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Lecture Notes - 6/29/99 and 7/1/99
Alex Huk
Visual Pathways

Visual Pathways: Lecture Notes

0. Introduction
1. Photoreceptor Signals
2. Retinal Ganglion Cells

The retinal ganglion cells represent the output of the retina. They exhibit several important properties that are characteristic of many visual neurons. Begin by reviewing the basic anatomy of a neuron: cell body ("soma"), dendrites (which receive inputs from other cells), and the axon (which sends outputs to other cells).

  • Action Potentials. As opposed to graded responses, ganglion cells fire action potentials. These electrical "spikes" are the basic way that information is transmitted in the brain, and so we'll spend some time understanding how they work.

    • Retinal ganglion cells have a negative resting potential (~ -70 mV). At this resting potential, there is a tension between the concentrations and charges of sodium and potassium ions inside out outside the neuron. Sodium ions would like to enter the neuron, while potassium ions would like to leave it.

    • When the ganglion cell receives a supra-threshold level of input from the bipolar cells, voltage-gated sodium channels suddenly open, and Na+ ions rush in.

    • This causes a sudden reversal of the charge (from negative to positive), and this propogates down the body (axon) of the neuron. [diagram of an action potential].

    • Then, the sodium channels close, and voltage-gated potassium channels open, which restores the cell to its (negative) resting potential.

    • Key points about action potentials:
      • Spikes are all-or-none, discrete, stereotyped events.
      • A certain threshold-level input must be achieved in order to produce a spike. Below that, no spikes.
      • Refractory period: after a neuron has fired an action potential, it cannot fire another until some time has passed.
      • The firing rate of the neuron (e.g., spikes per second) is what represents information about stimulus intensity within each neuron.

  • Receptive Fields. Retinal ganglion cells fire action potentials in response to certain types of retinal stimulation. The part of the retina that needs to be stimulated in order to elicit a spike is the retinal ganglion cell's receptive field.

    • The receptive field of a neuron can be defined, more generally, as:
      • The part of the visual world that the neuron is responsive to (that it "sees").
      • What the visual stimulus needs to be in order to elicit spikes.

    • Most retinal ganglion cells have concentric (or center-surround) receptive fields. [Retinal ganglion cell receptive field].

    • These receptive fields are divided into 2 parts (center/surround), one of which is excitatory ("ON"), the other inhibitory ("OFF"). For an ON/OFF center/surround cell, a spot of light shown on the inside (center) of the receptive field will elicit spikes, while light falling on the outside ring (surround) will suppress firing below the baseline rate. Opposite results for an OFF/ON cell.

    • In class, we will watch a film of the original researchers who first characterized receptive fields (Hubel and Weisel) defining the receptive field of retinal ganglion cells.

    • A perceptual consequence of these receptive field shapes can be seen in a Hermann grid. When you stare at a set of dark squares separated by a grid of white lines, you will see darkness at the intersections of the white lines.

      • Referring to the 2 following figures, note that at the intersections, there is relatively more light (white) falling on the inhibitory surround of ON/OFF receptive fields (compare to the amount of white falling on the inhibitory surround of receptive fields that are not centered over intersections. This relatively-larger amount of inhibition suppresses the firing of cells with receptive fields that overlie the intersections, decreasing their firing rates, and yielding the perception of darkness. [response at an intersection] [response off an intersection]

      • However, note that the darkness disappears at an intersection that you stare at directly. Why do you think this happens?

  • Lateral Inhibition. Visual neurons do not simply "pipe" the output of the retina through the visual pathways. Instad, the activity of a given neuron is affected by the activity of nearby neurons. Lateral inhibition in the retinal ganglion cells is a prime example.

    • When a retinal ganglion cell fires action potentials, it also inhibits the firing of nearby (lateral) ganglion cells.

    • Lateral inhibition performs edge (contrast) enhancement.

    • A perceptual example of this edge enhancement: Mach bands. The borders between light and dark parts of the image are exaggerated and appear as extra-light and extra-dark bars. Note that your perceptual experience can be predicted by the responses of the retinal ganglion cells.

Interlude. The Representation of the Visual Field in the Brain.

3. Parallel Pathways and the LGN
4. Gross Anatomy of the Cortex

Before we discuss visual cortex in detail, let's stop and get oriented in the brain as a whole.

  • As you know, there are left and right hemispheres of the brain. They are connected by a tract of fibers known as the corpus callosum.

  • In each hemisphere, there are 4 "lobes": frontal, temporal, parietal, occipital.

  • Visual information is processed primarily in the occipital lobes, but parallel pathways extend into the temporal and parietal lobes as information-processing becomes increasingly specialized.

Interlude. Spatial Organization in the Brain

The spatial organization of the brain often provides hints about what the brain does to transform sensory input to useful information for the guidance of action and thought. Spatial organization can be seen at many different levels:

  • Functional specialization: different types of information are processed in different parts of the brain (with varying degrees of separation).

  • Columnar architecture: within a brain area, neurons with similar (or complimentary) sensitivities lie close together, often in "columns" or "pinwheels".

  • Topography/Retinotopy: a "map" of the visual world (or, a map of the retina) is preserved in many visual brain areas. E.g., adjacent points of the visual world/retinal image are mapped onto (or processed by) adjacent neurons. Just as there is a "retinal image", there is a "neural image" in each visual area. People who study the visual system often use the existence of multiple retinotopic maps to localize different brain areas. A technique for localizing visual areas in humans using retinotopy measurements was developed here at Stanford.

5. Primary Visual Cortex: V1
Interlude. Defining and Separating Different Brain Areas

Brain areas can be differentiated according to 4 main criteria:

  • Function: physiology. Neurons in different parts of the brain are responsive to different aspects of the stimulus (= do different things).

  • Architecture: microanatomy can differ widely across brain areas. For example, V1 is also referred to as "striate cortex" because it has a series of stripes that run parallel to the surface; these stripes end abruptly at the end of V1.

  • Connections: different areas feed forward and also receive backward-reaching connections from distinct areas.

  • Topography: e.g., retinotopy. Each distinct visual area has its own retinotopic map.

Remember 'FACT' as a mnemonic.
6. Secondary Visual Areas

There are approximately 30 visual areas after V1. The functional specialization hypothesis drives much of the research about these areas. Some areas seem specialized for processing a certain aspect of visual information. E.g., MT - motion, V4 - color (?).

  • Cortical areas dedicated to vision are densely interconnected, and can seem quite confusing at first glance.

  • However, a more general organization is evident in a pair of parallel pathways.

    • What pathway. Temporal lobe; recognition of objects.

    • Where pathway. Parietal lobe; motion, spatial orientation, localization.

Interlude. Methods for Measuring Brain Activity
  • Methods of historical interest. Blood flow is selectively increased in parts of the brain that are active. Mosso (late 1800s) measured "brain pulsations" in infants, and found larger pulsations during periods of mental activity.

  • Electrophysiology. Can be done at the single-cell level in research animals. Invasive technique using an electrode implanted into cortex to measure neuronal spikes.

  • Optical imaging. Measure of blood flow. By opening the animal's skull, pointing a video camera at the exposed cortex, and collecting images that reflect the amount of oxygenated and deoxygenated blood researchers can measure the activity of a section of brain.

  • PET = "positron emission tomography". Subject is injected with a mild radioactive substance and lies in a PET scanner. Because blood is directed to areas of increased activity, more radioactive substance ends up in the active areas, and the PET scanner detects this.

  • fMRI = "functional magnetic resonance imaging". Just as a regular MRI machine can be used to image soft tissue in your knee, it can be fine-tuned to detect relative changes in the concentration of oxygenated and deoxygenated blood in the brain. Therefore, we can detect areas of increased neural activity. This technique is non-invasive (no surgery, no radiation), so it is the current technique of choice for measuring brain activity in humans.

  • EEG = "electroencephalograph". Electrodes placed on the scalp detect small changes in electrical activity. However, it is difficult to localize the source of these electrical signals.

Alex Huk
Last modified: Tue Jun 29 10:58:53 PDT