,m$ during the $60,ms$ delay.
- Cheetah's speed: $36,m/s$ (covers $2.25,m$ in $60,ms$ ).
- Antelope's speed: $25,m/s$ (covers $1.56,m$ in $60,ms$ ).
- Peregrine Falcon (Fastest Bird): $83,m/s$ (covers $5,m$ in $60,ms$ ).
- Rabbit speed: $11,m/s$ (covers $69,cm$ in $60,ms$ ).

Anatomical Structures Involved in Localisation and Motion

Retina: Contains orientation-selective ganglion cells and performs initial motion anticipation calculations.
Cortical Dorsal Stream: Often called the "Where" pathway; responsible for processing spatial location and motion.
Superior and Inferior Colliculus: Midbrain structures involved in sensory integration and orienting responses.
Superior Colliculus (SC) Details:
- Inputs: Receives signals from retinal ganglion cells, the auditory system, and the somatosensory system.
- Multisensory Integration: Combines information from different sensory modalities to create a unified spatial representation.
- Main Function: Regulation of saccadic movements.
- Histology: Consists of several layers visible through soma staining. The SO (Stratum Opticum) is an identifiable layer. Other layers receive input from the primary somatosensory cortex ( $S1$ ) and visual cortex ( $V1$ ).

Retinotopic Mapping

Definition: Retinotopic mapping is the organization whereby neighboring cells in the retina feed information to neighboring places in their target structures, such as the Lateral Geniculate Nucleus (LGN), the Superior Colliculus (SC), and the visual cortex.
Organization: Inputs exhibit a specific temporal-nasal organization.
Retinotopic Maps in Deeper Layers:
- Deeper layers of the Superior Colliculus contain neurons that spike immediately before a saccade.
- These motor neurons are also organized in maps aligned with the retinotopic maps of the sensory layers.
- Foveation Hypothesis: States that the interaction between these aligned sensory and motor maps initiates the orienting reflex.

Case Study: Hunting Circuitry in Zebrafish

Hunting Sequence:
1. Start
2. Slow J-turn (orienting toward prey)
3. Approach swim
4. Capture swim
5. Finish
Zebrafish Circuitry Components:
- Optic Tectum: The largest part of the fish brain, homologous to the mammalian superior colliculus.
- Visual Inputs: Processed through the Retina, AF7, AF10, and Pretectum.
- Integration Areas: Periventricular layer (PVL), Intertectal neurons, and Nucleus isthmi.
- Motor Command Pathway: Signals proceed from the Reticulospinal system and nMLF to Hindbrain spinal projection neurons, which trigger muscle contraction (e.g., right contraction and left relaxation results in a turn).

Cortical Processing of Motion: The Two-Stream Hypothesis

Dorsal Stream ("Where" Pathway):
- Origin: M-Ganglion cells in the retina.
- Path: Magno LGN $\rightarrow$ $V1$ $\rightarrow$ $V2$ $\rightarrow$ $V3$ $\rightarrow$ $MT$ (Middle Temporal area) $\rightarrow$ Parietal cortex.
- Function: Specialized for movement and spatial localization.
Ventral Stream ("What" Pathway):
- Origin: P-Ganglion cells in the retina.
- Path: Parvo LGN $\rightarrow$ $V1$ $\rightarrow$ $V2$ $\rightarrow$ $V4$ $\rightarrow$ $IT$ (Inferior Temporal cortex).
- Function: Specialized for color and form (object recognition).
Clinical Evidence (Patient LM):
- Following a stroke, Patient LM suffered from difficulty perceiving motion (akinetopsia), though object recognition and color perception remained intact.
- Practical Difficulties: She could not see coffee flowing and would let it spill because the fluid level did not seem to rise. She described people as being "suddenly here or there" without seeing them move. Cars would appear far away and then suddenly be very near.

Cellular Mechanisms of Direction Selectivity (DS)

Direction Selective (DS) Cells: These are retinal ganglion cells that respond specifically to movement in certain directions and are inhibited by movement in the opposite direction.
Morphology: DS cells exhibit highly asymmetric dendritic trees. Unlike the typical round shape of standard ganglion cells, the dendritic tree of a DS cell grows in one direction, which often indicates its preferred direction.
Synaptic Input Dynamics:
- DS cells receive excitatory inputs from bipolar cells and inhibitory inputs from amacrine cells.
- Preferred Direction: Excitation is large and rapid, while inhibition is smaller and delayed.
- Null Direction: Excitation is smaller and delayed, while inhibition is significantly larger and occurs earlier.

The Flash-Lag Illusion and Retinal Prediction

The Illusion: When an object is moving through space and a flash is presented exactly aligned with its current position, the moving object is perceived as being significantly ahead of the flash ("Flash Lag").
Cause: The retina does not just record light; it predicts the location of moving objects to compensate for the biological delays ( $60,ms$ ) in visual processing.