Using the Psychophysicists Microelectrode for Cognitive Psychology

By Peter J. Hills, Michael Pake

Cognitive psychologists have long sought ways to study processes within the brain and the particular function of its cells. In the past, neuroscientists measured directly the neural activity of cells in the monkey brain by drilling holes into the monkey’s skull. This unethical practice can’t be run on humans, and therefore researchers must find other techniques to record the brain’s activity.

Psychophysicists developed a technique called adaptation that reveals what the brain processes. Adaptation is when the response of a particular cell (or group of cells) is reduced following stimulation and therefore it allows cognitive psychologists to identify what particular cells do. That’s why adaptation is called a microelectrode (it’s not an electrode!).

Adapting to the world

One of the clearest examples of adaptation comes in the colour after-effect. The opponent-processing theory of colour suggests that colours are processed in opposition to each other. The after-effect of adaptation to red is green. The after-effect of adaptation to blue is yellow. The after-effect of black is white.

Stare at the dot on the figure on the left for 30 seconds. Then move your eyes quickly to the white

Stare at the dot on the figure on the left for 30 seconds. Then move your eyes quickly to the white page on the right. If you look to a blank wall, the afterimage should remain and can be brought back with a blink.

Understanding the mechanisms of the colour after-effect

The explanation for the colour after-effect is that cells in the eye selectively respond to one colour. When these cells are fatigued they stop responding, and the cells that code the opposite colour are relatively more active. This behaviour creates the perception of the opposite colour. By this logic, anything you can adapt to must have a neural population located in the brain.

What else can people adapt to?

You can adapt to spatial frequencies, size, motion and tilt. (The motion after-effect is demonstrated in a YouTube video.) Cognitive psychologists call these abilities low-level ‒ they’re abilities that occur early in cognitive processing and aren’t greatly affected by knowledge and experience. To cognitive psychologists’ surprise, you can adapt to higher-level stimuli ‒ such as shapes and even faces.

Locating faces

Consider the example of adaptation below. If you stare at the slightly distorted face in part (b) for 40 seconds and then look at the undistorted face in part (a), the latter appears distorted in the opposite direction. An understanding of adaptation would suggest that cells in the human brain process the height of the eyes. However, if you do the same exercise with the face in part (c), where the eyes are at different heights, do you get an after-effect when switching to the undistorted face in part (a)? Most adults don’t. This result shows that people don’t have cells in the brain that process the height of one eye. Further results with faces show that cells in the adult brain can adapt to particular facial identities, suggesting that humans have neural representations of the identity of someone.

Stare at the left face for one minute and then look at the middle face. Try it again but stare at t

Stare at the left face for one minute and then look at the middle face. Try it again but stare at the face on the right for one minute, then look to the middle face.

Understanding adaptation

These results suggest that people’s brains have neural populations for facial arrangements that exist normally. Interestingly, cells in the child’s brain can adapt to the stimuli in image c, indicating that the neural representation of faces develops slowly. This tendency allows psychologists to measure the development of the brain without drilling holes into the skull and directly measuring the response of particular cells.