Story of Psychology - Morton Hunt [300]
You can also experience the specificity of the vertical, horizontal, and oblique detector cells of your retina by staring fixedly at the center of this pattern:
FIGURE 30
A pattern that confuses the retina’s line-detector cells
The whirling and vibrating you see are probably due to the fact that when you look at the center, where rays of varied angles are close together, the eye’s continual movements cause the image on the retina to shift from one kind of angled line to another, sending a jumble of signals that confuses the cortical receptors of specialized directional sensitivity.
The line detection ability of specific neurons is also exemplified by the following two displays, in each of which one object “pops out” because its lines have a unique stimulus property for those neurons:
FIGURE 31
Line Detection: The one inconsistent figure in each set “pops out.”
The microelectrode technique enabled neurophysiologists to decipher the architecture of the visual cortex—the neurons are arranged vertically, about a hundred in a column, and in layers that run through the columns—and to measure the responses of neurons in every part of the visual cortex to a broad variety of stimuli. The result was a detailed picture of how different cells in different parts of the visual cortex distinguish among all sorts of shapes, contrasts in brightness, colors, movements, and depth cues. A neuron-to-neuron and column-to-column synaptic hook-up of immense complexity links the responses of all these cells, presenting the brain with a composite message of the coded information of what had been a retinal image.40
Where and how that assembled message is “seen” by the mind was not apparent, although it was clear, from much of the cognitive perception research, that the specialized responses of the visual cortical cells are not the final product, at least not in human beings. In simple animals the neuronal responses may be enough to produce appropriate action (either flight or attack). In human beings, the neural messages are often meaningless until they are interpreted by cognitive processes. In the case of the illusory triangle, the viewer’s mind, not cortical cells, supplies the missing parts of the figure. The same is true of many other incomplete or degraded images, where the viewer, consciously invoking higher mental processes, fills in the missing parts and sees what is not there. A case in point:
FIGURE 32
A degraded image. What is it?
At first, most people see this figure (by Irvin Rock) as a meaningless array of dark fragments. How the reversal to the white regions and to perception of the hidden word comes about is not known, but once it has been seen, the mind is almost unable to see the figure again as meaningless fragments.
Seeing Movement
The metaphor of the eye as camera implies that we see the world in snapshots, but our visual experience is one of unbroken movement. Indeed, the perception of our movement through the environment and the movement of things in the environment is one of the most important aspects of seeing. Vision without perception of movement would be almost valueless, perhaps even worse than no vision, to judge from a rare case reported in the journal Brain in 1983.
The patient was a woman who was admitted to a hospital after experiencing severe headaches, vertigo, nausea, and, worst of all, a disabling loss of the perception of movement. A brain scan and other tests showed that she had suffered damage to a part of the cerebral cortex outside the primary visual receiving area that is known to be crucial to movement perception.41 From the report:
[She had] a loss