week 3

Visual Pathways

• Primary Visual Pathways:

  • Information travels from the retina to the optic chiasm where some fibers cross.

  • Left visual field information goes to the right visual cortex, and right visual field information goes to the left visual cortex.

Hemispheric Representation

• Each hemifield is represented in the contralateral visual cortex.

• Retinotopic Mapping:

  • Visual cortex space is mapped topographically based on retinal stimuli.

  • Neurons responding to nearby stimuli in the visual field are also positioned close together in the cortex.

  • This spatial mapping persists through the system, though receptive fields become larger, reducing acuity with distance.

Photoreceptors and Acuity

• Fovea:

  • Area with densely packed photoreceptors providing the highest acuity.

  • Close to 1:1 mapping with retinal ganglion cells in the foveola, leading to clearer images at the occipital pole.

• Cortical magnification decreases as one moves from fovea to periphery, leading to reduced visual clarity in peripheral vision.

Processing in the Visual Pathway

• Information from the eye crosses over and is processed through the thalamus and the lateral geniculate nucleus (LGN) before reaching the primary visual cortex (V1).

• Visual Cortex Hierarchy:

  • V1 to V2 and V3; V4 associated with color perception.

  • Area MT or MST specializes in motion perception.

Receptive Fields in Visual Cortex

• V1 neurons focus on low-level features (edges, contrast) with small receptive fields allowing fine acuity.

• Neurons show orientation tuning, responding preferentially to specific stimuli arrangements (e.g., oriented bars).

Visual Perception Illusions

• Brightness Contrast Illusion:

  • Example: Identical gray circles appear different in brightness due to surrounding colors.

  • Represents how context influences luminance perception.

Lightness vs. Brightness vs. Luminance

• Lightness: Subjective perception of a surface.

• Brightness: How bright a light source appears.

• Luminance: Objective measurement of light intensity.

Inverse Problem of Perception

• The brain infers illumination, reflectance, and transmittance to derive spatial understanding.

• This generates challenges for accurate perception, leading to subjective experiences influenced by context (similar to average calculations in unclear situations).

Depth Perception

• Monocular cues (occlusion, size) and binocular cues (retinal disparity) help perceive depth despite a 2D retina.

  • Stereopsis: Depth perception based on different views from both eyes.

Binocular Rivalry

• Occurs when each eye perceives different images, leading to dominance of one over the other.

• Illustrates inference-based perception, where the brain makes selections of what to prioritize.

Peripheral Vision and Color Perception

• The perceptual experience in the peripheral vision shows lower resolution and color perception compared to central vision.

• Color Perception:

  • Relies on the reflected wavelength and the configuration of cone photoreceptors in the fovea.

Role of V4 in Color Perception

• Damage to V4 results in loss of color perception despite intact retinal and primary cortical functions.

• Neuroimaging studies show activation in V4 during color perception tasks.

Functional MRI (fMRI) Techniques

• fMRI assesses brain activity via blood flow changes, allowing insights into cognitive functions.

• BOLD Response: Measures changes in blood oxygen levels linked to neural activity, albeit indirectly.

Experimental Design in fMRI Studies

• Subtraction paradigm essential to isolate brain area activation. Control conditions are important to delineate specific effects.

• Recognizing the difference between whole-brain analysis versus region-of-interest analysis can mitigate false positives in large datasets.

Ventral vs. Dorsal Pathways

• Ventral Pathway (what stream): Involved in object recognition and form processing (V4, LOC).

• Dorsal Pathway (where stream): Responsible for spatial awareness and motion perception (MT).

• Double Dissociations: Provide evidence for the independent functions of these pathways; lesions in specific areas cause distinct cognitive deficits.

Object Recognition and Agnosia

• Damage to specific areas in the temporal lobe leads to different forms of agnosia, such as prosopagnosia (inability to recognize faces).

• Clinical examples highlight the functional specialization of brain regions and their role in complex perception and recognition processes.