Part 2: Consciousness, Sensation and Perception

Posted under Note Psychology

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Brain States and Consciousness

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Consciousness can be roughly defined by the awareness of ourselves and our environment. For a more specific definition and explanation, we shall resort to dual processing: the two-track mind.

The dual here refers to conscious and unconscious. The former is the mental processes that we can aware of, and the latter is also mental processes yet automatically piloted. Everything regarding consciousness, including the perception, memory, language, etc., works in these two tracks: A conscious deliberate high road and an unconscious, automatic low road. There are three important facts.

First, in our daily lives, the low road makes up a much higher proportion than the high road does. Even when someone is carrying out intense mental work, his/her brain activity may have increased no more than \(5\%\) compared to the baseline rate. Therefore, consciousness can be seen as the surface of our information-processing iceberg. The duality echoes the facts we learned about the nervous system, where most of the primitive survival functions (like breathing and heartbeat, etc.) are working without the computation power of the cerebral cortex.

Second, the consciousness may not arrive on time for the decision-making event. An unconscious mental process has to cross a threshold to become conscious, and it takes time to do so. As a result, the felt intention of move seems to be an effect of a mysterious cause, which leads to philosophical questions -- Is the brain ahead of the mind? Does the free mind exist?

The questions above can be explained by the duality. In phase one, a newly emerged mental process will remain unconscious and unfelt to the subject. The intensity of it keeps growing until reaching phase two, crossing the threshold, and being felt by the subject. The readiness potential read by certain equipment in phase one is not a showcase of god power, but an intrinsic nature of the growing-till-felt two-track procedure.

Third, the attention and inattention are selective. The selectivity is protection to our finite information-processing ability. Without it, our minds will be overflowed by the vast amount of stimuli.

In short, the expensive consciousness monitors new challenges, and the cheap unconsciousness takes care of routines. The former can overwrite the latter if necessary. And, our conscious mind is in one place at a time.

The two-track mind has far more implications than it was introduced in the book Psychology. In another book tilted Think: Fast and Slow, written by Daniel Kahneman extensively discusses it, termed as Two Systems.

System 1 is much like unconsciousness. It is fast, automatic, frequent, and emotional. It includes most of the primitive survival functions, such as localizing the source of a sound, determine the depth of an object, etc. It also includes some high-level functions like simple arithmetic within 10, facial recognition, and even a good chess move (for a chess master only). System 2 is slow, fatiguing, infrequent, logical, and calculating. Some examples include recognizing a sound, calculating two-digit multiplication, logical reasoning, and writing an academic paper.

  • System 1 and 2 are cheap and expensive, respectively.
  • Constantly running System 2 can consume the will power.
  • Proficiency can decrease the effort of a System 2 activity, or even affiliates it with System 1.

It is invaluable for an individual to intentionally run System 2 as much as possible. Because by doing so, the ability of thinking will be strengthened, the fewer biases will be made, the fewer efforts will be required to stay in System 2, and the more activities can be automatically operated in System 1.

Sensation and Perception

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Let us first clarify the terms. There are three terms that confuse me the most when I start to study psychology, which are sensation, perception, and cognition. They are explained in my own words as below.

  • Sensation refers to the process when stimuli are received by our particular sensory receptors and then our nervous systems. (Light \(\rightarrow\) retinal \(\rightarrow\) visual cortex.)
  • Perception refers to the process our brain organizes and interprets the received information, making it meaningful (Visual cortex \(\rightarrow\) other cortex\(\rightarrow\)colors, shapes, shapes, etc.)
  • Cognition refers to everything either down or top, low-level or high-level, i.e., the mental process itself.

The sensation and perception work through bottom-up processing and top-down processing. The former enables us to detect the very basic properties of the stimuli such as lines, angles, and colors. And the latter interprets the information detected by higher-level mental processes, which is largely influenced by our experience and expectations. Several relevant conceptions are discussed below.

Transduction is a necessary process related to the sensation. It refers to the process of transforming the stimuli into neural impulses that can be used by our brain.

Thresholding is an interesting property for sensation and perception. There are two types of thresholds, the absolute thresholds, and the difference thresholds.

  • The absolute threshold is the minimum stimulus energy required to be detected in \(50\%\) of the time.
    • Stimuli below the absolute threshold is called subliminal. It can influence the short-term subsequent mental process, but has little influence on long-term decision-making. Don't even think about subliminal persuasion by advertising.
  • The difference threshold is the minimum difference between two stimuli to be detected in \(50\%\) of the time.
    • According to Weber's law, the two stimuli must differ by a constant minimum percentage, not amount. It is harder to detect the difference between two sources of loud sound.

Sensory adaptation is the property to reduce sensitivity when a subject is exposed to constant stimulation. When we are staring at a spot, our eyes actually keep moving, so that the stimulation to our eyes' receptors are ensured. If the static is forced by equipment, the vision can vanish. The point for the sensory adaption is again to save the computational power. This leads to an interesting claim:

Perceptual set is the phenomenon that our perception can be influenced by our expectations, contexts, motivation, and emotions. It acts in a top-down manner. It recalls me an old saying in Chinese: You will see shits if there are shits in your heart, so as to the Buddha.

Both the sensory adaptation and perceptual set lead to an interesting conclusion:

We perceive the world not exactly as it is, but as it is useful for us to perceive it.

Vision: Sensory and Perceptual Processing

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As a humble computer science student who studies emotion analysis, I have to say the vision processing of the brain is dramatically relevant to my domain, both technically and physiologically. On the one hand, to better understand the emotional implications, I have to get familiar with the neural pathway from the physiological perspective. On the other hand, the technique I am using -- deep learning -- is largely inspired by the physiological fact. These two make interesting conjunction on my psychology learning path.

First, let us introduce the neural pathway for the information to travel from the eye to the brain. It is transmitted as below.

light energy reaches the retina's outer layer \(\rightarrow\) then the receptor cells (rods and cones) \(\rightarrow\) light energy triggers chemical changes \(\rightarrow\) neurotransmitters emits \(\rightarrow\) neural signals sparks \(\rightarrow\) nearby bipolar cells are activated \(\rightarrow\) nearby ganglion cells are activated \(\rightarrow\) impulses are transmitted along the axons of ganglion cells (optic nerve) \(\rightarrow\) impulses reaches thalamus \(\rightarrow\) information is distributed to areas like the visual cortex in the occipital lobe.

How Do We Perceive Color

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To answer this question, one must understand that the color itself is a subjective term. A tomato is red because it * reflects light wave whose band is in a certain range, not because the tomato is intrinsically red. And, the term red also comes from our culture, which is understandable* by humans only.

There are two theories to explain color perception, i.e., the three-color theory, and the opponent-process theory. The first one claims that the retina contains three different color receptors, which are sensitive to red, green, and blue, respectively. And the color is perceived by the combination of the three. However, the three-color theory failed to explain the phenomenon that people blind to red and green can still see yellow. The opponent-process theory claims that the retina can process color in an opposite grouped manner. Specifically, red and green, yellow and blue, and white and black, these three groups constitute the system. Some neurons can be turned on by red but turned off by green, some other neurons are the opposite. So far so forth for other groups.

Based on these two theories, color processing can be hypothesized as the two stages below.

  1. Light energy is received by the receptor in the retina, following the three-color theory.
  2. Neural impulse is perceived by the neural cortex in the occipital lobe, following the opponent-color theory.

Perceptual Constancy

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Perceptual constancy is a property that we tend to perceive something as unchanging even when illumination or retinal image is changed. It includes constancies on color, brightness, shape, and size, etc. Once again, this property echoes the very conclusion that our brain reassembles the information bricks of the external world for its own good.

Do not confuse perceptual set and perceptual constancy. The former weighs more on the influence of expectation, while the latter refers to the brain's intrinsic nature of making identical perceptions for the same type of stimuli.

Feature Detection

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Our eyes own other fascinating properties that greatly inspire the development of deep learning. Here I would like to introduce feature detection and parallel processing.

Feature detection is an operation which seeks to reveal a pattern of interest from given information. In fact, our brain will first destruct the raw information received, and then reassemble it. The reassembled information is called the feature.

There are different features in terms of types and levels. The types include visual, auditory, and olfactory, etc. (Here we focus on the visuals.) The levels of visual features, in a bottom-up manner, include line, corners, edges, angles, eyes, noses, faces, and particular objects.

Recall that the visual stimuli of light energy first reach the retina, received by the receptor cells, the rods, and cones. The receptors transduct the energy to chemical neurotransmitters and initialize the neural impulses in the bipolar cells. The latter then activate the neighboring ganglion cells, whereby the impulses are transmitted along the optic nerve, a bundle of ganglion cell axons, sent to the sensory hub called the thalamus, and finally arrive at the visual cortex.

The lowest level of features is detected and encoded by the ganglion cells. The features are then processed by the visual cortex in the occipital lobe to the next level. Then, the features are sent to corresponding areas for further processing. For example, features of faces will be sent somewhere in the temporal lobe to enable your facial recognition ability.

Parallel Processing

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Our brains are parallel processing! Given a visual scene, the brain decomposes it into subdimensions, such as motion, form, depth, and color, etc., and process them simultaneously. Our perceptions are the result of a weighted sum of all the subdimensions.


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Our mind works in a two-track manner: the consciousness and unconsciousness, for the sake of cost saving. Consciousness is but a surface of the information processing iceberg. The mental process has to cross a threshold to be felt.

Our mental process has System 1 and System 2, which are cheap and expensive, respectively. Staying in System 2 brings productivity, at the cost of will power consuming. Training to focus may reduce the difficulty to some extend.

Our sensation and perception work both as bottom-up and top-down. The brain will first destruct the raw informational bricks at the bottom, and then reassemble them for its own good. During which the experiences, expectations, contexts, and emotions, etc., are supervised atop.

The visual information processing includes five steps.

  1. The scene is there to be seen.
  2. Retinal processing: The retinal receptors receive and transduct the light energy, sending the impulses to the brain.
  3. Feature detection: The detector cells respond to specific features.
  4. Parallel processing: Brain cells works as multiple studios, to combine the information in subdimensions.
  5. Recognition: The brain interprets the reconstructed image.