While sensation is the bottom up processing that enables us to receive information from the environment, perception is the top down processing that gives the brain the ability to organize and interpret that information. Here we are going to talk about brightness perception, color perception, motion perception, pitch perception, depth perception and last but not least form perception.

  1. Brightness perception


      To understand brightness perception you should first know the difference between the terms brightness and illumination. While illumination is the physical measurement of the amount of light brightness is the subjective impression of the intensity of the light.


         When it comes to perceiving the light different factors are involved. However there are two factors that stand out from the others and these factors are adaptation and simultaneous brightness contrast.

        Adaption can be understood from the example of theater. When you step in to the theater you cannot see correctly but when you adapt to the situation it becomes easier to see. Biologically this is related to a photochemical in the rods called rhodopsin, before entering the theater rhodopsin absorbed the light outside which causes it to decompose into retinene and opsin in a process called bleaching. After the bleaching it takes time for the pigments to regenerate and this is why it takes you a little bit to adapt to the situation.


        Simultaneous brightness contrast on the other hand refers to the fact that a certain area appears brighter when surrounded by a darker stimulus than surrounded by a lighter stimulus.

   2.Color perception


        The perception of color is related to the wavelength of the light entering the eye. And when it comes to color perception there are two basic theories, Young-Helmholtz theory and Ewald Herring’s theory.


          Young-Helmholtz theory also known as trichromatic theory is a theory that suggests that the retina contains three different types of color receptors which are called “cones”. Each type of color receptors according to this theory is sensitive to different color and these colors are red, green and blue. So when the light enters the eye and hits the retina these receptors gets stimulated on various degrees and according to this theory it is the ratio of the activity of the receptors that determines the colors we see.

           Ewald Herring’s theory known as the opponent process theory of color vision on the other hand think that next to the colors given above “red, green and blue” there is an important color that should be added which is yellow. And as the name of the theory refers these colors are arranged in opposing pairs which gives us red and green, blue and yellow and next to these pairs of colors Herring added another opposing pair which is black and white.


         The reason why Herring came up with this theory is the most important and interesting part. Herring used the phenomena of afterimages which represents the visual sensation that appears after intense exposure to a certain stimulus. For example if you stare at a red image for a few minutes and then look to a white paper or a white wall you would see that same image but it will not be red, the color you will see actually is the opponent color of red which is green. Of course the same applies to the other pairs of colors.

   3.Motion perception


          It is quite obvious that real motion makes us see that a certain stimulus is moving, but what if I tell you that you can see light moving even if it is not.

       To explain that there are four basic concepts: Apparent motion, induced motion, Autokinetic effect and motion aftereffect.


         Apparent motion or what we can also call the phi phenomenon represents the idea that when two different light dots are flashed in succession with only few seconds apart, they tend to be perceived as one dot moving. This phenomenon was actually studied by Max Wertheimer one of the pioneers of Gestalt psychology.  

           Induced motion is the illusion of light movement that occurs when the background of the point of light moves.


         Autokinetic effect is when we see a dot of light in a dark room this illusion takes place and makes us see the dot of light like if it is moving.


         Motion aftereffect happens when we see a certain object moving for a long period of time and then stop. When the moving object stops in this case you will not perceive it that way rather you will see it moving in the opposite way.


  4.Pitch perception


          If you want to learn about the anatomy of the ear and how we hear biologically then you will be more interested in the part concerned with the auditory system in sensation.

        To understand pitch perception you should understand four important concepts about the dimensions of the sound which are frequency, intensity, pitch, loudness and timber. While frequency and intensity are objective dimensions of the sound, pitch, loudness and timbre are subjective dimensions of the sound.

         It should be known that frequency is the number of cycles in second measured in Hertz and that intensity is the amplitude of the sound wave measured in decibels. Pitch, loudness and timbre on the other hand are the subjective perception of the sound. Pitch is the subjective experience of frequency, loudness is the subjective experience of the intensity and timbre refers to the quality of the sound.


        Know that we are done with that we can go to pitch perception. Pitch perception can be explained by three theories: Helmholtz’s Place-Resonance theory, Frequency theory, Bekesy’s traveling wave.

          Helmholtz’s Place-Resonance theory is a theory that concentrates on the vibration of a part of the ear called the basilar membrane. According to this theory each different pitch causes a different place on the basilar membrane to vibrate which causes different hair cells to bend and there for results on the perception of different sounds.

          Frequency theory on the other hand says that the basilar membrane vibrates as a whole and that what determines the perception of the sound is the frequency of the vibration. For instance if a tone causes the basilar membrane to vibrate 200Hz, this vibration would cause nerve fibers in the auditory system to fire at 200 impulses per second. Hence the pitch is determined by the frequencies.    

         Bekesy’s traveling wave theory states that the basilar membrane vibrates as a whole.  However each part vibrates differently depending of the frequency. According to this theory high frequencies vibrate the part of the membrane near the cochlea more than the other parts and low frequencies vibrate maximally the part near the apex.


   5.Depth perception


          Depth perception is a quite amazing part of perception. The fact that we see a three dimensional image despite the fact that we have a two dimensional image reflection on our retina is mesmerizing.

          There are different cues that make us understand depth perception. Certain characteristics of the two dimensional image on the retina signal the three dimensionality of the object seen. In these cues we can find: interposition, relative size, linear perspective, texture gradient, motion parallax and binocular disparity.

          Interposition happen when we see an object A hiding an object B. when this happens we tend to see the hidden object , in this case object B further from object A or behind it. Relative size on the other hand as the name refers has a relationship with the size of the object seen; when we see an object far away its reflection on the retina appears smaller than the real size. So the comparison between that size and the real size indicates the distance between you and the object. Liner perspective indicate the fact that parallel lines appear to converge into the distance. Another concept is texture gradient which represents the sudden changes in distance or a change in direction as a certain pattern gets further. Motion parallax refers to the idea that when an observer moves, objects in a stationary environment appear to move. And last but not lest there is the concept of binocular disparity and this represents the fact that each eye sees an image that is slightly different than the other eye. If you did not believe this just put your figure in front of you eyes and look at it but only using one eye and then the other one if you do so you will see your finger changing its position according to this cue these different scenes that come from each eye give us depth perception when combined.   


  6.Form perception

        Form perception is related to how we abstract perceptual objects. The most important concept to think about when it comes to form perception is figure and ground where the figure is the object and the ground is the back ground. One of the famous examples where you cannot differentiate between the figure and the ground is the image given bellow if you concentrate on the white part of the image you will see a vase, but if you concentrate on the black part you will see two faces. Now that you have seen them both you will see the two images popping back and forth.



        Of course we cannot talk about perception without talking about Gestalt psychology since it plaid an important role in explaining the process of perception. According to Gestalt psychology there are four laws that explain form perception.

         1.Proximity: this law indicates that elements close to one another tend to be perceived together. This should be clear in the image bellow.


         2.Similarity: this law says that elements that are similar to one another tend to be perceived as a group. You can see that in the image bellow where the black dots tend to be perceived together and the white dots tend to be perceived together.


           3.Closure: this law represents our tendency to complete incomplete figures. In the image bellow you will see your tendency to see a circle and a square despite the fact that it is not complete.


           4.Good continuation: refers to the idea that elements that appear to follow the same direction tend to be grouped together.

       We can finish this part with an important idea. What you see and the way you see it is influenced by your perceptual set or in other words the psychological factors that determines the way you see the word like expectations, prejudice, stereotypes and even the mood that you are in right know. Hence what you see does not always represent the reality but it rather represents the way you want to see the world.

See Also:



Perception and experience