Visual Pathways in the Brain

• Perception of the external world relies on internal sensory representation and cumulative information gathered through senses and inherited knowledge.
• Sensory systems of living organisms are adapted to provide adaptive behavior in their ecological niches at a minimal cost due to natural selection.
• Each sensory channel is adapted to encode a constrained range of information from the environment.

Visual Pathways

• Ganglion cells from the retina project to the lateral geniculate nucleus (LGN) in the thalamus and the superior colliculus (SC) in the brain stem.
• Objects on one side are processed by the opposite brain hemisphere
• The superior colliculus (SC) triggers eye and head movements towards interesting visual objects, known as the "visual grasp reflex".
• The LGN projects to the primary visual cortex (V1 or striate cortex) via optic radiation.
• V1 sends projections to V2 via short fiber connections, and ultimately to over 30 distinct visual areas.
• Long fiber connections in the corpus callosum (CC) help bind the two sides of objects together.

Lateral Geniculate Nucleus (LGN)

• Composed of 6 layers, with 3 layers receiving information from the ipsilateral eye retina and 3 from the contralateral eye.
• Ganglion cells from the fovea project to parvocellular (P) layers, which provide fine details for object recognition.
• Peripheral retina ganglion cells project to magnocellular (M) layers, providing coarse information about location.
• M ganglion cells project to the superior colliculus, causing eye and head to turn towards interesting visual objects, which in turn places the object's image on the fovea which activates P cells, providing detailed information.
• Signals from each eye remain separate in the LGN and until the primary visual cortex.

Receptive Fields (RFs) in LGN

• Receptive field is the area of the retina where light stimuli changes the activity of a particular cell.

• Relay cells in LGN have two types of RFs:

  • ON center, OFF surround : measure relative brightness
  • OFF center, ON surround : measure relative darkness

• There are 4 types of "coloring" of the RFs, due to cells receiving signals from different types of cones.

• P and M relay cells produce different responses when light is turned on/off: tonic or phasic, good for detecting light changes.

• Receptive fields grow larger as eccentricity increases from the fovea (0 degrees).

• Weighting function of the receptive field can be described as the Difference-of-Gaussians (DoG) or the Laplacian-of-Gaussian (LoG).

• Relay cells exaggerate the contrast at borders, sending less redundant information to the CNS.

• The absolute amount of ambient light is largely irrelevant, the important thing is the ration in contrast between objects and surfaces.

Visual Cortex (V1)

• Located at the back of the head, also called area 17, or striate cortex.

• The fovea has an expanded representation accounting for about half of V1.

• The remaining peripheral retina is represented at the anterior half; things above the line of sight are represented on the lower half of the calcarine sulcus and vice versa.

• Like every cortical area, is made up of a thin sheet of grey matter near the surface.

• Has 6 layers; the cortex is called striate because of the prominent layer IVc where massive input from LGN ends.

• Divided into six functionally distinct layers, labeled 1 through 6.

• Layer 4 receives most visual input from the LGN and is further divided into 4 layers, labelled 4A, 4B, 4Ca, and 4Cß.

• Sublamina 4Ca receives magnocellular input while layer 4Cß receives input from parvocellular pathways.

• Estimated 150-200 million neurons in each hemisphere of the adult human primary visual cortex.

• It contains concentric cells of 4-th layer, simple, complex and hypercomplex cells.

• Receptive fields of cells in layer 4C are the same as those in LGN and ganglion cells.

Simple cells

• Have elongated receptive fields and are maximally sensitive to a line or edge of a particular orientation at a particular location of the retina.

• Are created from lower-order that have a radially symmetric receptive field with On-center and Off-surround.

• Each simple cell's receptive field is highly selective to a certain range of spatial frequency.

• Simple cells can discriminate small changes in spatial frequency, phase, and stimulus orientations, exhibiting a preferred orientation (tuning).

• In quantum mechanics, the Heisenberg uncertainty principle states that certain pairs of physical properties, such as position and momentum, cannot be simultaneously known to arbitrarily high precision

Complex Cells

• A main type of cortical neurons which can be found not only in V1 but also in the secondary visual cortex (V2), and Brodmann area 19 (V3).
• Have receptive fields which are similar to those of simple cells except the line can lie over a larger area of the retina (spatial invariance) and they fire more to moving lines.
• Tuning properties:
  • More than simple cells in numbers
  • Orientation selective
  • Larger spatial extension
  • Position insensitivity (invariance within RF)
  • Motion sensitivity
  • Nonlinear response.

Hypercomplex Cells

• Receptive fields of the end-stopped complex cell are similar to complex cells, except that they are maximally activated by lines of a particular length
• The activity is less for longer lines or shorter lines.
• Are able to resemble simple or complex cells in selectivity
• Characterized by a wide variety of subtypes depending on layers or spatial extent or sensitivity to wavelengths

V1 Organization

• Groups of neurons in V1 that are sensitive to color assembled in systematic columnar arrangement of "Cytochrome Oxidase Blobs".

• V1 Cells are single opponent

• In contrast V1 cells are mainly double opponent

• V1 has a retinotopic representation, forming a map of the eye in the brain, with adjacent areas in the eye mapped to adjacent sites in the brain.

• Precise correspondence between a location in V1 and the subjective visual field

• The retinotopic map is distorted, with the fovea having a very large representation.

• Changes of the visual image geometry from retina to V1 can be described by the conformal logarithmic transform.

• Input from left and right eyes (via LGN) enters at layer 4; one finds cells driven by input from one eye or the other but not both (monocular cells).

• V1 is a collection of orientation columns and electrode penetration parallel to the cortical surface encounters neurons whose orientation preference changes continuously.

• It can be found 3 overlapping maps of distribution of cells possessing different features:

  • Color blobs
  • Orientation selectivity
  • Eye dominance columns

• The non-random maps supports the hypercolumnar organization the visual cortex

V1 is a map of the retinotopically composed of a grid (1 mm by 1mm) of hypercolumns.

• Each hypercolumn analyses information from one small region of retina and adjacent hypercolumns analyse information from adjacent areas of retina.
• The map is distorted that devoted to the fovea representation.
• Each hypercolumn composed of two eye-dominance columns.

Each hypercolumn extracts information about color, orientation of line segments, and stereopsis: - Center of each hypercolumn cube one finds a region - Extracted color cells can be found in the blob, made up of sensitive double opponent cells with circular surround receptive fields - It radiates from the blobs,like spokes from the center of a wheel, ordered into pinwheels of the same orientation, and these extracted line segment cells are form but not color sensitive - Information about the eye is combined in binocular cells, located above and below the input layer 4C, extracting steropisis cues.

• Binocular cells contribute to the sense of visual depth

Input from the two eyes first converges onto cells in V1 where about 70% are binocularly driven and activated by particular retinal disparity. Activation of different binocular cells provide sense of depth giving images their 3 dimensional appearance. - The 'far' neuron is activated when the images are displaced inward - The 'in focus' cell is activated when there is no retinal disparity - The 'close' neuron is activated when the images are displaced outward

Complexity of neural representations increase with specialization of processing: the dorsal stream and the ventral stream.