Perception Laboratory: Visual Processing—Eye to Brain

Purpose: The purpose of this laboratory is to familiarize you with the visual pathways from the eye to the brain as well as the concept of visual receptive fields.

1. Visual Pathways from the Eyes through the Brain

Use the figure below to trace the pathways that process visual information from the eyes [Hint: Labeled A :-) ] to the visual cortex. Identify each structure on your worksheet and give a brief description of what happens at each structure.

To learn about the pathways and structures, you need to explore a few different web sites. First, go to Molavi's tutorials to work through the one on Basic Visual Pathways and then the one on Central Visual Pathways. When going through the Basic Visual Pathways tutorial, you can ignore the information on Meyer's Loop if you wish. However, place your answers to the first three lesion questions on your worksheet. (Don't peek at the answers before responding!) Next, work your way through George Mather's Visual Physiology site.

OK, now you have some further information about the primary visual cortex and secondary visual cortex. One good source for the primary visual cortex is provided by Matthew Schmolesky at the Webvision site. On your worksheets, list information you have learned about each of several different areas of the visual cortex. Try to write down information that goes beyond that found in your textbook.

2. The M, P, and K Pathways and the What and Where Pathways

As you've worked through the previous exercises, you will surely have seen reference to the M (Magno or Magnocellular) and P (Parvo or Parvocellular) pathways and possibly the K (Koniocellular) pathway. (And you will surely have read about P and M pathways in your textbook!) Just as a brief summary, and to expand a bit on the prior information, determine and then list on your worksheet the differences among the three pathways.

At some point in the processing of visual information, it seems likely that these pathways will no longer be distinct. (You may have noted Mather's comment to that effect when talking about the M and P pathways and V2.) In the early stages, however, the three pathways are quite distinct (though the K pathway is a bit difficult to find). How have researchers learned about the differing functions of the two pathways? You should be able to articulate a way to illustrate the differences in the two pathways, though you'd need to think in terms of non-human research. On your worksheet, provide a brief outline of how you would conduct research to illustrate the differences in function between the M, P, and K pathways.

You should also be able to find information about the structure and function of the What and Where pathways. Briefly distinguish between these two pathways.

4. Receptive Fields

In some of the earlier exercises, you should have noted mention of receptive fields. In general, you should think of a receptive field as a map of the portion of the retina that affects the activity of a particular neuron. Stimulation of the appropriate area of the retina can either increase or decrease the rate of firing of a neuron (from its normal rate of firing, called its maintained activity). Mapping receptive fields requires the use of single-cell recording, so the research is done with non-human organisms (particularly cats and monkeys). The basic message of all the research to date is that the closer to the eye (e.g., optic nerve, LGN) the simpler the receptive field and the further from the eye (e.g., secondary visual cortex) the more complex the receptive field. Receptive fields serve to define the various visual cells (simple cortical cell, complex cortical cell, etc.). To provide you with background on receptive fields, first look at Peter Kaiser's explanation of receptive fields. Next look at John Krantz's explanation of receptive fields.

At the early stages of the visual system (precortical), neurons have circular receptive fields. An inner circle will either cause excitation or inhibition of the neuron. That inner circular area will have an antagonistic surround, which is a surround that causes the opposite action in the neuron. Thus, these neurons are said to have receptive fields that are either on-center, off-surround or off-center, on-surround. As you will see later, when we discuss computational approaches to vision, these receptive fields are extraordinarily useful in the detection of edges!

In the visual cortex, neurons have more complex receptive fields. As you worked through Molavi's tutorial on Central Visual Pathways (F. Receptive Fields in V1), you should have noted her explanation of how input from neurons in the LGN can be combined to produce the more complex receptive fields found in the visual cortex.