lecture recording on 30 January 2025 at 14.24.07 PM

Visual Resolution and Angle

• Key Question: What is the smallest spatial detail that can be resolved?

• Example Detail: Simple grading with black and white patterns used for measurement.

• Measuring Size: Absolute size varies with distance; needed an independent measure.

• Visual Angle:

  • Defined as independent of distance.

  • Two objects (e.g., green bar and red bar) can appear the same size due to their visual angles despite actual size differences.

  • Visual angle correlates with the size perceived on the retina.

  • Example: A tree occupies a larger visual angle than a child, indicating perceived size despite physical distance.

Blindsight and Visual Pathways

• Blindsight phenomenon in patients with damage to the primary visual cortex:

  • Patients often report blindness but can still identify stimuli or point to locations with higher-than-chance accuracy.

  • Key Visual Pathways: 90% of projections to the lateral geniculate nucleus (LGN); 10% project to the superior colliculus.

• The superior colliculus is involved in eye movements and motion perception.

• Implications: Suggests cortical processing is necessary for conscious visual perception.

Primary Visual Cortex Characteristics

• Location: Occipital lobe, also referred to as Area V1 or Striate Cortex due to its striped appearance when stained.

• Retinotopic Mapping:

  • Spatial organization where neighboring points in the visual field correspond to neighboring points in the retina.

  • Preserves spatial relationships of objects in a person's visual field in cortex activity.

• Cortical Magnification:

  • Central 10 degrees of the visual field occupies about 50% of V1, showing high coding capacity for central vision compared to the periphery.

Neuronal Response in V1

• Role of Neurons:

  • Neurons sensitive to different spatial frequencies, crucial for perceiving layout and details.

  • Simple Cells: Respond to specific orientations of lines, combining input from retinal ganglion cells to detect orientation.

• Discovery: Made by researchers David Hubel and Torsten Wiesel, earning the Nobel Prize in 1981 for their work on orientation-selective neurons.

Habituation and Perception Effects

• Tilt Aftereffect:

  • When exposed to a pattern for an extended time, visual perception of orientation can temporarily shift.

  • Visual system adapts to a given orientation and can misinterpret later orientations as tilted.

  • Highlights the importance of populations of neurons over individual responses in perception.

Hypercolumns in Visual Cortex

• Discoveries by Hubel and Wiesel:

  • Neurons organized into 'hypercolumns' within V1 that respond to various orientations and input from both eyes.

  • Each hypercolumn processes information from the left and right eyes, contributing to depth perception.

Complex Cells in V1

• Structure and Function:

  • Complex cells respond to motion and can have larger receptive fields than simple cells.

  • Provide a broader range of perception in dynamic contexts (e.g., moving objects).

Monocular Cues for Depth Perception

• Occlusion: Identifies objects in front based on blocking perspective.

• Relative Size: Compares sizes to infer distance when the object is familiar.

• Familiar Size: Utilizes known sizes to gauge unknown distances.

• Relative Height: Objects higher in the visual field are perceived as farther away.

• Texture Gradient: Changes in texture density indicate depth.

• Linear Perspective: Parallel lines converge as they recede into the distance.

• Aerial Perspective: More distant objects appear hazy due to atmospheric scattering.

• Shading: Light source perception creates depth interpretation based upon shadowing.

Dynamic Cues for Depth Perception

• Motion Parallax: Observes that nearby objects move quickly across the visual field, while distant objects move slower when observers move in a given direction.

• Optic Flow: Related to moving towards a point, where objects closer to the focal point appear to flow by rapidly while faraway objects seem to recede more slowly.