9b: Cortical pathways

CORTICAL PATHWAYS

  • Definition: Cortical pathways refer to the neural pathways in the brain responsible for processing visual information.

  • Key Components:

    • V1 is the major cortical projection site for LGN output.

    • Beyond V1, many specialised visual cortical areas exist, approximately 30 linked to various aspects of visual processing.

  • Post-V1 Processing:

    • Two Main Pathways:

    • Ventral Pathway

    • Dorsal Pathway

    • Evidence suggests a potential third cortical pathway related to visual processing.

VENTRAL AND DORSAL STREAMS

  • Ventral Stream:

    • Projects to the Inferotemporal Cortex.

    • Includes cortical area V4.

    • Dominated by parvocellular (P cell) inputs, responsible for color and fine detail.

  • Dorsal Stream:

    • Projects to the Posterior-Parietal Cortex.

    • Includes V5 (MT) area involved in motion perception.

    • Dominated by magnocellular (M cell) inputs, responsible for motion and spatial awareness.

V4

  • Originally Thought To Be: A color processing area due to the presence of numerous color-sensitive cells.

  • Cell Arrangement: Cells organized into clusters that are selective for color and orientation.

  • Performance Variation: Color and orientation tuning become more precise when animals engage in difficult discrimination tasks.

  • Subsystems: Possible existence of color and form subsystems within V4.


  • involved in processing colour and form

  • colour-sensitive cells

  • organised into clusters

    • some clusters respond to specific colours

    • others respond to specific orientations

  • v4 is adaptive, especially when doing a difficult visual task (telling apart simiilar colours or shapes)

INFEROTEMPORAL CORTEX

  • Contains multiple subdivisions with specialised functions.

  • Cell Sensitivities:

    • Respond to shape, color, or texture within their large receptive fields.

    • FFA (Fusiform Face Area): Selective for face recognition, heavily functioning under parvocellular input.

V5 (MT)

  • Functionality:

    • Most cells in this region are selective to motion.

    • Plays a significant role in the processing of motion information.

POSTERIOR-PARIETAL CORTEX

  • Function:

    • Involved in locating objects in space.

    • Directs attention to specific areas of the visual field.

  • Input to this area is mainly magnocellular dominated.

VENTRAL AND DORSAL PATHWAYS: DISTINCTION

  • What vs. Where Distinction:

    • (Ungerleider & Mishkin)

    • What: Object vision, linked to the ventral pathway (temporal cortex), P cell dominated.

    • Where: Spatial vision, motion processing, linked to the dorsal pathway (parietal cortex), M cell dominated.

    • Dorsal pathway also contributes to motion processing.

EVIDENCE FOR DISTINCTION

  • Research Types:

    • Studies examining functional distinctions in ventral and dorsal streams.

    • Focus on investigations in primates, including humans and non-human species.

  • Humans:

    • Utilization of PET & fMRI studies.

    • Comparison of spatial vs. face-matching tasks revealing activity in occipital lobe:

    • Spatial tasks activate the posterior parietal region (dorsal, where and how).

    • Face-matching engages temporal lobe (ventral, what ).

    • Additional insights through clinical studies of discrete lesions indicating functional losses.

  • Non-Human Primates:

    • Single Cell Recording Studies reveal:

    • Ventral pathway sensitivity to color and shape (FFA for faces).

    • Dorsal pathway sensitivity to motion (not to patterns).

    • Lesion Studies:

    • Dorsal lesions in (V5) affect motion perception without impacting other qualities.

    • Ventral lesions (V4) impair color and pattern perception while sparing motion.

BEHAVIORAL TASKS

  • Object Discrimination Task: Animals learn to identify one object from a pair then select the new object presented (ventral impairments will affect).

  • Landmark Task: Animals identify the food well closer to a landmark object (e.g., a tall cylinder) (dorsal impairments will affect).

    • Which pathway lesion (dorsal or ventral) would affect performance on which task?

MULTIPLE PROCESSING PATHWAYS AND STAGES

  • Key Aspects of Brain Processing:

    • Presence of parallel pathways: Multiple processing streams exist to extract various information types.

    • Multiple processing stages ensure information transformation at each pathway stage.

    • Clear implications for understanding functional localization within the brain.

TRANSFORMATION AT THE CORTICAL LEVEL

  • Transformation of information progresses through the visual processing hierarchy:

    • From light captured by cones to retinal ganglion cells, then to LGN and finally V1.

    • Transformations Include:

    • Luminance -> Dots of light (variation of light) -> Bars/Edges (spatial frequency components) -> Motion/Texture.

  • Consideration of tuning properties of V1 cells indicates further processing needs to represent typical objects of interest.

SMALL RECEPTIVE-FIELD SIZE OF V1 CELLS

  • V1 cells possess small receptive fields, approximately 2 degrees of visual angle.

  • This limitation necessitates pooling outputs from several V1 cells to process larger objects effectively.


  • each v1 cells can only see a tiny part of the visual field (2 degrees)

  • combine/pooling signals from many cells to build a full picture

NEED FOR POOLING

  • Implication of Small RFs: Objects are generally larger than the receptive fields of individual cells, thus requiring:

    • Pooling outputs from multiple V1 cells for full object processing.

POOLING IN FORM PROCESSING

  • Stages of Processing (built in layers):

    • Local edge/line extraction -> Combination to form contours/outlines -> Formation of shapes -> Development of high-level representations (e.g., face recognition).

STIMULI FOR BEHAVIOURAL STUDIES

  • Logic of Behavioral Studies:

    • Design stimuli targeting specific brain areas/functions and assess performance based on their interactions with those stimuli.

    • Tasks designed need to necessitate pooling of localized information from V1 cells.

DETECTING SHAPES

  • Task Example: Determine if the narrow side of an ellipse points left or right. This examines:

    • How the network that extracts overall shape aligns with the contour's smoothness and alignment.

    • Insights from studies on long-term ecstasy users indicate impact on network orientation tuning.

NEUTRAL MODELS FOR CONTOUR INTEGRATION

  • Cells with varying orientation alignment weight inputs differently:

    • Cells having similar orientation tuning exhibit stronger connections and hence heavier weighting.

SEPARATE MOTION PATHWAY?

  • Discussion on whether the motion processing fits as an extension of the dorsal pathway or constitutes a separate pathway.

TESTING FOR SEPARATION OF MOTION PATHWAY

  • Testing methodologies can include functional imaging or single-cell recordings.

  • Behavioral measures should be considered to demonstrate spatial localization vs. motion pathways.

    • Referencing motion aftereffect demonstrations for experimental design.

MOTION AFTEREFFECT (MAE)

  • Definition: Viewing motion in one direction leads to a static image appearing to move in the opposite direction post-viewing; also known as the waterfall illusion.

  • Understanding of MAE highlights a distinction between perceived location and motion, suggesting separate processing pathways.

MOTION CELL LOCATIONS

  • Inquiry into whether motion-sensitive cells are located in the retina or cortex of humans.

  • Experimental approaches can help identify these sites through differences in functioning between retinal and cortical cells.

INTEROCULAR TRANSFER OF MAE

  • Adaptation to motion with one eye followed by testing the MAE with the other showcases:

    • Monocular cells in the retina vs. binocular cells in the cortex; demonstrating motion units are cortical.


Interocular Transfer Explained


•  Interocular transfer means:

  ⁠◦  You adapt to motion using one eye.

  ⁠◦  Then you test the MAE using the other eye.

•  If the second eye still sees the aftereffect, it means the adaptation happened after the point where both eyes’ signals are combined.


🧠 What This Reveals


•  Retinal cells are monocular — they only respond to input from one eye.

•  Cortical cells (especially in areas like V1 and MT) are binocular — they integrate input from both eyes.

•  So if MAE transfers between eyes, it must be happening in the cortex, not the retina. 


🧩 Conclusion


•  Interocular transfer of MAE shows that motion processing units are cortical, because only cortical neurons can compare and adapt across both eyes.