Hubel and Wiesel

In the 60s, the flow of information about the brain continues to increase, and this organ becomes to be more and more characterized.

The complexity and quantity of data increases rapidly, but the integration, and description about how the system actually works and processes stimuli was still a difficult task.

The outside world contains a huge amount of information, of different types, with many different details. Each discrete stimuli is complex on its own.
We have the ability to clearly differentiate information in our experience, because it has different qualities.

This world of colors, smells, tastes, sounds and dynamics, the outside world stimulation, is transformed in chemical substances, electromagnetic or pressure waves by our brain, in order to be processed.

If we continue with the apparent simplification, in the end, all the information we perceive, end up as an electrical impulse or a chemical substance in our brain.

How does this code, that the brain manages, contains all the detailed and precise information that we experience, and how can the process be so quick that we have an online experience?

For an investigator, understanding how information around us is transduced and codified in a nervous impulse, is an extremely important and complicated challenge.
And, It’s even more complicated to understand how the brain, later on, decodes the information in those impulses and chemical substances and understands the stimuli that produced the process in the first place.
That is: how the brain sees color red, and perceives a sweet flavor coming from the mouth.

Edgar Adrian, Nobel Prize laureate of 1932, was one of the scientist that continued the characterization of the neural code. He studied how the optical nerve emits pulses when detecting stimuli passing by the visual field of an anaesthetized toad.

Studying the nerves in the skin that encode sensibility of touch, he realized that the intensity of the external stimuli didn’t change the intensity of the electric pulse in the nerve. This is the “all or nothing” principle. The action potential doesn’t have grades.

Fig 1. Codification of intensity

However, the intensity of the external stimuli is, in fact, encoded in a particular way. The more intense the stimuli, the more frequent the electric pulses are, and less intense stimuli produce less frequency of pulses.

Fig 2. Types of adaptation. a) very fast b) fast c) slow.

This discovery was contrasted and supported by investigations in motor fibers. The adaptation processes were also revealed. When stimuli persisted in time, the frequency of the action potentials decreased. This explained why we habituate to a certain smell after a period of time, for example.


Receptors of different modalities have different adaptation patterns, and in the case of pain, while the stimulus is present, there is no adaptation.

Haldan K. Hartline, Nobel Prize in 1957, continued this line of investigation. He studied the processing of information about contrast in the horseshoe crab. How limits and borders are processed in the brain of these animals. These crabs have different nerves for each region of the visual field represented in a specific location of the retina.

Fig 3. Organization of Ommatidia in the retina.

The nerve that received the light directly was the one that shot the greater frequency of nervous impulses. Adjacent nerves also register information, but their emitted frequency was much smaller.
This comparison between the frequencies of adjacent ommatidiums codified information about contrast. This visual system was composed of interconnected ommatidia, which were able to inhibit among them, to send useful information.

The definitive study on how the visual system works, was introduced by David Hubel and Torsten Wiesl in the 60s.

They studied the visual system of cats in a profound way, and deservedly won the Nobel prize for his work, in 1981.

In their experiments, they stimulated a cat’s retina with a dynamic beam of light and measured the activity of specific neurons on the visual area of the brain. This single unit measure was possible through the tungsten electrode, ingenious tool they created.

Neurons in the visual cortex of the cat, activated, exclusively when the line of light was located in a specific area of the visual field represented in the retina.
The activation of the populating neurons of the visual cortex was dependent on the orientation of the stimulus. They would only activate to specific orientations.
This means, that neurons in the visual cortex, not only respond to contrast, as Hartline affirmed, they are also reactive to orientations and patterns.

Let’s track the entire path:

Fig 4. Visual pathway

Retinas in each eye, divided in areas corresponding to certain regions of the visual field, receive information. They send this information through the optic nerve to the visual region of the thalamus. From this point, information is sent to the primary cortex, located in the occipital lobe.

The primary visual cortex, is populated by neurons that are specialized in different aspects of the visual stimuli. Here the system acquires great complexity trough an interconnection with other regions of the brain.

When studying the visual cortex, Hubel and Wiesl found diversity in neurons, and a concrete hierarchy. They characterized three types of cells:

  • Simple cells, specialized in the light-darkness processing.
  • Complex cells, specialized in borders and movement.
  • Hypercomplex cells that were responsive to angles of direction.

This area of the brain, with the different types of specialized cells, could make a primitive but pretty complete representation of the visual stimuli.
They also found a particular organization of the neurons working with similar aspects of the stimuli. Organization composed by modules formed by columns. Each column represented a unit of computation. At the same time, columns were connecting between them in a horizontal manner, ensuring the contrast of the information.

Fig 5. a) Columns and modules; b) orientation columns; c) receptive fields in the retina

Wide connectivity with different areas of the brain was also present, this aspect is important in the reconstruction of the image perceived, inferences about the specific stimulus, given by memories, and information of other systems.

A part from these notorious contribution, Hubel and Wiesl defined the critical range of ages in which the development of the visual cortex can be modified by experience. They explained that we learn to see in the initial period after birth.

These two scientists made a great impact in the field, not only for the obvious detailed model they stablished, but also, because we can understand from this system, that reality is perceived through a personal and particular reconstruction of the outside world, where a contrast with information from previous experience is necessary.

Our view of the world (literally in this case) is limited, and enriched at the same time, by the organ inside our skull. Reality is inaccessible to us, and we are subject to our biased perception.

At the same time, the organization and processing of the system is an impressive machine.

References

  • Koch, C. (2004). The quest for consciousness: a neurobiological approach. Englewood, Colorado: Roberts and Company Publishers.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this:
close-alt close collapse comment ellipsis expand gallery heart lock menu next pinned previous reply search share star